# How long can afterglow last after a fireball?

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I was watching the YouTube video Perseid Meteor Shower - Mojave Desert, California which looks East before sunrise, presumably in mid-August of 2016. I've made a GIF of a few seconds near the beginning where a fireball appears at about00:03in the video, and a faint luminescent trail appears to drift and change shape. I've timed the two stars in Orion Bellatrix and Betelgeuse and the motion is about seven seconds of video to move about 30 minutes in R.A. (near declination 0), so based on that the three or four second visibility in the video works out to about 10-15 minutes.

Is this common? Am I interpreting what I'm seeing correctly? Is such luminosity visible by eye, or only using a camera with a good lens?

This is an persistent ionization trail. As the meteoroid goes through the upper atmosphere at high speed (sometimes more than 45km/s) electrons are stripped from atoms. As the electrons recombine, the gas glows.

Persistent trails are formed by brighter meteors, and can be visible to the naked eye. They may be visible for up to about 45 minutes, but that depends on the meteor. Generally brighter meteors have brighter trails.

The movement and streaking in the trail can reveal the presence of upper atmosphere winds. As radio waves bounce off the ionised trails, they can be used for long distance radio communictions.

Persistent trails were discussed by the "Bad Astronomer"

Image taken during the 1966 Leonid Meteor storm, produced by a Mag -6 meteor

## Gamma Ray Bursts and The Fireball Model

Gamma-Ray Bursts (GRBs) are some of the most energetic events in the universe. The energy that is released during a GRB is impressively high (the most powerful bursts can eject energy equal to over 9000 supernovae). These energy levels are so extreme that they cannot be created by thermal processes. So, what causes these high energy levels?

The Fireball Model is one of the few models that has been put forth to explain why GRBs tend to have such high energy levels. It also attempts to explain the time scales that govern them and why they generate an afterglow. More importantly, the model helps answer pressing questions about GRBs, like why they are so variable (liable to change) over short time scales. Ultimately, it seems that this variability is directly related to the high energy levels, as the variability indicates that it occurs over a very small area (with the emission of a GRB being on the order of 10^52 ergs, coming from a very small area, it was then theorized that a Lorentz Factor of

100 much be associated with the GRB).

The fireball model uses two different shock wave models to explain both the initial burst of gamma-rays and the extended afterglow that is detected after the GRB. To understand the fireball model, the data must be considered in its separate parts. First, there is the energy output. It can have a range of several orders of magnitude — from 10^49 all the way through to 10^54 ergs. Second, there is the burst duration, which can be as short as a few milliseconds and as long as several hours. It took many years before physicists were able to get close to determining exactly how GRBs operate, as many different theories were proposed, but they all struggled to explain all of the different characteristics that are observed between the different types of GRBs. In short, the fireball model must be able to encompass all of these variables in order to apply to all GRBs (and thus be a plausible model). Fortunately, this is something the model has excelled at throughout the years.

The name of the fireball model suggests the mechanism to which a GRB occurs — in a fireball of ultra-relativistic energy consisting of optically thin material with very few baryons. In essence, during the GRB event, the inner engine remains undetectable due to the optical thickness and the lack of a thermal profile due to the compactness of the inner engine. The internal shocks cause the detectable GRB, and the external shocks form the gradual afterglow.

## Fast radio burst 'afterglow' was actually a flickering black hole

Last February a team of astronomers reported detecting an afterglow from a mysterious event called a fast radio burst, which would pinpoint the precise position of the burst's origin, a longstanding goal in studies of these mysterious events. These findings were quickly called into question by follow-up observations. New research by Harvard astronomers Peter Williams and Edo Berger shows that the radio emission believed to be an afterglow actually originated from a distant galaxy's core and was unassociated with the fast radio burst.

"Part of the scientific process is investigating findings to see if they hold up. In this case, it looks like there's a more mundane explanation for the original radio observations," says Williams.

The new work has been accepted for publication in Astrophysical Journal Letters.

As their name suggests, fast radio bursts (or FRBs) are brief yet powerful spurts of radio energy lasting only a few milliseconds. The first ones were only identified in 2007. Their source has remained a mystery.

"We don't even know if they come from inside our galaxy or if they're extragalactic," explains Berger.

Most FRBs have been identified in archival data, making immediate follow-up impossible. The new event, FRB 150418, is only the second one to be identified in real time. Radio observations reported in Nature purportedly showed a fading radio afterglow associated with the FRB. That afterglow was used to link the FRB to a host galaxy located about 6 billion light-years from Earth.

In late February and March of this year, Williams and Berger investigated the supposed host galaxy in detail using the NSF's Jansky Very Large Array network of radio telescopes. The fantastic sensitivity of the VLA allowed the researchers to monitor the radio galaxy at the necessary cadence without having to disrupt the observatory's regular schedule of operations.

If the initial observations had been an afterglow, it should have completely faded away. Instead they found a persistent radio source whose strength varied randomly by a factor of three, often reaching levels that matched the initial brightness of the claimed afterglow. The initial study also saw this source, but unluckily missed any rebrightenings.

"What the other team saw was nothing unusual," states Berger. "The radio emission from this source goes up and down, but it never goes away. That means it can't be associated with the fast radio burst."

The emission instead originates from an active galactic nucleus that is powered by a supermassive black hole. Dual jets blast outward from the black hole, and complex physical processes within those jets create a constant source of radio waves.

The variations we see from Earth may be due to a process called "scintillation," where interstellar gases make an intrinsically steady radio beacon appear to flicker, just like Earth's atmosphere makes light from stars twinkle. The source itself might also be varying as the active galactic nucleus periodically gulps a little more matter and flares in brightness.

While the link between the fast radio burst and a specific galaxy has vanished, the astronomers remain optimistic for future studies.

"Right now the science of fast radio bursts is where we were with gamma-ray bursts 30 years ago. We saw these things appearing and disappearing, but we didn't know what they were or what caused them," says Williams.

"Now we have firm evidence for the origins of both short and long gamma-ray bursts. With more data and more luck, I expect that we'll eventually solve the mystery of fast radio bursts too," he adds.

## GRB 130603B: No Compelling Evidence for Neutron Star Merger

The near infrared (NIR) flare/rebrightening in the afterglow of the short hard gamma ray burst (SHB) 130603B measured with the Hubble Space Telescope (HST) and an alleged late-time X-ray excess were interpreted as possible evidence of a neutron star merger origin of SHBs. However, the X-ray afterglow that was measured with the Swift XRT and Newton XMM has the canonical behaviour of a synchrotron afterglow produced by a highly relativistic jet. The H-band flux observed with HST 9.41 days after burst is that expected from the measured late-time X-ray afterglow. The late-time flare/rebrightening of the NIR-optical afterglow of SHB 130603B could have been produced also by jet collision with an interstellar density bump. Moreover, SHB plus a kilonova can be produced also by the collapse of a compact star (neutron star, strange star, or quark star) to a more compact object due to cooling, loss of angular momentum, or mass accretion.

#### 1. Introduction

Stripped envelope supernova explosions and neutron star mergers in close binaries were originally suggested by Goodman et al. [1] as possible sources of cosmological gamma ray bursts. However, their proposed underlying mechanism—a spherical fireball produced by neutrino-antineutrino annihilation into electron positron pairs beyond the surface of the collapsing/merging star—turned out not to be powerful enough to produce GRBs observable at very large cosmological distances as indicated from analysis of the first 153 GRBs observed with the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory [2], which was launched in 1991. Consequently, Shaviv and Dar proposed [3] that highly relativistic jets of ordinary matter are probably ejected in such events and produce narrowly collimated GRBs by inverse Compton scattering of circumstellar light. They also suggested that short GRBs may also be produced by highly relativistic jets ejected in the phase transition of compact stars, such as neutron stars, strange stars, and quark stars, into more compact objects due to mass accretion or to cooling and loss of angular momentum via winds and radiation. After the discovery of GRB afterglows, Dar [4] proposed that they are highly beamed synchrotron radiation emitted by these highly relativistic jets in their collision with the interstellar matter.

By now, there is convincing evidence that long duration GRBs and their afterglows are produced mostly by highly relativistic jets launched in stripped envelope supernova explosions (mainly of type Ic), but, despite the enormous observational efforts, the origin of short duration GRBs remains unknown. In fact, the circumstantial evidence that has been claimed to link short hard GRBs (SHBs) with neutron star merger in close binaries, such as their location in both spiral and elliptical galaxies [5, 6] and the distribution of their location offsets relative to the center of their host galaxies, which extends to a distance of 100 kpc [6] and beyond (e.g., SHB 080503 with the lack of a coincident host galaxy down to 28.5 mag in deep Hubble Space Telescope imaging [7]), actually favours a phase transition in a single compact star [8] with a large natal kick velocity over merging neutron stars in neutron star binaries [1] whose velocities are much smaller [9].

A more direct observational evidence that SHBs are produced by neutron stars merger was proposed by Li and Paczynski [10]. These authors suggested that neutron star mergers may create significant quantities of neutron-rich radioactive nuclei whose decay should result in a faint transient in the days following the burst, a so-called kilonova or macronova.

Recently, the broad band afterglow of the SHB 130603B (Melandri et al. [11] and Golenetskii et al. [12]) that was measured with the Swift X-ray telescope (XRT), Newton XMM, HST, and ground-based optical and radio telescopes was interpreted by Tanvir et al. [13, 14], Berger et al. [15], and Fong et al. [16] as evidence supporting a neutron star merger origin of SHB 130603B. However, in this paper we show that the X-ray afterglow of SHB 130603B, which was measured with Swift XRT (Swift-XRT GRB light-curve repository [17]) and Newton XMM [16], had the canonical behaviour of a synchrotron afterglow produced by a highly relativistic jet propagating in a normal interstellar environment, as predicted by the cannonball model of GRBs [8, 18–20] long before its empirical discovery by Nousek et al. with Swift [21]. This canonical X-ray afterglow does not have a “mysterious late-time X-ray excess” as claimed in [16], and the flux observed in the NIR H-band with HST 9.6 days after burst [13, 14] is that expected from the measured late-time X-ray afterglow. Moreover, a fast decline of a late-time afterglow followed by a rebrightening/flare in the NIR and optical afterglow of a GRB can be produced by a jet colliding with a density bump in the interstellar medium [8], as was observed in several long duration GRBs, such as 030329 [22, 23] and 070311 [24], and SHBs such as 050724 [25] and 080503 [26]. The host galaxy of SHB 130603B at redshift

[7], as seen in high-resolution HST imaging, is a perturbed spiral galaxy due to interaction with another galaxy [27]. SHB 130603B was located in one of its tidally disrupted arms [27]. The interaction of the SHB jet with such a bumpy environment may have caused the flare/rebrightening in the NIR afterglow observed with the HST on day 9.41 [13, 14].

Furthermore, a late-time flare/rebrightening of a NIR-optical afterglow of SHB can be produced by either a jet collision with an interstellar density bump or a kilonova. However, SHB plus kilonova can be produced also by collapse of compact stars (neutron star, strange star, or quark star) to a more compact object due to cooling, loss of angular momentum, or mass accretion [3, 8, 28]. The distribution of pulsar velocities has a high velocity component due to single pulsars and a lower velocity component from pulsars in binaries and isolated millisecond pulsars [9]. Hence, single compact stars are more likely than neutron star binaries (neutron stars, neutron star-black hole, and neutron star-white dwarf binaries) to be found at the large observed offsets of several SHBs from the center/disk of their host galaxies or at far away distances where no nearby host candidate was found in very deep searches.

Finally, the star formation within the host, location of SHB 130603B on top of the tidally disrupted arm, strong absorption features, and large line of sight extinction that were observed indicate that the GRB progenitor was probably not far from its birth place [27], untypical of the rather long mean lifetime before neutron star merger due to gravitational wave emission estimated for the known neutron star binaries in our galaxy (see, e.g., [9] for a recent review).

#### 2. The X-Ray Afterglow of SHB 130603B

The conclusion of Fong et al. [16] that the X-ray afterglow of SHB 130603B shows “a mysterious late-time X-ray excess” was based on a standard fireball model analysis of its X-ray afterglow. The standard fireball model, however, predicts that the temporal index

of the afterglow of a conical jet that is parametrized as a smoothly broken power law,

## Ask Ethan: Why Doesn't The Afterglow Of The Big Bang Eventually Fade Away?

An illustration of the radiation background at various redshifts in the Universe. Note that the CMB . [+] isn't just a surface that comes from one point, but rather is a bath of radiation that exists everywhere at once.

Earth: NASA/BlueEarth Milky Way: ESO/S. Brunier CMB: NASA/WMAP

For the past 13.8 billion years, our Universe has been expanding, cooling, and gravitating. The hot Big Bang itself was, at least for our observable Universe, a one-time event that was the proverbial starting gun for everything that's happened since. As we expanded and cooled, we formed atomic nuclei, neutral atoms, stars, galaxies, and eventually, rocky planets like Earth. Yet somehow, as we look out into the Universe, we can still see the leftover glow originating from the Big Bang — the Cosmic Microwave Background (CMB) — even today. How is this possible? That's what Lothar Voigt wants to know, asking:

Why is the CMB washing over us continuously and not just as a one-time event at some point in our own past or future? If the Sun suddenly turned transparent, all the light would rush out and that’s then the end of it. Sunspots and all. What am I missing?

It's a deep question, but it represents a great opportunity to learn how our Universe truly works. Let's dive in.

The distances between the Sun and many of the nearest stars shown here are accurate, but only a very . [+] small number of stars are presently located within 10 light-years of us. The farther away a star is, the farther back into the past we find ourselves looking.

Andrew Z. Colvin / Wikimedia Commons

When we look out in our Universe at any object that emits light, we're not seeing that object as it exists today, right at this very moment, where the exact number of seconds have elapsed since the Big Bang as they have for us. Instead, we're seeing that object as it was in the past: back when that light was emitted. That light is then required to journey through the Universe until it arrives at our eyes.

When we see our Sun, we're not observing the light it's emitting right now, but rather the light it emitted 8 minutes and 20 seconds ago: the amount of time it takes light to traverse the Earth-Sun distance.

When we look at a star that's hundreds or thousands of light-years away, we're seeing it as it was hundreds or thousands of years ago perhaps Betelgeuse, at 640 light-years away, has gone supernova at some point in the past 640 years. But if it has, that light hasn't arrived.

Galaxies identified in the eXtreme Deep Field image can be broken up into nearby, distant, and . [+] ultra-distant components, with Hubble only revealing the galaxies it's capable of seeing in its wavelength ranges and at its optical limits. It's important to remember that the light we see is only the light arriving right now, after journeying through the vast expanse of space.

NASA, ESA, and Z. Levay, F. Summers (STScI)

And when we look at a distant galaxy, we're seeing light that's millions or even billions of years old. That light was:

• generated millions or billions of years ago,
• travels millions or billions of years through the expanding Universe,
• and arrives at our eyes.

If a star in that galaxy goes supernova, we observe the supernova when the light arrives: not before and not after. If new stars form, we observe the light from the formation only when it arrives, not before or after, and the light from the stars only after they form and it has time to arrive. When those stars die, their light ceases to be emitted, and hence, once it passes by us, we'll never see them again.

The details in the Big Bang’s leftover glow have been progressively better and better revealed by . [+] improved satellite imagery. We see the Big Bang's leftover glow in all directions in space at all times it never goes away.

NASA/ESA and the COBE, WMAP and Planck teams

On the other hand, light from the Big Bang is still visible today, even though the Big Bang itself occurred 13.8 billion years ago. If we had been around just 1 million years after the Big Bang, we would have been able to see that light as well, although it would be at higher energies, since the Universe would have expanded by a smaller amount and the light would have shorter wavelengths and hence higher temperatures.

The more time that goes by, the more we see that leftover light:

• decrease in temperature,
• decrease in the number density of photons,
• and decrease in importance relative to matter and dark energy.

Despite all of these changes, and despite the fact that the Big Bang only occurred at one instant in time (a very long time ago), that leftover glow — once known as the primeval fireball and now known as the Cosmic Microwave Background (CMB) — continues to persist.

The leftover glow from the Big Bang, the CMB, permeates the entire Universe. As a particle flies . [+] through space, it is constantly being bombarded by CMB photons. If the energy conditions are right, even the collision of a low-energy photon like this has an opportunity to create new particles.

Rather than view this as a puzzle, we should treat this as an opportunity to understand how the light from the CMB is different from the light arriving from stars, galaxies, and individual astrophysical sources of light. For everything else in the Universe — everything that creates light — that light is:

• created at a particular location in space,
• created at a particular moment in time,
• travels away from the source, through the (expanding) Universe, at the speed of light,
• and arrives at our eyes, the observer, only for that one instant.

For stars, galaxies, supernovae, cataclysmic events, gas clouds, flares, and any other source of radiation, these things are all true. But for the Big Bang's leftover glow, one very, very important thing is different. All of that radiation does come from a particular instant in time it does travel through the Universe at the speed of light it does arrive at our eyes at one particular instant. But it wasn't created at just one location in space.

If you look farther and farther away, you also look farther and farther into the past. The earlier . [+] you go, the hotter and denser, as well as less-evolved, the Universe turns out to be. The earliest signals can even, potentially, tell us about what happened prior to the moments of the hot Big Bang. Note that we "see" very similar representations of the Universe in all directions, and that as time goes on, we'll be seeing objects, locations, and surfaces whose light has yet to arrive.

NASA / STScI / A. Feild (STScI)

The biggest, hardest-to-understand difference about the Big Bang from everything else is that the Big Bang doesn't have a point-of-origin. It's not like a stellar event or explosion there's no location you can point to and say, "this is where the Big Bang happened: here, and nowhere else." What makes the Big Bang so special is that it occurred everywhere at once.

The Big Bang represents a moment in time, 13.8 billion years ago, when the Universe was in an ultra-hot, ultra-dense state, filled with matter, antimatter, and radiation. Everything that's occurred since that time has occurred in the aftermath of the Big Bang. The annihilation of antimatter (leaving just a tiny bit of normal matter behind), the formation of protons and neutrons, the fusion of light elements, the formation of neutral atoms, the first stars and galaxies, etc. All of that occurred everywhere throughout the Universe, but only as we move forward in time.

Our deepest galaxy surveys can reveal objects tens of billions of light years away, but there are . [+] more galaxies within the observable Universe we still have yet to reveal between the most distant galaxies and the cosmic microwave background, including the very first stars and galaxies of all. As the Universe continues to expand, the cosmic frontiers will recede to ever greater distances.

Sloan Digital Sky Survey (SDSS)

This is the key idea to understanding where this radiation comes from. When we see the Big Bang's leftover glow, we are seeing the light that is only — right now — arriving at our eyes after a 13.8 billion year journey. The radiation we observe was emitted not at the instant of the Big Bang itself, but from a point in time that occurred 380,000 years later: when electrons finally were able to stably bind themselves to protons (and other atomic nuclei) without immediately being blasted apart again.

Prior to that time, radiation bounces back-and-forth off of all the free electrons populating the Universe. Put simply, photons (particles of light) and electrons interact frequently and easily put technically, their cross-section is large. But once you form neutral atoms, and your light is low enough in energy, those neutral atoms then become transparent to that light.

At early times (left), photons scatter off of electrons and are high-enough in energy to knock any . [+] atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms, and instead simply free-stream, since they have the wrong wavelength to excite these atoms to a higher energy level.

E. Siegel / Beyond the Galaxy

So what does that light do? The same thing all light does: it travels through the Universe, at the speed of light, until it reaches something for it to interact with.

But here's the thing: that light is everywhere. That light — the light that we observe as making up the CMB — was emitted from all points in the Universe, everywhere, all at once, some 13.8 billion years ago. The light that was emitted from our location has been travelling away from us at the speed of light for the past 13.8 billion years, and owing to the expansion of the Universe, is now some 46 billion light-years away from us.

Similarly, the light that's arriving at our eyes today was emitted 13.8 billion years ago, and the "surface" we see where the CMB originates from (from our perspective) is now 46 billion light-years away.

The extent of the visible Universe now goes on for 46.1 billion light-years: the distance that light . [+] emitted at the instant of the Big Bang would be located from us today, after a 13.8 billion year journey. As time marches on, light that is still on its way to us will eventually arrive.

Wikipedia user Pablo Carlos Budassi

So what's happening? The CMB light that arrived a second ago was emitted from a spherical surface that was slightly closer to us than the CMB light that's arriving right now. The light that we observed the first time we detected the CMB more than half a century ago was even closer, while the light that we'll observe in the far future is still on its way, coming to us from a point we can not yet see, since that light hasn't arrived yet.

What this means is that the Universe, everywhere, right now, is filled with about 411 CMB photons for every cubic centimeter of space that we have. It also means, when we look at galaxies and other astronomical objects that are very far away, those objects were interacting with CMB photons that were:

• more numerous (because the Universe had expanded less),
• more energetic (because those photon wavelengths had been stretched less),
• and were at a higher temperature.

That last part is interesting, because radiation interacts with matter, and we can observe — and actually have observed — how the CMB was hotter in the past.

A 2011 study (red points) has given the best evidence to date that the CMB used to be higher in . [+] temperature in the past. The spectral and temperature properties of distant light confirms that we live in an expanding Universe where the Big Bang's leftover glow reaches all points at once.

P. Noterdaeme, P. Petitjean, R. Srianand, C. Ledoux and S. López, (2011). Astronomy & Astrophysics, 526, L7

So what's actually occurring? The CMB actually is washing over us right now, and this very moment is the only opportunity we'll ever have to see those specific CMB photons that are arriving at Earth today. It took a 13.8 billion year journey across the expanding Universe to bring them to our eyes, but they've arrived after the most cosmic voyage of all: from the Big Bang to us.

But before those photons arrived, there were photons arriving from slightly closer locations. And after those photons are done arriving, they'll be replaced by photons that are arriving from locations that are slightly further away. This will continue for all eternity, as while both the number density and energy of these photons will continue to drop, they'll never go away completely. The Big Bang filled the entire Universe with this omnidirectional bath of radiation. As long as we exist in this Universe, the Big Bang's leftover glow will always be with us.

## SOLAR SYSTEM/SUN, ATMOSPHERES, EVOLUTION OF ATMOSPHERES | Meteors

### Conditions in the Meteor Path after Deposition

Once the air is heated and meteoric matter has been deposited, the resulting pressure increase by more than a factor of 10 creates a shock wave which expands radially. Assuming that this expansion occurs adiabatically, then the pressure will equilibrate with the background atmosphere when the radius is about 70 m (example for a −12-magnitude Leonid). Even at the very low pressures of the upper mesosphere/lower thermosphere (<10 −8 hPa), the size and velocity (Mach 270) of such a meteoroid would create a turbulent wake (Reynolds number >2000).

At this stage, the meteor is detected in several manners. First, a wake of the forbidden ‘green line’ emission at 557 nm from the O ( 1 S– 1 D) transition is seen in fast meteors, even relatively faint ones. The emission is thought to be produced by the Barth mechanism. Intensity estimates show that about 15% of the O2 in the initial train is dissociated in a −12 magnitude Leonid.

Radar reflections have detected an extended wake out to 6 km and several seconds behind a meteoroid that contributes to nonspecular reflections at UHF and VHF frequencies. Plasma instabilities and turbulence are responsible for an anomalous cross-field diffusion of meteor trails in the Earth's magnetic field that can be up to an order of magnitude faster than the rate expected from ambipolar diffusion In bright fireballs, this wake is accompanied by an afterglow rich in metal atom line emissions, without the atmospheric emission lines that are typical for the meteor spectrum. The afterglow has been interpreted as due to secondary ablation from debris particles. Evidently, even a fast meteor can deposit solid debris in its path, if conditions are favorable. The ablation vapor cools from ∼4400 K to ∼1200 K in a few seconds. This afterglow is followed by a recombination emission phase that lasts tens of seconds. Mid-infrared emission remains detected for longer, even when the gas and dust has cooled to just 50 K over the ambient T ≈ 250 K a few minutes after the fireball.

A persistent chemiluminescence remains for Leonids <−3 magnitude, which is called the persistent train ( Figure 7 ). Those persistent trains can be visible at ∼13 magn/arcsec 2 for many (tens of) minutes. The dynamical mechanism is not yet fully understood. Typically, two bright bands of light are seen with various levels of billowing. The train spectrum shows sodium emission lines and a broad molecular band continuum identified as the FeO orange arc band. The luminous mechanism is thought to be the catalytic recombination of ambient ozone with oxygen atoms in the trail through the Chapman airglow mechanism:

Figure 7 . Persistent train of a Leonid fireball. Photo © R. Haas, Dutch Meteor Society.

where the branching ratio of reaction [11] to produce the Na (3 2 P) state (which then emits an orange photon at 589 nm) is about 10%. The FeO molecular emission band probably arises from

where reaction [12] is sufficiently exothermic to produce FeO in excited electronic states, leading to emission between 570 and 630 nm with about a 2% efficiency. Thus persistent trains serve as a model for natural airglow emission, in extreme conditions and with the reactive components separated. The trains also probe upper atmosphere winds, wind sheer, and diffusion rates.

On the relatively short time scale of the train (minutes rather than hours), and particularly in the presence of elevated concentrations of atomic oxygen, it is very unlikely that the metallic species would be able to form more stable reservoir compounds such as NaHCO3 or Fe(OH)2. Indeed, between 85 and 100 km the meteoric metals are overwhelmingly in the atomic form in the background atmosphere. The postulated formation of nanometer-sized recondensed meteoric vapor particles in the warm meteor wake has not yet been demonstrated. However, when the metal atoms settle to lower elevations, they quickly react chemically and can condense to form particles. Most of this fine-grained material is expected to be transported to one of the poles, following seasonal winds in the upper mesosphere.

## Fast radio burst “afterglow” was actually a flickering black hole

Observations by the NSF’s Jansky Very Large Array, pictured here, show that a suspected fast radio burst afterglow is actually radio emission from an active galactic nucleus. Image credit: NRAO. Last February a team of astronomers reported detecting an afterglow from a mysterious event called a fast radio burst, which would pinpoint the precise position of the burst’s origin, a longstanding goal in studies of these mysterious events. These findings were quickly called into question by follow-up observations. New research by Harvard astronomers Peter Williams and Edo Berger shows that the radio emission believed to be an afterglow actually originated from a distant galaxy’s core and was unassociated with the fast radio burst.

“Part of the scientific process is investigating findings to see if they hold up. In this case, it looks like there’s a more mundane explanation for the original radio observations,” says Williams.

The new work has been accepted for publication in Astrophysical Journal Letters.

As their name suggests, fast radio bursts (or FRBs) are brief yet powerful spurts of radio energy lasting only a few milliseconds. The first ones were only identified in 2007. Their source has remained a mystery.

“We don’t even know if they come from inside our galaxy or if they’re extragalactic,” explains Berger.

Most FRBs have been identified in archival data, making immediate follow-up impossible. The new event, FRB 150418, is only the second one to be identified in real time. Radio observations reported in Nature purportedly showed a fading radio afterglow associated with the FRB. That afterglow was used to link the FRB to a host galaxy located about 6 billion light-years from Earth.

In late February and March of this year, Williams and Berger investigated the supposed host galaxy in detail using the NSF’s Jansky Very Large Array network of radio telescopes. The fantastic sensitivity of the VLA allowed the researchers to monitor the radio galaxy at the necessary cadence without having to disrupt the observatory’s regular schedule of operations.

If the initial observations had been an afterglow, it should have completely faded away. Instead they found a persistent radio source whose strength varied randomly by a factor of three, often reaching levels that matched the initial brightness of the claimed afterglow. The initial study also saw this source, but unluckily missed any rebrightenings.

“What the other team saw was nothing unusual,” states Berger. “The radio emission from this source goes up and down, but it never goes away. That means it can’t be associated with the fast radio burst.”

The emission instead originates from an active galactic nucleus that is powered by a supermassive black hole. Dual jets blast outward from the black hole, and complex physical processes within those jets create a constant source of radio waves.

The variations we see from Earth may be due to a process called “scintillation,” where interstellar gases make an intrinsically steady radio beacon appear to flicker, just like Earth’s atmosphere makes light from stars twinkle. The source itself might also be varying as the active galactic nucleus periodically gulps a little more matter and flares in brightness.

While the link between the fast radio burst and a specific galaxy has vanished, the astronomers remain optimistic for future studies.

“Right now the science of fast radio bursts is where we were with gamma-ray bursts 30 years ago. We saw these things appearing and disappearing, but we didn’t know what they were or what caused them,” says Williams.

“Now we have firm evidence for the origins of both short and long gamma-ray bursts. With more data and more luck, I expect that we’ll eventually solve the mystery of fast radio bursts too,” he adds.

## How long can afterglow last after a fireball? - Astronomy

Follow-up observations of large numbers of gamma-ray burst (GRB) afterglows, facilitated by the Swift satellite, have produced a large sample of spectral energy distributions and light curves, from which their basic micro- and macro-physical parameters can in principle be derived. However, a number of phenomena have been observed that defy explanation by simple versions of the standard fireball model, leading to a variety of new models. Polarimetry can be a major independent diagnostic of afterglow physics, probing the magnetic field properties and internal structure of the GRB jets. In this paper we present the first high-quality multi-night polarimetric light curve of a Swift GRB afterglow, aimed at providing a well-calibrated data set of a typical afterglow to serve as a benchmark system for modelling afterglow polarization behaviour. In particular, our data set of the afterglow of GRB 091018 (at redshift z = 0.971) comprises optical linear polarimetry (R band, 0.13-2.3 d after burst) circular polarimetry (R band) and near-infrared linear polarimetry (Ks band). We add to that high-quality optical and near-infrared broad-band light curves and spectral energy distributions as well as afterglow spectroscopy. The linear polarization varies between 0 and 3 per cent, with both long and short time-scale variability visible. We find an achromatic break in the afterglow light curve, which corresponds to features in the polarimetric curve. We find that the data can be reproduced by jet break models only if an additional polarized component of unknown nature is present in the polarimetric curve. We probe the ordered magnetic field component in the afterglow through our deep circular polarimetry, finding P circ < 0.15 per cent (2σ), the deepest limit yet for a GRB afterglow, suggesting ordered fields are weak, if at all present. Our simultaneous R- and Ks-band polarimetry shows that dust-induced polarization in the host galaxy is likely negligible.

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## A New Era of Submillimeter GRB Afterglow Follow-Ups with the Greenland Telescope

Planned rapid submillimeter (submm) gamma-ray-bursts (GRBs) follow-up observations conducted using the Greenland Telescope (GLT) are presented. The GLT is a 12-m submm telescope to be located at the top of the Greenland ice sheet, where the high altitude and dry weather porvide excellent conditions for observations at submm wavelengths. With its combination of wavelength window and rapid responding system, the GLT will explore new insights on GRBs. Summarizing the current achievements of submm GRB follow-ups, we identify the following three scientific goals regarding GRBs: (1) systematic detection of bright submm emissions originating from reverse shock (RS) in the early afterglow phase, (2) characterization of forward shock and RS emissions by capturing their peak flux and frequencies and performing continuous monitoring, and (3) detections of GRBs at a high redshift as a result of the explosion of first generation stars through systematic rapid follow-ups. The light curves and spectra calculated by available theoretical models clearly show that the GLT could play a crucial role in these studies.

#### 1. Introduction

Gamma-ray bursts (GRBs) are among the most powerful explosions in the universe and are observationally characterized according to intense short flashes mainly in high-energy band (so-called “prompt emission”) and long-lasting afterglows observed in X-ray to radio bands. Both types of radiation are sometimes extremely bright and can be observed using small- and middle-aperture optical and near-infrared telescopes (e.g., [1, 2]). Because of their intense luminosity, the highest redshift (

) in the reionization epoch (

) has been observed and has a high possibility for discovery even at

[2]. Although there are various diversities (long/short duration of prompt

-ray radiation, X-ray flares associated with X-ray afterglows, and complex temporal evolutions of afterglows), optical afterglow and host galaxy observations indicate that the majority of long-duration GRBs occur as a result of the death of massive stars (e.g., [3]). Thus GRBs are unique and powerful means of observing explosions first generation stars (population III, POP-III). Understanding the diversity of the astrophysical entities that cause GRBs is the subject of ongoing study and represents one of the most prominent inquiries in modern astrophysics.

Using GRBs to investigate the high- universe requires an understanding of their radiation mechanisms. Confirming the existence of reverse shocks (RS) and ascertaining their typical occurrence conditions are critical. The GRB afterglow is believed to involve a relativistically expanding fireball (e.g., [4]). The Interstellar Medium (ISM) influences the fireball shell after it has accumulated, and considerable energy is transferred from the shell to the ISM. The energy transfer is caused by two shocks: a forward shock (FS) propagating into the ISM and an RS propagating into the shell. Millimeter/submillimeter (mm/submm) observations are the key elements in understanding the emission mechanism of GRB afterglows. They provide “clean” measurements of source intensity and are unaffected by scintillation and extinction. Hence, studies of submm properties of the afterglow are likely to enrich the understanding of GRB physics.

In this paper, we review the status and achievements of submm afterglow observations in Section 2. In Section 3, we introduce the Greenland Telescope (GLT) project and its advantages for GRB follow-up observations. We also expect the GRB follow-ups in the era of GLT in Section 4. On the basis of these advantages, we establish three scientific goals by introducing the expected light curves and spectra with the expected sensitivity limit of GLT in Section 5. Finally, we summarize this paper in Section 6.

#### 2. Status and Achievements of Submillimeter Afterglow Follow-Ups

Numbers of dedicated follow-up instruments of GRBs and afterglows have been developed in -ray, X-ray [5], optical, and near-infrared (e.g. [6–12]). Afterglow observations in X-ray and optical have been adequately covered from very early phase including some fractions of -ray prompt phase (e.g., [13–16]). In addition, more than 300 afterglow observations have been made at the cm wavelengths mainly using the Very Large Array (e.g., [17]). However, submm has lagged behind X-ray and optical. Figure 1 shows a schematic summary of achievements in GRB observations according to wavelength and time range. It is obvious that prompt afterglow observations are lacking at submm wavelengths. The numbers of submm-detected events have been limited as summarized in Figures 1 and 2. Figure 2 shows all of afterglow observations in submm bands (230 and 345 GHz) including upper limits. There have been only seven detections and three well-monitored events (GRB030329, GRB100418A, and GRB120326A) in the submm bands. Unlike X-ray and optical observations, afterglow monitoring in the submm band covers only the late phase of GRBs and misses their brightening phases. However, several of these observations, in conjunction with intensive X-ray and optical monitoring, as in the GRB 120326A case [18], have addressed crucial physical properties of afterglow. Hence, submm afterglow observations are crucial for understanding the nature of GRBs. In the following, we briefly summarize three well-monitored submm afterglow cases.

-ray to submm). (b) One of actual light curves in X-ray, optical, and submm with the earliest submm detection.

GRB030329. Because of the low redshift (

) origin and bright optical afterglow (

13 mag at 0.1 days), numerous follow-up observations were conducted at various wavelengths (e.g., [3, 19–31]). The 250-GHz monitoring follow-ups were performed by the MAMBO-2 bolometer array on the IRAM 30-m telescope [32] and the IRAM Plateau de Bure Interferometer (PdBI) [33]. The monitoring observations were conducted from 1.4 to 22.3 days after the burst. The light curve after

8 days exhibited a simple power-law decay with a decay index of

[32]. The value was consistent with that determined for the optical decay after

0.5 days and indicated a common physical effect [34]. These monitoring observations supported the two-component jet model, in which a narrow-angle jet is responsible for the high-energy emission and early optical afterglow the radio afterglow emission is powered by the wide-angle jet [32, 35].

GRB100418A. The Submillimeter Array (SMA, [36]) was used to observe the submm afterglow from

16 hours after the burst. Continuous monitoring proceeded until 5 days after the burst, at which point it became undetectable [37]. As shown in Figure 2, the submm light curve exhibited a significant fading (equivalent decay power-law index of

−1.3) between 1 and 2 days and then exhibited a plateau phase until 4 days. The X-ray and optical light curves showed a late bump peak at

GRB120326A. The SMA observation provided the fastest detection to date among the seven submm afterglows at 230 GHz (Figure 2). In addition, comprehensive monitoring in the X-ray and optical bands was also performed. These observations revealed that the temporal evolution and spectral properties in the optical bands were consistent with the standard FS synchrotron with jet collimation (6°.7). Furthermore, the X-ray and submm behavior indicated different radiation processes from the optical afterglow as shown in Figure 3. Introducing synchrotron self-inverse Compton radiation from RS is a possible solution that is supported by the detection and slow decay of the afterglow in the submm band. As shown in Figure 4, the light curve exhibited the slow temporal evolution (

s this evolution is consistent with the expected decay index of the RS with the

condition [18]. Here, and are the observing frequency and the characteristic synchrotron frequency of the RS. And because half of the events exhibit similar X-ray and optical properties (e.g., [39–41]) as the current event, GRB120326A constitutes a benchmark requiring additional rapid follow-ups conducted using submm instruments such as the SMA and the Atacama Large Millimeter/submm Array (ALMA).

s after the burst [18]. The red dashed line shows the forward shock synchrotron model spectrum calculated using the boxfit code [89] with the same parameters for the best modeling light curve shown in the subpanel. The blue dotted lines show the reverse shock synchrotron radiation and its self-inverse Compton component calculated based on Kobayashi et al. [72] using the observed values and model function for the forward shock component. Subpanel: X-ray, optical, and submm light curves of the GRB120326A afterglow. The grey dotted lines show the best analytical fitted functions described in the text. The orange solid line shows the best modeling function for the

-band light curve obtained with the numerical simulation using boxfit.

To enhance afterglow studies, submm monitoring observations from an early phase are required. Although rapid follow-ups have been performed using the SMA, as in the GRB130427A case (beginning 1.6 hours after the burst), these follow-ups have been still limited specifically, we have failed to detect the GRB130427A afterglow with an insufficient sensitivity (

10 mJy) for detecting the RS component [42–44] caused by poor weather conditions. The ongoing SMA programs have also been suffering from the poor weather conditions of the Mauna Kea site. The condition of the Mauna Kea site for the SMA and the James Clerk Maxwell Telescope (JCMT) is inferior for submm observations comparing with other sites such as the ALMA site. Five out of 13 observations with SMA (average responding time is 11.3 hours) were made under the poor or marginal weather conditions. Hence, weather condition for submm bands is one of the crucial points for the time critical observations. As demonstrated through JCMT observations [45–50], rapid follow-ups can be managed by using existing submm telescopes with suitable follow-up programs and observation system. The typical delay time of JCMT is hours scale (average of 59 min with 6 GRB observations). Therefore, the succession of the JCMT rapid response system is also desired in the future. In addition, the constructions of dedicated submm telescopes based on these experiences at the better observing site are required to conduct systematic rapid and dense continuous follow-ups (sometimes coordinated with several submm telescopes at the different longitude for covering light curve within a day).

#### 3. Greenland Telescope

The GLT is a state-of-the-art 12-m submm telescope to be located in the Summit Station in Greenland. The aims of the project are establishing a new submm very long baseline interferometer (VLBI) station for the first imaging of shadow of supermassive black holes in M87 [51] and exploring a new terahertz frequency window [52] and time-domain astronomy in submm (e.g., this paper). The expected first light of the GLT will be made in 2017/18.

##### 3.1. Site of the Greenland Telescope

The Summit Station is located on top of the Greenland ice sheet, at 72.5° N and 38.5° W (north of the Arctic Circle) at a 3,200-m altitude. The temperature is extremely low, especially in winter when the temperature reaches between −40°C and −60°C (the lowest temperature of −72°C has been recorded). Because of the combination of low temperature and high altitude, considerably low opacity is expected. In 2011, a tipping radiometer at 225 GHz was deployed to Greenland to monitor the sky and weather conditions at the Summit Station and measure opacities from October 2011 to March 2014. The best and the most frequent opacities at 225 GHz were 0.021 and 0.04, respectively [51]. The weather conditions are compatible with those at the ALMA site and significantly better than those of the Manna Kea site [53]. These low opacities and weather conditions are the advantage of the GLT in managing submm and time critical observations including GRB follow-ups with higher sensitivity (or short exposure cycle).

##### 3.2. Planned Instruments and Expected Sensitivities

The GLT will be initially equipped with VLBI receivers at 86, 230, and 345 GHz. Whether large single-dish receivers as second-generation instruments (e.g., submm heterodyne arrays, a THz HEB array, and bolometric spectrometer array) will be installed depends on the scientific merits of the instruments and is still under discussion. For GRB observations, three frequency bands of the VLBI receivers are appropriate and, therefore, the first generation receivers can be used for afterglow observations. The current expected sensitivities of VLBI receivers are 36 and 88 mJy with 1 s integration at 230 and 345 GHz, respectively [54]. These are at least two times better than those of SMA and JCMT. We note that minimum integration time would be less than 0.5 s, since duty cycle for positional switch will be 2 Hz. Longer integration will be achieved as the accumulation of the short integrated data points. Hence, the GLT with the receivers will provide sufficient sensitivities to detect GRB counterparts at the 230 GHz band with shorter exposure (e.g., 3

limit of 48.3 mJy with 5 s, 10.8 mJy with 100 s, 3.6 mJy with 15 min, and 1.8 mJy with 1 h). This shorter exposure cycle is one of advantages to characterized temporal evolutions of submm afterglows. The field-of-view (FOV) of the receivers will be 25′′ and 16′′ at 230 and 345 GHz, respectively. These relatively narrow FOV require a tiling observation for covering entire error region determined by hard X-ray instruments (e.g., Swift/BAT) or accurate position determination with X-ray afterglows.

We are also planning to install semiautomated responding system for the GRB alerts to manage the rapid GRB follow-ups with secure procedures at the extreme site. We enjoyed the prototype system at the Kiso observatory [23]. The pointing will be started using the position determined by -ray instruments and additional adjustment will be made responding to the position of X-ray afterglows. By combination use of this system and site advantages including visibility for targets as shown in Figure 5, we will be able to perform nearly real-time follow-ups for GRBs whose declinations are higher than 30 degrees. A continuous monitoring (nearly video mode) will be managed in the first one day.

(dark cyan), and 0 (cyan) deg., respectively.

#### 4. Expected GRB Follow-Up Observations in the GLT Era

To achieve successful observations, rapid follow-ups using the GLT will be coordinated through Swift and the planned Space-based Multiband Astronomical Variable Objects Monitor (SVOM) [55]. One of the obstacles to performing rapid follow-ups of Swift-detected GRBs is the poor visibility from ground-based instruments. Although Swift has enabled detecting between 100 and 150 events per year, only

10 GRBs per year are observable from the major astronomical observation sites (e.g., Mauna Kea, Chile) without any delay from their -ray triggers, because of visibility problems that occur when using ground-based instruments and the random pointing strategy associated with Swift observations. According to statistical data in 10 years of Swift observations, 10 to 12 events per year could be observed from the early phase of the afterglow by using currently existing telescopes within a 0–3-hour delay by maintaining a proper exposure time ( 3-4 hours). The current fraction of total Swift/BAT pointing time to around antisun directions (sun hour angle from 9 to 15 hours) is limited to 30–40% [56]. The ideal location of the GLT will enable solving this problem. As shown in Figure 5, GRBs located at a declination higher than 29 degrees will always exhibit an elevation angle higher than 12 degrees over days. Hence, in winter, the GLT will be able to begin rapid GRB follow-ups without any delay caused by unfavorable visibility and perform continuous submm monitoring over days. In addition, these observation conditions are advantageous for observing GRB afterglows that exhibit a rich diversity in various time ranges. In summer, the 86 GHz receiver will be used when weather conditions are unsuitable for observations at 230 and 345 GHz.

) will be a timely mission for conducting rapid GLT observations. The GRB detectors will observe antisun directions that enable ground-based instruments to begin follow-ups and continuous long-term monitoring of markedly early cases immediately after receiving GRB alerts. The GRB detection rate of SVOM will be

80 events per year, providing 10 to 20 events per year for rapid GLT follow-ups. In a typical GRB case, X-ray afterglows will be observed immediately through a SVOM X-ray telescope (MXT) [57], with the same strategy of X-ray observations of Swift. This provides a position accuracy of the counterpart of 2-3′′, which is sufficient to cover the position with the FOV of the GLT.

An additional crucial advantage of SVOM follow-ups is the capability of detecting X-ray flashes (XRFs) and X-ray-rich GRBs (XRRs) [58] and determining the prompt spectral peak energy

. Because of the slightly higher energy range of Swift/BAT (15–150 keV), sample collections of XRFs and related rapid follow-ups have been entirely terminated. The estimation of the Swift-detected GRBs (mainly XRRs and classical GRBs) has been provided by joint spectrum fittings of Swift/BAT and Suzaku/WAM [59]. Although spectral parameters of prompt emissions are adequately constrained by these joint fittings (e.g., [60–64]), the number of events is limited. This has caused a stagnation of the study of GRBs with prompt characterization. Because two of the prompt-emission-observing instruments onboard SVOM, ECLAIRs [65], and the Gamma Ray Monitor (GRM) [66] will cover the energy bands 4–150 keV and 30–5000 keV, respectively, the for most of SVOM-detected GRBs would be determined. In addition, numerous XRRs and XRFs would be detected with the estimation through ECLIAS and GRM. According to the HETE-2 observations [58], the numbers of XRFs, XRRs, and GRBs were 16, 19, and 10, respectively. The number of softer events (XRFs/XRRs) was considerably higher than that of classical GRBs. Because the lower-energy coverage of HETE-2 (2–400 keV) [67] was similar to that of SVOM, a high volume of XRF and XRR samples with measurements can be generated. This is likely to enhance the study of the origins of XRRs and XRFs by enabling the determination of their physical parameters. Hence, the GLT will be able to facilitate characterizing prompt and late-phase submm afterglows of all three types of bursts for the first time, providing crucial insights into the nature of XRFs and XRRs.

#### 5. Expected Science Cases

On the basis of the summary of submm afterglow observations and the GLT project, we established the following three scientific goals.

##### 5.1. Systematic Detection of Bright Submm Emissions

It is believed that RSs generate short bright optical flashes (e.g., [13]) and/or intense radio afterglows (e.g., [68]). According to the standard relativistic fireball model, RSs are expected to radiate emissions in the long wavelength bands (optical, infrared, and radio) by executing a synchrotron process in a particularly early afterglow phase (e.g., [69]).

Detecting this brief RS emission and measuring its magnitude would lead to constraints on several crucial parameters of the GRB ejecta, such as its initial Lorentz factor and magnetization [70]. Although RS emission has been detected in the optical wavelength in several GRBs, early optical observations for most GRBs have yielded no evidence of RS emission. The nondetection of RSs in optical bands could be an indicator of a magnetically dominated outflow. Another possible reason for the nondetection is that the typical RS synchrotron frequency is markedly below the optical band (e.g., [71]). Searching for RS emissions in the submm wavelength would test these possibilities. The comparison of early optical and submm temporal evolution would enable studying the composition of the GRB ejecta. If an RS component was regularly detected in GRBs of which the early optical light curve shows no evidence of RS emission, we would be able to claim that GRB ejecta are baryonic in nature. The detection of RS emissions in the submm band of most GRB would support the possibility that GRBs are baryonic flow.

One of the critical problems is that there has been no systematic submm observational study in the early afterglow phase of GRBs. As shown in Figure 1, the number of events that have been observed earlier than 1 day after bursts has remained at 16 for some time. The expected RS light curve for classical GRBs is fainter than 1 mJy at 1 day after bursts and therefore undetectable using currently existing submm telescopes, excpet for ALMA. Figure 6 shows the expected RS light curves based on Kobayashi [69] and Kobayashi et al. [72] with various magnetic energy densities of RS

and an initial Lorentz factor

in comparison with the GLT sensitivity limit. In most of the cases shown in Figure 6, the RS component faded away before s (

1 day). Hence, to detect and characterize RS components, rapid (

min scale) and continuous dense monitoring within 1 day is required. Although some of successful RS observations were made by SMA, CARMA, VLA, and others with their open use framework (e.g. [18, 42–44]), dedicated radio telescopes are strongly desired to make the systematic investigation. In addition, dense monitoring with the same wavelength is required to characterize the RS components, because sparse monitoring, even though rapid detection is included, failed to decode RS and FS components (e.g., [73]). Therefore, the use of the GLT would initiate the era of systematic submm follow-ups.

(0.001, 0.01, 0.1, and 0.3). Other physical parameters are fixed as

. (b) Expected RS light curves in the 230 GHz band with various initial Lorentz factor

. Other physical parameters are

limits with 900 s and 1 h exposure are indicated with dotted lines in both panels.

In Figure 6, cases of low initial Lorentz factors (20, 40) are provided showing XRRs and XRFs that are expected to be detected using the planned GRB satellite SVOM. The origin of the XRFs is not known. However, there are two major models, namely, (1) the failed GRBs or dirty fireball model (e.g., [74]) and (2) the off-axis jet model [75]. According to the dirty fireball model, low-inital-Lorentz-factor (

), GRBs produce a lower spectral peak energy in the prompt phase because of

dependence, and it is therefore natural to attribute this energy to XRRs and XRFs. The low Lorentz factors substantially delay the RS peak (Figure 6(b)). For the latter model, it is assumed that the viewing angle is considerably larger than the collimation angle of the jet, and the high ratio of X-ray to -ray fluence is caused by a relativistic beaming factor when a GRB is observed through off-axis direction. The key observable feature is the achromatic brightening optical afterglow light curves, of which the peak time depends on the viewing angle [76, 77]. Hence, identifying a delayed RS peak through rapid GLT monitoring and the prompt spectral characterization of SVOM will confirm and identify the origin of XRFs and XRRs.

##### 5.2. Characterization of Forward and Reverse Shock Emissions

The spectral characteristics of FS and RS synchrotron emissions are related as follows:

) and ) denote the mass and magnetization ratio parameters, respectively. Here, , , and are the Lorentz factors at the deceleration time, the fractions of magnetic energy of RS and FS, respectively. RS emission is typically expected to be considerably brighter (

100 times) than FS emission as shown in Figure 7. Therefore, the emission from RSs is sensitive to the properties of the fireball, and the broadband spectrum and light curve evolutions of FS/RS can provide critical clues to understanding GRBs.

. Synchrotron radiations from reverse and forward shock are indicated by red solid and blue dashed lines, respectively. A green hatched box indicates the frequency coverage of GLT.

Regarding FSs, afterglows can be described by synchrotron emissions from a decelerating relativistic shell that collides with an external medium. According to the FS synchrotron model, both the spectrum and light curve consist of several power-law segments with related indices (e.g., [78, 79]). The broadband spectrum is characterized according to the synchrotron peak frequency and the peak spectrum flux density . The peak spectrum flux is expected to occur at low frequencies (from X-ray to radio) over time (from minutes to several weeks) as

. The peak spectrum flux density is predicted to remain constant in the circumburst model, whereas it decreases as in the wind model. Therefore, determining the characterizing frequencies and peak fluxes by using temporal and spectrum observations provides direct evidence of the FS/RS shock synchrotron model and typical (or average) physical conditions of a fireball.

Snapshots of the broadband spectrum and continuous monitoring of light curves in the submm bands are essential to characterizing the radiation of afterglow by decoding each component. Figure 8 shows the expected light curve in the 230 GHz band at , 0.5, and 0.7. We fix the rest of parameters as explosion energy erg, circumburst number density cm −3 , the electron spectral index

, the electron energy density , and the magnetic energy density of FS . The brightening caused by the passing of the synchrotron peak frequency can be detected to determine the FS component in light curves. The GLT has also enough sensitivities to detect FS component for nearby (

) events and the monitoring determines their . The expected passing through time across the 230 GHz band is several s (Figure 8). Therefore, the submm band is suitable for decoding both RS and FS components. Some of closure relations for FS and RS [79] will also constrain components even if the light curve or spectrum observations are sparsely performed. For X-ray and OIR cases, the expected timescale is between several minutes and

2 hours after the burst. In this time range, detecting the peak frequency directly is difficult because several additional components (e.g., long-lasting prompt emission, X-ray flares, etc.) characterize this phase. For the MIR case, observations fully rely on the satellite-based instruments. In this case, timely follow-ups are difficult because of operation confinement and limitation of number of resources. Furthermore, the slow temporal evolution in the radio band makes it difficult to obtain simultaneous optical and X-ray segments. This creates uncertainty regarding whether we observe the same synchrotron components or not. Hence, the GLT will provide unique results for nearby events ( ) by facilitating continuous monitoring.

. Solid lines indicate the total of RS and FS. GLT has also enough sensitivity to characterize the FS components for nearby (

limits with 1 h and 3 h exposure are indicated with dotted lines.

Optical monitoring combined with the GLT will be required. As we describe above, RS components will be caught by submm observations. For characterizing FS components, multicolor optical monitoring is suitable because the temporal evolution and spectrum of optical afterglow around 1 day after bursts are well consistent with the expectation of the FS model. Figure 3 shows one of the most appropriate examples of the earliest submm afterglow detection procedures performed using the SMA and related optical monitoring [18]. Because of the rapid submm monitoring, the FS and RS components were separated. Conducting multifrequency monitoring by using the GLT requires rapid optical follow-ups, which will be conducted using our own optical network EAFON [80–82] and other ground-based optical telescopes, as numerous observations have been conducted.

##### 5.3. Discovering of First-Star Explosions by Using GRBs

The findings regarding a high-redshift GRB at [1] indicated the possibility of using GRBs to probe the processes and environments of star formation as far back in time as the earliest cosmic epochs. Numerous theoretical models (e.g., [83–85]) show that some POP-III stars generate GRBs as an end product. Hence, detecting the signals of high- GRBs has been a recent prominent objective in modern astrophysics.

One of the prospective methods of identifying POP-III GRBs is to detect RS components in the submm bands. Unlike OIR observations, submm observations provide clean measurements of the source intensity, unaffected by extinction. Because of the intense luminosity of the RSs, it is expected that the radiation from high- GRBs (

) can be observed if the GLT is used with the rapid response system. Inoue et al. [86] showed that the RS component of POP-III GRBs at and 30 in the 300 GHz band is substantially brighter than 1 mJy, and these bright RS components will be detectable by using the GLT. In addition, we simulated the expected RS light curves at , 15, and 30 based on Kobayashi [69] and Kobayashi et al. [72]. For this calculation, we assumed that because the progenitor stars might be considerably more massive than nearby events (e.g., [83]). Other physical parameters are fixed as , , , , and . As shown in Figure 9, the GLT has great potential to detect the high- GRBs even those at with the rapid responding, shorter exposure cycle, and continuous dense monitoring. These initial detections of the GLT in the early phase will provide the opportunity to conduct additional follow-ups using 30-m class telescopes such as the TMT. Because these large telescopes typically enable conducting follow-ups for a limited number of events, the target selections of the GLT observations will be critical.

, 15, and 30 at 230 GHz. Other physical parameters are fixed as

limits with 900 s and 1 h exposure are indicated with dotted lines. GLT with rapid follow-ups has sufficient potential to detect these higher-

The event rate of high- GRBs, which may be connected to the star-formation rate in the early universe, is not known. Wanderman and Piran [87] estimated that the event rate of high- ( ) GRBs might be

10 events per year per steradian on the basis of limited

100 Swift-detected long GRBs with known redshift and measured peak flux. Their estimation showed that Swift/BAT exhibited substantially high redshift fractions (

3.4% at ), whereas Swift and related follow-ups detected only a few events. Hence, uncertainty exists regarding the number of higher- events that Swift has detected thus, the frequency of such events is not thoroughly understood because appropriate follow-ups in long wavelength (e.g., IR, submm) have not been conducted. Actually,

GRB candidate [88] was also detected through Swift. Therefore, a continuous effort is required in this field of research, despite a success rate that is typically low. In addition, Wanderman and Piran [87] expected that SVOM will detect 0.1–7 events per year at . To detect these events, rapid follow-up coordination with submm instruments will be crucial, because it is impossible to identify higher- candidates within hours from bursts with limited observational information. Therefore, the installing of a rapid responding system in the GLT will enable us to perform high- GRB observations.

#### 6. Summary

We briefly summarized the current achievements of submm follow-up observations of GRBs. Although submm afterglow observations are critical to understanding the nature of GRBs (e.g., GRB030329 and GRB120326A), the number of successful observations has been limited. This is because of the lack of dedicated submm telescopes that has made it difficult to perform rapid follow-up.

Furthermore, we introduced the single-dish mode of GLT. The development is ongoing and the expected first light of the GLT will be made in 2017/18. The first light instrument will be the VLBI receivers at 86, 230, and 345 GHz. The expected sensitivities of the receivers are 36 and 88 mJy in 1 s integration at 230 and 345 GHz, respectively. The GLT enables rapid and continuous submm monitoring of GRB submm counterparts in the prompt phase.

According to the aforementioned situations and expected capabilities of the GLT, we established the following three key scientific goals regarding GRB studies: (1) systematic detection of bright submm emissions originating from RS in the early afterglow phase by conducting rapid follow-ups, (2) characterization of FS and RS emissions by capturing their peak flux and frequencies through continuous monitoring, and (3) detection of the first star explosions as a result of GRBs at a high redshift through systematic rapid follow-ups.

Detecting RS emissions and monitoring light curves in the submm band could lead to constraints on several crucial parameters of the GRB ejecta, such as the initial Lorentz factor and magnetization. We calculated the expected RS light curves by using various initial Lorentz factors and the magnetic energy densities of RS and showed that these light curves could be characterized through rapid follow-ups of the GLT. Determining the origins of XRRs and XRFs will also be a major focus of the GLT together with the SVOM mission.

In addition to characterizing RS components, the GLT will be able to detect FS components of nearby ( ) GRBs. Because the spectral characteristics of the FS and RS synchrotron emissions are related, characterizing both FS and RS components provides critical insights into GRBs. We generated an expected submm light curve at , 0.5, and 0.7 and showed that the GLT can separate RS and FS components through long-term and continuous monitoring, because RS and FS components dominate earlier than

These two RS science topics will enhance the GRB studies as the probe of a high- universe. Because of the existence of RS and its extreme luminosity, it is expected that the radiation from high- GRB ( ) can be observed if the GLT is used with a rapid (hours scale) response system. We simulated the expected RS light curves at , 15, and 30. These light curves showed that the GLT has sufficient sensitivity to detect and characterize these events. The future SVOM mission may provide the capability to detect GRBs at the establishment of close coordination using longer wavelength (e.g., infrared, submm) instruments will be crucial. Because the rapid identification of counterparts at long wavelengths is crucial for conducting additional further follow-ups using 30-m class telescopes such as the TMT will be used. Therefore, the GLT could play a crucial role in detecting high- GRBs.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

The Greenland Telescope (GLT) Project is a collaborative project between Academia Sinica Institute of Astronomy and Astrophysics, Smithsonian Astrophysical Observatory, MIT Haystack Observatory, and National Radio Astronomy Observatory. The authors would like to thank Shiho Kobayashi for the useful comments. The authors would also like to thank all members of the GLT single-dish science team organized at ASIAA. This work is partly supported by the Ministry of Science and Technology of Taiwan Grants MOST 103-2112-M-008-021- (Yuji Urata), 103-2112-M-001-038-MY2 (Keiichi Asada), and 102-2119-M-001-006-MY3 (Hiroyuki Hirashita).

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