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

Authors: Yuji Urata; Kuiyun Huang; Keiichi Asada; Hiroyuki Hirashita; Makoto Inoue; Paul T. P. Ho
Journal: Advances in Astronomy
 (2015)

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## Abstract

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.


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## Body

## 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 (z) in the reionization epoch (z≳8) has been observed and has a high possibility for discovery even at z>10 [2]. Although there are various diversities (long/short duration of prompt γ-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-z 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 Section2. 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 inγ-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 γ-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.Figure 1
(a) Schematic summary of the GRB observational achievements along with time from the bursts in individual wavelength (fromγ-ray to submm). (b) One of actual light curves in X-ray, optical, and submm with the earliest submm detection.
(a)
(b)Figure 2
Light curve summary of afterglow observations in submm bands (230 and 345 GHz). Red (230 GHz upper limits), blue (345 GHz upper limits), orange (GRB120326A), and purple (GRB100418A) points were obtained with the SMA. The SMA successfully monitored two of three well-observed submm afterglows, GRB100418A and GRB120326A.GRB030329. Because of the low redshift (z=0.168) 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 -1.68 [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 ~5 × 104 s [38].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 (αsubmm=-0.33) between 4×104 and 1×105 s; this evolution is consistent with the expected decay index of the RS with the νobs<νm,RSSync condition [18]. Here, νobs and νm,RSSync 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).Figure 3
Main panel: The spectral energy distribution of GRB120326A at6.42×104 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 r-band light curve obtained with the numerical simulation using boxfit.Figure 4
Submm light curve of GRB120326A with RS and FS model functions.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., 3σ 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 γ-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.Figure 5
An example of visibility plot in winter for the GRB that declination is 29 deg. Hatchs indicate the sun altitude lower than-18 (dark blue), -12 (blue), -6 (dark cyan), and 0 (cyan)  deg., respectively.


## 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., 3σ 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 γ-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.Figure 5
An example of visibility plot in winter for the GRB that declination is 29 deg. Hatchs indicate the sun altitude lower than-18 (dark blue), -12 (blue), -6 (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 throughSwift and the planned Space-based Multiband Astronomical Variable Objects Monitor (SVOM) [55]. One of the obstacles to performing rapid follow-ups ofSwift-detected GRBs is the poor visibility from ground-based instruments. AlthoughSwift 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 γ-ray triggers, because of visibility problems that occur when using ground-based instruments and the random pointing strategy associated withSwift observations. According to statistical data in 10 years ofSwift 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 totalSwift/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.SVOM (2020~) 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 ofSVOM 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 aSVOM X-ray telescope (MXT) [57], with the same strategy of X-ray observations ofSwift. 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 ofSVOM 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 Epeak. Because of the slightly higher energy range ofSwift/BAT (15–150 keV), sample collections of XRFs and related rapid follow-ups have been entirely terminated. The Epeak estimation of theSwift-detected GRBs (mainly XRRs and classical GRBs) has been provided by joint spectrum fittings ofSwift/BAT andSuzaku/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 onboardSVOM, ECLAIRs [65], and the Gamma Ray Monitor (GRM) [66] will cover the energy bands 4–150 keV and 30–5000 keV, respectively, the Epeak for most ofSVOM-detected GRBs would be determined. In addition, numerous XRRs and XRFs would be detected with the Epeak estimation through ECLIAS and GRM. According to theHETE-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 ofHETE-2 (2–400 keV) [67] was similar to that ofSVOM, a high volume of XRF and XRR samples with Epeak 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 Figure1, 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 ϵB,RS and an initial Lorentz factor Γ0 in comparison with the GLT sensitivity limit. In most of the cases shown in Figure 6, the RS component faded away before 1×105 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.Figure 6
(a) Expected RS light curves in the 230 GHz band with variousϵB,RS (0.001, 0.01, 0.1, and 0.3). Other physical parameters are fixed as z=1, E=3×1052, Γ=80, n=1, ϵe=0.3, and ϵB,FS=0.01. (b) Expected RS light curves in the 230 GHz band with various initial Lorentz factor Γ0. Other physical parameters are z=1, E=3×1052, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. The expected GLT 3-σ limits with 900 s and 1 h exposure are indicated with dotted lines in both panels.
(a)
(b)In Figure6, 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 (Γ0≪100), GRBs produce a lower spectral peak energy Epeak in the prompt phase because of Epeak∝Γ04 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 γ-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 ofSVOM 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:νm,FS~RB1/2RM2νm,RS, and Fmax⁡,RS~RMRB-1/2Fmax⁡,FS (e.g., [70]), where RM(=Γd2/Γ0) and RB(=ϵB,FS/ϵB,RS) denote the mass and magnetization ratio parameters, respectively. Here, Γd, ϵB,FS, and ϵB,RS 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.Figure 7
Expected spectrum at the deceleration time withz=1, E=1×1052, Γ=80, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. 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 νm,FS and the peak spectrum flux density Fmax⁡,FS. The peak spectrum flux is expected to occur at low frequencies (from X-ray to radio) over time (from minutes to several weeks) as νm,FS∝t-3/2. The peak spectrum flux density Fmax⁡,FS is predicted to remain constant in the circumburst model, whereas it decreases as Fmax⁡,FS∝t-1/2 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. Figure8 shows the expected light curve in the 230 GHz band at z=0.3, 0.5, and 0.7. We fix the rest of parameters as explosion energy E=3×1052 erg, circumburst number density n=1 cm−3, the electron spectral index p=2.1, the electron energy density ϵe=0.3, and the magnetic energy density of FS ϵB,FS=0.01. The brightening caused by the passing of the synchrotron peak frequency νm can be detected to determine the FS component in light curves. The GLT has also enough sensitivities to detect FS component for nearby (z≲0.7) events and the monitoring determines their νm,FS. The expected νm,FS passing through time across the 230 GHz band is several ×105 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 (z≲0.7) by facilitating continuous monitoring.Figure 8
Expected RS (dashed-dot) and FS (dashed) light curves in the 230 GHz band atz=0.3, 0.5, and 0.7 with E=3×1052 erg, n=1 cm−3, p=2.1, ϵe=0.3, and ϵB,FS=0.01. Solid lines indicate the total of RS and FS. GLT has also enough sensitivity to characterize the FS components for nearby (z≲0.7) GRBs. The expected GLT 3-σ 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. Figure3 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 atz=8.2 [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-z 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-z GRBs (z>10–30) 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 z=15 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 z=10, 15, and 30 based on Kobayashi [69] and Kobayashi et al. [72]. For this calculation, we assumed that E=1×1053 because the progenitor stars might be considerably more massive than nearby events (e.g., [83]). Other physical parameters are fixed as Γ=80, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. As shown in Figure 9, the GLT has great potential to detect the high-z GRBs even those at z=30 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.Figure 9
Expected light curves of GRB afterglows atz=10, 15, and 30 at 230 GHz. Other physical parameters are fixed as E=1×1053, Γ=80, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. The expected GLT 3-σ 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-z events.The event rate of high-z 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-z (z>7) GRBs might be ~10 events per year per steradian on the basis of limited ~100Swift-detected long GRBs with known redshift and measured peak flux. Their estimation showed thatSwift/BAT exhibited substantially high redshift fractions (~3.4% at z>7), whereasSwift and related follow-ups detected only a few z>7 events. Hence, uncertainty exists regarding the number of higher-z events thatSwift 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, z~9 GRB candidate [88] was also detected throughSwift. 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 thatSVOM will detect 0.1–7 events per year at z>10. To detect these events, rapid follow-up coordination with submm instruments will be crucial, because it is impossible to identify higher-z 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-z GRB observations.


## 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 Figure1, 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 ϵB,RS and an initial Lorentz factor Γ0 in comparison with the GLT sensitivity limit. In most of the cases shown in Figure 6, the RS component faded away before 1×105 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.Figure 6
(a) Expected RS light curves in the 230 GHz band with variousϵB,RS (0.001, 0.01, 0.1, and 0.3). Other physical parameters are fixed as z=1, E=3×1052, Γ=80, n=1, ϵe=0.3, and ϵB,FS=0.01. (b) Expected RS light curves in the 230 GHz band with various initial Lorentz factor Γ0. Other physical parameters are z=1, E=3×1052, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. The expected GLT 3-σ limits with 900 s and 1 h exposure are indicated with dotted lines in both panels.
(a)
(b)In Figure6, 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 (Γ0≪100), GRBs produce a lower spectral peak energy Epeak in the prompt phase because of Epeak∝Γ04 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 γ-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 ofSVOM 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:νm,FS~RB1/2RM2νm,RS, and Fmax⁡,RS~RMRB-1/2Fmax⁡,FS (e.g., [70]), where RM(=Γd2/Γ0) and RB(=ϵB,FS/ϵB,RS) denote the mass and magnetization ratio parameters, respectively. Here, Γd, ϵB,FS, and ϵB,RS 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.Figure 7
Expected spectrum at the deceleration time withz=1, E=1×1052, Γ=80, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. 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 νm,FS and the peak spectrum flux density Fmax⁡,FS. The peak spectrum flux is expected to occur at low frequencies (from X-ray to radio) over time (from minutes to several weeks) as νm,FS∝t-3/2. The peak spectrum flux density Fmax⁡,FS is predicted to remain constant in the circumburst model, whereas it decreases as Fmax⁡,FS∝t-1/2 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. Figure8 shows the expected light curve in the 230 GHz band at z=0.3, 0.5, and 0.7. We fix the rest of parameters as explosion energy E=3×1052 erg, circumburst number density n=1 cm−3, the electron spectral index p=2.1, the electron energy density ϵe=0.3, and the magnetic energy density of FS ϵB,FS=0.01. The brightening caused by the passing of the synchrotron peak frequency νm can be detected to determine the FS component in light curves. The GLT has also enough sensitivities to detect FS component for nearby (z≲0.7) events and the monitoring determines their νm,FS. The expected νm,FS passing through time across the 230 GHz band is several ×105 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 (z≲0.7) by facilitating continuous monitoring.Figure 8
Expected RS (dashed-dot) and FS (dashed) light curves in the 230 GHz band atz=0.3, 0.5, and 0.7 with E=3×1052 erg, n=1 cm−3, p=2.1, ϵe=0.3, and ϵB,FS=0.01. Solid lines indicate the total of RS and FS. GLT has also enough sensitivity to characterize the FS components for nearby (z≲0.7) GRBs. The expected GLT 3-σ 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. Figure3 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 atz=8.2 [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-z 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-z GRBs (z>10–30) 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 z=15 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 z=10, 15, and 30 based on Kobayashi [69] and Kobayashi et al. [72]. For this calculation, we assumed that E=1×1053 because the progenitor stars might be considerably more massive than nearby events (e.g., [83]). Other physical parameters are fixed as Γ=80, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. As shown in Figure 9, the GLT has great potential to detect the high-z GRBs even those at z=30 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.Figure 9
Expected light curves of GRB afterglows atz=10, 15, and 30 at 230 GHz. Other physical parameters are fixed as E=1×1053, Γ=80, n=1, ϵe=0.3, ϵB,RS=0.03, and ϵB,FS=0.01. The expected GLT 3-σ 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-z events.The event rate of high-z 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-z (z>7) GRBs might be ~10 events per year per steradian on the basis of limited ~100Swift-detected long GRBs with known redshift and measured peak flux. Their estimation showed thatSwift/BAT exhibited substantially high redshift fractions (~3.4% at z>7), whereasSwift and related follow-ups detected only a few z>7 events. Hence, uncertainty exists regarding the number of higher-z events thatSwift 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, z~9 GRB candidate [88] was also detected throughSwift. 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 thatSVOM will detect 0.1–7 events per year at z>10. To detect these events, rapid follow-up coordination with submm instruments will be crucial, because it is impossible to identify higher-z 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-z 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 factorsΓ0 and the magnetic energy densities of RS ϵB,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 theSVOM mission.In addition to characterizing RS components, the GLT will be able to detect FS components of nearby (z≲0.7) 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 z=0.3, 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 ~4 × 104 s and later than ~105 s, respectively.These two RS science topics will enhance the GRB studies as the probe of a high-z universe. Because of the existence of RS and its extreme luminosity, it is expected that the radiation from high-z GRB (z>10–30) can be observed if the GLT is used with a rapid (hours scale) response system. We simulated the expected RS light curves at z=10, 15, and 30. These light curves showed that the GLT has sufficient sensitivity to detect and characterize these events. The futureSVOM mission may provide the capability to detect GRBs at z>10; 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-z GRBs.



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*Source: 165030-2015-08-24.xml*