GRB and Transient Sciences
Yuji Urata 1 , Kuiyun Huang 2
1

Institute of Astronomy, National Central University, Chung-Li 32054, Taiwan
2
Center for General Education, Chung Yuan Christian University, Taoyuan 32023, Taiwan
urata@g.ncu.edu.tw

Abstract
We overview the science cases of Gamma-ray Burst and transients for the Next Generation Very Large Array(ngVLA). Based on ongoing SMA, ALMA and multi-wavelength observations, there are ﬁve science cases; (1)Energetics and acceleration efﬁciency of GRB and related transients, (2) probe of ﬁrst-generation stars, (3) GRBSN-SNR connection, (4) uniﬁcation of short GRBs as gravitational wave source, (5) relativistic transients as multimessenger astronomy.
Key words: gamma-ray burst: individual — Submillimeter: galaxies — X-rays: bursts

1. Introduction
Gamma-ray Bursts (GRBs) are among the most powerful explosions in the universe and are observationally characterizedaccording to intense short ﬂashes mainly occurring in the highenergy band (so-called prompt emission) and long-lived afterglows seen from the X-ray bands to radio bands. GRBs are
emitted in extremely relativistic jets with Lorentz factor larger
than ∼100 emanating from newly born black holes or magnetars, making it an ideal showcase of relativistic astrophysics(e.g., Mészáros & Rees 1997). Thanks to their exceptional
brightness they are visible up to very high redshifts, providing a unique window on the early Universe and the formationof the primordial stars (e.g., Tanvir et al. 2009; Bloom et al.
2009). The majority of long GRBs are believed to be produced
at the deaths of massive stars. It is therefore expected that
GRBs existed at redshifts where no luminous AGN or galaxies
had been formed yet. Long GRBs are excellent probes for theera of the ”ﬁrst star” formation and cosmic re-ionization. On
the other hand, Short-GRB have long been suspected on theoretical grounds to arise from compact object binary mergers(NS-NS or NS-BH). The unprecedented observation of shortGRB (GRB170817) coincident with the detection of gravitational wave (GW170817) from coalescing binary NSs in an
elliptical galaxy presents the long-awaited smoking gun thatbinary NS mergers give rise to short-GRB170817 (Abbott etal. 2017). Furthermore GRBs is one of the EM counterpart ofneutrino events (Murase & Ioka 2013). Therefore, GRBs havebeen playing one of the central roles of the multi-messenger
astronomy.
2. GRB afterglows and advantages
ter/submillmiter observationsofmillime-Characterizations of the radiation are the most fundamental observational approach for the GRB/transient studies.
Observations in radio bands are critical for characterizing theradiations from GRB afterglows and related transient phenomenon. GRBs are believed to result from the conversionof the kinetic energy of ultra-relativistic particles or possibly

the electromagnetic energy of a Poynting ﬂux to radiation inan optically thin region. In this case, the model accepts various
central engines (e.g. massive star explosion for long GRBs,
and mergers for short GRBs (Figure 1). This generic ”ﬁreball”
model (Mészáros & Rees 1997) has also been conﬁrmed by
the afterglow observations. Therefore, it is reasonable to expect that afterglows can be described by synchrotron emission
from a decelerating relativistic shell that collides with an external medium (external synchrotron shock model; Figure 1 &2).
Based on the external synchrotron shock model, both thespectrum and the light curve consist of several power-law segments with related indices (e.g., Sari et al. 1998). The broadband spectrum is characterized by the synchrotron peak frequency νm and the peak spectrum ﬂux density Fv,max asshown in Figure 2. These two parameters are key to themodel. The peak frequency is expected to fall at lower frequencies (X-ray to radio) over time (minutes to several days) asνm ∝ t−3/2 . The peak spectra ﬂux density Fv,max is predicted
to remain constant in the circumburst model, while decrease as
Fv,max ∝ t−1/2 in the wind model. Expected critical differences between short and long GRBs are (1) explosion energy
scale (short events could be much smaller e.g. E ∼ 1051−52erg), (2) circumburst density (events must be thin density condition e.g. n < 0.1 cm). Since characterizing frequency andpeak ﬂux are fully dependent on these two physical parameters,
constraining on Fv,max and νm using temporal and spectrum
observations are crucial. As shown in Figure 2, the radio bands(e.g. ngVLA, ALMA) are critical for characterizing the ﬂux
peak and its temporal evolutions.
Based on the external synchrotron shock modeling for actual afterglow observing data, physical parameters can be estimated (explosion energy E, circumburst number density n,observing angle θjet , synchrotron slope p, magnetic ﬁeld energy fraction ϵB , and accelerated particle energy density fraction ϵE ). Table 1 is the summary for several examples based on
multi-frequency observations (van Eerten et al. 2012; Cenko et
al. 2013; Urata et al. 2014; Urata et al. 2015b; Huang et al.
2017). Note that these parameter estimations were performedby assuming 100% of electrons are accelerated at the shocks.

2[Vol. ,

Fig. 1. The concept of GRB energetics (red box). Based on the standard “ﬁreball” model, various central engine (i.e. varisou kind of GRBs) exist.
The total energy of GRB could be estimated by observing prompt and afterglow observations. In the afterglow phase, the fraction of non-energizedelectrons are critical for estimating the total energy (Toma et al. 2008). Since the non-energized electrons do not emit obserable emission, the Faraday
depolarization is the key. In the purple box, the ﬁrst demostration of the Faraday depolarization is presented (Urata et al. 2019).

Table 1. Summary of burst parameters obtained by observations and numerical modeling

Parameters
Category
src
Epeak
(keV)
Eiso (erg)zθjet (rad)E (erg)n (cm−3 )θobs (rad)
p
ϵB
ϵe
Data

020903
XRF
3.3+1.8−1.0
49
1.4+18.0−0.7 × 10
0.251
0.10
5.9 × 1052
1.1
0.21
2.8
1.4 × 10−3
2.9 × 10−1
Opt, Radio

080330XRF(XRR?)< 88< 2.2 × 1052
1.51
0.12
2.3 × 1052
9.0
0.12
2.1
1.6 × 10−1
1.4 × 10−1
Opt990510GRB
423+42−42+0.3
2.1−0.3
× 10531.619
0.075
1.8 × 1053
0.03
0 (ﬁxed)
2.28
4.6 × 10−3
3.7 × 10−1
X,Opt,Radio

131030AGRB
406 ± 2253
3.0+2.0−0.2 × 10
1.293
0.15
3.4 × 1052
0.3
0 (ﬁxed)
2.1
4.4 × 10−2
2.7 × 10−1
Opt, ALMA

120326A
XRR
107.8+15.3−15.3
52
3.2+0.4−0.3 × 10
1.798
0.14
3.9 × 1052
1.0
0 (ﬁxed)2.5 (ﬁxed)
1.0 × 10−3
6.9 × 10−1
Opt

PTF11agg
on-axis orphan(?)−−
0.5< z <3.0
0.20
9 × 1052
0.001
0.19
3.0
4 × 10−2
2 × 10−1
Opt, RadioNo. ]

3

Fig. 2. Temporal evolution of forward shock synchrotron radiation with typical explosion parameters (left). One of example of observed SED evolution
using VLA and SMA (right).

Measurements of acceleration fraction are therefore required
for estimating actual energetics (§ 3).
3. Energetics and acceleration efﬁciency
The energetics of GRBs are fundamental physical parameters that cannot only reveal their progenitor systems but
also probe both the early and current states of the universe.
Although substantial observational efforts have been madesince the afterglow discovery, the total energies have been
estimated so far without considering non-energized (or nonaccelerated), cool electrons at the relativistic collisionlessshock (Figure 1) that do not emit observable radiation (Tomaet al. 2008), while the existence of such cool electrons is wellstudied for supernova remnants and solar winds (van Adelsberg
et al. 2008; Vink et al. 2015). As shown in Figure 1, a clear
method for identifying non-energized electrons in GRB afterglows is the measurement of their Faraday effect that suppresses the radio polarization but keeps the higher frequency(e.g. optical) one as emitted (Toma et al. 2008). The intensive studies of non-energized electrons in a number of afterglows will make a revolutionary change in the GRB progenitors, because diversity of GRB afterglows are likely linking
with types of progenitor stars.
Quite limited theoretical works investigated the fraction of
non-energzied electrons in GRB studies. One of the criticalreasons was the absence of a suitable method to constrain thefraction before the ALMA era. Hence, most related theoretical models simply assume that all the electrons are energizedby the ultra-relativistic shock and emit the observed afterglow(100% energized case). The measurement of Etot = Eγ +EAGof ∼ 1051 erg has been estimated for several events, and thestandard mass of progenitor star ∼ 30M ⊙ is preferred forclassical long GRBs. Here, Eγ and EAG are radiated energies in prompt and afterglow phases as synchrotron radi-

ations, respectively. However, a substantial fraction of theelectrons likely remain cool at the moment of the shock anddo not emit observable radiation (i.e., high synchrotron absorption)(Toma et al. 2008). In this case, however, the energy related to this non-energized electron component, hiddenenergy EAG,h , increases the required total energy, which is,
Etot = Eγ + EAG + EAG,h . Theoretically, the revised Etot
could be larger than 1052 erg (Mészáros & Rees 2010; Toma etal. 2011). In this extreme but quite expectable case, the standard scenario of the ∼ 30M⊙ progenitor would be inapplicable; however, extremely massive stars (e.g. ∼ 300M⊙ ) would
be required to be progenitors of such GRBs. Therefore, observationally identifying of the non-energized electron component
is essential and a unique approach to constraining the mass of
GRB progenitors.
Urata et al. (2019) demonstrated the ﬁrst linear polarization
detection on GRB171205A and the weak detection implied theexistence of non-energized cool electrons assuming optical linear polarization of 1% (the puple box of Figure 1). Following
the successful measurement, two more polarimetry have been
executed with coordinated optical polarimetry for precise measurements of non-energized electrons.
ngVLA can enhance this study for applying this new method
for various types of GRBs. The high sensitivity of ngVLA canapply the method for most of all types of GRBs including short
GRB afterglows that are hardly characterized using current instruments. Multi-frequency polarimetry at the same day with
ngVLA also can provide accurate measurements of the fractionof non-energized cool electrons by characterizing the polarization spectrum (Figure 1).
4. Characterization of High-z GRBs
GRBs are currently being exploited as probes of the early
Universe. Afterglow observational studies have revealed that

4[Vol. ,

Flux density [mJy]

0.1

0.01

10−3

10−4
0.1

1

10

Time after bursts [days]

100

Fig. 3. Expected afterglow light curves at z = 12. The dashed line
indicates the 5σ sensitivity of ngVLA. The black line indicates themodel function based on the numerical modeling for high-z (z = 8.3)
GRB090423 (Figure 5).

Fig. 5. IR K-band light curve of GRB090423 (z = 8.3) with external
synchrotron model. The green line indicates the best ﬁtted model basedon external forward shock synchrotron radiation. The corresponding
model lines for this event are also shown in Figure 3 & 4 with the blacklines.The red boxes indicate the observing window using autonomous
IR follow-ups by future satellite missions such as HiZ-GUNDAM and
THESEUS.

Flux density [mJy]

0.01

10−3

10−4
1

10

Time after burst [days]

100

Fig. 4. Expected afterglow light curves at z = 30. The dashed line
indicates the 5σ sensitivity of ngVLA. The black line indicates themodel function based on the numerical modeling for high-z (z = 8.3)
GRB090423 (Figure 5).

the majority of long GRBs occur as a result of the death of massive stars (e.g., Stanek et al. 2003). Since the highest-z eventsat the reionization epoch (z ∼9) have already been observed(Tanvir et al. 2009), and their discovery at z > 10 is highly possible (e.g., Bloom et al. 2009), long GRBs are unique and powerful means to study explosions of ﬁrst-generation stars and to
explore reionization and dusts creation history by GRB as lightsource (e.g., Totani et al. 2006; Perley et al. 2010; Jang et al.
2011). Hence, understanding of the physical properties of long
GRBs could probe the physical conditions of early Universe
and ﬁrst-generation stars.
The brightness of GRB afterglows are generally brighter
than other possible explosions from the ﬁrst generation stars
such as pair-instability supernova. Figure 3 shows the expected
afterglow light curves at z = 12 in the 16 GHz band basedon physical parameters obtained by numerical forward shockmodelings for high-z (e.g., Figure 5) and lower-z GRB after-

glows (e.g., Table 1). As described in §2, these light curveand SED monitoring will provide explosion physical parameters. Thanks to high sensitivity of ngVLA, the fraction of
non-energized cool electrons are also possible (described in§3). These physical parameters and total energetic measurements therefore enable to reveal the properties of ﬁrst generation stars.
The high signal-to-noise ratio (S/N) observations may beable to search for atomic and molecular absorption lines andthen enable to estimate redshift for these high-z GRBs (Inoueet al. 2007). Current redshift estimation of GRBs is relied on
Ly α break, metal absorption lines in afterglows spectrum andemissions lines from host galaxies through OIR spectroscopy.
Since OIR afterglow is generally faded out rapidly, timely OIR
spectroscopy is required (e.g. Figure 5). These observations
are sometimes affected by weather and available instrumentson large OIR telescopes. The lifetime of radio afterglows in
the ngVLA band is usually long (Figure 3) and high sensitivity
ngVLA observations would be able to measure redshifts once
the atomic and molecular absorption method is established.
The accurate position determined by ngVLA for high-z GRBs
also enable to perform deep IR searches for their host galaxies using JWST and future large missions (e.g. LUVOIR). The
host galaxies study at high-z using ngVLA may not be appropriate, because the long life time of radio afterglows.
In 2020th/2030th, two satellite missions, HiZ-GUNDAM(Yonetoku et al. 2020) and THESEUS (Amati et al. 2018)
are planned and would be able to provide GRB alerting up
to z ∼ 12 − 14 with reasonable position accuracies (i.e. arcsec level with IR counterparts or several arcmin without IR
counterparts) . These missions will select high-z GRBs based
on OIR photometric redshift method. In this case, these missions may not be able to provide timely GRB alerts for OIR
spectroscopy on very high-z events (e.g., only single detec-

No. ]

5tion in the red band). Although OIR follow-up may not beable to determine their redsfhits, the coordination with ngVLA
follow-ups would provide new approaches. By responding
to these high-z GRB position alerts, ngVLA can detect the
majority of GRB afterglows (Figure 3). In addition to thesesatellite missions, the planned Large Submillimeter Telescope(LST; Kawabe et al. 2016) would provide high-z GRBs up toz ∼ 30 by the high cadence radio time-domain surveys for radio ﬂashes originated from GRB reverse shock. The associatedafterglow from forward shock can be also observed by ngVLA(Figure 4).
5. GRB/SN-SNR connection
The long GRBs are originated from massive stellar explosions. This conﬁrmation was made by the optical spectroscopy
in the afterglow phase (∼10 days-several week) by identifying
SNe component resemble to the type Ic supernova and featurebroad lines implying fas moving ejecta with velocities ∼ 0.1c
(Mazzali et al. 2007; Hjorth & Bloom 2012). If there is an associated SNe, jets are not the only ejecta expelled in the GRB
events, a spherical outﬂow is also present. After the explosion,
the SN ejecta will remain in free expansion for a few decades,
and will sweep up material from the surrounding medium. The
SN ejecta interacts with the surrounding medium, accelerating particles to relativistic speeds and amplifying the magnetic
ﬁeld, producing radio synchrotron emission much like in a typical SN remnant.
The radio emission peaks when SNe has swept up an equivalent mass to the initial ejected mass, at the Sedov-Taylor time.
Due to the explosion velocity measured by optical spectrum
is large, the Sedov-Talyor time of GRB is expected to be 2
orders of magnitude shorter than typical supernova remnant(Barniol Duran & Giannios 2015). The expected time scale
is 10∼several 10 years, although it also depends on several
parameters as same as radiation from GRB jets. With thisshorter timescale than typical SNR, observations of SNR associated with GRB could be managed for ∼ 10 GRB samples
at z < 0.2 (Barniol Duran & Giannios 2015). Detecting theradio rebrightnening of long GRB caused by the associate SNtransform into an SNR would enrich the understanding of thestellar explosion.
Several efforts have been made for ﬁnding the radio radiations. VLA observed three nearby SNe associated GRB andconstrained on the density of the surrounding medium for various assumed values of the microphysical parameters related tothe magnetic ﬁeld and synchrotron-emitting electrons (Peters
et al. 2019). The ALMA observations targeted on host galaxies (around ∼ 10 years after the GRB) detected an unexpectedradio variability which is not explained by the external synchrotron shock radiation. Since the early phase of X-ray andoptical afterglows can be explained by the external synchrotron
shock model, the radio variability may be related with SNe
and/or SNR radiations (Huang et al. 2021 in prep.). Theseobservations are suffering from the sensitivity limitation of thecurrent instruments. Further detailed observations with highsensitivity are therefore required. ngVLA has two advantages;(i) characterization of their radiation and measuring physicalparameters by high sensitivity ﬂux measurements and (ii) di-

Fig. 6. Top: Expected SED temporal evolution of short GRB afterglow
at z = 1. Bottom: Expected light curves of short GRBs at various redshifts. The redshift range of currently observed short GRBs are 0∼2.5.
ngVLA can characterized majority of short GRBs.

rect imaging of the expansion with high sensitivity and angularresolution capabilities.
6. Short GRBs
Short GRBs are sudden ﬂashes of γ-rays with durations
shorter than 2 seconds. Their γ-ray properties as well as theirenvironments (e.g., ambient density and location in their hostgalaxies) are consistent with the popular scenario of neutronstar (NS) mergers. The ﬁrst GW event from a NS merger, GW
170817, was observed by LIGO and Virgo and associated withthe short GRB (Abbott et al. 2017). A bright kilonova emission was also discovered in UV, optical and near-IR bands.
Furthermore, the X-ray afterglow started to rise several days
after the GW trigger revealing the presence of a relativistic jet
viewed off-axis. These rich results are basically described by
the off-axis short GRB model with r-process elements synthesized in the ejecta (e.g., Alexander et al. 2017; Haggard et al.
2017; Lazzati et al. 2017; Murguia-Berthier et al. 2017; Ioka,& Nakamura 2018; Jin et al. 2018; Kathirgamaraju et al.
2018; Troja et al. 2018; Troja et al. 2019). However, the as-6[Vol. ,

Fig. 7. Expected short GRB afterglow light curves at z = 0.3 in the 100
GHz band along with jet viewing angles.

sociation of short GRBs and GWs is still unclear due to theatypical properties of GRB 170817A. After the discovery of
GW 170817/GRB 170817A, understanding the nature of short
GRBs, in general, becomes incredibly important. Estimationsof explosion parameters of on-axis short GRB are therefore
critical as the solid on-axis template for understanding of theoff-axis origin of GRB170817A. The broader community also
pays attention to these estimations, because of the connection
between short GRBs and GW transients.
As VLA, SMA and ALMA demonstrated multi-frequency
observations for long-GRBs, the radio observations have beenplaying a key role in deriving burst explosion physical parameters by characterizing of the synchrotron radiation (e.g., vanEerten et al. 2012; Urata et al. 2014; Urata et al. 2015a; Huang
et al. 2017). Using the same method, the uniﬁed picture of
the off-axis jet model for long GRBs was veriﬁed by examining the properties of X-ray Flash (XRFs; Urata et al. 2015b).
Unlike long GRBs, short GRB afterglows tend to faint possibly due to low circumburst density or smaller explosion energy. These properties make short GRB afterglows fainter thanthose of long GRBs and the observations of short GRB afterglows require higher sensitivities and much rapid response in
the higher frequency side. Figure 6 is the expected spectrum
temporal evolution at z = 1 and light curves at various redshiftusing lower circumburst density and smaller explosion energy(other micro physical parameters are identical to those of long
GRBs shown in Figure 2 left). Thanks to high sensitivity of
ngVLA, short GRB afterglows even at higher redshift (e.g.,z = 1) could be observed (Figure 6). Since the currently observed redshift range of short GRBs is 0 < z < 2.5 (average ofz ∼ 0.5), ngVLA could provide concrete on-axis template forunderstanding of off-axis short GRBs by observing majority ofshort events.
ngVLA would also be able to characterize off-axis events
Figure 7 shows expected short GRB afterglows along withviewing angles (i.e., off-axis short GRB orphan afterglows).
Since off-axis short GRBs in their prompt phase become softer
spectrum and fainter apparent luminosity, position alerts from
GRB satellites may not be available for these off-axis events.

Fig. 8. One of examples of unique stellar transient, AT2018cow as thecandidate of EM counterpart of neutrino. The radiation is characterized
by synchrotron radiation. As identical to the GRB external synchrotron
model, the peak frequency shifts to lower frequency side along withtime. Radio polarimetry is also the critical tool for revealing their nature (Huang et al. 2019).

In this case, coordinated observations with wide ﬁeld time domain surveys in OIR (e.g., LSST) and radio (e.g., SKA) are
required.
7. Transients as high energy cosmic-rays and neutrino
sources
The origin of the highest-energy cosmic rays is believed to
be extragalactic, but their acceleration sites remain unidentiﬁed. High-energy neutrinos are expected to be produced in ornear the acceleration sites when cosmic rays interact with matter and ambient light, producing charged mesons that decay
into neutrinos and other particles. Unlike cosmic rays, neutrinos can travel through the Universe unimpeded by interactions
with other particles and undeﬂected by magnetic ﬁelds, providing a means to identify and study the extreme environments
producing cosmic rays. The ﬁrst EM counterpart of high energy neutrinos was identiﬁed as blazar (IceCube Collaboration
et al. 2018b; IceCube Collaboration et al. 2018c). The measurement has a major impact on our view of the non-thermal
universe, but understanding cosmic accelerators require a substantial increase in the number of multi-messenger observations.
As IceCube and the multi-messenger community demonstrated, high-energy γ-ray observations using Fermi/LAT were
key to identifying the counterpart (IceCube Collaboration et al.
2018c). Although more than 60 neutrino events were followedup by the same method using Fermi after the 1st EM counterpart discovery in 2017, none of them were identiﬁed as
the 2nd neutrino counterpart. This indicates that the originof the events is not only blazar and the search method using
Fermi/LAT are inadequate. Other possible EM counterpartsof high-energy neutrinos are proposed to be tidal disruption
ﬂares (TDE; Wang et al. 2011; Wang & Liu 2016; Senno etNo. ]
al. 2017; Dai & Fang 2017; Lunardini & Winter 2017) and
GRBs (Murase & Ioka 2013) that also share similar ultrarelativistic jets. In addition to these transients, Huang et al.
(2019) also demonstrated with the ALMA polarimetry that the
new type of optical transient (i.e. fast-rising blue optical transient, AT2018cow type of stellar explosion) is possible counterpart. In fact, IceCube also reported the possible neutrinos detection from AT2018cow (IceCube Collaboration et al. 2018a).
All of these transients have the mm/submm counterparts invarious time scales and characterizing their synchrotron radiation would provide explosion parameters by identifying thesynchrotron peak frequency and the peak spectral ﬂux density(e.g., Zauderer et al. 2011; Huang et al. 2019). Figure 8 shows
one of the examples of radio ﬂux and polarimetric monitoringon AT2018cow. These observations revealed that new typesof transients discovered by the synoptic optical transient survey is a unique stellar explosion and possible PeV cosmic-ray
accelerator (Huang et al. 2019). The high sensitivity ngVLA
photometric and polarimetric monitoring can therefore reveal
the nature of various transients and promote multi-messenger
astronomy.
8. Synergy with other future missions
8.1. GRB satellite missions
There is no doubt that dedicated transient satellite missions
with multi-frequency capability are required for promoting
further GRB cosmology and multi-message astrophysics as
the Swift mission demonstrated. In 2020th and 2030th, HiZGUNDAM (Yonetoku et al. 2020) and THESEUS (Amati etal. 2018) are planned by JAXA and ESA, respectively. Theirautonomic IR observations from soon after explosions are key
to use GRBs as probe of the early universe. Localization ofshort GRBs (arcmin level) and its high-sensitivity IR (equivalent to ground-based 4-m class telescope) monitoring arealso essential to characterize GW transients and associatedkilo/macronova radiation caused by NS-NS or NS-BH mergers. Hence, coordinations with these missions are key to promote sciences described in §4 and §6.
These GRB missions will also provide other high energy
transients. One of the critical ﬁndings by the hard X-ray surveyof Swift was the ﬁrst TDE (Burrows et al. 2011). Followed by
the discovery, numbers of TDE have been identiﬁed by opticalwide ﬁeld surveys (e.g., Gezari et al. 2012). ngVLA followups will characterize their non-thermal radiation and promote
multi-messenger astronomy (§7).
8.2. OpticalTransient Surveys
Time-domain surveys in various wavelengths have been
making mysterious new transients discoveries. These results
are remarkable, and newly discovered transients are revolutionizing our knowledge of astronomy and astrophysics. Opticaluntargeted imaging surveys such as CRTS, PTF, Pan-STARRS1, ZTF (Drake et al. 2009; Law et al. 2009; Chambers et al.
2016; Bellm et al. 2019). have also been discovering new stellar explosions such as super-luminous super nova (SLSNe), energetic SNe, and fast-rising blue optical transient (i.e. FBOTsuch as AT2018cow in §7). Since SLSNe and energetic SNe
are suggested to have the common powerful central energy

7Fig. 9. Uniﬁcation picture of GRBs along with jet viewing alngle andLorentz factor (or power).The background ﬁgure is from Nakar, &Piran 2017.

source to that of GRBs (e.g., Pastorello et al. 2010; Kann etal. 2019; Izzo et al. 2020), the uniﬁcation picture of massive
stellar transient along with GRB jet viewing angle (identical tothat of AGN; Figure 9) may be studied by the direct imaging of
GRB-SNR (§5). As described in §7, the AT2018cow type newtransient is one of potential neutrino counterparts. In the era ofngVLA, numbers of optical time domain surveys using smaller
telescopes may be available. As AT2018cow was discoveredby the ATLAS 0.5 m telescope, these optical surveys would bevery powerful to promote science cases described in §5 & §7.
A coordination with radio transient surveys such as SKA
and LST may be enable to make further understanding of offaxis short GRB afterglow (i.e. orphan GRBs) related with GW
counterparts (§6). Although ngVLA can characterize the properties of off-axis short GRB afterglows (Figure 7), the ﬁndingof events must be performed by other instruments (e.g., Lambet al. 2018; Huang et al. 2020). Since GRB satellite missions
are inadequate for ﬁnding off-axis short GRBs, the wide ﬁeld
time domain surveys in OIR (e.g. LSST) and radio (e.g. SKA,
LST) are required. Especially, radio survey may be efﬁcientfor ﬁnding of off-axis short GRB orphan afterglows because of
sensitivities of instruments, life time of afterglows, and classiﬁcation of transients.

This work is supported by the Ministry of Science andTechnology of Taiwan grants MOST 105-2112-M-008-013MY3 (Y.U.).
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