Swift Observations of GRB 210104A

E. Troja (NASA/GSFC/UMCP), J.A. Kennea (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 11:26:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210104A (trigger=1015873) (Troja et al. GCN Circ. 29233). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 138° from the Sun (11.9 hours East) and 73° from the 71%-illuminated Moon. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.

Troja et al. (GCN Circ. 29233) reported the detection with UVOT of an optical afterglow. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

Standard analysis products for this burst are available at https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Palmer et al. (GCN Circ. 29251), the BAT ground-calculated position is RA, Dec = 103.771, 64.675 deg which is RA(J2000) = 06^h55^m04.9^s Dec(J2000) = +64°40'30.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 82%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-1 s and ends at ~T+45 s. Several prominent peaks occur at ~T+4 s, ~T+21 s, ~T+27 s, and ~T+34 s, respectively. T₉₀ (15-350 keV) is 32.06 ± 0.49 s (estimated error including systematics).

The time-averaged spectrum from T-1.00 to T+45.39 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.55 ± 0.04. The fluence in the 15-150 keV band is 8.8 ± 0.2 x 10^-6 erg cm^-2. This fluence is larger than that of 91.2% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+26.78 s in the 15-150 keV band is 12.1 ± 0.4 ph cm^-2 s^-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1015873/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 29260). We have analysed 13 ks of XRT data for GRB 210104A, from 69 s to 299.6 ks after the BAT trigger. The data comprise 1.8 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 29245).

The late-time light curve (Figure 2) (from T0+6.1 ks) can be modelled with a power-law decay with a decay index of α=1.36 ± 0.05.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.012 ± 0.028. The best-fitting absorption column is 1.10 ± 0.08 x 10²¹ cm^-2, in excess of the Galactic value of 5.1 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.92 (+0.11, -0.10) and a best-fitting absorption column of 1.24 (+0.31, -0.29) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (3.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.24 (+0.31, -0.29) x 10²¹ cm^-2
Galactic foreground: 5.1 x 10²⁰ cm^-2
Excess significance: 4.1 σ
Photon index: 1.92 (+0.11, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01015873.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210104A 71 s after the BAT trigger (Breeveld and Troja GCN Circ. 29247). A fading source is detected in all UVOT filters in the initial exposures, at the position given in Troja et al., (GCN Circ. 29233), and also seen by others (Xin et al., GCN Circ. 29235; Hosokawa et al., GCN Circ. 29237; Kim et al., GCN Circ. 29238; Lipunov et al., GCN Circ. 29239 and Horiuchi et al., GCN Circ. 29241). Table 3 gives preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc., 1358, 373). No correction has been made for the expected extinction in the Milky Way corresponding to a reddening of E_B-V of 0.04 mag. in the direction of the GRB (Schlegel et al. 1998).

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s^-1 illuminated-detector^-1. The vertical green dash-dotted lines show the T₅₀ interval, the vertical black dashed lines show the T₉₀ interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference

06^h55^m05.25^s +64°40'34.0" 1.10" UVOT-initial Troja et al. GCN Circ. 29233

06^h55^m05.38^s +64°40'34.1" 2.0" XRT-final UKSSDC

06^h55^m05.47^s +64°40'33.8" 1.4" XRT-enhanced Beardmore et al. GCN Circ. 29245

06^h55^m04.9^s +64°40'30.6" 1.0' BAT-refined Palmer et al. GCN Circ. 29251

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Xin et al. 29235 GWAC-F60A optical afterglow
observations Xinglong Obs.

Optical Hu et al. 29236 BOOTES-4/MET early optical afterglow
detection BOOTES-4 detection

Optical Hosokawa et al. 29237 MITSuME Akeno optical observation MITSuME Akeno detection

Optical Kim et al. 29238 Assy optical observations Assy-Turgen Obs. detection

Optical Lipunov et al. 29239 Fermi GRB 210104A: Global MASTER-Net
observations report MASTER

Optical Horiuchi et al. 29241 MITSuME Ishigaki optical observation MITSuME detection

Optical Lipunov et al. 29242 Swift GRB210104.88: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 29248 MASTER OT rebrightening detection MASTER detection

Optical Zhu et al. 29252 Nanshan/NEXT optical powerlaw decay Xinjiang Astro. Obs.

Optical Paek et al. 29254 DOAO optical afterglow observations DOAO

Optical Kumar et al. 29257 1.3m DFOT Optical Afterglow Detection DFOT detection

Optical Kim et al. 29265 Assy continued optical observations Assy-Turgen Obs. detection

Optical 29269 iTelescope optical upper limit iTelescope upper limits

Optical Anandagoda and Geier 29273 SARA-KP 0.9m Optical Afterglow
Detection SARA detection

Optical 29274 VIRT optical transient detection. Virgin Island Robotic Telescope detection

Optical Mao et al. 29275 GMG observations Gao-Mei-Gu

Optical Moskvitin and Vlasyuk 29277 SAO RAS optical observations SAO RAS detection

Optical Kim et al. 29283 Assy continued optical observations,
correction to GCN Circ. 29265 Assy-Turgen Obs. detection

Optical Belkin et al. 29286 Mondy optical observations, light curve Mondy light curve

Optical Nakamura et al. 29291 AROMA-N Optical Afterglow Detection AROMA-N detection

Optical Kann et al. 29318 Late-time CAHA 2.2m detection CAHA detection

Optical Dimple et al. 29337 3.6m DOT optical upper limit DOT upper limits

Optical Kool et al. 29405 ZTF21aagwbjr/AT2021buv: ZTF discovery
of a fast optical transient likely
associated with GRB 210104A

Optical Melandri and Rossi 29440 LBT observations LBT

Radio Smith et al. 29304 JCMT SCUBA-2 sub-mm observations SCUBA

Gamma-ray Fermi 29232 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Biltzinger et al. 29234 BALROG localization (Fermi Trigger
631452424 / GRB 210104477) BALROG

Gamma-ray Malacaria et al. 29246 Fermi GBM detection Fermi GBM E_peak=155±21 keV
T₉₀=32 seconds

Gamma-ray Frederiks et al. 29258 Konus-Wind detection Konus-Wind E_peak=157 (-18,+23) keV

Gamma-ray Cherry et al. 29268 CALET Gamma-Ray Burst Monitor detection CALET

Gamma-ray Nadella et al. 29299 AstroSat CZTI detection CZTI

Gamma-ray Svinkin et al. 29744 Konus-Wind detection Konus-Wind E_peak=240 (-35,+45) keV

Other Smartt et al. 29262 early ATLAS detections of the bright
afterglow detection

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter T_start(s) T_stop(s) Exp(s) Mag

white_FC 71 221 147 14.61 ± 0.02

white 562 582 20 16.23 ± 0.05

white 1512 1706 39 17.02 ± 0.05

b 538 558 20 16.59 ± 0.10

u_FC 283 533 246 15.28 ± 0.03

v 613 632 20 16.36 ± 0.16

uvw1 662 682 20 16.12 ± 0.16

uvm2 637 657 20 16.58 ± 0.25

uvw2 588 608 20 17.09 ± 0.26

RA (J2000)	Dec (J2000)	Error	Note	Reference
06^h55^m05.25^s	+64°40'34.0"	1.10"	UVOT-initial	Troja et al. GCN Circ. 29233
06^h55^m05.38^s	+64°40'34.1"	2.0"	XRT-final	UKSSDC
06^h55^m05.47^s	+64°40'33.8"	1.4"	XRT-enhanced	Beardmore et al. GCN Circ. 29245
06^h55^m04.9^s	+64°40'30.6"	1.0'	BAT-refined	Palmer et al. GCN Circ. 29251

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Xin et al.	29235	GWAC-F60A optical afterglow observations	Xinglong Obs.
Optical	Hu et al.	29236	BOOTES-4/MET early optical afterglow detection	BOOTES-4	detection
Optical	Hosokawa et al.	29237	MITSuME Akeno optical observation	MITSuME Akeno	detection
Optical	Kim et al.	29238	Assy optical observations	Assy-Turgen Obs.	detection
Optical	Lipunov et al.	29239	Fermi GRB 210104A: Global MASTER-Net observations report	MASTER
Optical	Horiuchi et al.	29241	MITSuME Ishigaki optical observation	MITSuME	detection
Optical	Lipunov et al.	29242	Swift GRB210104.88: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	29248	MASTER OT rebrightening detection	MASTER	detection
Optical	Zhu et al.	29252	Nanshan/NEXT optical powerlaw decay	Xinjiang Astro. Obs.
Optical	Paek et al.	29254	DOAO optical afterglow observations	DOAO
Optical	Kumar et al.	29257	1.3m DFOT Optical Afterglow Detection	DFOT	detection
Optical	Kim et al.	29265	Assy continued optical observations	Assy-Turgen Obs.	detection
Optical		29269	iTelescope optical upper limit	iTelescope	upper limits
Optical	Anandagoda and Geier	29273	SARA-KP 0.9m Optical Afterglow Detection	SARA	detection
Optical		29274	VIRT optical transient detection.	Virgin Island Robotic Telescope	detection
Optical	Mao et al.	29275	GMG observations	Gao-Mei-Gu
Optical	Moskvitin and Vlasyuk	29277	SAO RAS optical observations	SAO RAS	detection
Optical	Kim et al.	29283	Assy continued optical observations, correction to GCN Circ. 29265	Assy-Turgen Obs.	detection
Optical	Belkin et al.	29286	Mondy optical observations, light curve	Mondy	light curve
Optical	Nakamura et al.	29291	AROMA-N Optical Afterglow Detection	AROMA-N	detection
Optical	Kann et al.	29318	Late-time CAHA 2.2m detection	CAHA	detection
Optical	Dimple et al.	29337	3.6m DOT optical upper limit	DOT	upper limits
Optical	Kool et al.	29405	ZTF21aagwbjr/AT2021buv: ZTF discovery of a fast optical transient likely associated with GRB 210104A
Optical	Melandri and Rossi	29440	LBT observations	LBT
Radio	Smith et al.	29304	JCMT SCUBA-2 sub-mm observations	SCUBA
Gamma-ray	Fermi	29232	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Biltzinger et al.	29234	BALROG localization (Fermi Trigger 631452424 / GRB 210104477)	BALROG
Gamma-ray	Malacaria et al.	29246	Fermi GBM detection	Fermi GBM	E_peak=155±21 keV T₉₀=32 seconds
Gamma-ray	Frederiks et al.	29258	Konus-Wind detection	Konus-Wind	E_peak=157 (-18,+23) keV
Gamma-ray	Cherry et al.	29268	CALET Gamma-Ray Burst Monitor detection	CALET
Gamma-ray	Nadella et al.	29299	AstroSat CZTI detection	CZTI
Gamma-ray	Svinkin et al.	29744	Konus-Wind detection	Konus-Wind	E_peak=240 (-35,+45) keV
Other	Smartt et al.	29262	early ATLAS detections of the bright afterglow		detection

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	71	221	147	14.61 ± 0.02
white	562	582	20	16.23 ± 0.05
white	1512	1706	39	17.02 ± 0.05
b	538	558	20	16.59 ± 0.10
u_FC	283	533	246	15.28 ± 0.03
v	613	632	20	16.36 ± 0.16
uvw1	662	682	20	16.12 ± 0.16
uvm2	637	657	20	16.58 ± 0.25
uvw2	588	608	20	17.09 ± 0.26

Table 3. UVOT observations reported by Breeveld and Troja (GCN Circ. 29247). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

April 3, 2021