Swift Observations of GRB 210112A

E. Ambrosi (INAF-IASFPA), P.A. Evans (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 01:37:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210112A (trigger=1016881) (Ambrosi et al. GCN Circ. 29289). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 90° from the Sun (5.0 hours West) and 81° from the 2%-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.

Ambrosi et al. (GCN Circ. 29289) reported the discovery 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 Stamatikos et al. (GCN Circ. 29297), the BAT ground-calculated position is RA, Dec = 219.005, 33.058 deg which is RA(J2000) = 14^h36^m01.3^s Dec(J2000) = +33°03'30.4" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 89%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-10 s and ends at ~T+220 s. The three main peaks occur at ~T+1 s, ~T+70 s, and ~T+105 s, respectively, and the one at ~T+70 s is the largest peak. T₉₀ (15-350 keV) is 107.60 ± 13.02 s (estimated error including systematics).

The time-averaged spectrum from T-10.44 to T+218.47 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.45 ± 0.03. The fluence in the 15-150 keV band is 2.33 ± 0.03 x 10^-5 erg cm^-2. This fluence is larger than that of 98.1% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+68.86 s in the 15-150 keV band is 20.5 ± 0.6 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/1016881/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 29295). We have analysed 49 ks of XRT data for GRB 210112A, from 64 s to 1078.9 ks after the BAT trigger. The data comprise 859 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 29290).

The late-time light curve (Figure 2) (from T0+6.4 ks) can be modelled with a power-law decay with a decay index of α=1.36 ± 0.03.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.56 ± 0.03. The best-fitting absorption column is 2.34 ± 0.14 x 10²¹ cm^-2, in excess of the Galactic value of 9.5 x 10¹⁹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.02 ± 0.09 and a best-fitting absorption column of 2.9 (+0.4, -0.3) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.9 (+0.4, -0.3) x 10²¹ cm^-2
Galactic foreground: 9.5 x 10¹⁹ cm^-2
Excess significance: 13.0 σ
Photon index: 2.02 ± 0.09

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01016881.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210112A 83 s after the BAT trigger (Siegel and Ambrosi GCN Circ. 29303). A source consistent with the XRT position (Goad et al. GCN Circ. 29290) is detected in the initial UVOT exposures. 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 0.014 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

14^h36^m01.49^s +33°03'15.3" 0.50" UVOT-refined Siegel and Ambrosi GCN Circ. 29303

14^h36^m01.47^s +33°03'15.4" 1.4" XRT-final UKSSDC

14^h36^m01.45^s +33°03'15.0" 1.7" XRT-enhanced Goad et al. GCN Circ. 29290

14^h36^m01.3^s +33°03'30.4" 1.2' BAT-refined Stamatikos et al. GCN Circ. 29297

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Novichonok et al. 29292 Kitab a optical upper limit upper limits

Optical Kann et al. 29296 OSN Reddened Afterglow Detection Obs.de Sierra Nevada detection

Optical Kann et al. 29300 OSN second epoch, decay Obs.de Sierra Nevada

Optical Gupta et al. 29301 1.3m DFOT Optical upper limit Devasthal Opt.Tel. upper limits

Optical Pankov et al. 29302 Mondy optical observations Mondy

Optical Kann et al. 29319 Deep CAHA 2.2m detection CAHA detection

Optical Dimple et al. 29333 3.6m DOT optical detection DOT detection

Optical Rossi and Kann 29456 LBT host detection and correction to
GCN Circ. 29319 (CAHA observations) CAHA

Gamma-ray Ursi et al. 29293 AGILE detection AGILE

Gamma-ray Svinkin et al. 29315 Konus-Wind detection Konus-Wind E_peak=214 (-16,+18) keV
Duration=~143 seconds
Fluence=6.72(-0.65,+0.72)x10^-5erg cm^-2

Gamma-ray Zheng et al. 29326 Insight-HXMT/HE detection Insight-HXMT T₉₀=12.27 seconds

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 83 233 147 19.54 ± 0.10

v 626 1419 97 >18.5

b 551 1517 97 >19.1

u 296 1493 304 >20.2

w1 675 1468 78 >19.1

RA (J2000)	Dec (J2000)	Error	Note	Reference
14^h36^m01.49^s	+33°03'15.3"	0.50"	UVOT-refined	Siegel and Ambrosi GCN Circ. 29303
14^h36^m01.47^s	+33°03'15.4"	1.4"	XRT-final	UKSSDC
14^h36^m01.45^s	+33°03'15.0"	1.7"	XRT-enhanced	Goad et al. GCN Circ. 29290
14^h36^m01.3^s	+33°03'30.4"	1.2'	BAT-refined	Stamatikos et al. GCN Circ. 29297

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Novichonok et al.	29292	Kitab a optical upper limit		upper limits
Optical	Kann et al.	29296	OSN Reddened Afterglow Detection	Obs.de Sierra Nevada	detection
Optical	Kann et al.	29300	OSN second epoch, decay	Obs.de Sierra Nevada
Optical	Gupta et al.	29301	1.3m DFOT Optical upper limit	Devasthal Opt.Tel.	upper limits
Optical	Pankov et al.	29302	Mondy optical observations	Mondy
Optical	Kann et al.	29319	Deep CAHA 2.2m detection	CAHA	detection
Optical	Dimple et al.	29333	3.6m DOT optical detection	DOT	detection
Optical	Rossi and Kann	29456	LBT host detection and correction to GCN Circ. 29319 (CAHA observations)	CAHA
Gamma-ray	Ursi et al.	29293	AGILE detection	AGILE
Gamma-ray	Svinkin et al.	29315	Konus-Wind detection	Konus-Wind	E_peak=214 (-16,+18) keV Duration=~143 seconds Fluence=6.72(-0.65,+0.72)x10^-5erg cm^-2
Gamma-ray	Zheng et al.	29326	Insight-HXMT/HE detection	Insight-HXMT	T₉₀=12.27 seconds

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	83	233	147	19.54 ± 0.10
v	626	1419	97	>18.5
b	551	1517	97	>19.1
u	296	1493	304	>20.2
w1	675	1468	78	>19.1

Table 3. UVOT observations reported by Siegel and Ambrosi (GCN Circ. 29303). 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.

February 11, 2021