Swift Observations of GRB 210919A

A. Tohuvavohu (U. Toronto), M. Perri (SSDC and INAF-OAR) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 00:28:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located a short GRB 210919A (trigger=1073893) (Tohuvavohu et al. GCN Circ. 30846). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 96° from the Sun (6.4 hours West) and 104° from the 96%-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.

Rossi et al. (GCN Circ. 31453) determined a redshift of 0.2411 from LBT. 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 Barthelmy et al. (GCN Circ. 30863), the BAT ground-calculated position is RA, Dec = 80.242, 1.286 deg which is RA(J2000) = 05h20m58.1s Dec(J2000) = +01°17'08.8" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%.

The BAT light curve (Figure 1) showed a short pulse of a duration ~ 1 s. T90 (15-350 keV) is 0.16 ± 0.03 s (estimated error including systematics).

The time-averaged spectrum from T+0.05 to T+0.24 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.58 ± 0.27. The fluence in the 15-150 keV band is 8.8 ± 1.7 x 10-8 erg cm-2. This fluence is larger than that of 68% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.35 s in the 15-150 keV band is 1.5 ± 0.3 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/1073893/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 30854). We have analysed 1.5 ks of XRT data for GRB 210919A, from 105 s to 1.6 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210919A 100 s after the BAT trigger (Siegel and Tohuvavohu GCN Circ. 30857). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 30850) 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 EB-V of 0.141 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
05h21m01.05s +01°18'42.1" 4.7" XRT-final UKSSDC
05h21m01.08s +01°18'41.5" 4.6" XRT-enhanced Goad et al. GCN Circ. 30850
05h20m58.1s +01°17'08.8" 2.0' BAT-refined Barthelmy et al. GCN Circ. 30863

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30847 Swift GRB 210919A: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 30848 BOOTES-2/TELMA optical upper limit BOOTES-2 upper limits
Optical Strausbaugh and Cucchiara 30849 LCO Optical Upper Limit LCO upper limits
Optical Pankov et al. 30851 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Optical Perley et al. 30852 NOT upper limits NOT upper limits
Optical Takamatsu et al. 30855 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Kim et al. 30856 LOAO and LSGT optical upper limits LOAO upper limits
Optical Zhang et al. 30858 Keck NIRC2 Kp band upper limits Keck upper limits
Optical 30860 Lowell Discovery Telescope upper limits
(correction to GCN Circ. 30859)		 Lowell Discovery Telescope upper limits
Optical Pierel et al. 30868 DECam Upper Limits Dark Energy Camera upper limits
Optical Kann et al. 30883 CAHA 2.2m Observations: Afterglow/Host
Galaxy Candidate		 CAHA
Optical Kann et al. 30884 OSN 1.5m Source Confirmation Obs.de Sierra Nevada detection
Optical Hasuda and Sakamoto 30888 iTelescope T-11 upper limits iTelescope upper limits
Optical O'Connor et al. 30934 LDT optical observations and candidate
host galaxies		 Lowell Discovery Telescope
Optical Kann et al. 30983 FORS2/VLT observations VLT/FORS2
Optical Rossi et al. 31453 LBT spectroscopic redshift of the host
galaxy candidates		 LBT redshift
Optical Pankov et al. 31567 Maidanak optical observations Maidanak
X-ray Sakamoto et al. 30879 Chandra rapid ToO upper limit Chandra upper limits
Gamma-ray Minaev and Pozanenko 31566 SPI-ACS/INTEGRAL observation and
evaluation of Ep		 INTEGRAL Epeak=73 (11 -- 516) keV

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 100 249 147 >20.8
uFC 312 561 246 >19.9
white 100 1193 343 >21.4
v 643 1244 70 >18.7
b 568 1342 63 >19.3
u 312 1318 304 >19.8
w1 693 1293 58 >19.7
m2 1248 1268 19 >19.0
w2 618 1044 39 >19.4

Table 3. UVOT observations reported by Siegel and Tohuvavohu (GCN Circ. 30857). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

February 11, 2022