Swift Observations of GRB 200922A

M.J. Moss (GWU), A. D'Ai (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 12:06:46 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200922A (trigger=997024) (Moss et al. GCN Circ. 28471). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 105° from the Sun (7.8 hours East) and 50° from the 33%-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.

Moss et al. (GCN Circ. 28471) reported the discovery with UVOT of an optical afterglow. Izzo et al. (GCN Circ. 28478) reported the position from LCO for the optical afterglow of this GRB. 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 Sakamoto et al. (GCN Circ. 28484), the BAT ground-calculated position is RA, Dec = 296.954, -55.199 deg which is RA(J2000) = 19^h47^m49.0^s Dec(J2000) = -55°11'55.1" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 74%.

The mask-weighted light curve (Figure 1) shows a single FRED-like pulse that starts at ~T-2 s, peaks at ~T+0, and ends at ~T+16 s. T₉₀ (15-350 keV) is 10.30 ± 1.96 s (estimated error including systematics).

The time-averaged spectrum from T-1.91 to T+16.22 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.43 ± 0.26, and E_peak of 39.3 ± 5.7 keV (χ² 52.48 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.8 ± 0.1 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T-0.46 s in the 15-150 keV band is 11.4 ± 0.5 ph cm^-2 s^-1. This fluence is larger than that of 70% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 2.12 ± 0.06 (χ² 76.50 for 57 d.o.f.). 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/997024/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 28476). We have analysed 26 ks of XRT data for GRB 200922A, from 81 s to 354.9 ks after the BAT trigger. The data comprise 123 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 28473).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.150 (+0.023, -0.022).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.33 (+0.18, -0.17). The best-fitting absorption column is 1.6 (+0.5, -0.4) x 10²¹ cm^-2, in excess of the Galactic value of 4.7 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.21 (+0.24, -0.23) and a best-fitting absorption column of 2.5 (+0.9, -0.8) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.5 (+0.9, -0.8) x 10²¹ cm^-2
Galactic foreground: 4.7 x 10²⁰ cm^-2
Excess significance: 4.4 σ
Photon index: 2.21 (+0.24, -0.23)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00997024.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200922A 86 s after the BAT trigger (Siegel and Moss GCN Circ. 28482). We identify a fading source near the XRT position (Osborne et al. GCN Circ. 28473) and also at the location of a weak source in the DSS. 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 0.05 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

19^h47^m48.76^s -55°12'16.5" 0.5" UVOT GCN Circ. 28482

19^h47^m48.87^s -55°12'15.9" 1.5" XRT-final UKSSDC

19^h47^m48.87^s -55°12'15.9" 1.5" XRT-enhanced Evans et al. GCN Circ. 28483

19^h47^m49.0^s -55°11'55.1" 1.0' BAT-refined Sakamoto et al. GCN Circ. 28484

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 28477 Swift GRB 200922A: Global MASTER-Net
observations report MASTER

Optical Izzo et al. 28478 LCO observations LCO

Optical Malesani et al. 28492 chance superposition between optical
afterglow and archival star light curve

Optical Pozanenko et al. 28505 Chilescope optical observations

Gamma-ray Zhang et al. 28490 Insight-HXMT/HE detection Insight-HXMT T₉₀=1.87 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 86 236 147 14.37±0.02

white 4770 4970 196 17.51±0.04

white 28889 28909 19 18.62±0.20

v 3745 3945 196 17.35±0.10

v 9268 23149 1460 17.79±0.05

b 4566 4765 196 17.95±0.07

b 27982 28889 881 18.49±0.05

u 4360 4560 196 17.18±0.06

u 15921 16791 848 18.20±0.06

uvw1 4156 4355 196 17.23±0.10

uvw1 11089 15915 887 18.26±0.09

uvw1 50455 56029 225 18.97±0.27

uvm2 3950 4150 196 17.12±0.12

uvm2 10182 11081 885 18.19±0.10

uvm2 50396 55850 223 19.26±0.36

uvw2 3541 5089 307 17.14±0.09

uvw2 21653 22553 885 18.77±0.12

uvw2 50338 55672 223 >19.44

RA (J2000)	Dec (J2000)	Error	Note	Reference
19^h47^m48.76^s	-55°12'16.5"	0.5"	UVOT	GCN Circ. 28482
19^h47^m48.87^s	-55°12'15.9"	1.5"	XRT-final	UKSSDC
19^h47^m48.87^s	-55°12'15.9"	1.5"	XRT-enhanced	Evans et al. GCN Circ. 28483
19^h47^m49.0^s	-55°11'55.1"	1.0'	BAT-refined	Sakamoto et al. GCN Circ. 28484

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	28477	Swift GRB 200922A: Global MASTER-Net observations report	MASTER
Optical	Izzo et al.	28478	LCO observations	LCO
Optical	Malesani et al.	28492	chance superposition between optical afterglow and archival star		light curve
Optical	Pozanenko et al.	28505	Chilescope optical observations
Gamma-ray	Zhang et al.	28490	Insight-HXMT/HE detection	Insight-HXMT	T₉₀=1.87 seconds

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	86	236	147	14.37±0.02
white	4770	4970	196	17.51±0.04
white	28889	28909	19	18.62±0.20
v	3745	3945	196	17.35±0.10
v	9268	23149	1460	17.79±0.05
b	4566	4765	196	17.95±0.07
b	27982	28889	881	18.49±0.05
u	4360	4560	196	17.18±0.06
u	15921	16791	848	18.20±0.06
uvw1	4156	4355	196	17.23±0.10
uvw1	11089	15915	887	18.26±0.09
uvw1	50455	56029	225	18.97±0.27
uvm2	3950	4150	196	17.12±0.12
uvm2	10182	11081	885	18.19±0.10
uvm2	50396	55850	223	19.26±0.36
uvw2	3541	5089	307	17.14±0.09
uvw2	21653	22553	885	18.77±0.12
uvw2	50338	55672	223	>19.44

Table 3. UVOT observations reported by Siegel and Moss (GCN Circ. 28482). 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.

September 27, 2020