Swift Observations of GRB 221202A

R. Caputo (GSFC), P.A. Evans (U. Leicester) and P. Kuin (MSSL-UCL) for the Swift team

1. Introduction

At 20:06:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221202A (trigger=1142997) (Caputo et al. GCN Circ. 33005). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 98° from the Sun (11.5 hours West) and 80° from the 73%-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.

Kuin and Caputo (GCN Circ. 33016) 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

Analysis of the BAT data was reported by Barthelmy and Krimm (GCN Circ. 33015).

The mask-weighted light curve (Figure 1) shows a single peaked pulse.

The time-averaged spectrum from T+1.03 to T+12.44 s is best fit by a simple

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1142997/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 33010). We have analysed 14 ks of XRT data for GRB 221202A, from 82 s to 91.7 ks after the BAT trigger. The data comprise 211 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 Beardmore et al. (GCN Circ. 33008).

The late-time light curve (Figure 2) (from T0+4.4 ks) can be modelled with a power-law decay with a decay index of α=0.47 (+0.18, -0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.25 (+0.11, -0.10). The best-fitting absorption column is 3.94 (+0.29, -0.28) x 10²¹ cm^-2, in excess of the Galactic value of 2.1 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.70 (+0.32, -0.29) and a best-fitting absorption column of 2.0 ± 0.7 x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.6 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.0 ± 0.7 x 10²¹ cm^-2
Galactic foreground: 2.1 x 10²⁰ cm^-2
Excess significance: 4.5 σ
Photon index: 2.70 (+0.32, -0.29)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01142997.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221202A 102 s after the BAT trigger (Kuin and Caputo GCN Circ. 33016). A source consistent with the XRT position (Beardmore et al. GCN Circ. 33008) 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 0.027 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

05^h08^m19.85^s -59°49'57.0" 2.0" XRT-final UKSSDC

05^h08^m19.92^s -59°49'56.9" 2.2" XRT-enhanced Beardmore et al. GCN Circ. 33008

05^h08^m00.5^s -59°50'09.0" 2.1' BAT-refined Barthelmy and Krimm GCN Circ. 33015

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 33004 Swift GRB221202.84: Global MASTER-Net
observations report MASTER

Optical de Wet and Groot 33007 MeerLICHT upper limits MeerLICHT upper limits

Optical Strausbaugh and Cucchiara 33009 LCOGT Optical Upper Limits Las Cumbres Obs. Global Tele.Network upper limits

Optical Rossi et al. 33014 VLT/FORS2 Afterglow Discovery VLT/FORS2 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 102 733 186 20.66 ± 0.20

v 590 4820 216 >19.9

b 516 709 39 >19.8

u 260 857 285 >19.7

w1 640 832 39 >20.8

m2 615 808 39 >18.1

w2 566 4635 235 >20.0

RA (J2000)	Dec (J2000)	Error	Note	Reference
05^h08^m19.85^s	-59°49'57.0"	2.0"	XRT-final	UKSSDC
05^h08^m19.92^s	-59°49'56.9"	2.2"	XRT-enhanced	Beardmore et al. GCN Circ. 33008
05^h08^m00.5^s	-59°50'09.0"	2.1'	BAT-refined	Barthelmy and Krimm GCN Circ. 33015

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	33004	Swift GRB221202.84: Global MASTER-Net observations report	MASTER
Optical	de Wet and Groot	33007	MeerLICHT upper limits	MeerLICHT	upper limits
Optical	Strausbaugh and Cucchiara	33009	LCOGT Optical Upper Limits	Las Cumbres Obs. Global Tele.Network	upper limits
Optical	Rossi et al.	33014	VLT/FORS2 Afterglow Discovery	VLT/FORS2	detection

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	102	733	186	20.66 ± 0.20
v	590	4820	216	>19.9
b	516	709	39	>19.8
u	260	857	285	>19.7
w1	640	832	39	>20.8
m2	615	808	39	>18.1
w2	566	4635	235	>20.0

Table 3. UVOT observations reported by Kuin and Caputo (GCN Circ. 33016). 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.

December 4, 2022