Swift Observations of GRB 220813A

M.G. Bernardini (INAF-OAB), J.A. Kennea (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 19:23:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220813A (trigger=1120270) (Bernardini et al. GCN Circ. 32465). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 76° from the Sun (4.1 hours West) and 88° 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.

Bernardini et al. (GCN Circ. 32465) 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

BAT refined results are not available.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 32468). We have analysed 16 ks of XRT data for GRB 220813A, from 80 s to 179.2 ks after the BAT trigger. The data comprise 149 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 Osborne et al. (GCN Circ. 32466).

The late-time light curve (Figure 2) (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of α=1.02 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.53 ± 0.13. The best-fitting absorption column is 1.37 (+0.29, -0.27) x 10²¹ cm^-2, in excess of the Galactic value of 2.3 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.00 ± 0.13 and a best-fitting absorption column of 4.9 (+2.9, -2.6) x 10²⁰ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.9 (+2.9, -2.6) x 10²⁰ cm^-2
Galactic foreground: 2.3 x 10²⁰ cm^-2
Excess significance: 1.7 σ
Photon index: 2.00 ± 0.13

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01120270.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220813A 100 s after the BAT trigger (Kuin and Bernardini GCN Circ. 32470). A source consistent with the XRT position (Osborne et al., GCN Circ. 32466) 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 0.022 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^h26^m07.60^s -33°00'59.0" 0.52" UVOT-refined Kuin and Bernardini GCN Circ. 32470

05^h26^m07.56^s -33°01'00.6" 2.0" XRT-final UKSSDC

05^h26^m07.55^s -33°01'00.8" 2.0" XRT-enhanced Osborne et al. GCN Circ. 32466

05^h26^m00^s -32°59'06" 3' BAT-initial Bernardini et al. GCN Circ. 32465

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 32467 Swift GRB 220813A: Global MASTER-Net
observations report MASTER

Optical Fausey et al. 32471 VLT optical observations and possible
host galaxy VLT marginal detection

Gamma-ray Frederiks et al. 32472 Konus-Wind detection Konus-Wind E_peak=242 (-17,+23) keV

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 100 250 147 19.67 ± 0.11

v 589 1581 117 >18.3

b 515 1506 97 >19.6

u 258 1652 360 >20.6

w1 638 1630 117 >19.6

m2 613 1605 117 >20.1

w2 564 1556 117 >20.0

RA (J2000)	Dec (J2000)	Error	Note	Reference
05^h26^m07.60^s	-33°00'59.0"	0.52"	UVOT-refined	Kuin and Bernardini GCN Circ. 32470
05^h26^m07.56^s	-33°01'00.6"	2.0"	XRT-final	UKSSDC
05^h26^m07.55^s	-33°01'00.8"	2.0"	XRT-enhanced	Osborne et al. GCN Circ. 32466
05^h26^m00^s	-32°59'06"	3'	BAT-initial	Bernardini et al. GCN Circ. 32465

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	32467	Swift GRB 220813A: Global MASTER-Net observations report	MASTER
Optical	Fausey et al.	32471	VLT optical observations and possible host galaxy	VLT	marginal detection
Gamma-ray	Frederiks et al.	32472	Konus-Wind detection	Konus-Wind	E_peak=242 (-17,+23) keV

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	100	250	147	19.67 ± 0.11
v	589	1581	117	>18.3
b	515	1506	97	>19.6
u	258	1652	360	>20.6
w1	638	1630	117	>19.6
m2	613	1605	117	>20.1
w2	564	1556	117	>20.0

Table 3. UVOT observations reported by Kuin and Bernardini (GCN Circ. 32470). 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.

August 16, 2022