Swift Observations of GRB 210723A

T. Sbarrato (INAF-OAB), A.P. Beardmore (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 14:46:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210723A (trigger=1061284) (Sbarrato et al. GCN Circ. 30485). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 53° from the Sun (0.1 hours West) and 122° from the 100%-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.

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 Krimm and Barthelmy (GCN Circ. 30495).

The mask-weighted light curve (Figure 1) shows a single-pulse structure that

The time-averaged spectrum from T-4.59 to T+66.64 s is best fit

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1061284/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 30493). We have analysed 15 ks of XRT data for GRB 210723A, from 95 s to 155.5 ks after the BAT trigger. The data comprise 267 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.52 (+0.35, -0.23). At T+153 s the decay flattens to an α of 1.66 (+0.10, -0.08) before breaking again at T+957 s to a final decay with index α=1.07 ± 0.07.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.75 (+0.11, -0.10). The best-fitting absorption column is 1.25 (+0.14, -0.13) x 10²² cm^-2, in excess of the Galactic value of 7.7 x 10²¹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.75 (+0.25, -0.24) and a best-fitting absorption column of 1.6 (+0.4, -0.3) x 10²² cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.4 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+0.4, -0.3) x 10²² cm^-2
Galactic foreground: 7.7 x 10²¹ cm^-2
Excess significance: 4.3 σ
Photon index: 1.75 (+0.25, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01061284.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210723A 99 s after the BAT trigger (LaPorte and Sbarrato GCN Circ. 30528). No optical afterglow consistent with the XRT position (Evans GCN Circ. 30486) 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 0.962 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

08^h06^m53.08^s -32°53'08.5" 2.0" XRT-final UKSSDC

08^h06^m53.12^s -32°53'06.4" 1.9" XRT-enhanced Evans GCN Circ. 30486

08^h06^m53.4^s -32°53'35.8" 1.0' BAT-refined Krimm and Barthelmy GCN Circ. 30495

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 30491 Swift GRB 210723A: Global MASTER-Net
observations report MASTER

Gamma-ray Lesage et al. 30499 Fermi GBM Observation Fermi GBM E_peak=113±5 keV
T₉₀=43 seconds
Fluence=7.6±0.2x10^-6erg cm^-2
(68^th percentile for long GRBs)

Gamma-ray Asaoka et al. 30506 CALET Gamma-Ray Burst Monitor detection CALET

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_FC 99 249 147 >20.6

u_FC 312 562 246 >19.2

white 99 1364 369 >20.6

v 641 1241 78 >18.2

b 567 1339 78 >19.3

u 312 1315 304 >19.4

w1 691 1290 58 >19.0

m2 666 1266 78 >18.8

w2 617 1217 78 >19.5

RA (J2000)	Dec (J2000)	Error	Note	Reference
08^h06^m53.08^s	-32°53'08.5"	2.0"	XRT-final	UKSSDC
08^h06^m53.12^s	-32°53'06.4"	1.9"	XRT-enhanced	Evans GCN Circ. 30486
08^h06^m53.4^s	-32°53'35.8"	1.0'	BAT-refined	Krimm and Barthelmy GCN Circ. 30495

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	30491	Swift GRB 210723A: Global MASTER-Net observations report	MASTER
Gamma-ray	Lesage et al.	30499	Fermi GBM Observation	Fermi GBM	E_peak=113±5 keV T₉₀=43 seconds Fluence=7.6±0.2x10^-6erg cm^-2 (68^th percentile for long GRBs)
Gamma-ray	Asaoka et al.	30506	CALET Gamma-Ray Burst Monitor detection	CALET

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	99	249	147	>20.6
u_FC	312	562	246	>19.2
white	99	1364	369	>20.6
v	641	1241	78	>18.2
b	567	1339	78	>19.3
u	312	1315	304	>19.4
w1	691	1290	58	>19.0
m2	666	1266	78	>18.8
w2	617	1217	78	>19.5

Table 3. UVOT observations reported by LaPorte and Sbarrato (GCN Circ. 30528). 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.

July 28, 2021