Swift Observations of GRB 210722A

P. D'Avanzo (INAF-OAB), K.L. Page (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 20:54:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210722A (trigger=1061223) (D'Avanzo et al. GCN Circ. 30475). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 97° from the Sun (6.3 hours West) and 98° from the 98%-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.

D'Avanzo et al. (GCN Circ. 30475) reported the discovery with UVOT of an optical afterglow. Thoene et al. (GCN Circ. 30487) determined a redshift of 1.145 from GTC. 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. 30489), the BAT ground-calculated position is RA, Dec = 27.031, -6.347 deg which is RA(J2000) = 01h48m07.4s Dec(J2000) = -06°20'49.9" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 43%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-5 s, peaks at ~T0, and ends at ~T+30 s, with some weak tail emission that lasts until ~T+60 s. T90 (15-350 keV) is 50.20 ± 10.54 s (estimated error including systematics).

The time-averaged spectrum from T-4.40 to T+59.59 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.62 ± 0.14. The fluence in the 15-150 keV band is 2.5 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 67% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.00 s in the 15-150 keV band is 2.2 ± 0.5 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/1061223/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 30483). We have analysed 6.3 ks of XRT data for GRB 210722A, from 91 s to 47.1 ks after the BAT trigger. The data comprise 294 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+4.5 ks) can be modelled with a power-law decay with a decay index of α=1.31 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.20 ± 0.05. The best-fitting absorption column is 4.2 (+1.0, -0.9) x 1020 cm-2, in excess of the Galactic value of 2.2 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.84 (+0.11, -0.10) and a best-fitting absorption column of 3.5 (+2.4, -1.2) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (3.8 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210722A 94 s after the BAT trigger (Siegel and D'Avanzo GCN Circ. 30482). A source consistent with the XRT position 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 0.027 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
01h48m07.28s -06°20'49.7" 0.42" UVOT-refined Siegel and D'Avanzo GCN Circ. 30482
01h48m07.26s -06°20'51.3" 1.4" XRT-final UKSSDC
01h48m07.26s -06°20'51.3" 2.0" XRT-refined Page et al. GCN Circ. 30483
01h48m07.4s -06°20'49.9" 1.2' BAT-refined Barthelmy et al. GCN Circ. 30489

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zhu et al. 30476 Nanshan/NEXT optical afterglow
observations		 Xinjiang Astro. Obs. detection
Optical Pankov et al. 30478 Assy optical detection Assy-Turgen Obs. detection
Optical Lipunov et al. 30480 Swift GRB 210722A: Global MASTER-Net
observations report 		MASTER
Optical Kumar et al. 30481 GIT optical detection and preliminary
analysis		 GROWTH-India Tel. detection
Optical Butler et al. 30484 RATIR Optical Observations RATIR
Optical Thoene et al. 30487 Redshift from OSIRIS/GTC GTC redshift
Optical Pankov et al. 30488 Kitab optical observations, light curve light curve
Optical Jelinek et al. 30492 D50 optical detection D50 detection
Optical Pankov et al. 30494 continued Assy optical observations Assy-Turgen Obs. detection
Optical Fu et al. 30504 Nanshan/NEXT further optical
observations		 Xinjiang Astro. Obs.
Optical Kim et al. 30505 continued Assy optical observations Assy-Turgen Obs. detection
Optical Klotz et al. 31754 TAROT Reunion observatory optical light
curve		 TAROT light curve
Gamma-ray Wood 30490 Fermi GBM observation Fermi GBM Epeak=160±30 keV
T90=62 seconds
Fluence=5.8±0.6x10-6erg cm-2
(brighter than 61% of long GRBs)

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 94 244 147 14.31 ± 0.02
v 637 656 19 14.52 ± 0.06
b 563 583 20 14.65 ± 0.04
u 308 557 246 13.68 ± 0.02
w1 686 705 20 14.08 ± 0.06
w2 4908 4971 62 17.08 ± 0.14

Table 3. UVOT observations reported by Siegel and D'Avanzo (GCN Circ. 30482). 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.

March 14, 2022