Swift Observations of GRB 210822A

K.L. Page (U. Leicester), M. Perri (SSDC and INAF-OAR) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 09:18:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210822A (trigger=1069788) (Page et al. GCN Circ. 30677). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 148° from the Sun (10.2 hours East) and 35° 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.

Page et al. (GCN Circ. 30677) reported the discovery with UVOT of an optical afterglow. Zhu et al. (GCN Circ. 30692) determined a redshift of 1.736 from NOT. 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 Lien et al. (GCN Circ. 30689), the BAT ground-calculated position is RA, Dec = 304.461, 5.265 deg which is RA(J2000) = 20h17m50.7s Dec(J2000) = +05°15'54.9" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts and peaks at T0. The main pulse structure ends at ~T+20 s, followed by a long tail lasting till ~T+400 s. T90 (15-350 keV) is 180.8 ± 42.7 s (estimated error including systematics).

The time-averaged spectrum from T-0.37 to T+402.66 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.30 ± 0.04. The fluence in the 15-150 keV band is 2.0 ± 0.04 x 10-5 erg cm-2. This fluence is larger than that of 97.1% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.00 s in the 15-150 keV band is 27.7 ± 0.7 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/1069788/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 30691). We have analysed 29 ks of XRT data for GRB 210822A, from 64 s to 522.6 ks after the BAT trigger. The data comprise 1.2 ks in Windowed Timing (WT) mode (the first 9 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 Evans et al. (GCN Circ. 30682).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=0.92 ± 0.09. At T+145 s the decay steepens to an α of 3.1 (+4.9, -1.0). The light curve breaks again at T+157 s to a decay with α=1.090 (+0.015, -0.014), before a final break at T+7430 s s after which the decay index is 1.77 (+0.08, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.810 ± 0.017. The best-fitting absorption column is 3.6 ± 0.6 x 1021 cm-2, at a redshift of 1.736, in addition to the Galactic value of 1.5 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.71 (+0.10, -0.09) and a best-fitting absorption column of 3.4 (+3.6, -3.3) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10-11 (4.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.5 x 1021 cm-2
Intrinsic column: 3.4 (+3.6, -3.3) x 1021 cm-2 at z=1.736
Photon index: 1.71 (+0.10, -0.09)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01069788.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210822A 85 s after the BAT trigger (Siegel and Page GCN Circ. 30710). A fading source consistent with the XRT position and the previously reported UVOT source is 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 0.162 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
20h17m45.0s +05°17'00.1" UVOT-initial Page et al. GCN Circ. 30677
20h17m45.01s +05°17'00.5" 1.9" XRT-final UKSSDC
20h17m45.01s +05°17'00.6" 2.0" XRT-enhanced Evans et al. GCN Circ. 30682
20h17m50.7s +05°15'54.9" 1.0' BAT-refined Lien et al. GCN Circ. 30689

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 30679 KAIT Optical Afterglow Confirmation KAIT detection
Optical Butler et al. 30680 RATIR Optical Detections RATIR detection
Optical Lipunov et al. 30681 Swift GRB 210822A: Global MASTER-Net
observations report 		MASTER
Optical Fu et al. 30684 Nanshan/NEXT optical observations Xinjiang Astro. Obs. detection
Optical Zhu et al. 30692 NOT photometry and spectroscopy NOT spectroscopy
Optical Romanov and Lane 30701 Abbey Ridge Observatory optical
afterglow observation		 Abbey Ridge Obs.
Optical Guelbenzu et al. 30703 GROND observations GROND detection
Optical Belkin et al. 30712 TSHAO optical observation Zeiss-1000 detection
Optical Belkin et al. 30714 TSHAO continued optical observation Zeiss-1000 detection
Optical Kann et al. 30723 CAHA 2.2m observations CAHA
Radio Laskar et al. 30713 VLA detection VLA detection
Gamma-ray Wang et al. 30678 GECAM detection GECAM
Gamma-ray Ohno et al. 30687 Fermi-LAT detection Fermi LAT Emax=1.0 GeV
Gamma-ray Prasad et al. 30693 AstroSat CZTI detection CZTI
Gamma-ray Frederiks et al. 30694 Konus-Wind detection Konus-Wind Epeak=398 (-29,+30) keV
Gamma-ray Wang et al. 30696 GECAM refined analysis GECAM
Gamma-ray Ohno et al. 30697 Detection by GRBAlpha GRBAlpha
Gamma-ray Kobayashi et al. 30744 CALET Gamma-Ray Burst Monitor detection CALET

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 (fc) 86 236 147 13.63±1.10
white 577 1200 205 15.35±0.02
white 4965 5058 91 17.43±0.05
white 34405 35230 804 19.15±0.05
white 149533 190336 1894 20.64±0.11
v 627 1250 77 15.07±0.05
v 27494 28215 702 18.68±0.13
b 553 1176 58 15.31±0.04
b 4760 4960 196 17.56±0.06
b 33493 34400 885 19.18±0.08
u (fc) 298 548 245 13.78±0.03
u 700 1304 39 14.86±0.05
u 4555 4755 196 16.85±0.05
u 57947 58635 670 19.73±0.19
uvw1 676 1299 77 15.28±0.07
uvw1 39434 96567 1775 19.90±0.22
uvm2 824 1274 38 >17.66
uvm2 92215 93015 788 >20.06
uvw2 602 1225 77 17.21±0.18
uvw2 21659 64335 1469 20.40±0.34

Table 3. UVOT observations reported by Siegel and Page (GCN Circ. 30710). 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 31, 2021