Swift Observations of GRB 210820A

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 17:38:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210820A (trigger=1069551) (Beardmore et al. GCN Circ. 30664). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 127° from the Sun (8.6 hours West) and 76° 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.

Beardmore et al. (GCN Circ. 30664) reported the discovery with UVOT of an optical afterglow. Butler et al. (GCN Circ. 30672) reported the position from RATIR for the optical afterglow of this GRB. 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 Laha et al. (GCN Circ. 30675), the BAT ground-calculated position is RA, Dec = 20.622, 4.642 deg which is RA(J2000) = 01h22m29.4s Dec(J2000) = +04°38'30.6" with an uncertainty of 3.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 48%.

The mask-weighted light curve (Figure 1) shows many weak overlapping pulses that start at ~T0 and end at ~T+260 s. T90 (15-350 keV) is 196.8 ± 51.4 s (estimated error including systematics).

The time-averaged spectrum from T+0.78 to T+258.97 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.26 ± 0.22. The fluence in the 15-150 keV band is 1.5 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 52% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+10.92 s in the 15-150 keV band is 0.8 ± 0.2 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/1069551/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 30668). We have analysed 13 ks of XRT data for GRB 210820A, from 131 s to 162.5 ks after the BAT trigger. The data comprise 339 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 30667).

The late-time light curve (Figure 2) (from T0+5.9 ks) can be modelled with a power-law decay with a decay index of α=1.3 (+0.8, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.77 ± 0.03. The best-fitting absorption column is 1.20 ± 0.11 x 1021 cm-2, in excess of the Galactic value of 3.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.28 (+0.25, -0.13) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.9 x 10-11 (3.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.6 (+4.3, -0.0) x 1020 cm-2
Galactic foreground: 3.6 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.28 (+0.25, -0.13)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01069551.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210820A 131 s after the BAT trigger (LaPorte and Beardmore GCN Circ. 30686). 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 0.030 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
01h22m33.48s +04°36'28.3" 0.45" UVOT-refined LaPorte and Beardmore GCN Circ. 30686
01h22m33.51s +04°36'27.5" 2.5" XRT-final UKSSDC
01h22m33.51s +04°36'27.5" 2.5" XRT-enhanced Beardmore et al. GCN Circ. 30667
01h22m29.4s +04°38'30.6" 3.4' BAT-refined Laha et al. GCN Circ. 30675

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30666 Swift GRB 210820A: Global MASTER-Net
observations report 		MASTER
Optical Fu et al. 30669 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Zhu et al. 30670 NOT optical upper limit NOT upper limits
Optical Butler et al. 30672 RATIR Optical Detections RATIR detection
Optical Strausbaugh and Cucchiara 30673 LCO Optical Upper Limit LCO upper limits
Optical Blazek et al. 30674 LT optical upper limit Liverpool Telescope upper limits
Optical Komesh et al. 30688 NUTTelA-TAO / BSTI Early Optical Limits
(Preliminary)		 NUTTelA upper limits
Optical Guelbenzu et al. 30695 GROND observations GROND detection
Optical Kann et al. 30731 CAHA 2.2m Optical Detection CAHA detection
Optical Belkin et al. 30757 TSHAO optical upper limit Zeiss-1000 upper limits

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
v 437 1684 156 18.86 ± 0.24
b 387 555 39 18.57 ± 0.19
u 131 381 246 17.60 ± 0.06
w1 784 1733 58 >19.0

Table 3. UVOT observations reported by LaPorte and Beardmore (GCN Circ. 30686). 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.

September 4, 2021