Swift Observations of GRB 210818A

A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) for the Swift team

1. Introduction

At 01:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210818A (trigger=1069169) (Beardmore et al. GCN Circ. 30652). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 100° from the Sun (-0.0 hours West) and 66° from the 76%-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. 30652) 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

As reported by Krimm et al. (GCN Circ. 30659), the BAT ground-calculated position is RA, Dec = 148.949, -87.029 deg which is RA(J2000) = 09h55m47.7s Dec(J2000) = -87°01'45.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%.

The mask-weighted light curve (Figure 1) shows many overlapping pulses on top of a general FRED-like structure. The burst emission starts at ~T0, peaks at ~T+2 s, and ends at ~T+130 s. T90 (15-350 keV) is 73.56 ± 1.55 s (estimated error including systematics).

The time-averaged spectrum from T-0.69 to T+128.97 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 ± 0.03. The fluence in the 15-150 keV band is 2.6 ± 0.03 x 10-5 erg cm-2. This fluence is larger than that of 98.8% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.20 s in the 15-150 keV band is 11.2 ± 0.4 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/1069169/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 30660). We have analysed 10 ks of XRT data for GRB 210818A, from 65 s to 127.7 ks after the BAT trigger. The data comprise 188 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 30653).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.04 ± 0.20. At T+111 s the decay steepens to an α of 6.8 (+0.7, -0.6). The light curve breaks again at T+157 s to a decay with α=0.64 (+0.22, -0.26), before a final break at T+456 s s after which the decay index is 1.48 (+0.06, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 0.86 ± 0.06. The best-fitting absorption column is 2.5 (+0.5, -0.4) x 1021 cm-2, in excess of the Galactic value of 1.7 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.63 (+0.14, -0.10) and a best-fitting absorption column of 1.84 (+0.55, -0.10) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.4 x 10-11 (5.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.84 (+0.55, -0.10) x 1021 cm-2
Galactic foreground: 1.7 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.63 (+0.14, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01069169.

4. UVOT Observations and Analysis

UVOT results are not available.

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
09h55m28.15s -87°01'28.8" 0.62" UVOT-initial Beardmore et al. GCN Circ. 30652
09h55m28.50s -87°01'30.9" 2.0" XRT-final UKSSDC
09h55m28.23s -87°01'30.9" 2.2" XRT-enhanced Osborne et al. GCN Circ. 30653
09h55m47.7s -87°01'45.3" 1.0' BAT-refined Krimm et al. GCN Circ. 30659

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30661 Swift GRB 210818A: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 30662 Swift GRB210818.68: Global MASTER-Net
observations report 		MASTER
Gamma-ray Ursi et al. 30654 AGILE detection AGILE
Gamma-ray Prasad et al. 30655 AstroSat CZTI detection CZTI
Gamma-ray Frederiks et al. 30657 Konus-Wind detection Konus-Wind Epeak=1107 (-71,+78) keV

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

August 20, 2021