Swift Observations of GRB 210930A

V. D'Elia (SSDC and INAF-OAR), A. D'Ai (INAF-IASFPA) and A. Belles (PSU) for the Swift team

1. Introduction

At 02:46:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210930A (trigger=1075565) (D'Elia et al. GCN Circ. 30897). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 52° from the Sun (0.7 hours East) and 69° from the 40%-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

As reported by Krimm et al. (GCN Circ. 30917), the BAT ground-calculated position is RA, Dec = 197.415, 48.630 deg which is RA(J2000) = 13h09m39.5s Dec(J2000) = +48°37'47.1" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T+6 s, peaks at ~T+9 s, and ends at ~T+20 s. T90 (15-350 keV) is 11.81 ± 1.94 s (estimated error including systematics).

The time-averaged spectrum from T+5.84 to T+19.24 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.58 ± 0.19. The fluence in the 15-150 keV band is 4.0 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 17% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+8.40 s in the 15-150 keV band is 0.7 ± 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/1075565/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 30902). We have analysed 12 ks of XRT data for GRB 210930A, from 143 s to 161.6 ks after the BAT trigger. The data comprise 63 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. 30900).

The late-time light curve (Figure 2) (from T0+17.1 ks) can be modelled with a power-law decay with a decay index of α=1.01 (+0.33, -0.27).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 4.04 (+0.30, -0.27). The best-fitting absorption column is 1.46 (+0.18, -0.16) x 1022 cm-2, in excess of the Galactic value of 9.6 x 1019 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.3 ± 0.4 and a best-fitting absorption column of 6.2 (+2.4, -2.0) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10-11 (7.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.2 (+2.4, -2.0) x 1021 cm-2
Galactic foreground: 9.6 x 1019 cm-2
Excess significance: 5.0 σ
Photon index: 2.3 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01075565.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210930A 144 s after the BAT trigger (Belles and D'Elia GCN Circ. 30903). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 30900) 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 0.010 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
13h09m36.46s +48°37'43.0" 2.2" XRT-final UKSSDC
13h09m36.45s +48°37'42.8" 2.4" XRT-enhanced Osborne et al. GCN Circ. 30900
13h09m39.5s +48°37'47.1" 1.6' BAT-refined Krimm et al. GCN Circ. 30917

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30901 Swift GRB 210930A: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 30909 CAHA 2.2m telescope optical limit CAHA 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
whiteFC 144 293 147 >19.9
uFC 302 551 246 >19.6
white 144 1722 411 >20.6
v 631 1773 136 >18.7
b 559 1873 134 >19.6
u 302 1848 363 >19.7
w1 681 1824 136 >19.5
m2 655 1450 39 >18.7
w2 607 1747 117 >20.4

Table 3. UVOT observations reported by Belles and D'Elia (GCN Circ. 30903). 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.

October 2, 2021