Swift Observations of GRB 210712A

B. Sbarufatti (PSU), D.N. Burrows (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 09:43:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210712A (trigger=1059881) (Sbarufatti et al. GCN Circ. 30447). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 59° from the Sun (1.0 hours West) and 70° from the 6%-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 Ukwatta et al. (GCN Circ. 30460), the BAT ground-calculated position is RA, Dec = 97.322, -35.376 deg which is RA(J2000) = 06h29m17.3s Dec(J2000) = -35°22'32.2" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 71%.

The mask-weighted light curve (Figure 1) shows a series of peaks. The first is from roughly T-40 to T-30. Then the strongest peak is from T-5 to T+15. This is followed by a string of weaker peaks from T+20 to T+120. The spacecraft slewed away from the burst location by T+700 s. T90 (15-350 keV) is 136.41 ± 63.58 s (estimated error including systematics).

The time-averaged spectrum from -39.76 to 174.18 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.00 ± 0.53, and Epeak of 51.8 ± 11.6 keV (χ2 70.95 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.0 ± 0.3 x 10-6 erg cm-2 and the 1-s peak flux measured from T+0.09 s in the 15-150 keV band is 2.3 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 71% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.85 ± 0.11 (χ2 80.11 for 57 d.o.f.). 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/1059881/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 30457). We have analysed 18 ks of XRT data for GRB 210712A, from 86 s to 222.9 ks after the BAT trigger. The data comprise 164 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 30453).

The late-time light curve (Figure 2) (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of α=1.13 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.52 ± 0.06. The best-fitting absorption column is 1.10 (+0.22, -0.21) x 1021 cm-2, consistent with the Galactic value of 8.9 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.98 (+0.13, -0.09) 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 3.3 x 10-11 (4.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.9 (+2.9, -0.0) x 1020 cm-2
Galactic foreground: 8.9 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.98 (+0.13, -0.09)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01059881.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210712A 91 s after the BAT trigger (Siegel and Sbarufatti GCN Circ. 30456). No optical afterglow consistent with the enhanced XRT position (Evans et al. GCN Circ. 30453) 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 0.076 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
06h29m19.64s -35°22'17.8" 2.1" XRT-final UKSSDC
06h29m19.63s -35°22'18.0" 2.1" XRT-enhanced Evans et al. GCN Circ. 30453
06h29m17.3s -35°22'32.2" 1.7' BAT-refined Ukwatta et al. GCN Circ. 30460

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 30455 BOOTES-3/YA optical upper limit BOOTES-3 upper limits
Gamma-ray Bissaldi 30643 Fermi GBM observation Fermi GBM Epeak=230±50 keV
T90=8 seconds
Fluence=1.51±0.17x10-6erg cm-2
(23rd percentile for 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
whiteFC 91 241 147 >19.5
uFC 304 554 246 >19.6
white 91 604 167 >19.5
b 560 579 19 >18.0
u 304 554 246 >19.6
w2 609 614 4 >17.5

Table 3. UVOT observations reported by Siegel and Sbarufatti (GCN Circ. 30456). 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.

August 14, 2021