Swift Observations of GRB 211207A

E. Ambrosi (INAF-IASFPA), P.A. Evans (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 20:52:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211207A (trigger=1088376) (Ambrosi et al. GCN Circ. 31182). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 93° from the Sun (7.0 hours West) and 128° from the 18%-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.

Ambrosi et al. (GCN Circ. 31182) reported the discovery with UVOT of an optical afterglow. Schneider et al. (GCN Circ. 31188) determined a redshift of 2.272 from VLT. 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 Sakamoto and Barthelmy (GCN Circ. 31189), the BAT ground-calculated position is RA, Dec = 149.601, -24.360 deg.

The BAT light curve (Figure 1) showed a complex structure with a duration of about 4 s. T90 (15-350 keV) is 3.73 ± 0.87 s (estimated error including systematics).

The time-averaged spectrum from T-1.30 to T+2.97 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.72 ± 0.30. The fluence in the 15-150 keV band is 1.9 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 6.1% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.15 s in the 15-150 keV band is 0.9 ± 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/1088376/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 31185). We have analysed 19 ks of XRT data for GRB 211207A, from 87 s to 149.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 31184).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.95 (+0.06, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.02 (+0.25, -0.24). The best-fitting absorption column is 1.6 (+1.2, -1.0) x 1022 cm-2, at a redshift of 2.272, in addition to the Galactic value of 4.9 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 4.9 x 1020 cm-2
Intrinsic column: 1.6 (+1.2, -1.0) x 1022 cm-2 at z=2.272
Photon index: 2.02 (+0.25, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01088376.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 211207A 85 s after the BAT trigger (Siegel and Ambrosi GCN Circ. 31204). A source consistent with the XRT position (Osborne et al. GCN Circ. 31184) 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 0.050 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
09h58m29.79s -24°21'31.3" 0.43" UVOT-refined Siegel and Ambrosi GCN Circ. 31204
09h58m29.69s -24°21'32.2" 2.1" XRT-final UKSSDC
09h58m29.71s -24°21'34.8" 2.3" XRT-enhanced Osborne et al. GCN Circ. 31184
09h58m24.1s -24°21'37.3" 2.3' BAT-refined Sakamoto and Barthelmy GCN Circ. 31189

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31183 Swift GRB 211207A: Global MASTER-Net
observations report 		MASTER
Optical Schneider et al. 31188 VLT/X-shooter redshift VLT redshift
Gamma-ray Fletcher 31290 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=70 keV

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 85 234 147 17.52 ± 0.04
v 627 1595 117 >18.5
b 553 743 36 19.08 ± 0.31
u 297 546 246 18.15 ± 0.09
w1 677 1470 97 >18.7
m2 652 1272 39 >18.7
w2 776 1570 78 >19.0

Table 3. UVOT observations reported by Siegel and Ambrosi (GCN Circ. 31204). 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.

December 24, 2021