Swift Observations of GRB 211221A

K.L. Page (U. Leicester), M. Capalbi (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 20:48:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211221A (trigger=1090472) (Page et al. GCN Circ. 31273). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 155° from the Sun (11.3 hours East) and 48° from the 94%-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 Barthelmy et al. (GCN Circ. 31289), the BAT ground-calculated position is RA, Dec = 78.906, 0.902 deg.

The BAT light curve (Figure 1) showed a complex peak structure with a duration of about 20 s. There may be hints of precursor activity and low-level post-peak emission from the GRB as seen in the mask weighted light curve. The post-peak emission lasts beyond the available event data (which ends at T+963). T90 (15-350 keV) is 671.67 ± 105.96 s (estimated error including systematics).

The time-averaged spectrum from T-129.40 to T+688.36 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.48 ± 0.13. The fluence in the 15-150 keV band is 4.0 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 78% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.27 s in the 15-150 keV band is 0.5 ± 0.1 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/1090472/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Capalbi et al. (GCN Circ. 31281).

We have analysed 5.9 ks of XRT data for GRB 211221A, from 44 s to 73.7 ks after the BAT trigger. The data comprise 52 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 31275).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.02 (+0.05, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.59 (+0.31, -0.29). The best-fitting absorption column is 1.1 (+0.4, -0.3) x 1022 cm-2, in excess of the Galactic value of 1.4 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.2 x 10-11 (9.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.1 (+0.4, -0.3) x 1022 cm-2
Galactic foreground: 1.4 x 1021 cm-2
Excess significance: 5.0 σ
Photon index: 1.59 (+0.31, -0.29)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01090472.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 211221A 63 s after the BAT trigger (Siegel and Page GCN Circ. 31317). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 31275) 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 0.143 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

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
05h15m35.39s +00°54'52.4" 1.9" XRT-final UKSSDC
05h15m35.39s +00°54'52.2" 2.2" XRT-enhanced Evans et al. GCN Circ. 31275
05h15m37.4s +00°54'06.2" 2.6' BAT-refined Barthelmy et al. GCN Circ. 31289

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31274 Swift GRB 211221A: Global MASTER-Net
observations report 		MASTER
Optical Malesani et al. 31276 NOT optical limits NOT upper limits
Optical Groot et al. 31277 MeerLICHT optical limits MeerLICHT upper limits
Optical Hu et al. 31279 BOOTES-4/MET optical upper limit BOOTES-4 upper limits
Optical Komesh et al. 31285 NUTTelA-TAO / BSTI Early Optical Limits
(Preliminary)		 NUTTelA upper limits
Gamma-ray Fletcher 31291 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=230 keV
Other Fouad and Takey 31503 No V & R detection, KAO telescope detection

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 63 213 147 >20.7
uFC 275 525 246 >20.1
white 63 1010 317 >21.1
v 606 799 39 >18.6
b 531 724 39 >19.4
u 275 872 285 >20.0
w1 655 848 39 >18.1
m2 631 650 19 >17.8
w2 582 774 39 >19.0

Table 3. UVOT observations reported by Siegel and Page (GCN Circ. 31317). 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.

January 21, 2022