Swift Observations of GRB 201221A

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 07:09:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201221A (trigger=1013852) (Page et al. GCN Circ. 29096). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 49° from the Sun (3.7 hours West) and 116° from the 43%-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.

Malesani et al. (GCN Circ. 29100) reported the position from VLT for the optical afterglow of this GRB. Malesani et al. (GCN Circ. 29100) determined a redshift of 5.70 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 Barthelmy et al. (GCN Circ. 29116), the BAT ground-calculated position is RA, Dec = 214.488, -45.406 deg which is RA(J2000) = 14h17m57.1s Dec(J2000) = -45°24'20.8" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 42%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-7 s and ends at ~T+40 s. T90 (15-350 keV) is 44.5 ± 6.2 s (estimated error including systematics).

The time-averaged spectrum from T-7.63 to T+41.72 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.40 ± 0.15. The fluence in the 15-150 keV band is 1.9 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 57% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.16 s in the 15-150 keV band is 1.0 ± 0.3 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/1013852/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 29108). We have analysed 19 ks of XRT data for GRB 201221A, from 143 s to 241.2 ks after the BAT trigger. The data comprise 320 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 29097).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.59 ± 0.07, followed by a break at T+10.6 ks to an α of 1.75 (+0.24, -0.21).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.45 (+0.16, -0.15). The best-fitting absorption column is 7.3 (+8.0, -6.2) x 1022 cm-2, at a redshift of 5.7, in addition to the Galactic value of 9.3 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 4.9 x 10-11 (4.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 9.3 x 1020 cm-2
Intrinsic column: 7.3 (+8.0, -6.2) x 1022 cm-2 at z=5.7
Photon index: 1.45 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01013852.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201221A 141 s after the BAT trigger (Breeveld and Page GCN Circ. 29109). No optical afterglow consistent with the optical position (Malesani et al. GCN Circ. 29100) 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 0.11 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
14h17m55.22s -45°24'58.1" 1.5" XRT-final UKSSDC
14h17m55.24s -45°24'58.3" 1.5" XRT-enhanced Beardmore et al. GCN Circ. 29105
14h17m57.1s -45°24'20.8" 1.6' BAT-refined Barthelmy et al. GCN Circ. 29116

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29098 Swift GRB 201221A: Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 29099 FRAM-Auger optical limit FRAM upper limits
Optical Malesani et al. 29100 VLT/X-shooter afterglow discovery and
spectroscopic redshift z = 5.7		 VLT redshift
Optical D'Avanzo et al. 29149 REM early-time afterglow detection REM detection
Gamma-ray Veres 29110 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=129±28 keV
Fluence=3.30±0.27x10-4erg cm-2
Gamma-ray Veres 29111 correction to GCN Circ. 29110 - Fermi GBM
Sub-Threshold Detection		 Fermi GBM
Gamma-ray Frederiks et al. 29150 Konus-Wind detection and joint
Konus-Wind + Swift-BAT spectral
analysis		 Konus-Wind Epeak=194 (-63,+81) 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
whiteFC 141 291 147 >20.0
uFC 300 549 246 >19.1
white 141 951 257 >20.2
v 629 4762 236 >18.8
b 555 748 39 >18.3
u 300 5271 357 >19.5
w1 679 5172 236 >19.4
m2 654 4967 236 >19.2
w2 605 4557 235 >19.4

Table 3. UVOT observations reported by Breeveld and Page (GCN Circ. 29109). 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.

December 24, 2020