Swift Observations of GRB 191221B

S. Laha (GSFC/UMBC/CRESST), P. D'Avanzo (INAF-OAB) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 20:39:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191221B (trigger=945521) (Laha et al. GCN Circ. 26534). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 93° from the Sun (7.6 hours West) and 60° from the 22%-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.

Laha et al. (GCN Circ. 26534) reported the discovery with UVOT of an optical afterglow. Lipunov et al. (GCN Circ. 26537) reported the position from MASTER for the optical afterglow of this GRB. Kuin (GCN Circ. 26538) determined a redshift of 1.19, and Vielfaure et al. (GCN Circ. 26553) determined a redshift of 1.148 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 et al. (GCN Circ. 26562), the BAT ground-calculated position is RA, Dec = 154.831, -38.152 deg which is RA(J2000) = 10h19m19.3s Dec(J2000) = -38°09'07.1" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 12%.

The mask weighted light curve (Figure 1) shows complex structures with a duration of ~30 s. The overall structure starts at ~T-10 s and lasts till ~T+20s. There is some hint of weak emission extending up to ~ T+60 s. T90 (15-350 keV) is 48.00 ± 16.00 s (estimated error including systematics).

The time-averaged spectrum from T-15.28 to T+96.72 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.24 ± 0.05. The fluence in the 15-150 keV band is 1.9 ± 0.1 x 10-5 erg cm-2. This fluence is larger than that of 96.8% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). the fluence of this burst is among the brightest ~3% of all BAT-detected GRBs (based on fluences from the BAT GRB catalog https://swift.gsfc.nasa.gov/results/batgrbcat/). The 1-s peak photon flux measured from T+8.22 s in the 15-150 keV band is 4.8 ± 0.7 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/945521/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 26559). We have analysed 55 ks of XRT data for GRB 191221B, from 74 s to 1020.9 ks after the BAT trigger. The data comprise 1.3 ks 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 Beardmore et al. (GCN Circ. 26540).

The late-time light curve (Figure 2) (from T0+4.9 ks) can be modelled with an initial power-law decay with an index of α=1.02 (+0.08, -0.10), followed by a break at T+38.8 ks to an α of 1.90 (+0.13, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.079 ± 0.014. The best-fitting absorption column is consistent with the Galactic value of 8.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.87 (+0.09, -0.05) 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 3.5 x 10-11 (4.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 8.6 x 1020 cm-2
Intrinsic column: 8.6 (+10.1, -0.0) x 1020 cm-2 at z=1.148
Photon index: 1.87 (+0.09, -0.05)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00945521.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191221B 92 s after the BAT trigger (Kuin and Laha GCN Circ. 26574). A source consistent with the XRT position (Beardmore et al. GCN Circ. 26540) and ALMA position (Laskar et al., GCN Circ. 26564) was 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 0.08 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
10h19m19.24s -38°09'27.64" 0.42" UVOT-refined Kuin and Laha GCN Circ. 26574
10h19m19.17s -38°09'27.5" 1.4" XRT-final UKSSDC
10h19m19.18s -38°09'27.3" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 26540
10h19m19.3s -38°09'07.1" 1.1' BAT-refined Sakamoto et al. GCN Circ. 26562

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26536 Swift GRB 191221B: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 26537 MASTER OT detection MASTER detection
Optical Gokuldass et al. 26551 VIRT Optical Upper Limits Virgin Island Robotic Telescope upper limits
Optical Gokuldass et al. 26552 Correction to GCN26551 - VIRT OT
observation		 Virgin Island Robotic Telescope
Optical Vielfaure et al. 26553 VLT/X-shooter redshift VLT redshift
Optical Lipunov et al. 26555 MASTER VWFC OT yearly detection MASTER detection
Optical Strausbaugh and Cucchiara 26560 LCO Optical Detection LCO detection
Optical Gendre et al. 26561 Zadko observatory - Gingin optical
observations		 Zadko detection
Optical 26565 iTelescope optical observations iTelescope
Optical Kong 26566 iTelescope T17 detection iTelescope detection
Radio Laskar et al. 26564 ALMA detection ALMA detection
Radio Laskar et al. 26580 ATCA detection of the radio afterglow ATCA detection
Gamma-ray Longo et al. 26549 AGILE/MCAL observations AGILE
Gamma-ray Gaikwad et al. 26567 AstroSat CZTI detection CZTI T90=11.16 seconds
Gamma-ray Xue et al. 26573 Insight-HXMT/HE detection Insight-HXMT T90=8.85 seconds
Gamma-ray Frederiks et al. 26576 Konus-Wind observation Konus-Wind Epeak=377 (-29,+30) keV
Gamma-ray Sugita et al. 26616 CALET Gamma-Ray Burst Monitor detection CALET
Other 26990 MeerKAT observation

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 92 241 147 see below
u 304 554 246 12.30 ± 0.02
b 560 579 20 13.54 ± 0.04
uvw2 610 630 20 13.20 ± 0.06
v 635 655 20 13.42 ± 0.05
uvm2 659 679 20 12.79 ± 0.06
uvw1 684 704 20 12.82 ± 0.05

Table 3. UVOT observations reported by Kuin and Laha (GCN Circ. 26574). 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.

February 6, 2020