Swift Observations of GRB 191220A

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

1. Introduction

At 13:29:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191220A (trigger=945344) (Laha et al. GCN Circ. 26516). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 52° from the Sun (0.9 hours East) and 86° from the 35%-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 Palmer et al. (GCN Circ. 26535), the BAT ground-calculated position is RA, Dec = 281.335, 26.679 deg which is RA(J2000) = 18h45m20.3s Dec(J2000) = +26°40'43.1" with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 19%.

The mask weighted light curve (Figure 1) shows complex structures with a duration of ~50 s, with a hint of some weak emission lasting till ~ T+ 240 s. The overall structure starts at ~ T-2 s and lasts till ~ T+60 s. T90 (15-350 keV) is 175.55 ± 125.93 s (estimated error including systematics).

The time-averaged spectrum from T-2.46 to T+248.42 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.12 ± 0.40. The fluence in the 15-150 keV band is 1.1 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 41% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+6.69 s in the 15-150 keV band is 1.1 ± 0.4 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/945344/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 26524). We have analysed 14 ks of XRT data for GRB 191220A, from 138 s to 91.5 ks after the BAT trigger. The data comprise 38 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 26515).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=3.7 (+1.1, -0.8), followed by a break at T+396 s to an α of 0.48 ± 0.06.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.15, -0.12). The best-fitting absorption column is consistent with the Galactic value of 1.5 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 3.4 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.5 ± 0.3 x 1021 cm-2
Galactic foreground: 1.5 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.00 (+0.15, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00945344.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191220A 138 s after the BAT trigger (Kuin and Laha GCN Circ. 26526). No optical afterglow consistent with the XRT position 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 0.14 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
18h45m31.15s +26°39'21.3" 2.0" XRT-final UKSSDC
18h45m31.15s +26°39'21.2" 2.3" XRT-enhanced Osborne et al. GCN Circ. 26522
18h45m20.3s +26°40'43.1" 3.0' BAT-refined Palmer et al. GCN Circ. 26535

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26514 Swift GRB191220.56: Global MASTER-Net
observations report 		MASTER
Optical Moskvitin 26519 SAO RAS possible OT detection SAO RAS possible detection
Optical Belkin et al. 26525 AbAO optical observations Abastumani Astro. Obs.

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
uFC 138 387 246 >19.8
u 138 6726 1061 >20.0
w1 446 6521 687 >19.6
m2 420 6316 687 >19.9
w2 396 6766 719 >21.1

Table 3. UVOT observations reported by Kuin and Laha (GCN Circ. 26526). 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 22, 2019