Swift Observations of GRB 191228A

M.G. Bernardini (INAF-OAB) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 00:01:19 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191228A (trigger=946476) (Bernardini et al. GCN Circ. 26601). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 86° from the Sun (5.9 hours East) and 65° from the 3%-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 Ukwatta et al. (GCN Circ. 26607), the BAT ground-calculated position is RA, Dec = 5.341, -8.650 deg, which is RA(J2000) = 00h21m21.9s Dec(J2000) = -08°38'58.3" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 90%.

The mask-weighted light curve (Figure 1) ] shows 3 broad peaks starting at T-50 s with peaks at ~T+5, ~T+55, and ~T+130 s, and ending at T+190 s. T90 (15-350 keV) is 166.9 ± 10.2 s (estimated error including systematics).

The time-averaged spectrum from T-39.2 to T+138.9 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.06 ± 0.14. The fluence in the 15-150 keV band is 2.0 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 58% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+54.59 s in the 15-150 keV band is 0.6 ± 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/946476/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 26609). We have analysed 7.7 ks of XRT data for GRB 191228A, from 127 s to 48.0 ks after the BAT trigger. The data comprise 216 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of α=1.58 (+0.13, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 ± 0.05. The best-fitting absorption column is 3.09 (+0.24, -0.23) x 1021 cm-2, in excess of the Galactic value of 3.8 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.14 (+0.16, -0.15) and a best-fitting absorption column of 4.0 ± 0.7 x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (6.1 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191228A 131 s after the BAT trigger (Gropp and Bernardini GCN Circ. 26611). 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 0.04 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
00h21m23.24s -08°40'58.2" 1.4" XRT-final UKSSDC
00h21m23.25s -08°40'57.9" 1.4" XRT-enhanced Evans et al. GCN Circ. 26606
00h21m21.9s -08°38'58.3" 2.4' BAT-refined Ukwatta et al. GCN Circ. 26607

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26600 Swift GRB191228.00: Global MASTER-Net
observations report 		MASTER
Optical Xu et al. 26603 UKIRT NIR upper limit UKIRT upper limits
Optical Mueller et al. 26604 ePESSTO+ NTT optical upper limit PESSTO upper limits
Optical Pozanenko et al. 26615 Mondy optical upper limit Mondy upper limits
Optical Strausbaugh and Cucchiara 26630 Early Optical Upper Limits from LCO LCO upper limits
Optical Jelinek et al. 26653 FRAM-ORM optical limit FRAM upper limits

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 131 281 147 >20.3
uFC 289 539 246 >19.6
white 131 1009 327 >20.6
v 619 811 39 >18.2
b 545 737 39 >19.2
u 289 712 265 >19.6
w1 668 860 39 >18.4

Table 3. UVOT observations reported by Gropp and Bernardini (GCN Circ. 26611). 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 8, 2020