Swift Observations of GRB 191031D

V. D'Elia (SSDC), B. Sbarufatti (PSU) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 21:23:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191031D (trigger=932608) (D'Elia et al. GCN Circ. 26112). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 86° from the Sun (4.5 hours East) and 71° from the 17%-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 Sakamoto et al. (GCN Circ. 26133), the BAT ground-calculated position is RA, Dec = 283.269, 47.645 deg which is RA(J2000) = 18h53m04.5s Dec(J2000) = +47°38'40.9" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 37%.

The mask-weighted light curve (Figure 1) shows a multi-peak structure from T-0.05 s to T+0.3 s with a harder first pulse and softer second pulse. There is no sign of extended emission. T90 (15-350 keV) is 0.29 ± 0.05 s (estimated error including systematics).

The time-averaged spectrum from T-0.03 to T+0.31 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.80 ± 0.15. The fluence in the 15-150 keV band is 4.1 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 92% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.36 s in the 15-150 keV band is 4.3 ± 0.4 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The duration and hardness of GRB 191031D place it squarely within the distribution of short, hard bursts detected by BAT. Using the 4-ms binned light curve (Figure 1) , the spectral lag of the burst is 0.2 ms ± 2 ms for the 50-100 keV to 15-25 keV bands, and 0.4 ms ± 3 ms for the 100-350 keV to 25-50 keV bands. These values are consistent with a short GRB.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/932608/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 26124). We have analysed 6.2 ks of XRT data for GRB 191031D, from 101 s to 17.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.04 (+0.22, -0.21).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.90 (+0.26, -0.16). The best-fitting absorption column is consistent with the Galactic value of 6.6 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 3.3 x 10-11 (3.8 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191031D 93 s after the BAT trigger (Gropp and D'Elia GCN Circ. 26128). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 26117) 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 0.08 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
18h53m09.57s +47°38'38.5" 1.8" XRT-final UKSSDC
18h53m09.48s +47°38'38.6" 1.9" XRT-enhanced Osborne et al. GCN Circ. 26117
18h53m04.5s +47°38'40.9" 1.4' BAT-refined Sakamoto et al. GCN Circ. 26133

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Butler et al. 26121 RATIR Optical and NIR Observations RATIR upper limits
Optical Watson et al. 26122 COATLI Optical Observations COATLI
Optical Ogawa et al. 26125 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Paterson et al. 26134 MMT Binospec Optical Upper Limit MMT upper limits
Optical Dichiara and Troja 26147 Gemini optical upper limit Gemini upper limits
Radio Laskar et al. 26120 VLA 9.8 GHz upper limit VLA upper limits
X-ray Escorial et al. 26175 Chandra upper limit Chandra upper limits
X-ray D'Avanzo et al. 26841 XMM-Newton upper limit XMM upper limits
Gamma-ray Mailyan and Meegan 26118 Fermi GBM detection Fermi GBM Epeak=856±53 keV
T90=0.256 seconds
Fluence=4.36±0.09x10-6erg cm-2
(97 percentile for short GRBs)
Gamma-ray Ursi et al. 26123 AGILE/MCAL observations AGILE
Gamma-ray Frederiks et al. 26126 Konus-Wind observation Konus-Wind Epeak=370 (-46,+58) keV
Duration=~0.35 seconds
Fluence=2.72(-0.27,+0.31)x10-6erg cm-2
Gamma-ray Gaikwad et al. 26129 AstroSat CZTI detection CZTI T90=0.35 seconds
Gamma-ray Frederiks 26149 Konus-Wind observation of GRB 191031D:
correction to GCN Circ. 26126		 Konus-Wind Epeak=735 (-92,+112) keV
Fluence=5.72(-0.63,+0.69)x10-6erg cm-2
Gamma-ray Shimizu et al. 26240 CALET Gamma-Ray Burst Monitor detection CALET
Other Mailyan 26114 Correction in name: GRB 191031D (not
GRB 191031C)

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 93 242 147 >20.8
uFC 306 555 246 >20.2
white 93 6694 575 >21.1
v 636 5863 294 >19.9
b 561 6683 471 >20.4
u 306 6477 697 >20.3
w1 686 6273 452 >19.8
m2 661 6067 294 >19.6
w2 612 5658 294 >19.9

Table 3. UVOT observations reported by Gropp and D'Elia (GCN Circ. 26128). 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 22, 2020