Swift Observations of GRB 191031C

E. Ambrosi (INAF-IASFPA), D.N. Burrows (PSU) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 18:43:54 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191031C (trigger=932595) (Ambrosi et al. GCN Circ. 26108). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 82° from the Sun (6.7 hours West) and 92° from the 16%-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.

Gropp and Ambrosi (GCN Circ. 26127) reported the detection with UVOT of an optical afterglow. 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. 26132), the BAT ground-calculated position is RA, Dec = 115.880, -62.322 deg which is RA(J2000) = 07h43m31.2s Dec(J2000) = -62°19'20.1" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 24%.

The mask-weighted light curve (Figure 1) shows several peaks. The first begins around T-40 s and runs until T-20 s. The second, brightest, peak begins immediately after, peaks around T+5 s, and decays to background by T+120 s. There are several weak peaks superimposed on this one, the largest coming around T+45 s. T90 (15-350 keV) is 93.2 ± 19.0 s (estimated error including systematics).

The time-averaged spectrum from T-37.98 to T+112.66 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.60 ± 0.07. The fluence in the 15-150 keV band is 7.2 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 88% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.04 s in the 15-150 keV band is 4.6 ± 0.5 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/932595/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 26130). We have analysed 9.8 ks of XRT data for GRB 191031C, from 87 s to 173.3 ks after the BAT trigger. The data comprise 1.6 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 26109).

The late-time light curve (Figure 2) (from T0+6.6 ks) can be modelled with a power-law decay with a decay index of α=1.19 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.99 (+0.08, -0.06). The best-fitting absorption column is 1.87 (+0.28, -0.08) x 1021 cm-2, consistent with the Galactic value of 1.8 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.79 (+0.12, -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 3.9 x 10-11 (4.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.8 (+0.4, -0.0) x 1021 cm-2
Galactic foreground: 1.8 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.79 (+0.12, -0.05)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00932595.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191031C 91 s after the BAT trigger (Gropp and Ambrosi GCN Circ. 26127). A source consistent with the XRT position (Goad et al. GCN Circ. 26115) 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 0.17 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
07h43m28.92s -62°19'29.9" 1.7" XRT-final UKSSDC
07h43m28.92s -62°19'28.8" 1.8" XRT-enhanced Goad et al. GCN Circ. 26115
07h43m31.2s -62°19'20.1" 1.3' BAT-refined Palmer et al. GCN Circ. 26132

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Fermi 26107 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Fermi 26111 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Fletcher et al. 26119 Fermi GBM detection Fermi GBM Epeak=107±9 keV
T90=78 seconds
Fluence=1.33±0.06x10-5erg cm-2
(81st percentile for long GRBs)
Gamma-ray Luo et al. 26146 Insight-HXMT/HE detection Insight-HXMT T90=47.315 seconds

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
u 307 556 245 >19.71
white 92 242 147 20.27± 0.32
white 862 1011 147 >20.39

Table 3. UVOT observations reported by Gropp and Ambrosi (GCN Circ. 26127). 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.

November 3, 2019