Swift Observations of GRB 191031A

A. D'Ai (INAF-IASFPA) for the Swift team

1. Introduction

At 00:36:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191031A (trigger=932435) (D'Ai et al. GCN Circ. 26101). Swift did not slew due to an observing constraint. At the time of the trigger, the initial BAT position was 27° from the Sun (1.3 hours East) and 33° from the 11%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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 Markwardt et al. (GCN Circ. 26131), the BAT ground-calculated position is RA, Dec = 233.490, 6.109 deg which is RA(J2000) = 15h33m57.7s Dec(J2000) = +06°06'32.9" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 46%.

The mask-weighted light curve (Figure 1) shows a single pulse with some overlying structure. The emission starts at ~T-8 s, peaks at T+3 s, and decays to background by ~T+18 s. A pre-planned spacecraft slew took the burst location out of the field of view by T+95 s. T90 (15-350 keV) is 19.1 ± 6.3 s (estimated error including systematics).

The time-averaged spectrum from T-11.08 to T+16.87 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.38 ± 0.19. The fluence in the 15-150 keV band is 9.0 ± 1.1 x 10-7 erg cm-2. This fluence is larger than that of 36% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+3.48 s in the 15-150 keV band is 1.0 ± 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/932435/BA/.

3. XRT Observations and Analysis

XRT refined results are not available.

4. UVOT Observations and Analysis

UVOT results are not available.

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.

RA (J2000) Dec (J2000) Error Note Reference
15h33m57.7s +06°06'32.9" 2.0' BAT-refined Markwardt et al. GCN Circ. 26131

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Watson et al. 26103 COATLI Optical Observations COATLI
Optical Lipunov et al. 26113 Swift GRB191031.89: Global MASTER-Net
observations report 		MASTER
Gamma-ray Fermi 26102 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Lesage et al. 26116 Fermi GBM detection Fermi GBM Epeak=218±57 keV
T90=200 seconds

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

November 2, 2019