Swift Observations of GRB 191024A

K.L. Page (U. Leicester), A. Melandri (INAF-OAB) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 04:42:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191024A (trigger=931242) (Page et al. GCN Circ. 26063). Swift slewed to the burst after a delay to clear the Earth limb observing constraint. At the time of the trigger, the initial BAT position was 100° from the Sun (10.8 hours East) and 122° from the 21%-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 Laha et al. (GCN Circ. 26071), the BAT ground-calculated position is RA, Dec = 10.129, -67.138 deg which is RA(J2000) = 00h40m30.9s Dec(J2000) = -67°08'17.1" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 90%.

The mask-weighted light curve (Figure 1) shows a single-pulsed structure that starts and peaks at ~T0, and ends at ~T+8 s. T90 (15-350 keV) is 7.08 ± 0.98 s (estimated error including systematics).

The time-averaged spectrum from T-0.96 to T+7.45 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.78 ± 0.14. The fluence in the 15-150 keV band is 4.7 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 18% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.31 s in the 15-150 keV band is 1.6 ± 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/931242/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 26068). We have analysed 11 ks of XRT data for GRB 191024A, from 837 s to 60.1 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 26064).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.90 (+0.16, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.7, -0.6). The best-fitting absorption column is 4.1 (+4.3, -2.9) x 1021 cm-2, in excess of the Galactic value of 2.7 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 4.1 x 10-11 (6.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.1 (+4.3, -2.9) x 1021 cm-2
Galactic foreground: 2.7 x 1020 cm-2
Excess significance: 2.2 σ
Photon index: 1.9 (+0.7, -0.6)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00931242.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191024A 833 s after the BAT trigger (Breeveld and Page GCN Circ. 26070). No optical afterglow consistent with the XRT position (Beardmore et al., GCN Circ. 26064) 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 0.02 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
00h40m27.53s -67°08'42.2" 2.2" XRT-final UKSSDC
00h40m27.53s -67°08'42.2" 2.4" XRT-enhanced Beardmore et al. GCN Circ. 26064
00h40m30.9s -67°08'17.1" 1.3' BAT-refined Laha et al. GCN Circ. 26071

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26065 MASTER optical observation MASTER
Optical Bolmer and Schady 26066 GROND early upper limits GROND upper limits
Optical Xu et al. 26067 VLT/X-shooter optical observations VLT
Optical Strausbaugh and Cucchiara 26069 Afterglow Upper Limits with LCO LCO 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 833 982 147 >21.2
white 833 13531 360 >22.0
v 989 6893 274 >19.9
b 1087 13389 1043 >21.6
u 1062 19531 716 >21.3
w1 1038 19072 1160 >21.1
m2 6898 7098 197 >20.2
w2 1137 1506 39 >19.7

Table 3. UVOT observations reported by Breeveld and Page (GCN Circ. 26070). 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.

October 26, 2019