Swift Observations of GRB 191106A

F.E. Marshall (NASA/GSFC), K.L. Page (U. Leicester) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 14:15:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191106A (trigger=933515) (Marshall et al. GCN Circ. 26177). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 76° from the Sun (3.2 hours East) and 87° from the 70%-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 Barthelmy et al. (GCN Circ. 26198), the BAT ground-calculated position is RA, Dec = 269.356, 46.046 deg which is RA(J2000) = 17h57m25.4s Dec(J2000) = +46°02'44.7" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 95%.

The mask-weighted light curve (Figure 1) shows many overlapping pulses. The burst emission starts at ~T-1.3 s and ends at ~T+2.4 s. T90 (15-350 keV) is 3.42 ± 0.35 s (estimated error including systematics).

The time-averaged spectrum from T-1.31 to T+2.38 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.89 ± 0.17. The fluence in the 15-150 keV band is 3.2 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 13% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.02 s in the 15-150 keV band is 1.1 ± 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/933515/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 26190). We have analysed 12 ks of XRT data for GRB 191106A, from 56 s to 75.6 ks after the BAT trigger. The data comprise 30 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.72 (+0.11, -0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.3 (+0.6, -0.5). The best-fitting absorption column is 4.1 (+3.2, -2.2) x 1021 cm-2, in excess of the Galactic value of 4.4 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 3.4 x 10-11 (6.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.1 (+3.2, -2.2) x 1021 cm-2
Galactic foreground: 4.4 x 1020 cm-2
Excess significance: 2.7 σ
Photon index: 2.3 (+0.6, -0.5)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00933515.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191106A 132 s after the BAT trigger (Oates and Marshall GCN Circ. 26199). No optical afterglow consistent with the XRT position (Page et al., GCN Circ. 26190) 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 0.06 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
17h57m20.66s +46°02'05.5" 3.7" XRT-final UKSSDC
17h57m20.55s +46°02'06.9" 2.3" XRT-refined Page et al. GCN Circ. 26190
17h57m25.4s +46°02'44.7" 1.7' BAT-refined Barthelmy et al. GCN Circ. 26198

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26178 MASTER optical observation MASTER
Optical Hu et al. 26179 BOOTES-4/MET optical limit BOOTES-4 upper limits
Optical Belkin et al. 26180 Mondy optical upper limit Mondy upper limits
Optical Zhu et al. 26191 NOT optical upper limit NOT upper limits
Gamma-ray Gaikwad et al. 26201 AstroSat CZTI detection CZTI T90=3.6 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
whiteFC 4987 5137 147 >20.0
uFC 132 381 246 >20.1
white 4987 6775 541 >21.0
v 437 7186 549 >19.9
b 387 12533 1264 >21.3
u 132 11621 1631 >20.8
w1 486 7510 464 >20.0
m2 461 7391 510 >20.0
w2 412 13245 1223 >20.6

Table 3. UVOT observations reported by Oates and Marshall (GCN Circ. 26199). 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.

November 9, 2019