Swift Observations of GRB 191101A

A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 21:08:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191101A (trigger=932742) (D'Ai et al. GCN Circ. 26135). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 67° from the Sun (2.3 hours East) and 72° from the 26%-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 Stamatikos et al. (GCN Circ. 26167), the BAT ground-calculated position is RA, Dec = 251.845, 43.740 deg which is RA(J2000) = 16h47m22.8s Dec(J2000) = +43°44'24.5" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 47%.

The mask-weighted light curve (Figure 1) shows a single, rather weak pulse, extending from ~T-20 s, peaking at T+0 s, dropping sharply at T+25 s, followed by low-level extended emission out to ~T+140 s. T90 (15-350 keV) is 137.95 ± 22.83 s (estimated error including systematics).

The time-averaged spectrum from T-20.97 to T+143.53 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.78 ± 0.18. The fluence in the 15-150 keV band is 1.9 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 57% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-1.00 s in the 15-150 keV band is 1.3 ± 0.3 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/932742/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 26144). We have analysed 14 ks of XRT data for GRB 191101A, from 92 s to 51.2 ks after the BAT trigger. The data comprise 190 s in Windowed Timing (WT) mode (the first 5 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 26137).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of α=0.53 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.24 ± 0.05. The best-fitting absorption column is 1.03 ± 0.11 x 1021 cm-2, in excess of the Galactic value of 1.3 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.00 (+0.11, -0.10) and a best-fitting absorption column of 7.9 (+2.6, -2.4) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.9 (+2.6, -2.4) x 1020 cm-2
Galactic foreground: 1.3 x 1020 cm-2
Excess significance: 4.4 σ
Photon index: 2.00 (+0.11, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00932742.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191101A 110 s after the BAT trigger (LaPorte and D'Ai GCN Circ. 26164). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 26140) 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 0.01 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
16h47m20.93s +43°44'26.6" 1.4" XRT-final UKSSDC
16h47m20.94s +43°44'27.5" 1.8" XRT-enhanced Beardmore et al. GCN Circ. 26140
16h47m22.8s +43°44'24.5" 1.7' BAT-refined Stamatikos et al. GCN Circ. 26167

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26139 Swift GRB191101.88: Global MASTER-Net
observations report 		MASTER
Optical Gokuldass et al. 26142 VIRT Optical Upper Limits Virgin Island Robotic Telescope upper limits
Optical Kong 26145 iTelescope optical limit iTelescope 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 110 259 147 >21.1
uFC 268 517 246 >20.3
white 110 5863 589 >21.6
v 597 1588 110 >19.6
b 523 5658 294 >21.2
u 268 5453 540 >20.9
w1 646 5248 294 >19.3
m2 622 5043 300 >19.8
w2 573 1564 117 >19.3

Table 3. UVOT observations reported by LaPorte and D'Ai (GCN Circ. 26164). 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 5, 2019