Swift Observations of GRB 191011A

S.L. Laha (GSFC/UMBC/CRESST), J.P. Osborne (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:35:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191011A (trigger=928924) (Laha et al. GCN Circ. 25988). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 136° from the Sun (10.1 hours West) and 54° from the 94%-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.

Laha et al. (GCN Circ. 25988) reported the detection with UVOT of an optical afterglow. Malesani et al. (GCN Circ. 25991) determined a redshift of 1.722 from VLT. 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 Ukwatta et al. (GCN Circ. 26003), the BAT ground-calculated position is RA, Dec = 44.729, -27.853 deg which is RA(J2000) = 02h58m55.0s Dec(J2000) = -27°51'12.4" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a short pulse that starts and peaks at ~T0, and ends at ~T+2 s. The short pulse is followed by some weak emission that lasts until ~T+8 s. T90 (15-350 keV) is 7.37 ± 0.95 s (estimated error including systematics).

The time-averaged spectrum from T-0.21 to T+8.04 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.94 ± 0.17. The fluence in the 15-150 keV band is 3.3 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 15% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.18 s in the 15-150 keV band is 1.8 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

Using the 16-ms binned light curve (Figure 1) , the spectral lag of the short pulse is 30 (+45, -34) ms for the 50-100 keV to 15-25 keV bands, and 20 (+40, -44) ms for the 25-50 keV to 15-25 keV bands. These values are consistent with both short and long GRBs due to the large uncertainties.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/928924/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 25998). We have analysed 20 ks of XRT data for GRB 191011A, from 84 s to 109.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 25995).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.16 (+0.15, -0.21), followed by a break at T+855 s to an α of 1.32 ± 0.08.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.81 (+0.15, -0.14). The best-fitting absorption column is 4.5 (+3.4, -3.0) x 1021 cm-2, at a redshift of 1.722, in addition to the Galactic value of 1.5 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 3.6 x 10-11 (4.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.5 x 1020 cm-2
Intrinsic column: 4.5 (+3.4, -3.0) x 1021 cm-2 at z=1.722
Photon index: 1.81 (+0.15, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00928924.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191011A 78 s after the BAT trigger (Kuin and Laha GCN Circ. 26002). A source consistent with the XRT position (Evans et al. GCN Circ. 25995) 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
02h58m54.79s -27°50'43.4" 0.73" UVOT-refined Kuin and Laha GCN Circ. 26002
02h58m54.71s -27°50'43.4" 1.4" XRT-final UKSSDC
02h58m54.72s -27°50'43.4" 1.5" XRT-enhanced Evans et al. GCN Circ. 25995
02h58m55.0s -27°51'12.4" 1.5' BAT-refined Ukwatta et al. GCN Circ. 26003

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 25989 Swift GRB191011.19: Global MASTER-Net
OT detection		 MASTER detection
Optical Lipunov et al. 25990 Swift GRB191011.19: Global MASTER-Net
observations report 		MASTER
Optical Malesani et al. 25991 VLT X-shooter redshift from RRM
Observations		 VLT redshift
Optical Guelbenzu and Schady 25992 GROND Detection of the Optical/NIR
afterglow		 GROND detection
Optical Lipunov et al. 25996 MASTER OT retraction. MASTER retraction
Optical Watson et al. 26001 RATIR Optical and NIR Observations RATIR upper limits
Gamma-ray Fermi 25987 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Burgess et al. 25993 BALROG localization (Fermi Trigger
592461363 / GRB 191011192)		 BALROG
Gamma-ray Bissaldi and Veres 26000 Fermi GBM observation Fermi GBM Epeak=89±23 keV
T90=13 seconds
Fluence=4.33±0.50x10-7erg cm-2
(4.0 percentile for long GRBs)
Other Guelbenzu 25994 correction of reported magnitudes

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 78 228 147 20.27 ± 0.16
v 620 11672 1211 >20.2
b 545 7080 510 21.03 ± 0.27
u 290 6875 697 20.86 ± 0.27
w1 670 6670 510 >20.3
m2 1073 12198 767 >21.1
w2 1024 7491 432 >20.9

Table 3. UVOT observations reported by Kuin and Laha (GCN Circ. 26002). 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.

October 13, 2019