Swift Observations of GRB 191123A

A. Tohuvavohu (U. Toronto), P.A. Evans (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 10:38:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191123A (trigger=939079) (Tohuvavohu et al. GCN Circ. 26279). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 49° from the Sun (1.5 hours West) and 30° from the 13%-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.

Tohuvavohu et al. (GCN Circ. 26279) reported the discovery with UVOT of an optical afterglow. 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. 26286), the BAT ground-calculated position is RA, Dec = 215.289, 22.823 deg which is RA(J2000) = 14h21m09.3s Dec(J2000) = +22°49'23.5" with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%.

The mask-weighted light curve (Figure 1) shows a pair of pulses, each with some sub- structure. The first starts at ~T-10 s, peaks at T+0 and decays by T+40 s. The second pulse starts at T+110 s, peaks soon afterward and decays by T+180 s. T90 (15-350 keV) is 275.1 ± 102.4 s (estimated error including systematics).

The time-averaged spectrum from -2.1 to 288.1 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.67 ± 0.25. The fluence in the 15-150 keV band is 1.2 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 45% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+111.77 s in the 15-150 keV band is 1.2 ± 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/939079/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 26285).

We have analysed 6.2 ks of XRT data for GRB 191123A, from 76 s to 18.5 ks after the BAT trigger. The data comprise 540 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 26284).

The late-time light curve (Figure 2) (from T0+5.6 ks) can be modelled with a power-law decay with a decay index of α=1.00 ± 0.15.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.52 ± 0.05. The best-fitting absorption column is 6.7 (+1.6, -1.5) x 1020 cm-2, in excess of the Galactic value of 3.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.98 (+0.12, -0.11) and a best-fitting absorption column of 4.8 (+2.6, -1.8) 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 (3.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.8 (+2.6, -1.8) x 1020 cm-2
Galactic foreground: 3.0 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.98 (+0.12, -0.11)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00939079.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191123A 78 s after the BAT trigger (Kuin and Tohuvavohu GCN Circ. 26282). A candidate optical afterglow was 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 0.04 mag. in the direction of the GRB (Schlegel et al. 1998).

The Swift/UVOT began settled observations of the field of GRB 191123A 78 s after the BAT trigger (Kuin and Tohuvavohu GCN Circ. 26287). A source consistent with the XRT position (Goad et al. GCN Circ. 26284) 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 0.04 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
14h21m02.88s +22°51'36.1" 0.44" UVOT-refined Kuin and Tohuvavohu GCN Circ. 26287
14h21m02.84s +22°51'36.0" 1.4" XRT-final UKSSDC
14h21m02.85s +22°51'35.7" 1.7" XRT-enhanced Goad et al. GCN Circ. 26284
14h21m09.3s +22°49'23.5" 2.2' BAT-refined Laha et al. GCN Circ. 26286

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical 26283 iTelescope optical upper limits 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
white 78 228 147 18.25 ± 0.05
v 620 2111 175 18.28 ± 0.19
b 546 1168 58 19.18 ± 0.25
u 290 6376 501 >20.4
w1 669 6171 372 >19.9
m2 1074 5966 216 >19.8
w2 595 12186 944 >20.9

Table 3. UVOT observations reported by Kuin and Tohuvavohu (GCN Circ. 26287). 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.

November 25, 2019