Swift Observations of GRB 191122A

B. Sbarufatti (PSU), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 13:32:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191122A (trigger=938899) (Sbarufatti et al. GCN Circ. 26269). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 128° from the Sun (11.8 hours East) and 127° 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 Krimm et al. (GCN Circ. 26275), the BAT ground-calculated position is RA, Dec = 54.290, -32.176 deg which is RA(J2000) = 03h37m09.7s Dec(J2000) = -32°10'35.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) shows a single FRED-shaped pulse rising from T-20 s to T+0 s, then decaying out to roughly T+250 s. T90 (15-350 keV) is 153.3 ± 38.3 s (estimated error including systematics).

The time-averaged spectrum from -11.2 to 245.4 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.24 ± 0.28, and Epeak of 89.2 ± 41.9 keV (χ2 69.11 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.6 ± 0.3 x 10-6 erg cm-2 and the 1-s peak flux measured from T+6.44 s in the 15-150 keV band is 3.1 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 87% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.67 ± 0.07 (χ2 76.48 for 57 d.o.f.). 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/938899/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 26274). We have analysed 13 ks of XRT data for GRB 191122A, from 85 s to 109.8 ks after the BAT trigger. The data comprise 180 s in Windowed Timing (WT) mode (the first 7 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 Evans et al. (GCN Circ. 26270).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.29 ± 0.05. At T+866 s the decay steepens to an α of 1.94 (+1.22, -0.13). The light curve breaks again at T+6095 s to a decay with α=-1.5000000 (+1.4530000, -0.0000024), before a final break at T+9099 s s after which the decay index is 2.5 ± 0.4.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.06 (+0.04, -0.03). The best-fitting absorption column is consistent with the Galactic value of 1.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.15 (+0.08, -0.07) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.4 x 10-11 (5.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 10.0 (+16.2, -0.0) x 1019 cm-2
Galactic foreground: 1.0 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.15 (+0.08, -0.07)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00938899.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191122A 104 s after the BAT trigger (Kuin and Sbarufatti GCN Circ. 26277). No optical afterglow consistent with the XRT position (Evans GCN Circ. 26270,26272) 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
03h37m04.77s -32°11'12.1" 1.4" XRT-final UKSSDC
03h37m04.77s -32°11'11.9" 1.6" XRT-enhanced Evans et al. GCN Circ. 26272
03h37m09.7s -32°10'35.4" 1.0' BAT-refined Krimm et al. GCN Circ. 26275

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Sun et al. 26271 CHES Optical Observations
Optical Hu et al. 26273 BOOTES-4/MET optical limit BOOTES-4 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 104 253 147 >21.1
uFC 316 566 246 >20.4
white 104 11801 1305 >22.0
v 647 6097 432 >20.2
b 572 10938 1121 >21.9
u 316 6696 642 >20.5
w1 696 6508 413 >20.1
m2 846 6303 413 >21.2
w2 797 5893 413 >20.1

Table 3. UVOT observations reported by Kuin and Sbarufatti (GCN Circ. 26277). 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 24, 2019