Swift Observations of GRB 191029A

P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 11:15:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191029A (trigger=932058) (Evans et al. GCN Circ. 26088). Swift did not slew immediately because of an observing constraint. At the time of the trigger, the initial BAT position was 114° from the Sun (9.3 hours East) and 102° from the 2%-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 Lien et al. (GCN Circ. 26110), the BAT ground-calculated position is RA, Dec = 352.140, -40.464 deg which is RA(J2000) = 23h28m33.6s Dec(J2000) = -40d -27' -49.8" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 46%.

The mask-weighted light curve (Figure 1) shows a pair of weak pulses spanning the period from approximately T-5 to T+35 seconds. T90 (15-350 keV) is 39.5 ± 7.9 s (estimated error including systematics).

The time-averaged spectrum from T-1.05 to T+45.85 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.77 ± 0.21. The fluence in the 15-150 keV band is 1.1 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 41% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.90 s in the 15-150 keV band is 1.1 ± 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/932058/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 26094). We have analysed 20 ks of XRT data for GRB 191029A, from 3.1 ks to 206.2 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. 26095).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.89 ± 0.09.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.81 (+0.23, -0.19). The best-fitting absorption column is 3.5 (+5.3, -1.7) x 1020 cm-2, consistent with the Galactic value of 1.8 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 3.5 x 10-11 (3.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.5 (+5.3, -1.7) x 1020 cm-2
Galactic foreground: 1.8 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.81 (+0.23, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00932058.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191029A 4.3 ks after the BAT trigger (Oates and Evans GCN Circ. 26097). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 26095) 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
23h28m38.78s -40°30'04.1" 1.5" XRT-final UKSSDC
23h28m38.74s -40°30'03.9" 1.5" XRT-enhanced Evans et al. GCN Circ. 26095
23h28m33.6s -40°27'49.8" 1.8' BAT-refined Lien et al. GCN Circ. 26110

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical 26091 iTelescope optical upper limits iTelescope upper limits
Optical Hu et al. 26092 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
white 4280 22783 1598 >22.1
v 4691 15599 1082 >20.2
b 5510 28520 2353 >21.7
u 5305 21337 1082 >20.9
w1 5100 17046 723 >20.4
m2 4895 16504 1082 >20.6
w2 4486 11309 868 >20.7

Table 3. UVOT observations reported by Oates and Evans (GCN Circ. 26097). 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 1, 2019