Swift Observations of GRB 191019A

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

1. Introduction

At 15:12:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191019A (trigger=930285) (Simpson et al. GCN Circ. 26031). Swift did not slew due to an observing constraint. At the time of the trigger, the initial BAT position was 129° from the Sun (9.1 hours East) and 117° from the 70%-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.

LaPorte and Simpson (GCN Circ. 26053) reported the detection with UVOT of an optical afterglow. Perley et al. (GCN Circ. 26062) reported the position from NOT for the optical afterglow of this GRB. Fynbo et al. (GCN Circ. 26041) determined a redshift of 0.248 from NOT. 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. 26046), the BAT ground-calculated position is RA, Dec = 340.035, -17.340 deg which is RA(J2000) = 22h40m08.3s Dec(J2000) = -17°20'23.9" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 46%.

The mask-weighted light curve (Figure 1) shows a complex structure with many overlapping pulses that starts at ~T0 and ends at ~T+90 s. The burst went out of the BAT FOV at ~T+400 s. T90 (15-350 keV) is 64.35 ± 4.45 s (estimated error including systematics).

The time-averaged spectrum from T-0.83 to T+84.28 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.25 ± 0.05. The fluence in the 15-150 keV band is 1.0 ± 0.03 x 10-5 erg cm-2. This fluence is larger than that of 92.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.14 s in the 15-150 keV band is 5.7 ± 0.4 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/930285/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 26045). We have analysed 24 ks of XRT data for GRB 191019A, from 3.3 ks to 566.7 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 26043).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.28 (+0.17, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 ± 0.4. The best-fitting absorption column is 9 (+12, -9) x 1020 cm-2, at a redshift of 0.248, in addition to the Galactic value of 3.3 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 3.3 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 3.3 x 1020 cm-2
Intrinsic column: 9 (+12, -9) x 1020 cm-2 at z=0.248
Photon index: 2.0 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00930285.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191019A 3294 s after the BAT trigger (LaPorte and Simpson GCN Circ. 26053). A fading source consistent with the XRT position (Evans et al. GCN Circ. 26034) 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
22h40m05.86s -17°19'41.3" 1.7" XRT-final UKSSDC
22h40m05.93s -17°19'40.9" 2.4" XRT-enhanced Osborne et al. GCN Circ. 26043
22h40m08.3s -17°20'23.9" 1.0' BAT-refined Krimm et al. GCN Circ. 26046

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26032 MASTER optical observation MASTER
Optical Lipunov et al. 26033 Swift GRB191019.63: Global MASTER-Net
observations report 		MASTER
Optical Reva et al. 26036 TSHAO optical observations, probable
afterglow and host galaxy		 Zeiss-1000
Optical Perley et al. 26039 NOT optical observations NOT
Optical Fynbo et al. 26041 NOT optical spectroscopy of candidate
counterpart		 NOT spectroscopy
Optical Guelbenzu 26042 GROND observations GROND detection
Optical Lipunov et al. 26044 Swift GRB191019.67: Global MASTER-Net
observations report 		MASTER
Optical Zhu et al. 26059 Nanshan-0.6m optical observations Xinjiang Astro. Obs.
Optical Perley et al. 26062 NOT optical afterglow and host
association confirmation		 NOT
Optical Gendre et al. 26098 Zadko Telescope optical observations Zadko light curve

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
b 4541 4741 196
uvm2 5361 5561 196 >19.20
u 4336 4536 196 >19.43
v 5156 5356 196 >18.75
uvw1 4131 4331 196 >19.13
uvw2 4952 5151 196 >19.28
white 4746 4946 196 19.66± 0.20
white 27982 28668 668 20.27± 0.18

Table 3. UVOT observations reported by LaPorte and Simpson (GCN Circ. 26053). 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 31, 2019