Swift Observations of GRB 191004B

S.B. Cenko (GSFC), A. Tohuvavohu (U. Toronto) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 21:33:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191004B (trigger=927839) (Cenko et al. GCN Circ. 25947). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 123° from the Sun (9.4 hours West) and 106° from the 42%-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.

Cenko et al. (GCN Circ. 25947) reported the discovery with UVOT of an optical afterglow. Lipunov et al. (GCN Circ. 25948) reported the position from MASTER for the optical afterglow of this GRB. D'Elia et al. (GCN Circ. 25956) determined a redshift of 3.503 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 Stamatikos et al. (GCN Circ. 25955), the BAT ground-calculated position is RA, Dec = 49.200, -39.637 deg, which is RA(J2000) = 03^h16^m47.9^s Dec(J2000) = -39°38'15.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%.

The mask-weighted light curve (Figure 1) shows multiple overlapping peaks starting at T-1 s, peaking at ~T+7 s, and ending at ~T+45 s. T₉₀ (15-350 keV) is 37.7 ± 14.9 s (estimated error including systematics).

The time-averaged spectrum from T-1.45 to T+56.17 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.10 ± 0.08. The fluence in the 15-150 keV band is 2.5 ± 0.1 x 10^-6 erg cm^-2. This fluence is larger than that of 67% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+2.75 s in the 15-150 keV band is 5.0 ± 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/927839/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 25961). We have analysed 11 ks of XRT data for GRB 191004B, from 63 s to 252.4 ks after the BAT trigger. The data comprise 142 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 25951).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.9 (+0.6, -0.7), followed by a break at T+94.6 s to an α of 1.073 (+0.031, -0.027).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.75 (+0.09, -0.07). The best-fitting absorption column is 1.4 (+6.8, -1.4) x 10²¹ cm^-2, at a redshift of 3.503, in addition to the Galactic value of 1.9 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.54 (+0.21, -0.14) and a best-fitting absorption column of 2 (+178, -2) x 10²⁰ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.9 x 10²⁰ cm^-2
Intrinsic column: 2 (+178, -2) x 10²⁰ cm^-2 at z=3.503
Photon index: 1.54 (+0.21, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00927839.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191004B 69 s after the BAT trigger (LaPorte and Cenko GCN Circ. 25978). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 25951) 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 E_B-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

03^h16^m49.10^s -39°38'03.9" 0.43" UVOT-refined LaPorte and Cenko GCN Circ. 25978

03^h16^m49.01^s -39°38'05.4" 1.7" XRT-final UKSSDC

03^h16^m48.96^s -39°38'06.1" 2.0" XRT-enhanced Beardmore et al. GCN Circ. 25951

03^h16^m47.9^s -39°38'15.0" 1.0' BAT-refined Stamatikos et al. GCN Circ. 25955

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 25948 Swift GRB191004.9: Global MASTER-Net
OT detection MASTER detection

Optical Strausbaugh and Cucchiara 25949 Optical Afterglow Detection with LCO LCO detection

Optical Heintz et al. 25954 NOT optical photometry NOT detection

Optical D'Elia et al. 25956 VLT/X-shooter redshift VLT redshift

Optical Watson et al. 25958 COATLI Optical Observations COATLI

Optical Schady et al. 25960 GROND Detection of the Optical/NIR
Afterglow GROND detection

Optical Strausbaugh and Cucchiara 25977 Continued Optical Afterglow Monitoring
with LCO LCO detection

Gamma-ray Svinkin et al. 25974 Konus-Wind observation Konus-Wind E_peak=172 (-21,+27) keV
Duration=~5.5 seconds
Fluence=4.13(-0.40,+0.45)x10^-6erg cm^-2

Gamma-ray Svinkin 25976 Correction to Konus-Wind GCN Circ. 25974 Konus-Wind

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

Filter T_start(s) T_stop(s) Exp(s) Mag

b 38697 38997 295 >20.42

uvm2 44827 45727 885 >20.25

u 294 387 91 >19.04

v 43916 44822 885 >20.07

uvw1 45734 46281 538 >19.96

uvw2 39915 40544 618 >20.09

white 82 232 147

white 39003 39909 885 20.51± 0.13

RA (J2000)	Dec (J2000)	Error	Note	Reference
03^h16^m49.10^s	-39°38'03.9"	0.43"	UVOT-refined	LaPorte and Cenko GCN Circ. 25978
03^h16^m49.01^s	-39°38'05.4"	1.7"	XRT-final	UKSSDC
03^h16^m48.96^s	-39°38'06.1"	2.0"	XRT-enhanced	Beardmore et al. GCN Circ. 25951
03^h16^m47.9^s	-39°38'15.0"	1.0'	BAT-refined	Stamatikos et al. GCN Circ. 25955

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	25948	Swift GRB191004.9: Global MASTER-Net OT detection	MASTER	detection
Optical	Strausbaugh and Cucchiara	25949	Optical Afterglow Detection with LCO	LCO	detection
Optical	Heintz et al.	25954	NOT optical photometry	NOT	detection
Optical	D'Elia et al.	25956	VLT/X-shooter redshift	VLT	redshift
Optical	Watson et al.	25958	COATLI Optical Observations	COATLI
Optical	Schady et al.	25960	GROND Detection of the Optical/NIR Afterglow	GROND	detection
Optical	Strausbaugh and Cucchiara	25977	Continued Optical Afterglow Monitoring with LCO	LCO	detection
Gamma-ray	Svinkin et al.	25974	Konus-Wind observation	Konus-Wind	E_peak=172 (-21,+27) keV Duration=~5.5 seconds Fluence=4.13(-0.40,+0.45)x10^-6erg cm^-2
Gamma-ray	Svinkin	25976	Correction to Konus-Wind GCN Circ. 25974	Konus-Wind

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
b	38697	38997	295	>20.42
uvm2	44827	45727	885	>20.25
u	294	387	91	>19.04
v	43916	44822	885	>20.07
uvw1	45734	46281	538	>19.96
uvw2	39915	40544	618	>20.09
white	82	232	147
white	39003	39909	885	20.51± 0.13

Table 3. UVOT observations reported by LaPorte and Cenko (GCN Circ. 25978). 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 8, 2019