Swift Observations of GRB 191004A

S.B. Cenko (GSFC), J.D. Gropp (PSU) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 18:07:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191004A (trigger=927825) (Cenko et al. GCN Circ. 25945). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 134° from the Sun (10.6 hours West) and 99° from the 41%-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.

Lipunov et al. (GCN Circ. 25944) reported the position from MASTER for the optical afterglow of this GRB. LaPorte and Cenko (GCN Circ. 25972) reported the detection 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 Sakamoto et al. (GCN Circ. 25953), the BAT ground-calculated position is RA, Dec = 31.716, -36.932 deg, which is RA(J2000) = 02^h06^m51.8^s Dec(J2000) = -36°55'54.0" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) has two overlapping peaks starting at ~T-0.2 s, peaking at ~T_zero, and ending at ~3.1 s. T₉₀ (15-350 keV) is 2.44 ± 0.08 s (estimated error including systematics).

The time-averaged spectrum from T-0.09 to T+2.58 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.35 ± 0.08. The fluence in the 15-150 keV band is 1.3 ± 0.1 x 10^-6 erg cm^-2. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.09 s in the 15-150 keV band is 7.8 ± 0.5 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/927825/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 25957). We have analysed 22 ks of XRT data for GRB 191004A, from 67 s to 493.8 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 25950).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.76 ± 0.08, followed by a break at T+2884 s to an α of 2.39 (+0.20, -0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.04 (+0.21, -0.13). The best-fitting absorption column is 2.0 (+3.8, -0.5) x 10²⁰ cm^-2, consistent with the Galactic value of 1.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.81 (+0.14, -0.13) and a best-fitting absorption column of 6.0 (+3.4, -3.1) x 10²⁰ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.0 (+3.4, -3.1) x 10²⁰ cm^-2
Galactic foreground: 1.5 x 10²⁰ cm^-2
Excess significance: 2.4 σ
Photon index: 1.81 (+0.14, -0.13)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00927825.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191004A 84 s after the BAT trigger (LaPorte and Cenko GCN Circ. 25972). A fading source consistent with the XRT position (Osborne et al. GCN Circ. 25950) 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 0.01 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

02^h06^m40.38^s -36°56'01.9" 1.0" UVOT ?

02^h06^m40.44^s -36°55'59.2" 1.7" XRT-final UKSSDC

02^h06^m40.42^s -36°55'58.9" 1.7" XRT-enhanced Osborne et al. GCN Circ. 25950

02^h06^m51.8^s -36°55'54.0" 1.1' BAT-refined Sakamoto et al. GCN Circ. 25953

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 25944 Swift GRB191004.75: Global MASTER-Net
OT detection MASTER detection

Optical Heintz et al. 25952 NOT optical upper limits NOT upper limits

Optical Schady et al. 25959 GROND Upper limits GROND upper limits

Optical Strausbaugh and Cucchiara 25971 Optical Detection with LCO LCO detection

Optical Schady 26324 correction to reported GROND upper
limits GROND upper limits

Gamma-ray Svinkin et al. 25973 Konus-Wind observation Konus-Wind E_peak=157 (-37,+95) keV
Duration=~5 seconds
Fluence=2.00(-0.41,+0.62)x10^-6erg cm^-2

Gamma-ray Yamaoka et al. 25981 CALET Gamma-Ray Burst Monitor detection CALET

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 582 775 38 >18.83

uvm2 680 873 38 >18.00

u 327 750 265 >19.60

v 656 1104 58 >18.06

uvw1 705 898 38 >17.89

uvw2 632 1080 58 >18.28

white 115 1054 333 19.78± 0.16

white 5567 12438 956 >21.24

RA (J2000)	Dec (J2000)	Error	Note	Reference
02^h06^m40.38^s	-36°56'01.9"	1.0"	UVOT	?
02^h06^m40.44^s	-36°55'59.2"	1.7"	XRT-final	UKSSDC
02^h06^m40.42^s	-36°55'58.9"	1.7"	XRT-enhanced	Osborne et al. GCN Circ. 25950
02^h06^m51.8^s	-36°55'54.0"	1.1'	BAT-refined	Sakamoto et al. GCN Circ. 25953

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	25944	Swift GRB191004.75: Global MASTER-Net OT detection	MASTER	detection
Optical	Heintz et al.	25952	NOT optical upper limits	NOT	upper limits
Optical	Schady et al.	25959	GROND Upper limits	GROND	upper limits
Optical	Strausbaugh and Cucchiara	25971	Optical Detection with LCO	LCO	detection
Optical	Schady	26324	correction to reported GROND upper limits	GROND	upper limits
Gamma-ray	Svinkin et al.	25973	Konus-Wind observation	Konus-Wind	E_peak=157 (-37,+95) keV Duration=~5 seconds Fluence=2.00(-0.41,+0.62)x10^-6erg cm^-2
Gamma-ray	Yamaoka et al.	25981	CALET Gamma-Ray Burst Monitor detection	CALET

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
b	582	775	38	>18.83
uvm2	680	873	38	>18.00
u	327	750	265	>19.60
v	656	1104	58	>18.06
uvw1	705	898	38	>17.89
uvw2	632	1080	58	>18.28
white	115	1054	333	19.78± 0.16
white	5567	12438	956	>21.24

Table 3. UVOT observations reported by LaPorte and Cenko (GCN Circ. 25972). 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.

December 3, 2019