Swift Observations of GRB 181003A

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 05:11:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181003A (trigger=865179) (Beardmore et al. GCN Circ. 23295). Swift did not slew immediately to the burst due to an observing constraint. At the time of the trigger, the initial BAT position was 124° from the Sun (9.1 hours West) and 79° from the 41%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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. 23306), the BAT ground-calculated position is RA, Dec = 52.481, -33.991 deg which is RA(J2000) = 03^h29^m55.4^s Dec(J2000) = -33°59'25.8" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a weak structure that starts at ~T-10 s and ends at ~ T+20 s. The burst went out of the BAT field of view at ~T+97 s. T₉₀ (15-350 keV) is 21.5 ± 4.8 s (estimated error including systematics).

The time-averaged spectrum from T-11.20 to T+18.01 s is best fit by a power law with an exponential cutoff. This fit gives a photon index -0.20 ± 1.12, and E_peak of 56.5 ± 14.4 keV (χ² 52.40 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 4.2 ± 0.7 x 10^-7 erg cm^-2 and the 1-s peak flux measured from T+1.86 s in the 15-150 keV band is 0.6 ± 0.1 ph cm^-2 s^-1. This fluence is larger than that of 18% 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 1.54 ± 0.19 (χ² 63.13 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/865179/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 23305). Six uncatalogued X-ray sources are detected consistent with being within 296 arcsec of the Swift/BAT position, of which one ("Source 1") is fading with 3-sigma significance, and is therefore likely the GRB afterglow. The source is fading with α >0.8.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181003A 3095 s after the BAT trigger (LaPorte and Beardmore GCN Circ. 23303). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 23296) 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

03^h30^m06.89^s -33°57'52.8" 6.3" XRT-final UKSSDC

03^h30^m06.89^s -33°57'52.8" 6.3" XRT Tohuvavohu et al. GCN Circ. 23305

03^h29^m55.4^s -33°59'25.8" 1.7' BAT-refined Krimm et al. GCN Circ. 23306

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 23299 MASTER optical observation MASTER

Optical Guidorzi et al. 23300 LCO Cerro Tololo observations LCO marginal detection

Optical Watson et al. 23307 RATIR Optical and NIR Observations RATIR upper limits

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 5040 5240 196 >20.83

uvm2 3452 3652 196 >19.46

u 3862 3902 39 >19.19

v 3427 3447 196 >19.79

uvw1 3658 3857 196 >19.85

uvw2 5451 5597 144 >19.37

white 3091 3240 294 >21.73

RA (J2000)	Dec (J2000)	Error	Note	Reference
03^h30^m06.89^s	-33°57'52.8"	6.3"	XRT-final	UKSSDC
03^h30^m06.89^s	-33°57'52.8"	6.3"	XRT	Tohuvavohu et al. GCN Circ. 23305
03^h29^m55.4^s	-33°59'25.8"	1.7'	BAT-refined	Krimm et al. GCN Circ. 23306

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	23299	MASTER optical observation	MASTER
Optical	Guidorzi et al.	23300	LCO Cerro Tololo observations	LCO	marginal detection
Optical	Watson et al.	23307	RATIR Optical and NIR Observations	RATIR	upper limits

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
b	5040	5240	196	>20.83
uvm2	3452	3652	196	>19.46
u	3862	3902	39	>19.19
v	3427	3447	196	>19.79
uvw1	3658	3857	196	>19.85
uvw2	5451	5597	144	>19.37
white	3091	3240	294	>21.73

Table 3. UVOT observations reported by LaPorte and Beardmore (GCN Circ. 23303). 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.

October 5, 2018