Swift Observations of GRB 131002A

B.N. Barlow (High Point University), K.L. Page (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 06:54:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 131002A (trigger=572721) (Barlow et al. GCN Circ. 15283). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 90° from the Sun (4.3 hours East) and 86° from the 8%-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.

Breeveld and Barlow (GCN Circ. 15294) reported the detection with UVOT of an optical afterglow. Guidorzi (GCN Circ. 15282) reported the position from FTN for the optical afterglow of this GRB. 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 http://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Sakamoto et al. (GCN Circ. 15302), the BAT ground-calculated position is RA, Dec = 253.103, 82.036 deg which is RA(J2000) = 16h52m24.8s Dec(J2000) = +82°02'08.3" with an uncertainty of 3.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 71%.

The mask-weighted light curve (Figure 1) shows two peaks separated by ~45 seconds. The first short peak starts at ~T0 and lasts for ~1 s, the second long peak starts at ~T+40 and ends at ~T+60. T90 (15-350 keV) is 55.59 ± 3.98 s (estimated error including systematics).

The time-averaged spectrum from T-0.20 to T+59.67 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.63 ± 0.19. The fluence in the 15-150 keV band is 6.4 ± 0.8 x 10-7 erg cm-2. This fluence is larger than that of 27% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+43.17 s in the 15-150 keV band is 1.3 ± 0.1 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/572721/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 15292). We have analysed 27 ks of XRT data for GRB 131002A, from 56 s to 197.3 ks after the BAT trigger. The data comprise 636 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of α=1.17 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.07 (+0.18, -0.17). The best-fitting absorption column is 1.1 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 5.6 x 1020 cm-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.1 ± 0.4 x 1021 cm-2
Galactic foreground: 5.6 x 1020 cm-2
Excess significance: 2.5 σ
Photon index: 2.07 (+0.18, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00572721.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 131002A 77 s after the BAT trigger (Breeveld and Barlow GCN Circ. 15294). A fading source consistent with the optical position (Guidorzi GCN Circ. 15282) 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 0.08 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.

XRT light curve

Figure 2. The XRT light curve. Any data from a cross-hatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
16h52m52.94s +82°03'14.8" 1.7" XRT-final UKSSDC
16h52m53.02s +82°03'15.6" 1.8" XRT-enhanced Osborne et al. GCN Circ. 15287
16h52m24.8s +82°02'08.3" 3.9' BAT-refined Sakamoto et al. GCN Circ. 15302

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi 15282 Faulkes Telescope North optical afterglow FTN
Optical Elenin et al. 15290 ISON-NM optical observations ISON-NM detection
Gamma-ray von Kienlin and Meegan 15300 Fermi GBM observation Fermi GBM detection

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 77 227 147 17.94 0.06
white 870 1020 147 19.40 0.11
v 620 1071 59 18.89 0.36
b 546 739 39 19.41 0.31
u 290 7169 736 >21.1
w1 669 6964 510 >20.7
m2 645 13293 939 >21.1
w2 770 11860 1199 >21.5

Table 3. UVOT observations reported by Breeveld and Barlow (GCN Circ. 15294). 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 5, 2013