Swift Observations of GRB 181002A

J.D. Gropp (PSU) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 11:26:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181002A (trigger=865036) (Gropp et al. GCN Circ. 23290). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 123° from the Sun (8.2 hours West) and 35° from the 49%-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.

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 Cummings et al. (GCN Circ. 23294), the BAT ground-calculated position is RA, Dec = 64.739, 15.248 deg which is RA(J2000) = 04h18m57.4s Dec(J2000) = +15°14'51.6" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 70%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-30 s and ends at ~T+45 s. T90 (15-350 keV) is 63.4 ± 12.4 s (estimated error including systematics).

The time-averaged spectrum from T-28.38 to T+43.62 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.15 ± 0.23. The fluence in the 15-150 keV band is 8.8 ± 1.3 x 10-7 erg cm-2. This fluence is larger than that of 35% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+11.39 s in the 15-150 keV band is 0.6 ± 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/865036/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp (GCN Circ. 23301).

We have analysed 935 s of XRT data for GRB 181002A, from 142 s to 1.1 ks after the BAT trigger. The data comprise 95 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. 23292).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=4.15 (+0.18, -0.16).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.60 (+0.18, -0.17). The best-fitting absorption column is 4.0 (+0.7, -0.6) x 1021 cm-2, in excess of the Galactic value of 2.8 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.8 x 10-11 (6.7 x 10-11) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 4.0 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 2.8 x 1021 cm-2
Excess significance: 3.2 σ
Photon index: 2.60 (+0.18, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00865036.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181002A 144 s after the BAT trigger (Emery and Gropp GCN Circ. 23297). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 23292) 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 0.60 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
04h18m59.19s +15°12'46.4" 2.0" XRT-final UKSSDC
04h18m59.19s +15°12'46.4" 2.0" XRT-enhanced Beardmore et al. GCN Circ. 23292
04h18m57.4s +15°14'51.6" 1.8' BAT-refined Cummings et al. GCN Circ. 23294

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23293 MASTER optical observation MASTER
Optical Ulaczyk et al. 23298 GOTO optical limits Gravitational-wave Optical Transient Observer upper limits
Optical Mazaeva et al. 23308 Mondy optical upper limit Mondy upper limits

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 144 294 147 >20.8
uFC 302 552 246 >20.3
white 144 1008 334 >20.7
v 632 1058 58 >17.9
b 558 751 39 >18.6
u 302 726 265 >20.3

Table 3. UVOT observations reported by Emery and Gropp (GCN Circ. 23297). 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 6, 2018