Swift Observations of GRB 181202A

J.K. Cannizzo (NASA/UMBC), A.P. Beardmore (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 06:36:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181202A (trigger=874334) (Cannizzo et al. GCN Circ. 23482). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 59° from the Sun (2.2 hours East) and 90° from the 26%-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.

Cannizzo et al. (GCN Circ. 23482) reported the discovery 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 Ukwatta et al. (GCN Circ. 23489), the BAT ground-calculated position is RA, Dec = 280.737, 27.976 deg which is RA(J2000) = 18h42m56.9s Dec(J2000) = +27°58'33.4" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts at ~T0 and ends at ~T+7 s. The two peaks occur at ~T+1 s and ~T+6 s, respectively. T90 (15-350 keV) is 6.56 ± 0.61 s (estimated error including systematics).

The time-averaged spectrum from T-0.26 to T+6.76 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.61 ± 0.24. The fluence in the 15-150 keV band is 2.5 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 9.7% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.69 s in the 15-150 keV band is 0.9 ± 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/874334/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 23487). We have analysed 14 ks of XRT data for GRB 181202A, from 102 s to 161.8 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 23483).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.81 ± 0.06.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.62 (+0.21, -0.14). The best-fitting absorption column is 1.77 (+0.90, -0.10) x 1021 cm-2, consistent with the Galactic value of 1.7 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 4.3 x 10-11 (5.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.77 (+0.90, -0.10) x 1021 cm-2
Galactic foreground: 1.7 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.62 (+0.21, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00874334.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181202A 86 s after the BAT trigger (Siegel and Cannizzo GCN Circ. 23498). A source consistent with the XRT position (Evans et al. GCN Circ. 23483) 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 0.14 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
18h42m56.38s +27°57'35.0" 0.47" UVOT-refined Siegel and Cannizzo GCN Circ. 23498
18h42m56.40s +27°57'34.5" 1.5" XRT-final UKSSDC
18h42m56.38s +27°57'34.7" 1.8" XRT-enhanced Evans et al. GCN Circ. 23483
18h42m56.9s +27°58'33.4" 2.0' BAT-refined Ukwatta et al. GCN Circ. 23489

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23490 Global MASTER-Net optical observations MASTER upper limits
Optical Watson et al. 23496 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 Tstart(s) Tstop(s) Exp(s) Mag
v 623 1540 117 >18.7
b 342 591 246 19.57 ± 0.18
u (fc) 86 336 246 17.67 ± 0.07
w1 672 1441 97 >19.6
m2 647 1560 112 >22.2
w2 598 1516 117 >20.5

Table 3. UVOT observations reported by Siegel and Cannizzo (GCN Circ. 23498). 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 4, 2018