Swift Observations of GRB 181203A

E. Troja (NASA/GSFC/UMCP), A. Melandri (INAF-OAB) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 12:08:47 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181203A (trigger=874475) (Troja et al. GCN Circ. 23493). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 108° from the Sun (9.4 hours West) and 92° from the 15%-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.

Schady and Bolmer (GCN Circ. 23516) reported the position from GROND 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 https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Barthelmy et al. (GCN Circ. 23500), the BAT ground-calculated position is RA, Dec = 108.341, -39.783 deg which is RA(J2000) = 07h13m21.8s Dec(J2000) = -39°46'57.1" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 83%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-2 s, peaks at ~T+1 s, and ends at ~T+16 s. T90 (15-350 keV) is 15.30 ± 1.75 s (estimated error including systematics).

The time-averaged spectrum from T-1.96 to T+15.77 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.79 ± 0.15. The fluence in the 15-150 keV band is 7.8 ± 0.7 x 10-7 erg cm-2. This fluence is larger than that of 32% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.02 s in the 15-150 keV band is 1.1 ± 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/874475/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 23502). We have analysed 13 ks of XRT data for GRB 181203A, from 60 s to 201.9 ks after the BAT trigger. The data comprise 9 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 Goad et al. (GCN Circ. 23494).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.74 (+0.27, -0.26).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.84 (+0.29, -0.17). The best-fitting absorption column is consistent with the Galactic value of 2.3 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 3.9 x 10-11 (5.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 ± 0.8 x 1021 cm-2
Galactic foreground: 2.3 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.84 (+0.29, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00874475.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181203A 79 s after the BAT trigger (Marshall and Troja GCN Circ. 23505). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 23494) 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 0.24 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
07h13m23.62s -39°48'06.7" 1.6" XRT-final UKSSDC
07h13m23.66s -39°48'06.7" 2.0" XRT-enhanced Goad et al. GCN Circ. 23494
07h13m21.8s -39°46'57.1" 1.3' BAT-refined Barthelmy et al. GCN Circ. 23500

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 23513 BOOTES-3/YA optical observations BOOTES-3
Optical Schady and Bolmer 23516 GROND detection of the afterglow GROND 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
whiteFC 79 229 147 >20.7
uFC 293 543 246 >19.4
white 79 1370 373 >21.8
v 624 1421 97 >18.6
b 549 1346 78 >20.0
u 293 1495 324 >19.7
w1 673 1470 97 >19.0
m2 648 1445 97 >18.5
w2 599 1396 97 >19.7

Table 3. UVOT observations reported by Marshall and Troja (GCN Circ. 23505). 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.

December 8, 2018