Swift Observations of GRB 181126A

E. Sonbas (Adiyaman Univ.), J.P. Osborne (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 09:54:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181126A (trigger=873539) (Sonbas et al. GCN Circ. 23453). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 79° from the Sun (6.0 hours West) and 67° from the 87%-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 Stamatikos et al. (GCN Circ. 23463), the BAT ground-calculated position is RA, Dec = 152.350, -29.698 deg which is RA(J2000) = 10h09m24.0s Dec(J2000) = -29°41'52.9" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 68%.

The mask-weighted light curve (Figure 1) shows two short pulses. The first pulse starts at ~ T-0.1 s, peaks at ~T0, and ends at ~T+0.2 s. The second pulse starts at ~T+1.6 s, peaks at ~T+1.8 s, and ends at ~T+2.2 s. T90 (15-350 keV) is 2.09 ± 0.12 s (estimated error including systematics).

The time-averaged spectrum from T-0.07 to T+2.16 s is best fit by a power law with an exponential cutoff. This fit gives a photon index -0.13 ± 0.75, and Epeak of 83.8 ± 24.2 keV (χ2 66.64 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.1 ± 0.4 x 10-7 erg cm-2 and the 1-s peak flux measured from T+1.22 s in the 15-150 keV band is 2.7 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 13% 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 1.22 ± 0.15 (χ2 79.71 for 57 d.o.f.). All the quoted errors are at the 90% confidence level.

Using the 8-ms binned light curve (Figure 1) , the spectral lag for the 100-350 keV to 25-50 keV bands is 26 (+19, -15) ms for the first pulse, and 10 (+21, -13) ms for the second pulse. These lag values are intermediate between short and long GRBs, but also are consistent with short GRBs when considering the large error bars. In addition, no extended emission is found.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/873539/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 23459). We have analysed 11 ks of XRT data for GRB 181126A, from 82 s to 35.2 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 23454).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.99 (+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 2.7 (+0.8, -0.7). The best-fitting absorption column is 9.9 (+6.7, -4.3) x 1021 cm-2, in excess of the Galactic value of 8.1 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10-11 (1.2 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.9 (+6.7, -4.3) x 1021 cm-2
Galactic foreground: 8.1 x 1020 cm-2
Excess significance: 3.4 σ
Photon index: 2.7 (+0.8, -0.7)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00873539.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181126A 80s after the BAT trigger (Breeveld and Sonbas GCN Circ. 23460). No optical afterglow consistent with the XRT position (Beardmore et al., GCN Circ. 23455) 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 0.07 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
10h09m23.68s -29°41'12.7" 1.9" XRT-final UKSSDC
10h09m23.64s -29°41'14.9" 2.3" XRT-enhanced Beardmore et al. GCN Circ. 23455
10h09m24.0s -29°41'52.9" 1.5' BAT-refined Stamatikos et al. GCN Circ. 23463

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Tanvir and Fong 23458 deep Gemini imaging Gemini
Optical De 23462 No Ks-band afterglow detection with
Keck I		 Keck detection
Gamma-ray Bissaldi 23457 Fermi GBM observation Fermi GBM Epeak=90±25 keV
T90=0.6 seconds
Fluence=4.8±0.5x10-7erg cm-2
(64th percentile for short GRBs)

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 80 230 147 >20.4
white 80 1181 342 >20.5
v 622 1069 58 >18.0
b 1147 1167 20 >18.2
u 292 1142 285 >20.1
w1 671 1118 58 >20.2
m2 646 1093 58 >19.0
w2 597 1044 58 >18.4

Table 3. UVOT observations reported by Breeveld and Sonbas (GCN Circ. 23460). 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.

November 28, 2018