Swift Observations of GRB 181123B

A.Y. Lien (GSFC/UMBC), D.N. Burrows (PSU) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 05:33:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181123B (trigger=873186) (Lien et al. GCN Circ. 23432). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 64° from the Sun (3.6 hours West) and 118° from the 100%-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.

Fong et al. (GCN Circ. 23439) reported the position from Gemini 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

BAT refined results are not available.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 23435). We have analysed 11 ks of XRT data for GRB 181123B, from 95 s to 58.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23433).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.40 (+0.12, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.99 (+0.30, -0.20). The best-fitting absorption column is 3.9 (+6.5, -0.8) x 1020 cm-2, consistent with the Galactic value of 3.1 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 3.2 x 10-11 (3.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.9 (+6.5, -0.8) x 1020 cm-2
Galactic foreground: 3.1 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.99 (+0.30, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00873186.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181123B 84 s after the BAT trigger (Oates and Lien GCN Circ. 23437). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23434) 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 0.03 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
12h17m28.02s +14°35'52.7" 1.5" XRT-final UKSSDC
12h17m28.05s +14°35'52.4" 1.8" XRT-enhanced Osborne et al. GCN Circ. 23434
12h17m23s +14°35'22" 3' BAT-initial Lien et al. GCN Circ. 23432

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Fong et al. 23439 Gemini-North optical detection Gemini detection
Optical Paterson and Fong 23440 Keck near-infrared imaging Keck
Optical Lipunov et al. 23444 Global MASTER Net optical inspection MASTER
Optical Paterson et al. 23461 Keck further near-infrared imaging Keck
Radio Anderson et al. 23467 ATCA 5/9 GHz rapid-response radio
observation		 ATCA
Gamma-ray Yi et al. 23452 Insight-HXMT/HE detection Insight-HXMT T90=0.23 seconds

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 84 233 147 >20.3
uFC 296 546 246 >19.5
white 84 6529 727 >21.0
v 626 10922 1228 >19.8
b 551 6324 432 >20.1
u 296 6119 659 >20.1
w1 4278 12566 1113 >20.2
m2 5509 11827 1082 >20.5
w2 5100 6734 393 >19.8

Table 3. UVOT observations reported by Oates and Lien (GCN Circ. 23437). 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