Swift Observations of GRB 181023A

F.E. Marshall (NASA/GSFC), A. D'Ai (INAF-IASFPA) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 02:04:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181023A (trigger=868427) (Marshall et al. GCN Circ. 23366). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 51° from the Sun (2.7 hours East) and 121° from the 97%-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 Ukwatta et al. (GCN Circ. 23377), the BAT ground-calculated position is RA, Dec = 248.322, 19.581 deg which is RA(J2000) = 16h33m17.4s Dec(J2000) = +19°34'51.2" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows a complex structure with some weak overlapping pulses that starts at ~T-30 s and ends at ~T+40 s. Note that the burst entered the BAT FOV at T-48 s, and hence there might be additional emission beforehand. T90 (15-350 keV) is 58.0 ± 8.8 s (estimated error including systematics).

The time-averaged spectrum from T-27.62 to T+36.68 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.74 ± 0.24. The fluence in the 15-150 keV band is 1.3 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+32.18 s in the 15-150 keV band is 1.0 ± 0.3 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/868427/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 23373). We have analysed 15 ks of XRT data for GRB 181023A, from 214 s to 133.2 ks after the BAT trigger. The data comprise 493 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23369).

The late-time light curve (Figure 2) (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of α=1.7 (+1.5, -1.2).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 ± 0.08. The best-fitting absorption column is 2.3 ± 0.3 x 1021 cm-2, in excess of the Galactic value of 6.0 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 4.1 x 10-11 (5.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 ± 0.3 x 1021 cm-2
Galactic foreground: 6.0 x 1020 cm-2
Excess significance: 9.0 σ
Photon index: 1.79 ± 0.08

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00868427.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181023A 200 s after the BAT trigger (LaPorte and Marshall GCN Circ. 23376). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23369) 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 0.05 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
16h33m18.97s +19°34'13.7" 1.4" XRT-final UKSSDC
16h33m18.89s +19°34'14.2" 1.7" XRT-enhanced Osborne et al. GCN Circ. 23369
16h33m17.4s +19°34'51.2" 2.0' BAT-refined Ukwatta et al. GCN Circ. 23377

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Han et al. 23368 KAIT Optical Upper Limit KAIT 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 200 350 147 >20.1
white 200 2078 470 >20.8
v 356 2128 194 >19.4
b 455 2054 175 >20.3
u 430 2029 175 >19.6
w1 406 2005 175 >19.6
m2 554 1803 78 >19.2
w2 679 2104 175 >19.2

Table 3. UVOT observations reported by LaPorte and Marshall (GCN Circ. 23376). 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 25, 2018