Swift Observations of GRB 181022A

F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 17:30:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181022A (trigger=868312) (Marshall et al. GCN Circ. 23364). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 130° from the Sun (10.0 hours West) and 55° from the 96%-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. 23365), the BAT ground-calculated position is RA, Dec = 56.808, -29.374 deg which is RA(J2000) = 03h47m13.9s Dec(J2000) = -29°22'26.5" with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-2 s and ends at ~T+6 s. T90 (15-350 keV) is 6.74 ± 2.30 s (estimated error including systematics).

The time-averaged spectrum from T-1.65 to T+5.65 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.78 ± 0.41. The fluence in the 15-150 keV band is 1.9 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 6.1% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.40 s in the 15-150 keV band is 0.4 ± 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/868312/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 23374).

We have analysed 2.0 ks of XRT data for GRB 181022A, from 106 s to 5.4 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.73 ± 0.24.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.7 ± 0.5. The best-fitting absorption column is 2.9 (+2.9, -1.9) x 1021 cm-2, in excess of the Galactic value of 8.8 x 1019 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.4 x 10-11 (5.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.9 (+2.9, -1.9) x 1021 cm-2
Galactic foreground: 8.8 x 1019 cm-2
Excess significance: 2.5 σ
Photon index: 1.7 ± 0.5

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00868312.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181022A 78 s after the BAT trigger (LaPorte and Marshall GCN Circ. 23367). No optical afterglow consistent with the XRT position 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 0.01 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
03h47m09.88s -29°22'54.4" 3.7" XRT-final UKSSDC
03h47m10.09s -29°22'47.2" 2.1" XRT-refined Melandri et al. GCN Circ. 23374
03h47m13.9s -29°22'26.5" 2.6' BAT-refined Stamatikos et al. GCN Circ. 23365

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Martone et al. 23370 LCO Sutherland observations LCO
Optical Schady and Bolmer 23371 GROND upper limits GROND upper limits
Optical Iida et al. 23372 MITSuME Akeno optical upper limits MITSuME Akeno 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
uFC 98 347 246 >20.5
v 78 4922 288 >19.8
u 98 5429 503 >20.4
w1 661 5332 274 >20.1
m2 4927 5127 197 >20.2
w2 356 4717 442 >20.8

Table 3. UVOT observations reported by LaPorte and Marshall (GCN Circ. 23367). 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 24, 2018