Swift Observations of GRB 181010A

A. Melandri (INAF-OAB), P.A. Evans (U. Leicester) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 05:55:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181010A (trigger=866434) (Melandri et al. GCN Circ. 23309). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 133° from the Sun (9.5 hours West) and 143° from the 2%-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.

Melandri et al. (GCN Circ. 23309) reported the discovery with UVOT of an optical afterglow. J-B. Vielfaure et al. (GCN Circ. 23315) determined a redshift of 1.39 from VLT. 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 Lien et al. (GCN Circ. 23321), the BAT ground-calculated position is RA, Dec = 52.574, -23.023 deg, which is RA(J2000) = 03h30m17.7s Dec(J2000) = -23°01'22.7" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%.

The mask-weighted light curve (Figure 1) shows several overlapping peaks starting at ~T-5s, peaking at ~T+8 s, and returning to baseline at ~T+15 s. T90 (15-350 keV) is 16.4 ± 2.2 s (estimated error including systematics).

The time-averaged spectrum from T-4.04 to T+15.21 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.52 ± 0.15. The fluence in the 15-150 keV band is 6.9 ± 0.7 x 10-7 erg cm-2. This fluence is larger than that of 30% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+7.43 s in the 15-150 keV band is 1.4 ± 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/866434/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 23316). We have analysed 19 ks of XRT data for GRB 181010A, from 77 s to 126.7 ks after the BAT trigger. The data comprise 77 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 23313).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=8.0 (+0.0, -3.1). At T+86.5 s the decay flattens to an α of 0.738 ± 0.021. The light curve breaks again at T+11.9 ks to a decay with α=3.7 (+2.6, -0.7), and again at T+16.5 ks s to α=-1.5 (+0.0, -0.7), before a final break at T+22.4 ks s after which the decay index is 1.06 (+0.21, -0.18).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+0.09, -0.08). The best-fitting absorption column is 2.00 (+0.32, -0.29) x 1022 cm-2, at a redshift of 1.39, in addition to the Galactic value of 1.9 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 (5.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.9 x 1020 cm-2
Intrinsic column: 2.00 (+0.32, -0.29) x 1022 cm-2 at z=1.39
Photon index: 1.79 (+0.09, -0.08)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00866434.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181010A 96 s after the BAT trigger (Emery and Melandri GCN Circ. 23317). A source consistent with the XRT position (Evans et al. GCN Circ. 23313) 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
03h30m16.89s -23°02'15.4" 0.49" UVOT-refined Emery and Melandri GCN Circ. 23317
03h30m16.82s -23°02'16.6" 1.4" XRT-final UKSSDC
03h30m16.87s -23°02'18.4" 1.8" XRT-enhanced Evans et al. GCN Circ. 23313
03h30m17.7s -23°01'22.7" 1.4' BAT-refined Lien et al. GCN Circ. 23321

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gorbovskoy et al. 23310 MASTER-Argentina OT detection MASTER detection
Optical Martone et al. 23311 LCO Cerro Tololo observations LCO detection
Optical Troja et al. 23312 RATIR Optical and NIR Afterglow
Detection
RATIR detection
Optical J-B. Vielfaure et al. 23315 VLT/X-shooter redshift VLT redshift
Optical Reynolds et al. 23319 ePESSTO NTT optical observations PESSTO detection
Optical D'Avanzo et al. 23323 REM optical and NIR afterglow detection REM detection
Optical Schady and Bolmer 23326 GROND afterglow detection GROND detection
Gamma-ray von Kienlin 23320 Fermi GBM observation Fermi GBM Epeak=280±80 keV
T90=60 seconds
Fluence=6.13±0.98x10-7erg cm-2
(7th percentile for long 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 96 246 147 19.42 ± 0.09
white 589 6359 247 >21.0
v 3867 5503 393 >19.4
b 564 6323 413 >20.9
u 309 6117 639 >20.6
uvw1 4277 5912 393 >20.1
uvm2 4072 5707 393 >20.7
uvw2 5098 5298 197 >20.5

Table 3. UVOT observations reported by Emery and Melandri (GCN Circ. 23317). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

October 12, 2018