Swift Observations of GRB 180115A

J.K. Cannizzo (NASA/UMBC), J.A. Kennea (PSU) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 04:16:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180115A (trigger=805318) (Cannizzo et al. GCN Circ. 22335). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 71° from the Sun (5.0 hours East) and 92° from the 3%-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.

Cannizzo et al. (GCN Circ. 22335) reported the discovery with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 22346) determined a redshift of 2.487 from GTC. 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 Barthelmy et al. (GCN Circ. 22348), the BAT ground-calculated position is RA, Dec = 12.040, -15.632 deg which is RA(J2000) = 00h48m09.6s Dec(J2000) = -15°37'55.0" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 57%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~ T+25 s and ends at ~T+70 s. The main peak occurs at ~T+66 s. T90 (15-350 keV) is 40.9 ± 3.0 s (estimated error including systematics).

The time-averaged spectrum from T+26.49 to T+70.28 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.66 ± 0.22. The fluence in the 15-150 keV band is 7.6 ± 1.1 x 10-7 erg cm-2. This fluence is larger than that of 31% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+65.96 s in the 15-150 keV band is 0.6 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 22338). We have analysed 24 ks of XRT data for GRB 180115A, from 137 s to 259.1 ks after the BAT trigger. The data comprise 108 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22337).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=3.6 (+0.6, -0.5). At T+275 s the decay flattens to an α of 0.65 (+0.05, -0.06) before breaking again at T+8090 s to a final decay with index α=1.32 (+0.13, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.24 (+0.17, -0.10). The best-fitting absorption column is consistent with the Galactic value of 1.7 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.95 ± 0.10 and a best-fitting absorption column of 2.3 (+2.4, -2.3) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10-11 (3.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.7 x 1020 cm-2
Intrinsic column: 2.3 (+2.4, -2.3) x 1021 cm-2 at z=2.487
Photon index: 1.95 ± 0.10

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00805318.

4. UVOT Observations and Analysis

UVOT results are not available.

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
00h48m09.27s -15°37'49.9" 0.43" UVOT-refined Marshall and Cannizzo GCN Circ. 22357
00h48m09.32s -15°37'50.5" 1.4" XRT-final UKSSDC
00h48m09.30s -15°37'50.3" 1.7" XRT-enhanced Goad et al. GCN Circ. 22337
00h48m09.6s -15°37'55.0" 1.8' BAT-refined Barthelmy et al. GCN Circ. 22348

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Murata et al. 22340 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Cano et al. 22345 NOT detection of optical counterpart NOT detection
Optical de Ugarte Postigo et al. 22346 Redshift from OSIRIS/GTC GTC redshift
Other Cunningham et al. 22362 SEDM Observations

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 140 290 147 17.61 ± 0.04
white 753 773 20 16.95 ± 0.07
white 5663 5863 197 19.18 ± 0.08
v 628 648 20 16.50 ± 0.16
b 554 574 20 16.93 ± 0.11
u 299 548 246 16.32 ± 0.04
w1 1109 13068 1165 >20.7
m2 4743 12296 1279 >21.8
w2 5769 7329 318 >20.2

Table 3. UVOT observations reported by Marshall and Cannizzo (GCN Circ. 22357). 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.

January 19, 2018