Swift Observations of GRB 180706A

M. Stamatikos (OSU/NASA/GSFC), A. D'Ai (INAF-IASFPA) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 08:24:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180706A (trigger=846395) (Stamatikos et al. GCN Circ. 22909). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 64° from the Sun (5.1 hours East) and 112° from the 50%-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.

Stamatikos et al. (GCN Circ. 22909) reported the discovery with UVOT of an optical afterglow. 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 Sakamoto et al. (GCN Circ. 22921), the BAT ground-calculated position is RA, Dec = 181.687, 66.044 deg which is RA(J2000) = 12h06m45.0s Dec(J2000) = +66°02'40.1" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 86%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts at ~T-20 s and ends at ~T+50 s. The smaller peak occurs at ~T+2 s, and the larger peak happens at ~T+28 s. T90 (15-350 keV) is 42.7 ± 8.7 s (estimated error including systematics).

The time-averaged spectrum from T-18.77 to T+50.99 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.90 ± 0.08. The fluence in the 15-150 keV band is 2.4 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 65% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+27.82 s in the 15-150 keV band is 3.2 ± 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/846395/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 22917). We have analysed 18 ks of XRT data for GRB 180706A, from 77 s to 184.2 ks after the BAT trigger. The data comprise 169 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 Osborne et al. (GCN Circ. 22916).

The late-time light curve (Figure 2) (from T0+5.5 ks) can be modelled with a power-law decay with a decay index of α=0.92 (+0.08, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.11 ± 0.09. The best-fitting absorption column is 1.65 ± 0.18 x 1021 cm-2, in excess of the Galactic value of 1.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.09 ± 0.15 and a best-fitting absorption column of 5.3 (+3.2, -2.9) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10-11 (3.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.3 (+3.2, -2.9) x 1020 cm-2
Galactic foreground: 1.4 x 1020 cm-2
Excess significance: 2.2 σ
Photon index: 2.09 ± 0.15

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00846395.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180706A 95 s after the BAT trigger (Oates and Stamatikos GCN Circ. 22922). A source consistent with the XRT position (Osborne et al. GCN Circ. 22916) and the COATLI optical detection (Watson et al. GCN Circ. 22910) 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 0.02 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
12h06m34.34s +66°02'13.6" 0.77" UVOT-initial Stamatikos et al. GCN Circ. 22909
12h06m34.17s +66°02'12.6" 1.4" XRT-final UKSSDC
12h06m34.22s +66°02'13.7" 1.7" XRT-enhanced Osborne et al. GCN Circ. 22916
12h06m45.0s +66°02'40.1" 1.1' BAT-refined Sakamoto et al. GCN Circ. 22921

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Watson et al. 22910 COATLI Optical Detection COATLI detection
Optical Volnova et al. 22923 TSHAO optical observations Zeiss-1000 detection
Optical Ulaczyk et al. 22924 GOTO optical limits Gravitational-wave Optical Transient Observer upper limits
Optical Kann et al. 22989 OSN detection Obs.de Sierra Nevada detection
Gamma-ray Bissaldi 22919 Fermi GBM observation Fermi GBM Epeak=53±4 keV
T90=38 seconds
Fluence=3.3±0.2x10-6erg cm-2
(44th 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 95 245 147 19.88 ± 0.34
white 587 607 20 18.11 ± 0.21
v 637 1582 117 >18.5
b 563 756 39 18.34 ± 0.30
u 308 557 246 17.90 ± 0.12
w1 686 1459 78 18.44 ± 0.33
m2 661 1606 117 >18.8
w2 613 1558 117 >19.1

Table 3. UVOT observations reported by Oates and Stamatikos (GCN Circ. 22922). 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.

July 22, 2018