Swift Observations of GRB 180704A

E. Sonbas (Adiyaman Univ.), Z. Liu (NAOC / U. Leicester) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 05:36:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180704A (trigger=846088) (Sonbas et al. GCN Circ. 22885). Swift did not immediately slew to the burst due to an Earth limb constraint. At the time of the trigger, the initial BAT position was 62° from the Sun (4.7 hours West) and 83° from the 71%-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 Markwardt et al. (GCN Circ. 22894), the BAT ground-calculated position is RA, Dec = 32.605, 69.965 deg, which is RA(J2000) = 02h10m25.1s Dec(J2000) = +69°57'54.9" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 7%.

The mask-weighted light curve (Figure 1) shows several peaks, the first starting at ~T-21 s, and the lagest peak at ~T+1 s, with a long tail out to ~T+40 s. T90 (15-350 keV) is 19.7 ± 1.4 s (estimated error including systematics).

The time-averaged spectrum from T-17.90 to T+4.24 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 ± 0.10. The fluence in the 15-150 keV band is 6.2 ± 0.4 x 10-6 erg cm-2. This fluence is larger than that of 86% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.04 s in the 15-150 keV band is 9.6 ± 1.2 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/846088/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Liu et al. (GCN Circ. 22895). We have analysed 21 ks of XRT data for GRB 180704A, from 3.2 ks to 68.0 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 22890).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.84 (+0.13, -0.22), followed by a break at T+9616 s to an α of 1.70 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.94 (+0.13, -0.12). The best-fitting absorption column is 8.4 ± 1.1 x 1021 cm-2, in excess of the Galactic value of 4.9 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.7 x 10-11 (8.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.4 ± 1.1 x 1021 cm-2
Galactic foreground: 4.9 x 1021 cm-2
Excess significance: 5.4 σ
Photon index: 1.94 (+0.13, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00846088.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180704A 3255 s after the BAT trigger (Emery et al. GCN Circ. 22893). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 22890) 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 0.87 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
02h10m38.50s +69°57'50.4" 1.4" XRT-final UKSSDC
02h10m38.51s +69°57'50.7" 1.5" XRT-enhanced Evans et al. GCN Circ. 22890
02h10m25.1s +69°57'54.9" 1.3' BAT-refined Markwardt et al. GCN Circ. 22894

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 22886 MASTER optical observation MASTER
Gamma-ray Sharma et al. 22901 AstroSat CZTI detection CZTI T90=26 seconds
Gamma-ray Frederiks et al. 22918 Konus-Wind observation Konus-Wind Epeak=183 (-11,+33) keV

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 3255 5455 537 >20.8
v 4233 10567 1082 >19.8
b 3618 5253 393 >19.9
u 3413 5048 393 >19.6
uvw1 4643 4843 197 >19.2
uvm2 4438 11190 804 >20.3
uvw2 4028 9654 1082 >20.7

Table 3. UVOT observations reported by Emery et al. (GCN Circ. 22893). 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.

July 7, 2018