Swift Observations of GRB 190718A

V. D'Elia (SSDC), B. Sbarufatti (PSU) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 04:41:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190718A (trigger=915091) (D'Elia et al. GCN Circ. 25082). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 141° from the Sun (9.4 hours West) and 30° from the 98%-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 Palmer et al. (GCN Circ. 25097), the BAT ground-calculated position is RA, Dec = 336.605, -41.184 deg which is RA(J2000) = 22h26m25.2s Dec(J2000) = -41°11'02.7" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a main multi-peaked structure that starts at ~ T0 and ends at ~T+17 s. In addition, there is a weak pulse at ~T+700 s, which is at the same time as the large flare seen by the XRT. T90 (15-350 keV) is 720.0 ± 35.8 s (estimated error including systematics).

The time-averaged spectrum from T+0.1 to T+17.1 s (i.e., the duration of the main emission) is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.38 ± 0.26. The fluence in the 15-150 keV band is 3.6 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 15% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The spectral analysis for the total burst duration from ~T0 to ~T+720 s does not produce reasonable fits due to the large quiescent period from ~T+17 s to ~T+700 s. The 1-s peak photon flux measured from T+0.08 s in the 15-150 keV band is 1.0 ± 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/915091/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 25093). We have analysed 8.2 ks of XRT data for GRB 190718A, from 70 s to 121.3 ks after the BAT trigger. The data comprise 349 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 25084).

The late-time light curve (Figure 2) (from T0+4.6 ks) can be modelled with a power-law decay with a decay index of α=1.20 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.63 (+0.15, -0.14). The best-fitting absorption column is 7.0 (+4.2, -3.8) x 1020 cm-2, in excess of the Galactic value of 1.2 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 (4.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.0 (+4.2, -3.8) x 1020 cm-2
Galactic foreground: 1.2 x 1020 cm-2
Excess significance: 2.5 σ
Photon index: 1.63 (+0.15, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00915091.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190718A 70 s after the BAT trigger (Oates and D'Elia GCN Circ. 25089). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 25084) 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
22h26m18.27s -41°12'16.0" 1.4" XRT-final UKSSDC
22h26m18.26s -41°12'16.1" 1.7" XRT-enhanced Evans et al. GCN Circ. 25084
22h26m25.2s -41°11'02.7" 2.4' BAT-refined Palmer et al. GCN Circ. 25097

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 25083 Swift GRB190718.2 Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 25085 FRAM-Auger optical limit FRAM 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
whiteFC 70 219 147 >20.9
uFC 282 532 246 >20.0
white 70 1184 353 >21.0
v 612 1061 78 >18.3
b 538 1159 58 >19.1
u 282 4874 307 >20.0
w1 661 4847 235 >19.6

Table 3. UVOT observations reported by Oates and D'Elia (GCN Circ. 25089). 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 20, 2019