Swift Observations of GRB 180823A

B. Sbarufatti (PSU), A. Tohuvavohu (PSU) and S. Sebzda (PSU) for the Swift team

1. Introduction

At 19:04:32 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180823A (trigger=855434) (Sbarufatti et al. GCN Circ. 23169). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 56° from the Sun (3.9 hours East) and 97° from the 94%-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 Lien et al. (GCN Circ. 23175), the BAT ground-calculated position is RA, Dec = 210.358, 14.901 deg which is RA(J2000) = 14h01m25.9s Dec(J2000) = +14°54'03.2" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 92%.

The available data starts as the burst is rising, but it appears that it includes the entire burst. The mask-weighted light curve (Figure 1) shows a double-peaked structure, starting at ~T-43 seconds, with comparably sized peaks at ~T+5 and ~T+20 s, and returning to near background by T+70 seconds, with a fainter tail out past T+100 seconds. A spacecraft slew took the burst location out of the BAT field of view around T+450 seconds. T90 (15-350 keV) is 80.3 ± 6.9 s (estimated error including systematics).

The time-averaged spectrum from T-37.5 to T+72.0 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.58 ± 0.05. The fluence in the 15-150 keV band is 7.8 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 89% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+18.33 s in the 15-150 keV band is 2.3 ± 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/855434/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 23171). We have analysed 14 ks of XRT data for GRB 180823A, from 331 s to 101.9 ks after the BAT trigger. The data comprise 46 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. 23170).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.02 ± 0.04.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.06 (+0.18, -0.17). The best-fitting absorption column is 1.2 (+0.5, -0.4) x 1021 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 3.2 x 10-11 (4.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.2 (+0.5, -0.4) x 1021 cm-2
Galactic foreground: 1.2 x 1020 cm-2
Excess significance: 4.1 σ
Photon index: 2.06 (+0.18, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00855434.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180823A 381 s after the BAT trigger (Sebzda and Sbarufatti GCN Circ. 23173). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 23170) 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
14h01m25.39s +14°53'33.0" 1.4" XRT-final UKSSDC
14h01m25.40s +14°53'32.8" 1.6" XRT-enhanced Goad et al. GCN Circ. 23170
14h01m25.9s +14°54'03.2" 1.0' BAT-refined Lien et al. GCN Circ. 23175

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Butler et al. 23172 RATIR Optical and NIR Observations RATIR 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
white 381 417 36 >19.1
white 381 4476 380 >20.9
v 4686 10356 927 >19.2
b 4071 4271 197 >19.7
u 3866 4066 197 >19.8
m2 10361 10803 435 >19.8
w2 4482 4681 197 >20.0

Table 3. UVOT observations reported by Sebzda and Sbarufatti (GCN Circ. 23173). 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.

August 26, 2018