Swift Observations of GRB 180623A

K.L. Page (U. Leicester), J.A. Kennea (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 16:41:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180623A (trigger=844015) (Page et al. GCN Circ. 22828). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 126° from the Sun (8.2 hours East) and 50° from the 82%-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.

Kuin and Page (GCN Circ. 22831) reported the detection 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 Barthelmy et al. (GCN Circ. 22833), the BAT ground-calculated position is RA, Dec = 214.528, -60.268 deg which is RA(J2000) = 14h18m06.8s Dec(J2000) = -60°16'05.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 63%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-60 s and ends at ~T+115 s. The three main peaks occur at ~T0, ~T+25 s, and ~T+60 s, respectively. T90 (15-350 keV) is 114.9 ± 6.8 s (estimated error including systematics). Note that the burst entered the BAT FOV around ~ T-90 s. Therefore, there might be additional burst emission beforehand.

The time-averaged spectrum from T-61.8 to T+114.1 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.45 ± 0.04. The fluence in the 15-150 keV band is 1.2 ± 0.02 x 10-5 erg cm-2. This fluence is larger than that of 93.7% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+59.11 s in the 15-150 keV band is 5.6 ± 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/844015/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 22830). We have analysed 10 ks of XRT data for GRB 180623A, from 83 s to 75.6 ks after the BAT trigger. The data comprise 90 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. 22829).

The late-time light curve (Figure 2) (from T0+4.5 ks) can be modelled with an initial power-law decay with an index of α=1.53 (+0.20, -0.19), followed by a break at T+15.0 ks to an α of 3.4 (+1.1, -0.8).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.73 (+0.20, -0.19). The best-fitting absorption column is 6.4 ± 0.7 x 1022 cm-2, in excess of the Galactic value of 1.5 x 1022 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.07 (+0.26, -0.25) and a best-fitting absorption column of 6.7 ± 0.9 x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 8.9 x 10-11 (2.9 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.7 ± 0.9 x 1022 cm-2
Galactic foreground: 1.5 x 1022 cm-2
Excess significance: 10.1 σ
Photon index: 2.07 (+0.26, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00844015.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180623A 86 s after the BAT trigger (Kuin and Page GCN Circ. 22831). A source consistent with the XRT position (Evans et al. GCN Circ. 22829) 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 7.04 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
14h18m06.93s -60°15'12.5" 1.4" XRT-final UKSSDC
14h18m06.91s -60°15'12.7" 1.5" XRT-enhanced Evans et al. GCN Circ. 22829
14h18m06.8s -60°16'05.6" 1.0' BAT-refined Barthelmy et al. GCN Circ. 22833

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Kozlova et al. 22856 Konus-Wind observation Konus-Wind Epeak=233 (-33,+48) keV
Duration=~114 seconds
Fluence=3.15(-0.24,+0.28)x10-5erg cm-2

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 86 236 150 >20.68
white 878 997 119 20.04 ± 0.24
v 629 6296 432 >19.2
b 18042 18207 162 19.86 ± 0.28
u 299 17732 2252 >21.6
w1 678 16819 1318 >20.8
m2 653 6501 432 >20.0
w2 604 6092 432 >20.0

Table 3. UVOT observations reported by Kuin and Page (GCN Circ. 22831). 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.

June 27, 2018