Swift Observations of GRB 180924A

T.N. Ukwatta (LANL), A. Melandri (INAF-OAB) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 15:22:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180924A (trigger=863186) (Ukwatta et al. GCN Circ. 23261). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 110° from the Sun (8.8 hours West) and 66° from the 100%-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 Stamatikos et al. (GCN Circ. 23264), the BAT ground-calculated position is RA, Dec = 49.207, -58.524 deg, which is RA(J2000) = 03h16m49.7s Dec(J2000) = -58°31'27.6" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a couple peaks starting at ~T-90 s, peaking at ~T=50 and ~T0 s, and returning to baseline ~T+30 s. T90 (15-350 keV) is 95.1 ± 10.9 s (estimated error including systematics).

The time-averaged spectrum from T-85.15 to T+26.35 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.94 ± 0.14. The fluence in the 15-150 keV band is 1.4 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 49% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+2.73 s in the 15-150 keV band is 0.6 ± 0.1 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/863186/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 23266). We have analysed 11 ks of XRT data for GRB 180924A, from 82 s to 189.9 ks after the BAT trigger. The data comprise 141 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. 23262).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.75 (+0.28, -0.25). At T+166 s the decay steepens to an α of 2.81 (+1.48, -0.29). The light curve breaks again at T+476 s to a decay with α=-1.42 (+2.05, -0.08), before a final break at T+895 s s after which the decay index is 1.05 (+0.14, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 (+0.14, -0.13). The best-fitting absorption column is 1.0 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 2.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.88 (+0.20, -0.19) and a best-fitting absorption column of 6.9 (+5.2, -4.5) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (4.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.9 (+5.2, -4.5) x 1020 cm-2
Galactic foreground: 2.0 x 1020 cm-2
Excess significance: 1.8 σ
Photon index: 1.88 (+0.20, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00863186.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180924A 83 s after the BAT trigger (Siegel and Ukwatta GCN Circ. 23277). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 23262) 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 0.03 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
03h16m47.85s -58°31'57.1" 1.5" XRT-final UKSSDC
03h16m47.87s -58°31'57.3" 1.7" XRT-enhanced Evans et al. GCN Circ. 23262
03h16m49.7s -58°31'27.6" 1.4' BAT-refined Stamatikos et al. GCN Circ. 23264

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23267 MASTER inspection MASTER

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 83 233 147 >20.8
uFC 241 491 246 >19.7
white 83 542 167 >20.9
v 572 592 19 >17.1
b 497 517 19 >18.4
u 241 491 246 >19.7

Table 3. UVOT observations reported by Siegel and Ukwatta (GCN Circ. 23277). 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.

September 28, 2018