Swift Observations of GRB 180828A

A.P. Beardmore (U. Leicester), S.J. LaPorte (PSU) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 18:57:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located a source which is either GRB 180828A or a previously-unknown Galactic transient (trigger=856977) (Beardmore et al. GCN Circ. 23182). Swift slewed immediately to the source location. At the time of the trigger, the initial BAT position was 114° from the Sun (7.5 hours East) and 92° from the 95%-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. 23188), the BAT ground-calculated position is RA, Dec = 268.704, -25.791 deg which is RA(J2000) = 17h54m49.1s Dec(J2000) = -25°47'28.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 19%.

The mask-weighted light curve (Figure 1) shows a few overlapping pulses that starts at ~ T-7 s and ends at ~T+2 s, with some weak tail emission that lasts till ~ T+30 s. T90 (15-350 keV) is 14.0 ± 8.0 s (estimated error including systematics).

The time-averaged spectrum from T-7.29 to T+28.71 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.89 ± 0.06. The fluence in the 15-150 keV band is 8.8 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 91.2% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.29 s in the 15-150 keV band is 17.8 ± 0.9 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/856977/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by LaPorte et al. (GCN Circ. 23185). We have analysed 8.8 ks of XRT data for GRB 180828A, from 105 s to 40.0 ks after the BAT trigger. The data comprise 32 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.21 (+0.05, -0.04).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+1.4, -1.1). The best-fitting absorption column is 2.4 (+1.3, -0.9) x 1023 cm-2, in excess of the Galactic value of 1.4 x 1022 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 1.1 x 10-10 (7.4 x 10-10) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 2.4 (+1.3, -0.9) x 1023 cm-2
Galactic foreground: 1.4 x 1022 cm-2
Excess significance: 4.0 σ
Photon index: 2.2 (+1.4, -1.1)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00856977.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180828A 124 s after the BAT trigger (Emery and Beardmore GCN Circ. 23190). No optical afterglow consistent with the XRT position (LaPorte et al., GCN Circ. 23185) or the position of the source detected outside the enhanced XRT error region by RATIR (Butler et al., GCN Circ. 23189) 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 Milky Way extinction.

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
17h54m52.35s -25°47'52.6" 1.8" XRT-final UKSSDC
17h54m52.28s -25°47'53.0" 2.0" XRT-enhanced Evans et al. GCN Circ. 23184
17h54m49.1s -25°47'28.3" 1.0' BAT-refined Markwardt et al. GCN Circ. 23188

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Butler et al. 23189 RATIR Optical and NIR Observations RATIR upper limit
Gamma-ray Roberts and Meegan 23183 Fermi GBM observation Fermi GBM Epeak=358±9 keV
T90=8.3 seconds
Gamma-ray Khanam et al. 23191 AstroSat CZTI detection CZTI T90=8.9 seconds
Gamma-ray Kozlova et al. 23193 Konus-Wind observation Konus-Wind Epeak=341 (-21,+24) keV
Fluence=4.41(-0.17,+0.17)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 124 23102 1057 >21.8
v 16882 39624 1367 >20.3
b 5752 28839 1439 >21.7
u 5547 34577 1709 >21.1
uvw1 5342 33925 1082 >20.7
uvm2 5137 40314 1756 >21.0
uvw2 11215 38709 2087 >21.2

Table 3. UVOT observations reported by Emery and Beardmore (GCN Circ. 23190). 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 31, 2018