Swift Observations of GRB 180812A

A.Y. Lien (GSFC/UMBC), P. D'Avanzo (INAF-OAB) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 08:22:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180812A (trigger=852903) (Lien et al. GCN Circ. 23129). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 79° from the Sun (6.9 hours East) and 78° from the 1%-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.

LaPorte and Lien (GCN Circ. 23138) 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 Ukwatta et al. (GCN Circ. 23136), the BAT ground-calculated position is RA, Dec = 245.839, 74.674 deg which is RA(J2000) = 16h23m21.3s Dec(J2000) = +74°40'27.0" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T0 and ends at ~T+17 s. T90 (15-350 keV) is 16.51 ± 0.92 s (estimated error including systematics).

The time-averaged spectrum from T-0.30 to T+16.63 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 ± 0.27. The fluence in the 15-150 keV band is 8.0 ± 1.4 x 10-7 erg cm-2. This fluence is larger than that of 33% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+15.62 s in the 15-150 keV band is 1.9 ± 0.4 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 23134). We have analysed 17 ks of XRT data for GRB 180812A, from 114 s to 126.8 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 23131).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.09 ± 0.04.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.76 (+0.16, -0.15). The best-fitting absorption column is 7.1 (+4.3, -2.7) x 1020 cm-2, consistent with the Galactic value of 4.4 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 3.7 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.1 (+4.3, -2.7) x 1020 cm-2
Galactic foreground: 4.4 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.76 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00852903.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180812A 112 s after the BAT trigger (LaPorte and Lien GCN Circ. 23138). A fading source consistent with the XRT position (Goad et al. GCN Circ. 23131) 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 0.04 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
16h23m20.45s +74°39'52.9" 0.50" UVOT-refined LaPorte and Lien GCN Circ. 23138
16h23m20.33s +74°39'52.7" 1.4" XRT-final UKSSDC
16h23m20.32s +74°39'53.0" 1.7" XRT-enhanced Goad et al. GCN Circ. 23131
16h23m21.3s +74°40'27.0" 2.3' BAT-refined Ukwatta et al. GCN Circ. 23136

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Carrasco et al. 23130 BOOTES-5/JGT optical afterglow detection BOOTES detection
Optical Watson et al. 23132 COATLI Detection and Light Curve COATLI light curve
Gamma-ray von Kienlin et al. 23135 Fermi GBM detection Fermi GBM Epeak=110±25 keV
T90=38 seconds
Fluence=1.2±0.2x10-6erg cm-2
(18th percentile for long GRBs)

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 140 289 431 19.81 ± 0.17
white 1551 2092 77 >20.85
v 600 6610 372 >18.9
b 526 5994 352 >20.0
u 270 5789 598 >19.8
w1 650 6857 211 >19.5
m2 6616 6815 197 >20.2
w2 6206 6406 197 >20.2

Table 3. UVOT observations reported by LaPorte and Lien (GCN Circ. 23138). 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.

August 14, 2018