Swift Observations of GRB 180805A

P. D'Avanzo (INAF-OAB) and J.A. Kennea (PSU) for the Swift team

1. Introduction

At 09:04:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180805A (trigger=851829) (D'Avanzo et al. GCN Circ. 23074). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 69° from the Sun (2.2 hours East) and 119° from the 44%-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 Palmer et al. (GCN Circ. 23081), the BAT ground-calculated position is RA, Dec = 167.575, -45.327 deg which is RA(J2000) = 11h10m18.1s Dec(J2000) = -45°19'35.8" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at peaks at ~T0, and ends at ~T+1.9 s. T90 (15-350 keV) is 1.68 ± 0.41 s (estimated error including systematics).

The time-averaged spectrum from T+0.04 to T+1.90 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.58 ± 0.32. The fluence in the 15-150 keV band is 1.1 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 76% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.13 s in the 15-150 keV band is 1.1 ± 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/851829/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 23084).

We have analysed 4.4 ks of XRT data for GRB 180805A, from 93 s to 12.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 23075).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.32 ± 0.14.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.50 (+0.54, -0.25). The best-fitting absorption column is consistent with the Galactic value of 1.7 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10-11 (5.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 ± 2.2 x 1021 cm-2
Galactic foreground: 1.7 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.50 (+0.54, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00851829.

4. UVOT Observations and Analysis

UVOT results are not available.

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
11h10m15.76s -45°19'56.7" 2.0" XRT-final UKSSDC
11h10m15.90s -45°19'56.0" 2.5" XRT-enhanced Beardmore et al. GCN Circ. 23075
11h10m18.1s -45°19'35.8" 2.1' BAT-refined Palmer et al. GCN Circ. 23081

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Table 2. No GCN Circulars from other observatories were found.

August 7, 2018