Swift Observations of GRB 180510B

A. Tohuvavohu (PSU) and B. Sbarufatti (INAF-OAB/PSU) for the Swift team

1. Introduction

At 20:15:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located possible GRB 180510B (trigger=831816) (Tohuvavohu et al. GCN Circ. 22696). Due to an observing constraint Swift did not slew immediately to the burst. At the time of the trigger, the initial BAT position was 84° from the Sun (2.0 hours East) and 82° from the 24%-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.

Tohuvavohu et al. (GCN Circ. 22696) reported the discovery with UVOT of an optical afterglow. Schady (GCN Circ. 22701) reported the position from GROND for the optical afterglow of this GRB. Palmerio et al. (GCN Circ. 22702) determined a redshift of 1.305 from VLT. 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. 22705), the BAT ground-calculated position is RA, Dec = 77.965, -62.320 deg which is RA(J2000) = 05h11m51.6s Dec(J2000) = -62°19'13.0" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 41%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-20 s and peaks at ~T+1 s. The main pulse ends at ~ T+40 s, and there is some low-level emission that lasts until ~T+140 s. T90 (15-350 keV) is 134.3 ± 48.2 s (estimated error including systematics).

The time-averaged spectrum from T-19.92 to T+137.10 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.00 ± 0.17. The fluence in the 15-150 keV band is 2.1 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 60% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.96 s in the 15-150 keV band is 1.2 ± 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/831816/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 22708). We have analysed 3.3 ks of XRT data for GRB 180510B, from 3.0 ks to 90.3 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. 22698).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.42 ± 0.12.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.83 ± 0.12. The best-fitting absorption column is 1.3 (+0.4, -0.3) x 1021 cm-2, in excess of the Galactic value of 2.5 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 (4.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.3 (+0.4, -0.3) x 1021 cm-2
Galactic foreground: 2.5 x 1020 cm-2
Excess significance: 5.0 σ
Photon index: 1.83 ± 0.12

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00831816.

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
05h11m52.64s -62°19'26.3" 0.76" UVOT-initial Tohuvavohu et al. GCN Circ. 22696
05h11m52.57s -62°19'27.0" 1.5" XRT-final UKSSDC
05h11m52.56s -62°19'26.8" 1.5" XRT-enhanced Beardmore et al. GCN Circ. 22698
05h11m51.6s -62°19'13.0" 1.7' BAT-refined Barthelmy et al. GCN Circ. 22705

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Klotz et al. 22700 TAROT La Silla observatory optical
observations		 TAROT
Optical Schady 22701 GROND detection in all bands GROND detection
Optical Palmerio et al. 22702 VLT/X-shooter observations VLT detection

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

May 12, 2018