Swift Observations of GRB 180727A

A.P. Beardmore (U. Leicester), V. D'Elia (ASDC) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 14:15:28 UT, the Swift Burst Alert Telescope triggered and located GRB180727A (Trigger #850231) (Beardmore et al. GCN Circ. 23043). 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 Lien et al. (GCN Circ. 23047), the BAT ground-calculated position is RA, Dec = 346.639, -63.066 deg which is RA(J2000) = 23h06m33.3s Dec(J2000) = -63°03'57.9" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%.

The mask-weighted light curve (Figure 1) shows a single peak of duration ~1 second, peaking at T+0 s. T90 (15-350 keV) is 1.1 ± 0.2 s (estimated error including systematics).

The time-averaged spectrum from T-0.1 to T+1.3 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.59 ± 0.51, and Epeak of 67.6 ± 14.8 keV (χ2 43.23 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.7 ± 0.2 x 10-7 erg cm-2 and the 1-s peak flux measured from T-0.05 s in the 15-150 keV band is 3.6 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 87% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.60 ± 0.11 (χ2 57.46 for 57 d.o.f.). 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/850231/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 23045). We have analysed 9.0 ks of XRT data for GRB 180727A, from 58 s to 96.9 ks after the BAT trigger. The data comprise 24 s in Windowed Timing (WT) mode (the first 7 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=2.4 (+0.3, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 ± 0.4. The best-fitting absorption column is 2.1 (+1.8, -1.4) x 1021 cm-2, in excess of the Galactic value of 2.1 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 3.9 x 10-11 (5.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.1 (+1.8, -1.4) x 1021 cm-2
Galactic foreground: 2.1 x 1020 cm-2
Excess significance: 2.2 σ
Photon index: 1.9 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00850231.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180727A 77 s after the BAT trigger (Marshall and Beardmore GCN Circ. 23062). No optical afterglow consistent with the enhanced XRT position (D'Elia et al., GCN Circ. 23045) 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 0.02 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
23h06m39.92s -63°03'06.6" 1.7" XRT-final UKSSDC
23h06m39.92s -63°03'06.6" 1.7" XRT-refined D'Elia et al. GCN Circ. 23045
23h06m33.3s -63°03'57.9" 1.1' BAT-refined Lien et al. GCN Circ. 23047

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Veres 23044 Fermi GBM observation Fermi GBM Epeak=69±4 keV
T90=1.1 seconds
Fluence=3.32±0.18x10-7erg cm-2
(51st percentile for short 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
whiteFC 77 227 147 >20.9
uFC 290 303 12 >18.4
white 4280 5915 393 >21.7
v 4690 6312 379 >19.6
b 4075 5710 393 >21.2
u 5305 5505 197 >20.3
uvw1 5101 5301 197 >20.0
uvm2 4895 5095 197 >21.0
uvw2 4485 6122 393 >21.4

Table 3. UVOT observations reported by Marshall and Beardmore (GCN Circ. 23062). 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 3, 2018