Swift Observations of GRB 181027A

A.Y. Lien (GSFC/UMBC) for the Swift team

1. Introduction

At 01:48:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181027A (trigger=868984) (Lien et al. GCN Circ. 23383). Swift could not slew to the burst due to an observing constraint. At the time of the trigger, the initial BAT position was 46° from the Sun (0.9 hours East) and 130° from the 93%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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. 23384), the BAT ground-calculated position is RA, Dec = 225.361, 30.937 deg which is RA(J2000) = 15h01m26.7s Dec(J2000) = +30°56'12.6" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a weak emission followed by a bright peak. The emission starts at T-18.4 s, peaks at T+47.5 s and ends at T+85.4 s. T90 (15-350 keV) is 81.16 ± 10.55 s (estimated error including systematics).

The time-averaged spectrum from T-18.45 to T+85.44 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.42 ± 0.09. The fluence in the 15-150 keV band is 2.8 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 70% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+47.81 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/868984/BA/.

3. XRT Observations and Analysis

XRT refined results are not available.

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.

RA (J2000) Dec (J2000) Error Note Reference
15h01m26.7s +30°56'12.6" 1.1' BAT-refined Barthelmy et al. GCN Circ. 23384

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Veres and Bissaldi 23385 Fermi GBM detection Fermi GBM Epeak=183±27 keV
T90=90 seconds
Fluence=7.76±0.57x10-6erg cm-2
(68th percentile for long GRBs)

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

October 29, 2018