Swift Observations of GRB 191227A

A. Melandri (INAF-OAB) for the Swift team

1. Introduction

At 01:39:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191227A (trigger=946344) (Melandri et al. GCN Circ. 26595). Swift did not slew immediately due to an observing constraint. At the time of the trigger, the initial BAT position was 42° from the Sun (2.9 hours East) and 32° from the 1%-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 Stamatikos et al. (GCN Circ. 26608), the BAT ground-calculated position is RA, Dec = 319.172, -16.717 deg which is RA(J2000) = 21h16m41.4s Dec(J2000) = -16°43'02.5" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 13%.

The mask-weighted light curve (Figure 1) shows many overlapping pulses that starts from ~T-30 s and ends at ~T+105 s. Several obvious peaks can be seen at ~T0, ~T+3 s, ~T+5 s, and ~T+11 s, respectively. T90 (15-350 keV) is 58.5 ± 14.4 s (estimated error including systematics).

The time-averaged spectrum from T-30.55 to T+105.93 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.30 ± 0.05. The fluence in the 15-150 keV band is 2.5 ± 0.1 x 10-5 erg cm-2. This fluence is larger than that of 98.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+5.28 s in the 15-150 keV band is 21.8 ± 1.3 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/946344/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
21h16m41.4s -16°43'02.5" 1.2' BAT-refined Stamatikos et al. GCN Circ. 26608

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26594 Swift GRB191227.07: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 26596 Swift GRB191227A: Global MASTER-Net OT
detection		 MASTER detection
Optical Adachi et al. 26597 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Gamma-ray Fermi 26593 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Dirirsa et al. 26598 Marginal detection by Fermi-LAT Fermi LAT
Gamma-ray Poolakkil and Meegan 26599 Fermi GBM detection Fermi GBM Epeak=362±10 keV
T90=25 seconds
Fluence=7.30±0.05x10-5erg cm-2
(96.9 percentile for long GRBs)
Gamma-ray Svinkin et al. 26614 Konus-Wind observation Konus-Wind Epeak=275 (-26,+30) keV
Duration=~52 seconds
Fluence=1.02(-0.08,+0.09)x10-4erg cm-2
Gamma-ray Du et al. 26618 Insight-HXMT/HE observation Insight-HXMT T90=20.65 seconds

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

January 1, 2020