Swift Observations of GRB 191218A

E. Troja (NASA/GSFC/UMCP) for the Swift team

1. Introduction

At 02:42:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191218A (trigger=944916) (Troja et al. GCN Circ. 26480). Swift did not slew to this burst due to a Sun constraint. At the time of the trigger, the initial BAT position was 35° from the Sun (2.4 hours East) and 132° from the 63%-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 Markwardt et al. (GCN Circ. 26495), the BAT ground-calculated position is RA, Dec = 301.070, -40.277 deg which is RA(J2000) = 20h04m16.7s Dec(J2000) = -40°16'38.8" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 9%.

The mask weighted light curve (Figure 1) shows two peaks at ~T+4 s and ~T+45 s. The overall structure starts at ~T-2 s and lasts till ~T+58 s. T90 (15-350 keV) is 44.67 ± 0.73 s (estimated error including systematics).

The time-averaged spectrum from T-0.23 to T+46.03 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.12 ± 0.10. The fluence in the 15-150 keV band is 7.8 ± 0.4 x 10-6 erg cm-2. This fluence is larger than that of 89% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.66 s in the 15-150 keV band is 6.3 ± 0.9 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/944916/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
20h04m16.7s -40°16'38.8" 1.4' BAT-refined Markwardt et al. GCN Circ. 26495

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Gaikwad et al. 26482 AstroSat CZTI detection CZTI T90=51.03 seconds
Gamma-ray Ridnaia et al. 26506 Konus-Wind observation Konus-Wind Epeak=330 (-44,+61) keV
Duration=~78.8 seconds
Fluence=3.92(-0.37,+0.45)x10-5erg cm-2

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

December 20, 2019