Swift Observations of GRB 181224A

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

1. Introduction

At 13:36:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181224A (trigger=878911) (Melandri et al. GCN Circ. 23554). Swift did not slew to the burst because of the Moon constraint. At the time of the trigger, the initial BAT position was 144° from the Sun (9.8 hours West) and 16° from the 95%-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 Krimm et al. (GCN Circ. 23555), the BAT ground-calculated position is RA, Dec = 125.949, 6.194 deg which is RA(J2000) = 08h23m47.7s Dec(J2000) = +06°11'39.9" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 80%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~ T-12 s, peaks at ~ T+1 s, one ends at ~ T+4 s. T90 (15-350 keV) is 12.5 ± 4.2 s (estimated error including systematics).

The time-averaged spectrum from T-12.04 to T+3.30 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.27 ± 0.20. The fluence in the 15-150 keV band is 4.9 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 19% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.48 s in the 15-150 keV band is 1.0 ± 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/878911/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
08h23m47.7s +06°11'39.9" 1.9' BAT-refined Krimm et al. GCN Circ. 23555

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Table 2. No GCN Circulars from other observatories were found.

December 26, 2018