Swift Observations of GRB 200909B

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

1. Introduction

At 23:17:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200909B (trigger=995288) (Melandri et al. GCN Circ. 28406). Swift did not slew to the location due to an observing constraint. At the time of the trigger, the initial BAT position was 37° from the Sun (0.4 hours East) and 112° from the 54%-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 Laha et al. (GCN Circ. 28408), the BAT ground-calculated position is RA, Dec = 175.171, -31.199 deg which is RA(J2000) = 11h40m41.1s Dec(J2000) = -31°11'54.7" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 35%.

The mask-weighted light curve (Figure 1) shows a single peak that stars at T-6 s, peaks at T+1 s, and ends at T+20 s. A hint of an extended emission is visible until T+40 s. T90 (15-350 keV) is 16.9 ± 2.2 s (estimated error including systematics).

The time-averaged spectrum from T-5.6 to T+14.2 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.60 ± 0.14. The fluence in the 15-150 keV band is 1.2 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 45% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.44 s in the 15-150 keV band is 1.3 ± 0.3 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/995288/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
11h40m41.1s -31°11'54.7" 1.4' BAT-refined Laha et al. GCN Circ. 28408

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

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

September 11, 2020