Swift Observations of GRB 200903A

A.P. Beardmore (U. Leicester) for the Swift team

1. Introduction

At 00:44:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200903A (trigger=994389) (Beardmore et al. GCN Circ. 28350). Swift did not slew immediately due to an observing constraint. At the time of the trigger, the initial BAT position was 43° from the Sun (0.1 hours East) and 138° from the 99%-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 Ukwatta et al. (GCN Circ. 28360), the BAT ground-calculated position is RA, Dec = 164.313, 50.488 deg which is RA(J2000) = 10^h57^m15.1^s Dec(J2000) = +50°29'15.7" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 67%.

The mask-weighted light curve (Figure 1) shows a precursor at ~T-19 s, followed by the main structure with several overlapping pulses that starts at ~T-5 s and ends at ~T+20 s. The burst went out of the BAT FOV at T+64 s. T₉₀ (15-350 keV) is 24.15 ± 7.68 s (estimated error including systematics).

The time-averaged spectrum from T-19.18 to T+22.13 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.15 ± 0.28, and E_peak of 107.4 ± 65.8 keV (χ² 45.20 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.8 ± 0.1 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+0.14 s in the 15-150 keV band is 3.8 ± 0.3 ph cm^-2 s^-1. This fluence is larger than that of 70% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.56 ± 0.07 (χ² 51.88 for 57 d.o.f.). 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/994389/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

10^h57^m15.1^s +50°29'15.7" 1.0' BAT-refined Ukwatta et al. GCN Circ. 28360

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 28351 Swift GRB200903.03: Global MASTER-Net
observations report MASTER

Gamma-ray Fermi 28349 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Shimizu et al. 28357 CALET Gamma-Ray Burst Monitor detection CALET

Gamma-ray Lesage and Meegan 28362 Fermi GBM detection Fermi GBM E_peak=111.4±8.9 keV
T₉₀=26.9 seconds
Fluence=4.6±0.2x10^-6erg cm^-2
(55^th percentile for long GRBs)

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
10^h57^m15.1^s	+50°29'15.7"	1.0'	BAT-refined	Ukwatta et al. GCN Circ. 28360

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	28351	Swift GRB200903.03: Global MASTER-Net observations report	MASTER
Gamma-ray	Fermi	28349	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Shimizu et al.	28357	CALET Gamma-Ray Burst Monitor detection	CALET
Gamma-ray	Lesage and Meegan	28362	Fermi GBM detection	Fermi GBM	E_peak=111.4±8.9 keV T₉₀=26.9 seconds Fluence=4.6±0.2x10^-6erg cm^-2 (55^th percentile for long GRBs)

September 5, 2020