Swift Observations of GRB 210306B

E. Ambrosi (INAF-IASFPA) for the Swift team

1. Introduction

At 09:32:14 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210306B (trigger=1036024) (Ambrosi et al. GCN Circ. 29601). Swift did not slew to the burst because of Sun constraint. At the time of the trigger, the initial BAT position was 22° from the Sun (1.0 hours West) and 79° from the 46%-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 Lien et al. (GCN Circ. 29612), the BAT ground-calculated position is RA, Dec = 331.813, 10.152 deg which is RA(J2000) = 22h07m15.1s Dec(J2000) = +10°09'06.8" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%.

The BAT light curve (Figure 1) showed a multi-peaked structure that starts at T-2 s and ends at ~T+25 s. T90 (15-350 keV) is 24.32 ± 2.45 s (estimated error including systematics).

The time-averaged spectrum from T-2.42 to T+24.63 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.38 ± 0.20. The fluence in the 15-150 keV band is 1.7 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 55% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.04 s in the 15-150 keV band is 2.4 ± 0.5 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/1036024/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
22h07m15.1s +10°09'06.8" 1.9' BAT-refined Lien et al. GCN Circ. 29612

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Lesage et al. 29637 Fermi GBM detection Fermi GBM Epeak=73±8 keV
T90=15 seconds
Fluence=1.3±0.1x10-6erg cm-2
(19th percentile for long GRBs)

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

March 10, 2021