Swift Observations of GRB 210928A

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

1. Introduction

Axelsson et al. (GCN Circ. 30912) reported the GRB detection. 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.

2. BAT Observations and Analysis

BAT did not observe this burst.

3. XRT Observations and Analysis

We have analysed 12 ks of XRT data for the -detected burst GRB 210928A, from 67.4 ks to 630.8 ks after the trigger. The data are entirely in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.3 (+0.4, -0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.4 (+0.6, -0.3). The best-fitting absorption column is 2.342 (+3.358, -0.021) x 10²¹ cm^-2, consistent with the Galactic value of 2.3 x 10²¹ cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.8 x 10^-11 (5.5 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.342 (+3.358, -0.021) x 10²¹ cm^-2
Galactic foreground: 2.3 x 10²¹ cm^-2
Excess significance: <1.6 σ
Photon index: 1.4 (+0.6, -0.3)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00014850.

4. UVOT Observations and Analysis

UVOT results are not available.

Figure 1. The BAT light curve is not available.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference

16^h23^m15.10^s -17°52'33.4" 4.2" XRT-final UKSSDC

16^h23^m15.06^s -17°52'33.5" 2.9" XRT Page GCN Circ. 30919

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Chen et al. 30918 Kinder SLT-40cm optical limits Lulin upper limits

Gamma-ray Axelsson et al. 30912 Fermi-LAT detection Fermi LAT E_max=6 GeV

Gamma-ray Fletcher and Meegan 30913 Fermi GBM observation Fermi GBM E_peak=465±10 keV
T₉₀=24 seconds
Fluence=4.925±0.052x10^-5erg cm^-2
(brighter than 95% of long GRBs)

Gamma-ray Ridnaia et al. 30921 Konus-Wind detection Konus-Wind E_peak=361 (-33,+38) keV
Duration=~27.3 seconds
Fluence=6.79(-0.66,+0.71)x10^-5erg cm^-2

Other Rhodes et al. 30924 MeerKAT detection detection

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
16^h23^m15.10^s	-17°52'33.4"	4.2"	XRT-final	UKSSDC
16^h23^m15.06^s	-17°52'33.5"	2.9"	XRT	Page GCN Circ. 30919

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Chen et al.	30918	Kinder SLT-40cm optical limits	Lulin	upper limits
Gamma-ray	Axelsson et al.	30912	Fermi-LAT detection	Fermi LAT	E_max=6 GeV
Gamma-ray	Fletcher and Meegan	30913	Fermi GBM observation	Fermi GBM	E_peak=465±10 keV T₉₀=24 seconds Fluence=4.925±0.052x10^-5erg cm^-2 (brighter than 95% of long GRBs)
Gamma-ray	Ridnaia et al.	30921	Konus-Wind detection	Konus-Wind	E_peak=361 (-33,+38) keV Duration=~27.3 seconds Fluence=6.79(-0.66,+0.71)x10^-5erg cm^-2
Other	Rhodes et al.	30924	MeerKAT detection		detection

October 7, 2021