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GCN Circular 28808

Subject
GRB 201029A: Swift-XRT refined Analysis
Date
2020-10-30T10:11:05Z (4 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
M. Perri (ASDC), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), B. Sbarufatti
(PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), K.L. Page (U. Leicester),
A.P. Beardmore (U. Leicester) and S. Dichiara (NASA/GSFC/UMCP) reporton
behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 201029A (Dichiara et al.
GCN Circ. 28803), from 128 s to 34.7 ks after the  BAT trigger. The
data comprise 764 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 28806).

The light curve is dominated by flares. The first flare covers
~T0+200-650 s. The second flare started at T0+1100s and was still
ongoing when the GRB entered Earth eclipse at T0+1850 s. The third
flare began at T0+11.5 ks, was still ongoing at T0+23 ks and finished
at some point before T0+30.5 ks, although large observing gaps in this
window prevent acccurate measurements of the flare and it is possible
that there are multiple flares at this time rather than a single
long-lived flare. This behaviour, of multiple flares dominating the XRT
light curve for tens of kiloseconds, is reminiscent of GRBs 050904,
121027A and 130925A (see
https://www.swift.ac.uk/xrt_curves/allcurves.php for a mugshot gallery
of all XRT GRB afterglows).

This flaring activity makes it difficult to model the underlying
behaviour. Excluding all of the above times, the light curve can be
modelled with a power-law decay with a decay index of alpha=0.79
(+0.05, -0.06). These formal errors are likely underestimates, since
they depend on how accurate the estimates of flare times are, which are
themselves subject to proper modelling of the underlying emission.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	 of 1.65 (+/-0.05). The
best-fitting absorption column is  4.9 (+/-0.3) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.97 (+/-0.12) and a
best-fitting absorption column of 2.9 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.7 x 10^-11 (5.4 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 9.6 sigma
Photon index:	     1.97 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
0.79, the count rate at T+24 hours will be 0.024 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.2 x
10^-13 (1.3 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team
automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01003002.

This circular is an official product of the Swift-XRT team.
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