[vsnet-grb-info 8002] GRB 090621: Swift-XRT Team refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9543
SUBJECT: GRB 090621: Swift-XRT Team refined analysis
DATE:    09/06/21 12:18:05 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

K.L. Page (U. Leicester) & P.A. Curran (UCL-MSSL) report on behalf of the 
Swift-XRT team:

We have analysed the first four orbits of XRT data obtained for GRB 
090621 (Curran et al., GCN Circ. 9540), comprising 192 s of Windowed 
Timing (WT) mode and 6.4 ks of Photon Counting (PC) mode. The refined XRT 
position is RA, Dec = 11.01982, 61.94115, which is equivalent to

RA (J2000) =  00 44 04.76 
Dec(J2000) = +61 56 28.1

with an uncertainty of 3.6 arcsec (radius, 90% containment).

The X-ray light-curve shows a large increase in emission peaking at 264 s 
after the initial trigger, corresponding to the time of the second BAT 
trigger (Curran et al., GCN Circ. 9540). The underlying decay, however, 
can be parameterised as a single power-law with alpha = 0.81 +/- 0.04.

As is common, there is clear spectral evolution during the flare. A 
spectrum extracted from the PC mode data after the flare subsides (500 s 
after the trigger) can be modelled with a power-law of Gamma = 2.07 
+0.37/-0.35 and a total absorbing column of (1.5 +/- 0.4)x10^22 cm^-2; the 
Galactic column in this direction is 5.6x10^21 cm^-2 (Kalberla et al. 
2005). Using this spectrum, the counts to observed (unabsorbed) flux 
conversion factor is 6.3x10^-11 (1.5x10^-10) erg cm^-2 count^-1.

If the light-curve continues to decay with alpha ~ 0.81, the predicted 
count rate at 24 hours is 0.015 count s^-1, corresponding to an observed 
(unabsorbed) flux of 9.5x10^-13 (2.2x10^-12) erg cm^-2 s^-1.

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