GCN Circular 16428

M. De Pasquale (IASF-Palermo) reports on behalf of the Swift-XRT team:

We have analysed 3.0 ks of XRT data for the Fermi/GBM-detected
burst GRB 140620A (Fitzpatrick et al., GCN circ 16426
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 ), from 37.6
ks to 43.9 ks after the Fermi/GBM trigger. The data are entirely
in Photon Counting (PC) mode.
An X-ray source is detected within the Fermi/GBM error circle
at the position of the iPTF transient (Kasliwal et al., GCN circ.
16425
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 ). Using 2966 s of PC mode data and 2 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 281.87089,
+49.73093 which is equivalent to:

RA (J2000): 18h 47m 29.01s
Dec(J2000): +49d 43' 51.4"

with an uncertainty of 2.2 arcsec (radius, 90% confidence). This
is 0.4 arcsec from the iPTF position.

The late-time light curve (from T0+37.6 ks) is consistent with a
constant source of mean count rate 1.2e-01 ct/sec. A power-law fit
formally gives an index of -0.1 (+2.4, -1.3). Further observations
might be needed to confirm that this source is the X-ray afterglow
of 140620A.

A spectrum formed from the PC mode data can be fitted with an
absorbed power-law with a photon spectral index of 2.04 (+0.26,
-0.24). The best-fitting absorption column is 1.4 (+0.7, -0.6) x
10^21 cm^-2, in excess of the Galactic value of 5.2 x 10^20 cm^-2
(Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum is 3.3 x 10^-11
(4.3 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:

Total column:         1.4 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 5.2 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index:         2.04 (+0.26, -0.24)

If the light curve continues to evolve with a power-law index
of -0.1, the count rate at T+24 hours will be 0.13 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x
10^-12 (5.6 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/00020387.

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