GCN Circular 18039

V. D'Elia (ASDC), L. Izzo (URoma/ICRA), K. Page (U Leicester) and E. Sonbas (NASA/GSFC/Adiyaman Univ.) report on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for the possible GRB 150720A (Sonbas et al.  GCN Circ. 18037
Not found
 
 
), from 138 s to 40 ks after the BAT trigger. The data comprise 96 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 119.58144, -28.26249 which is equivalent to:

RA (J2000): 07 58 19.55
Dec(J2000): -28 15 45.0

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

The late time light curve can be modelled with a power-law decay with a decay index of alpha ~ 1.5.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.16 (+0.10, -0.10). The best-fitting absorption column is 3.1 (+0.6, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 5.72 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.7 x 10^-11 (1.6 x 10^-10) erg cm^-2 count^-1.

A summary of the WT-mode spectrum is thus:
Total column:	     8.8 (+0.6, -0.6) x 10^21 cm^-2
Galactic foreground: 5.72 x 10^21 cm^-2
Excess significance: 5.1 sigma
Photon index:	     3.16 (+0.10, -0.10)

If the light curve continues to decay with a power-law decay index of 1.5, the count rate at T+24 hours will be 1.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x 10^-14 (2.9 x 10^-13) erg cm^-2 s^-1.

Given the light curve and spectrum behaviour, we believe that this is a real GRB and not a galactic transient.

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