TITLE: GCN CIRCULAR NUMBER: 41379 SUBJECT: GRB 250814A: Swift-XRT refined Analysis DATE: 25/08/14 21:31:37 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), R. Brivio (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 250814A, from 163 s to 40.1 ks after the trigger. The data comprise 184 s in Windowed Timing (WT) mode (the first 10 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 306.30792, 48.39903 which is equivalent to:

RA (J2000): 20h 25m 13.90s Dec(J2000): +48d 23' 56.5"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an index of alpha=1.49 (+0.27, -0.28), followed by a break at T+338 s to an alpha of 3.15 (+0.26, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.33 (+0.24, -0.22). The best-fitting absorption column is 1.2 (+0.4, -0.3) x 10^22 cm^-2, in excess of the Galactic value of 6.5 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.9 (+/-0.4) and a best-fitting absorption column of 1.0 (+0.4, -0.3) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.5 x 10^-11 (9.3 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.0 (+0.4, -0.3) x 10^22 cm^-2 Galactic foreground: 6.5 x 10^21 cm^-2 Excess significance: 1.8 sigma Photon index: 1.9 (+/-0.4)

If the light curve continues to decay with a power-law decay index of 3.15, the count rate at T+24 hours will be 1.3 x 10^-7 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.0 x 10^-18 (1.2 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01342439.

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

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