TITLE: GCN CIRCULAR NUMBER: 39336 SUBJECT: GRB 250215A: Swift-XRT refined Analysis DATE: 25/02/15 11:39:14 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

We have analysed 1.7 ks of XRT data for the Einstein Probe/WXT-detected burst GRB 250215A, from 9.5 ks to 16.1 ks after the Einstein Probe/WXT trigger, also detected by Fermi/GBM (GCN Circ. 39327) and INTEGRAL (GCN Circ. 39331). The data are entirely in Photon Counting (PC) mode.

We detect a bright, previously uncatalogued X-ray source at the following coordinates: RA/Dec(J2000) = 156.33119, -27.69834, which is equivalent to

RA (J2000): 10 25 19.48 Dec(J2000): -27 41 54.0

with an uncertainty of 3.6 arcsec (radius, 90% confidence). This source is 6 arcsec from the FXT position (GCN Circ. 39329) and 0.35 arcsec from the NOT optical counterpart (GCN Circ. 39330).

The light curve can be modelled with a power-law decay with a decay index of alpha=1.1 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.95 (+0.23, -0.22). The best-fitting absorption column is 1.2 (+0.7, -0.5) x 10^21 cm^-2, consistent with the Galactic value of 6.8 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 3.5 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.2 (+0.7, -0.5) x 10^21 cm^-2 Galactic foreground: 6.8 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.95 (+0.23, -0.22)

If the light curve continues to decay with a power-law decay index of 1.1, the count rate at T+24 hours will be 0.038 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x 10^-12 (1.7 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/00019554.

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

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