TITLE: GCN CIRCULAR NUMBER: 39820 SUBJECT: GRB 250321A: Swift-XRT refined Analysis DATE: 25/03/21 18:23:30 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for the Swift-BAT-detected burst GRB 250321A, from 108 s to 39.4 ks after the Swift-BAT trigger. The data comprise 9 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. Using 1662 s of PC mode data and 3 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 = 295.09844, +21.04038 which is equivalent to:

RA (J2000): 19h 40m 23.62s Dec(J2000): +21d 02' 25.4"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This position is 72 arcsec from the Swift-BAT position.

The light curve can be modelled with a power-law decay with a decay index of alpha=1.13 (+0.11, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+1.3, -1.0). The best-fitting absorption column is 3.7 (+3.4, -2.2) x 10^22 cm^-2, in excess of the Galactic value of 1.1 x 10^22 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 7.3 x 10^-11 (2.4 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 3.7 (+3.4, -2.2) x 10^22 cm^-2 Galactic foreground: 1.1 x 10^22 cm^-2 Excess significance: 2.0 sigma Photon index: 2.2 (+1.3, -1.0)

If the light curve continues to decay with a power-law decay index of 1.13, the count rate at T+24 hours will be 6.9 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x 10^-14 (1.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01297508.

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

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