TITLE: GCN CIRCULAR NUMBER: 38256 SUBJECT: GRB 241115A: Swift-XRT afterglow detection DATE: 24/11/16 21:27:36 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the Swift-BAT-detected burst GRB 241115A, collecting 11.3 ks of Photon Counting (PC) mode data between T0+92 s and T0+102.8 ks.

An uncatalogued X-ray source is detected consistent with being within 296 arcsec of the Swift-BAT position and is above the RASS 3-sigma upper limit at this position and fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 2620 s of PC mode data and 5 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 = 86.77097, -0.67462 which is equivalent to:

RA (J2000): 05h 47m 05.03s Dec(J2000): -00d 40' 28.6"

with an uncertainty of 2.2 arcsec (radius, 90% confidence). This position is 50 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 (+/-0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 (+/-0.4). The best-fitting absorption column is 1.7 (+0.6, -0.5) x 10^22 cm^-2, in excess of the Galactic value of 3.3 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 5.9 x 10^-11 (1.2 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.7 (+0.6, -0.5) x 10^22 cm^-2 Galactic foreground: 3.3 x 10^21 cm^-2 Excess significance: 4.5 sigma Photon index: 2.0 (+/-0.4)

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01267921. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/01267921.

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

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