TITLE: GCN CIRCULAR NUMBER: 36631 SUBJECT: GRB 240606B: Swift-XRT afterglow detection DATE: 24/06/07 03:12:47 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), A.P. Beardmore (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-GUANO-detected burst GRB 240606B, collecting 4.0 ks of Photon Counting (PC) mode data between T0+22.9 ks and T0+35.1 ks.
Four uncatalogued X-ray sources are detected consistent with being within 493 arcsec of the BAT-GUANO position, of which one ("Source 1") is fading with 2.1 sigma significance and is undetected by eRosita to deeper upper limits. It is likely the afterglow.
Using 3507 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 = 40.29507, -60.61201 which is equivalent to:
RA (J2000): 02h 41m 10.82s Dec(J2000): -60d 36' 43.2"
with an uncertainty of 2.4 arcsec (radius, 90% confidence). This position is 53 arcsec from the BAT-GUANO position.
The light curve can be modelled with a power-law decay with a decay index of alpha=1.2 (+0.9, -1.0).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.7 (+0.4, -0.3). The best-fitting absorption column is 6 (+11, -4) x 10^20 cm^-2, consistent with the Galactic value of 2.4 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 4.1 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 6 (+11, -4) x 10^20 cm^-2 Galactic foreground: 2.4 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.7 (+0.4, -0.3)
If the light curve continues to decay with a power-law decay index of 1.2, the count rate at T+24 hours will be 0.013 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x 10^-13 (5.7 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/00021695. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021695.
This circular is an official product of the Swift-XRT team.
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