TITLE: GCN CIRCULAR NUMBER: 37334 SUBJECT: GRB 240828B: Swift-XRT afterglow detection DATE: 24/08/29 04:13:41 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), C. Salvaggio (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:
Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected burst GRB 240828B, collecting 2.9 ks of Photon Counting (PC) mode data between T0+13.5 ks and T0+25.0 ks.
Two uncatalogued X-ray sources are detected, of which one ("Source 1") is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 2142 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 = 63.45995, +16.71561 which is equivalent to:
RA (J2000): 04h 13m 50.39s Dec(J2000): +16d 42' 56.2"
with an uncertainty of 2.6 arcsec (radius, 90% confidence). This position is 5.5 arcmin from the SVOM/ECLAIRs position.
The light curve can be modelled with a power-law decay with a decay index of alpha=1.8 (+/-0.9).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.5, -0.4). The best-fitting absorption column is 6.2 (+3.7, -2.7) x 10^21 cm^-2, in excess of the Galactic value of 2.9 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 4.7 x 10^-11 (7.2 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 6.2 (+3.7, -2.7) x 10^21 cm^-2 Galactic foreground: 2.9 x 10^21 cm^-2 Excess significance: 1.9 sigma Photon index: 1.8 (+0.5, -0.4)
If the light curve continues to decay with a power-law decay index of 1.8, the count rate at T+24 hours will be 3.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x 10^-13 (2.6 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/00021706. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021706.
This circular is an official product of the Swift-XRT team.
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