GCN Circular 41644

P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi  (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250901a, collecting 2.0 ks of Photon Counting (PC) mode data between T0+6.2
ks and T0+12 ks after the trigger. We have detected 1 source.  This has been
automatically classified as an known X-ray source.

Known X-ray sources
-------------------

  Source 1 (SWIFT J181210.9+184216):
  ==================================
    RA (J2000.0):   273.0457  =  18 12 10.97
    Dec (J2000.0):  +18.7047  =  +18 42 16.9
    Error:	    5.5 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    2.1 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    0.0570 +/- 0.0097 ct s^-1
    Mean flux:	    (2.06 +/- 0.35)e-12 erg cm^-2 s^-1
    Peak rate:	    0.061 +/- 0.015 ct s^-1
    Peak flux:	    (2.21 +/- 0.54)e-12 erg cm^-2 s^-1
    ECF:	    3.62e-11 erg cm^-2 ct^-1
		      assuming NH=4.95e+21 cm^-2, gamma=2.27
		      determined from a spectral fit.
    This matches a catalogued X-ray source 1RXS J181210.7+184228
    in the ROSAT/RASSFSC catalogue. Details:
    Separation:     12.4" from the XRT source
    Cat Rate:	    2.0e-02 +/- 8.7e-03 ct s^-1 
    Cat Flux:	    6.7e-13 +/- 2.8e-13 erg cm^-2 s^-1 (0.3-10 keV)
		      so the source  is 2.5-sigma above the catalogued flux.
    The source may be fading, at the 0.9-sigma level.
    A SIMBAD object `1RXS J181210.7+184228' is 12.4" away.
    There are 3 2MASS objects within the source's 3-sigma error radius.

All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits  from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.

The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP/EP_FIELD00067.

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