GCN Circular 41330

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 EP250812a, collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.5
ks and T0+3.4 ks after the trigger. A likely counterpart has been found. The
position is consistent with the uncatalogued X-ray source detected by EP-FXT (Zhang
et al., GCN 41327
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 ). The details of this source are:

  Source 1 (SWIFT J020358.1-091907):
  ==================================
    RA (J2000.0):   30.9921  =	02h 03m 58.10s
    Dec (J2000.0):  -9.3187  =	-09d 19' 07.3"
    Error:	    3.7 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    67 arcsec from the Einstein Probe/WXT position.
    Mean rate:	    0.1069 +/- 0.0091 ct s^-1
    Mean flux:	    (4.00 +/- 0.34)e-12 erg cm^-2 s^-1
    Peak rate:	    0.203 +/- 0.046 ct s^-1
    Peak flux:	    (7.6 +/- 1.7)e-12 erg cm^-2 s^-1
    ECF:	    3.75e-11 erg cm^-2 ct^-1
		      assuming NH=4.71e+20 cm^-2, gamma=1.83
		      determined from a spectral fit.
    LSXPS UL:	    1.5e-03 ct/sec, (0.3-10 keV)
		      so the source is 4.4-sigma above this 3-sigma upper limit.
    The source may be fading, at the 2.6-sigma level.


We have detected a total of 3 sources.	These have been automatically classified as
follows:
  * 1 likely counterpart
  * 0 candidate counterparts
  * 0 uncatalogued X-ray sources
  * 2 known X-ray sources

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

  Source 2 (SWIFT J020340.7-092134):
  ==================================
    RA (J2000.0):   30.9196  =	02h 03m 40.70s
    Dec (J2000.0):  -9.3596  =	-09d 21' 34.6"
    Error:	    5.8 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    4.8 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    (8.5 [+3.0, -2.5])e-3 ct s^-1
    Mean flux:	    (2.42 [+0.87, -0.71])e-13 erg cm^-2 s^-1
    Peak rate:	    (8.5 [+3.0, -2.5])e-3 ct s^-1
    Peak flux:	    (2.42 [+0.87, -0.71])e-13 erg cm^-2 s^-1
    ECF:	    2.86e-11 erg cm^-2 ct^-1
		      assuming NH=4.16e+20 cm^-2, gamma=2.23
		      determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J020340.7-092134
    in the LSXPS catalogue. Details:
    Separation:     1.0" from the XRT source
    Cat Rate:	    4.6e-03 +/- 8.6e-04 ct s^-1 
    Cat Flux:	    1.3e-13 +/- 2.5e-14 erg cm^-2 s^-1 (0.3-10 keV)
		      so the source  is 1.3-sigma above the catalogued flux.
    There is no evidence for fading.
    A SIMBAD object `[VV2006] J020341.0-092134' is 3.4" away.

  Source 4 (SWIFT J020404.7-091504):
  ==================================
    RA (J2000.0):   31.0199  =	02h 04m 04.78s
    Dec (J2000.0):  -9.2513  =	-09d 15' 04.7"
    Error:	    5.8 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    4.0 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    (6.6 [+2.8, -2.2])e-3 ct s^-1
    Mean flux:	    (2.14 [+0.92, -0.72])e-13 erg cm^-2 s^-1
    Peak rate:	    (6.6 [+2.8, -2.2])e-3 ct s^-1
    Peak flux:	    (2.14 [+0.92, -0.72])e-13 erg cm^-2 s^-1
    ECF:	    3.25e-11 erg cm^-2 ct^-1
		      assuming NH=2.48e+20 cm^-2, gamma=1.94
		      determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J020405.1-091509
    in the LSXPS catalogue. Details:
    Separation:     7.3" from the XRT source
    Cat Rate:	    5.6e-03 +/- 1.5e-03 ct s^-1 
    Cat Flux:	    1.8e-13 +/- 4.9e-14 erg cm^-2 s^-1 (0.3-10 keV)
		      so the source  is 0.3-sigma above the catalogued flux.
    There is no evidence for fading.

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_FIELD00058.

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