GCN Circular 40662

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 EP250610a, collecting 2.1 ks of Photon Counting (PC) mode data between T0+2.8
ks and T0+9.4 ks after the trigger. A candidate counterpart has been found. The
details of this source are:

  Source 1 (SWIFT J163325.5+383159):
  ==================================
    RA (J2000.0):  248.3564  =	16h 33m 25.54s
    Dec (J2000.0): +38.5332  =	+38d 31' 59.5"
    Error:	   4.6 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   2.2 arcmin from the Einstein Probe/WXT position.
    Mean rate:	   0.0111 +/- 0.0027 ct s^-1
    Mean flux:	   (4.4 +/- 1.1)e-13 erg cm^-2 s^-1
    Peak rate:	   0.0111 +/- 0.0027 ct s^-1
    Peak flux:	   (4.4 +/- 1.1)e-13 erg cm^-2 s^-1
    ECF:	   3.96e-11 erg cm^-2 ct^-1, assuming NH=1.07e+20 cm^-2,
		   gamma=1.73; determined from a spectral fit.
    LSXPS UL:	   2.4e-03 ct/sec, (0.3-10 keV)
	 so the source is 3.1-sigma above this 3-sigma upper limit.
    There is no evidence for fading.


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

Uncatalogued X-ray sources
--------------------------

  Source 2 (SWIFT J163309.1+383226):
  ==================================
    RA (J2000.0):  248.2883  =	16h 33m 09.19s
    Dec (J2000.0): +38.5406  =	+38d 32' 26.2"
    Error:	   7.1 (arcsec, radius, 90% confidence).
    Detect flag:   REASONABLE
    Distance:	   78 arcsec from the Einstein Probe/WXT position.
    Mean rate:	   (2.4 [+1.6, -1.1])e-3 ct s^-1
    Mean flux:	   (1.02 [+0.70, -0.49])e-13 erg cm^-2 s^-1
    Peak rate:	   (2.4 [+1.6, -1.1])e-3 ct s^-1
    Peak flux:	   (1.02 [+0.70, -0.49])e-13 erg cm^-2 s^-1
    LSXPS UL:	   3.1e-03 ct/sec, (0.3-10 keV)
	 so the source is not above this 3-sigma upper limit.
    There is no evidence for fading.
    There is 1 2MASS object 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.

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