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GCN Circular 40592

Subject
EP#01709177494: Swift-XRT counterpart detection of known source
Date
2025-05-31T17:47:34Z (5 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
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-IASFPA), 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 EP#01709177494, collecting 2.0 ks of Photon Counting (PC) mode data between
T0+388 s and T0+6.2 ks after the trigger. A candidate counterpart has been found.
The details of this source are:

  Source 1 (SWIFT J170930.3-263917):
  ==================================
    RA (J2000.0):  257.3766  =	17h 09m 30.38s
    Dec (J2000.0): -26.6548  =	-26d 39' 17.3"
    Error:	   3.5 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   20 arcsec from the Einstein Probe/WXT position.
    Mean rate:	   11.35 +/- 0.22 ct s^-1
    Mean flux:	   (4.665 +/- 0.088)e-10 erg cm^-2 s^-1
    Peak rate:	   18.7 +/- 3.1 ct s^-1
    Peak flux:	   (7.7 +/- 1.3)e-10 erg cm^-2 s^-1
    ECF:	   4.11e-11 erg cm^-2 ct^-1, assuming NH=4.40e+21 cm^-2,
		   gamma=1.98; determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J170930.1-263918
    in the LSXPS catalogue. Details:
      Separation:  3.2" from the XRT source
      Cat Rate:    1.2e+00 +/- 9.0e-03 ct s^-1 
      Cat Flux:    4.7e-11 +/- 3.7e-13 erg cm^-2 s^-1 (0.3-10 keV)
	     so the source  is 5.7-sigma above the catalogued flux.
    There is no evidence for fading.
    A SIMBAD object `RX J1709.5-2639' is 8.3" away.
    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.