GCN Circular 40877

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 SVOM/ECLAIRs-detected GRB
250628B/sb25062804, collecting 1.4 ks of Photon Counting (PC) mode data between
T0+2.9 ks and T0+8.9 ks after the trigger. We have detected 2 sources, outside the
SVOM/ECLAIRs error circle. These have been automatically classified as follows:
  * 0 likely counterparts
  * 0 candidate counterparts
  * 1 uncatalogued X-ray source
  * 1 known X-ray source

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

  Source 2 (SWIFT J221311.1-454427):
  ==================================
    RA (J2000.0):  333.2963  =	22h 13m 11.11s
    Dec (J2000.0): -45.7408  =	-45d 44' 26.9"
    Error:	   6.8 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   12.0 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	   (7.7 [+3.9, -3.0])e-3 ct s^-1
    Mean flux:	   (2.9 [+1.5, -1.1])e-13 erg cm^-2 s^-1
    Peak rate:	   (7.7 [+3.9, -3.0])e-3 ct s^-1
    Peak flux:	   (2.9 [+1.5, -1.1])e-13 erg cm^-2 s^-1
    ECF:	   3.78e-11 erg cm^-2 ct^-1, assuming NH=2.76e+22 cm^-2,
		   gamma=2.97; determined from a spectral fit.
    RASS UL:	   1.0e-01 ct s^-1 (converted to XRT; 0.3-10 keV)
	 so the source is not above this 3-sigma upper limit.
    There is no evidence for fading.


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

  Source 1 (SWIFT J221222.2-452220):
  ==================================
    RA (J2000.0):  333.0927  =	22h 12m 22.25s
    Dec (J2000.0): -45.3724  =	-45d 22' 20.6"
    Error:	   6.3 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   11.9 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	   0.0141 [+0.0050, -0.0041] ct s^-1
    Mean flux:	   (3.16 [+1.12, -0.91])e-13 erg cm^-2 s^-1
    Peak rate:	   0.0141 [+0.0050, -0.0041] ct s^-1
    Peak flux:	   (3.16 [+1.12, -0.91])e-13 erg cm^-2 s^-1
    ECF:	   2.24e-11 erg cm^-2 ct^-1, assuming NH=1.46e+20 cm^-2,
		   gamma=3.23; determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J221222.5-452221
    in the LSXPS catalogue. Details:
      Separation:  3.6" from the XRT source
      Cat Rate:    2.4e-02 +/- 2.1e-03 ct s^-1 
      Cat Flux:    5.3e-13 +/- 4.7e-14 erg cm^-2 s^-1 (0.3-10 keV)
	     so the source is not above the catalogued flux.
    There is no evidence for fading.
    A SIMBAD object `TYC 8438-268-1' is 3.7" 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.

We did not detect any sources consistent with the five EP-FXT candidates reported by
Zhang et al. (GCN #40874
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 ).

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/SVOM.

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