GCN Circular 41617

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 source
GRB 250831A (trigger sb25083102), collecting 1.5 ks of Photon Counting (PC) mode
data between T0+3.0 ks and T0+4.7 ks after the trigger. A likely counterpart has
been found. The details of this source are:

  Source 1 (SWIFT J013907.0-175644):
  ==================================
    RA (J2000.0):   24.7795  =	01 39 07.08
    Dec (J2000.0):  -17.9457  =  -17 56 44.5
    Error:	    3.5 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    3.4 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    5.60 +/- 0.16 ct s^-1
    Mean flux:	    (1.571 +/- 0.046)e-10 erg cm^-2 s^-1
    Peak rate:	    9.9 +/- 2.2 ct s^-1
    Peak flux:	    (2.78 +/- 0.61)e-10 erg cm^-2 s^-1
    ECF:	    2.81e-11 erg cm^-2 ct^-1
		      assuming NH=1.22e+21 cm^-2, gamma=2.40
		      determined from a spectral fit.
    LSXPS UL:	    6.6e-03 ct/sec, (0.3-10 keV)
		      so the source is 4.6-sigma above this 3-sigma upper limit.
    There is no evidence for fading.

GCN Circs. 41615
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  and 41616
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  noted that the high-proper-motion M-dwarf binary star
system G 272-61 was within the SVOM field of view; with a proper motion of ~3.3
arcsec yr^-1, its position appears consistent with this XRT source.

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

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

  Source 2 (SWIFT J013909.3-175732):
  ==================================
    RA (J2000.0):   24.7888  =	01 39 09.31
    Dec (J2000.0):  -17.9590  =  -17 57 32.4
    Error:	    5.4 (arcsec, radius, 90% confidence).
    Detect flag:    POOR
    Distance:	    3.2 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    0.242 +/- 0.015 ct s^-1
    Mean flux:	    (6.73 +/- 0.42)e-12 erg cm^-2 s^-1
    Peak rate:	    0.392 +/- 0.082 ct s^-1
    Peak flux:	    (1.09 +/- 0.23)e-11 erg cm^-2 s^-1
    ECF:	    2.78e-11 erg cm^-2 ct^-1
		      assuming NH=8.33e+20 cm^-2, gamma=2.42
		      determined from a spectral fit.
    LSXPS UL:	    5.7e-04 ct/sec, (0.3-10 keV)
		      so the source is 4.8-sigma above this 3-sigma upper limit.
    There is no evidence for fading.


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

  Source 3 (SWIFT J013932.8-175532):
  ==================================
    RA (J2000.0):   24.8869  =	01 39 32.86
    Dec (J2000.0):  -17.9257  =  -17 55 32.5
    Error:	    8.1 (arcsec, radius, 90% confidence).
    Detect flag:    POOR
    Distance:	    2.9 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    (-1.0 +/- -1.0)e0 ct s^-1
    Mean flux:	    (-2.2 +/- -2.2)e-24 erg cm^-2 s^-1
    Peak rate:	    (3.0 [+2.2, -1.6])e-3 ct s^-1
    Peak flux:	    (6.6 [+4.9, -3.7])e-27 erg cm^-2 s^-1
    ECF:	    2.24e-24 erg cm^-2 ct^-1
		      assuming NH=3.77e+20 cm^-2, gamma=98.61
		      determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J013933.3-175540
    in the LSXPS catalogue. Details:
    Separation:     10.3" from the XRT source
    Cat Rate:	    9.9e-04 +/- 2.4e-04 ct s^-1 
    Cat Flux:	    2.2e-27 +/- 5.5e-28 erg cm^-2 s^-1 (0.3-10 keV)
		      so the source  is 1.0-sigma above the catalogued flux.
    There is no evidence for fading.
    A SIMBAD object `SDSS J013934.38-175541.9' is 23.8" away.
    There are 2 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/SVOM/SVOM_FIELD00032.

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