sb25081602

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
sb25081602, collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.8 ks
and T0+7.3 ks after the trigger. We have detected 2 sources.  These have been
automatically classified as follows:
  * 0 likely counterparts
  * 0 candidate counterparts
  * 0 uncatalogued X-ray sources
  * 2 known X-ray sources

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

  Source 1 (SWIFT J051406.2-400234):
  ==================================
    RA (J2000.0):   78.5261  =	05h 14m 06.26s
    Dec (J2000.0):  -40.0430  =  -40d 02' 34.8"
    Error:	    3.5 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    3.7 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    3.58 +/- 0.14 ct s^-1
    Mean flux:	    (1.474 +/- 0.058)e-10 erg cm^-2 s^-1
    Peak rate:	    6.7 +/- 1.5 ct s^-1
    Peak flux:	    (2.75 +/- 0.62)e-10 erg cm^-2 s^-1
    ECF:	    4.11e-11 erg cm^-2 ct^-1
		      assuming NH=9.61e+20 cm^-2, gamma=1.65
		      determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J051406.5-400237
    in the LSXPS catalogue. Details:
    Separation:     4.4" from the XRT source
    Cat Rate:	    5.0e+00 +/- 2.1e-02 ct s^-1 
    Cat Flux:	    2.1e-10 +/- 8.6e-13 erg cm^-2 s^-1 (0.3-10 keV)
		      so the source  is 1.1-sigma above the catalogued flux.
    There is no evidence for fading.
    This is 3.05" from the SIMBAD position of 4U 0513-40, noted by  
      Magnani et al. (GCN Circ. 41399
Loading...
 
 ).
    There are 10 2MASS objects within the source's 3-sigma error radius.

  Source 2 (SWIFT J051413.9-395933):
  ==================================
    RA (J2000.0):   78.5583  =	05h 14m 13.99s
    Dec (J2000.0):  -39.9927  =  -39d 59' 33.7"
    Error:	    5.8 (arcsec, radius, 90% confidence).
    Detect flag:    REASONABLE
    Distance:	    2.1 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    (-1.0 +/- -1.0)e0 ct s^-1
    Mean flux:	    (-2.2 +/- -2.2)e-11 erg cm^-2 s^-1
    Peak rate:	    (4.5 [+2.6, -2.1])e-3 ct s^-1
    Peak flux:	    (9.6 [+5.6, -4.4])e-14 erg cm^-2 s^-1
    ECF:	    2.16e-11 erg cm^-2 ct^-1
		      assuming NH=1.21e+22 cm^-2, gamma=9.85
		      determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J051414.3-395936
    in the LSXPS catalogue. Details:
    Separation:     4.6" from the XRT source
    Cat Rate:	    0.0e+00 +/- 2.3e-04 ct s^-1 
    Cat Flux:	    0.0e+00 +/- 5.1e-15 erg cm^-2 s^-1 (0.3-10 keV)
		      so the source  is 1.9-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/SVOM/SVOM_FIELD00030.

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

F. Magnani (CPPM), D. Dornic (CPPM), O. Godet (IRAP), H. Goto (Kanazawa Univ.) report on behalf of the SVOM mission team:

At 2025-08-16T15:31:20 UTC (T0), SVOM/ECLAIRs triggered and located a likely X-ray transient (SVOM burst-id sb25081602).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was only detected by the Count-Rate Trigger (CRT), which produced a sequence of 3 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 9.53 in the [5-20] keV energy band over a time window of 20.40 seconds starting at 2025-08-16T15:31:10.

The localization of the best alert is R.A., Dec. 78.514, -39.982 degrees:
R.A. (J2000) = 5h14m03.43s
Dec. (J2000) = -39d58m56.04s
 with a 90% confidence level (C.L.) radius of 8.33 arcmin (including systematic error of 2 arcmin added in quadrature).

The LMXB 4U 0513-40 is located at 3.75 arcmin from the ECLAIRs position. We note that there are other X-ray sources within the ECLAIRs error circle.  

SVOM did not slew.

No X-ray observation could be performed by SVOM/MXT for the time being. No optical observation could be performed by SVOM/VT for the time being.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is F. Magnani (francesco.magnani.work@gmail.com).
Please contact the BA by email if you require additional information.