sb25041603

Z. Wang (GXU), W. Zheng (UCB), S. Schanne (CEA), M.-G. Bernardini (INAF/LUPM) report on behalf of the SVOM mission team:

SVOM/ECLAIRs triggered on the X-ray transient labelled sb25041603 (SVOM burst-id sb25041603) starting at 2025-04-16T06:28:04 UTC (Tb).

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 Image Trigger (IMT), which produced 1 alert with the signal-to-noise-ratio (SNR) of 7.25 in the 5-8 keV energy band over a time window of 655.36 seconds starting at Tb.

The localization of the best alert is R.A., Dec 184.1075, 52.6309 degrees:

RA (J2000) = 12h16m25.81s
Dec (J2000) = 52d37m51.11s

with a 90% C.L. radius of 10.81 arcmin (including systematic error of 2 arcmin added in quadrature).

We notice the presence of several X-ray transient candidate sources in this error-box, among which the cataclismic variable V* EG UMa located at 9.3 arcmin, but also several quasars, the closest one being 2MASS J12161363+5242459 located at 5.3 arcmin.

The detection significance was below the Slew threshold, so no immediate slew was performed on this burst.

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.

The Burst Advocate (BA) on shift for this burst is Ziqi Wang (zq.wang@st.gxu.edu.cn).
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.

Francis Fortin (IRAP), Diego López-Cámara (UNAM), Jean-Grégoire Ducoin (CPPM), Damien Dornic (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan Watson (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM) and ZiQi Wang (GXU) report:

We observed the field of SVOM/ECLAIRs sb25041603 (Wang et al. GCN 40147
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 ) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope operated from the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. We obtained 50 exposures of 60 s in the i-band starting at 06:39:55 UT on 2025-04-16 (12 min after the onboard trigger, and 16 s after the alert reception).

In the stacked image, we do not detect any new source at the ECLAIRs source position (Wang et al. GCN 40147
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 ) down to the following 5-sigma limit:

i > 22.89 mag (AB)

The data were coadded with the Colibrí pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometric calibration was performed using nearby stars from the PanSTARRS DR1 catalog and the magnitude is not corrected for Galactic extinction.

We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.

P. Maggi (ObAS), D. Götz (CEA), H. Goto (Kanazawa University/CEA), M. Moita (CEA), C. Plasse (CEA), F. Robinet (IJCLab), C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team:

SVOM/ECLAIRs triggered on the X-ray transient labelled sb25041603 (SVOM burst-id sb25041603, Wang et al. GCN 40147
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 ). The field was observed by SVOM/MXT starting at T0 = 2025-04-16T08:57:43 (2.5 hour after trigger time Tb). MXT observed during 3 orbits for 3.0 ks effective exposure.
Using the full X-band dataset, we find a source located at RA=183.889, Dec=52.6534

RA (J2000) = 12h15m33

Dec (J2000) = +52d39m12.3

with a statistical 90% C.L. radius of 39", to which a 35” systematic uncertainty is to be added in quadrature. That position matches the M4V star [StKM 2-809](https://simbad.cds.unistra.fr/simbad/sim-id?Ident=%40568434&Name=StKM%202-809), and exclude the potential counterparts V* EG UMa and 2MASS J12161363+5242459 mentioned in GCN 40147
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 .

The spectrum is modelled by an absorbed thermal component with a Gaussian distribution of emission measure (`Tbabs*gadem` in xspec). The absorption column is NH = 5 (<10) x 1e20 /cm2 (90% C.L. uncertainties) and a broad distribution of temperature with a mean value of 2.7 (+/- 0.8) keV. The observed average flux in the 0.3-5 keV band is 2.2 (+/-0.4) x1e-11 erg/cm2/s. At the parallactic distance of 25 pc of StKM 2-809, this translates to a luminosity of 1.6e30 erg/s, consistent with a flare from a low-mass main sequence star. Over the 3 orbits the source was fading, from 3.1e-11 erg/cm2/s in the first half to 0.3e-11 erg/cm2/s in the second half. We conclude that SVOM/sb25041603 is likely not a GRB but a flare from the M4V star StKM 2-809.

The Space 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. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE.

The SVOM point of contact for this trigger is Ziqi Wang (zq.wang@st.gxu.edu.cn).
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this trigger.