TITLE: GCN CIRCULAR NUMBER: 40158 SUBJECT: SVOM/sb25041603 is not a GRB DATE: 25/04/17 07:50:32 GMT FROM: SVOM_group svomgroup@bao.ac.cn
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). 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%20...), and exclude the potential counterparts V* EG UMa and 2MASS J12161363+5242459 mentioned in GCN 40147.
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.
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