SVOM follow-up of the X-ray transient EP250916a
ATel #17396; A. Coleiro (APC, France), P. Maggi (ObAS, F), F. Cangemi (APC, F), C. Lachaud (APC, F)
on 18 Sep 2025; 12:02 UT
Credential Certification: Alexis Coleiro (alexis.coleiro@u-paris.fr)
Subjects: X-ray, Transient
Following the detection of the X-ray transient EP250916a by Einstein Probe/WXT on September 16, 2025 (ATel #17395), we conducted a Target-of-Opportunity (ToO) observation with the SVOM space mission on September 17, 2025 from 08:35:03 UT to 13:26:16 UT.
SVOM/MXT (0.2-10 keV) observed the source with a total exposure time of 7.0 ks. MXT detected a bright source at RA=268.2440 deg and Dec=-35.3198 deg (J2000), with a positional uncertainty of 27 arcsec at 90% confidence level, including a 25 arcsec systematic uncertainty added in quadrature. Spectral fitting with an absorbed power-law model (tbabs*powerlaw) yields Nh=0.32(+/-0.03)e22 cm^-2 and a photon index of 1.96(+/-0.14), with a 0.2-10 keV energy flux of 3.5e-10 erg/s/cm^2, confirming the brightening of the source since the first EP/WXT detection on September 16. The lightcurve does not show any significant variation of the counting rate over the duration of the observation.
SVOM/ECLAIRs observed the field with a total exposure time of 5.6 ks over the 4-150 keV energy band. The source is not significantly detected. A 3sigma upper limit of 1.75e-10 erg/s/cm^2 (4-10 keV) was derived with the same spectral model fitted on the MXT data. This upper limit is consistent with the MXT 4-10 keV extrapolated flux of 1.47e-10 erg/s/cm^2.
At the time of the SVOM observation, the SVOM/VT optical telescope was not taking data.
SVOM will continue monitoring the source within the next few days. The next ToO observation is scheduled on September 18 from 13:42:19 UT to 16:44:52 UT.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP.
