SVOM/ECLAIRs detection of a possible X-ray outburst of the High-Mass X-ray Binary LS V +44 17 / RX J0440.9+4431
ATel #17712; S. Le Stum (APC, France), M. Brunet (IRAP, F), A. Coleiro (APC, F), O. Godet (IRAP, F), P. Maggi (ObAS, F), D. Götz (CEA, F), F. Cangemi (APC, F), L. Tao (IHEP, China), L. Zhang (IHEP, C)
on 6 Mar 2026; 17:47 UT
Credential Certification: Alexis Coleiro (alexis.coleiro@u-paris.fr)
Subjects: X-ray, Binary, Neutron Star
On Tuesday, March 3, 2026 at 07:37:42 UT (Tb), the onboard trigger software of the SVOM/ECLAIRs telescope detected and localized a long duration soft X-ray transient at R.A. = 70.117 deg; Dec. = 44.650 deg (J2000) with an uncertainty radius of 9.4 arcmin in the 5-20 keV energy band over a time window of 327.68 s starting at Tb (best alert SNR=8.37).
Following this detection, SVOM autonomously slewed to the trigger direction, and observations were performed with MXT (0.2 - 10 keV) starting at 07:54:30 UT for a total exposure of 1.9 ks. A single X-ray source was detected at a position consistent with the BeHMXB LS V +44 17, suggesting that the detected outburst is associated with this source.
A preliminary joint spectral analysis of the simultaneous ECLAIRs and MXT data shows that the source is significantly detected up to approximately 25 keV. The 0.4 - 120 keV spectrum is reasonably well fitted by an absorbed cutoff power-law model (chi-square = 54.6/46 dof) with a photon index of 0.74 (+/-0.60), a cutoff energy of 8.0 (-3.0, +6.8) keV and a hydrogen column density of 1.7 (+/-0.6) e22 cm^-2.
The integrated absorbed flux in the 0.4 - 25 keV band is 1.3e-9 erg/s/cm^2, corresponding to a luminosity of 9.3e35 erg/s assuming a distance of 2.44 kpc (Fortin et al., 2023, A&A 671, A149).
The measured flux in the 17-60 keV band is 3.2e-10 erg/s/cm^2. For comparison, Ferrigno et al., 2013, A&A, 553, A103 reported a persistent flux level of 1.35e-11 erg/s/cm^2 in the same energy band, suggesting that SVOM detected the onset of a new outburst of LS V +44 17.
In addition, although LS V +44 17 has been in the field-of-view of ECLAIRs during 36 pointings since February 1, 2026, it was not significantly detected in the ECLAIRs Quick Look Analysis prior to March 3. For instance, on February 27, with an exposure of 8.4 ks, the ECLAIRs 3 sigma upper limit on the source energy flux was 9.1e-11 erg/s/cm^2 in the 4-10 keV band, assuming the same spectral model as the one fitted on March 3. Between March 3 and 5, the source has been regularly detected in ECLAIRs observations, and no significant variability has been observed.
No targeted Target-of-Opportunity observations with SVOM can be performed on this source due to solar constraints. Further multi-wavelength follow-up observations are encouraged to investigate the evolution of this possible outburst.
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.
