SVOM/ECLAIRs detection of the ongoing outburst from the HMXB XTE J1859+083

ATel #17770; F. Cangemi (APC, France), A. Coleiro (APC, F), M. Dennefeld (IAP, F), O. Godet (IRAP, F), D. Götz (CEA, F), S. Guillot (IRAP, F), S. Le Stum (APC, F), H.-L. Li (NAOC, China), S. Schanne (CEA, F), L. Tao (IHEP, C), L.-P. Xin (NAOC, C), L. Zhang (IHEP, C)
on 30 Apr 2026; 09:35 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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On Monday, April 27, 2026, at 13:58:46.114 UT, the onboard trigger software of the SVOM/ECLAIRs telescope detected and localized a source at R.A. = 284.818 deg and Dec. = 8.363 deg (J2000), with an uncertainty radius of 6.67 arcmin (90% confidence level) in the 5-20 keV energy band (SNR = 12.10).

Following this detection, SVOM autonomously slewed to the trigger direction, and observations were performed with MXT (0.2-10 keV) starting at 14:11:52.222 UT, for a total exposure of 13.7 ks. The MXT position R.A. = 284.765 deg, Dec. = 8.249 (J2000) deg with an uncertainty radius of 40.61 arcsec (90% confidence level) consistent with the source XTE J1859+083.

A preliminary joint spectral analysis of the simultaneous ECLAIRs and MXT data shows that the 0.6-100 keV spectrum is well fitted by an absorbed cutoff power-law model (chi-square = 289.32/266 dof) with a photon index of 1.33 (+/-0.16), a cutoff energy of 21 (+8, -4) keV and a hydrogen column density of 1.92 (+/-0.16) e22 cm-2. The absorbed flux in the 0.6-100 keV energy band is 2.0 (+/-0.4) e-09 erg/cm2/s. All errors are at the 90% confidence level.

Preliminary SVOM/VT analysis does not reveal any significant variation in the VT_B band flux of the Be-star companion candidate associated with this source (Salganik et al. 2021) during the observations lasting for about 8 hours. In the VT_R band, the flux also shows no variation; however, the photometry is contaminated by a nearby star located at R.A. = 284.756792 deg, Dec. = 8.246000 deg (J2000).

The onset of the outburst was first reported by MAXI on March 24 (ATel #17730). This is the first time the source has entered an outburst state since 2015 (ATels #7034, #7037, #7045, #7425). XTE J1859+083 is a high-mass X-ray binary (HMXB) hosting an accreting neutron star, identified as an X-ray pulsar with a Be-type companion. The relatively hard spectrum and the presence of a cutoff at about 20 keV are consistent with those typically observed in accretion-powered pulsars of this class.

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.