SVOM/ECLAIRs likely detection of the new XTE J1946+274 outburst confirming the MAXI trigger
ATel #16719; J. Rodriguez, N. Dagoneau, S. Schanne, D. Turpin (CEA Saclay, F), L. Bouchet O. Godet, S. Guillot (IRAP, Toulouse, F), F. Cangemi, A. Coleiro (APC, Paris, F), & SVOM JSWG: Jian-Yan Wei (NAOC, China), Bertrand Cordier (CEA, F), Shuang-Nan Zhang (IHEP, CN), Stephane Basa (LAM, F), Jean-Luc Attéia (IRAP, F), Arnaud Claret (CEA, F), Zi-Gao Dai (USTC, CN), Frederic Daigne (IAP, F), Jin-Song Deng (NAOC, CN), Andrea Goldwurm (APC, F), Diego Götz (CEA, F), Xu-Hui Han (NAOC, CN), Cyril Lachaud (APC, F), En-Wei Liang (GXU, CN), Yu-Lei Qiu (NAOC, CN), Susanna Vergani (Obs. Paris, F), Jing Wang (NAOC, CN), Chao Wu (NAOC, CN), Li-Ping Xin (NAOC, CN), Bing Zhang (UNLV, CN)
on 19 Jul 2024; 08:38 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)
Subjects: X-ray, Gamma Ray, Neutron Star, Transient, Pulsar
We report on the detection of an X-ray source by the ECLAIRs telescope onboard the Space-based multi-band astronomical Variable Objects Monitor (SVOM), currently in its early commissioning phase, during an observation centered on Cyg X-1 starting on MJD 60509 at 2024-07-18T07:15:48 UTC.
The source was automatically detected by the ECLAIRs on-board trigger software for the first time at 2024-07-18T07:37:41 after 22 minutes of exposure with a SNR 9.46 in the 8-50 keV energy range, and the information was sent to ground in real-time through the SVOM VHF network. No flux can be given as the SVOM/ECLAIRs telescope is still in early commissioning stage.
The position of the source is RA=296.33, DEC=27.308 (J2000) with a statistical uncertainty of about 10 arcmin. The systematic uncertainties are currently under evaluation, but at this early stage of the mission, it is safe to consider a total error of about 30 arcmin on the position.
The same source was detected again several times in the following orbits and is still detected during a new observation of the Cyg X-1 region on 2024-07-19. The information given here are those of the first trigger.
The center of the ECLAIRs trigger position is 5.6 arcmin away from the Be-high mass X-ray binary XTE J1946+274, i.e., well within the position uncertainties. Renewed activity from the source was reported by the MAXI transient alert on 2024-07-17 (ATel #16717, see also http://maxi.riken.jp/alert/novae/0508473980/0508473980.htm ), and we thus tentatively associate the ECLAIRs trigger source to XTE J1947+274.
XTE J1946+274 is a transient HXMB hosting a 15.8s spinning neutron star with a 172d orbital period. The source underwent periods of outbursts in 1998 (discovery outburst), 2010, 2018 and 2021. It is known for showing cyclotron absorption features in its hard X-ray spectrum, and periods of spin up and spin down of the neutron star. See e.g. Devaraj et al. 2024 (MNRAS, 527, 11015).
Multi-wavelength observations of this new phase of activity are strongly encouraged.
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.