Swift discovers an X-ray source in the neutron star binary candidate J235456.73+335625.9
ATel #15742; Mouyuan Sun (XMU), Shanshan Weng (NJNU), Junfeng Wang (XMU), Wei-Min Gu (XMU), Tuan Yi (XMU)
on 3 Nov 2022; 15:54 UT
Credential Certification: Mouyuan Sun (msun88@xmu.edu.cn)
Subjects: Optical, Ultra-Violet, X-ray, Binary, Neutron Star
We have requested Swift TOO observations on a newly identified close binary system, LAMOST J235456.73+335625.9 (hereafter J2354; RA=23h54m56.88s, DEC=33d56m25.7s), which, according to the LAMOST Doppler spectroscopic observations, is a nearby (with a Gaia DR3 distance of 127.7 pc) single-lined spectroscopic binary that hosts a compact object. The compact object is either an extremely near neutron star in binaries or a massive but cold white dwarf (Zheng et al. 2022; arXiv: 2210.04685). The Swift TOO observations were performed twice on Oct. 24, 2022, with a total exposure time of 1, 140 seconds. The Swift UV spectral energy distribution (SED) is consistent with being produced by the chromospheric activities of the visible K7 star in J2354. The corresponding XRT observations were performed in Photon Counting mode. We, for the first time, detect an X-ray source in the optical position of J2354 and the XRT count rate is 0.03 cps. Our preliminary analysis suggests that the low signal-to-noise (S/N) XRT spectrum can be marginally fitted by a very soft power law model, a blackbody model, or an APEC model. Among the three models, the power-law fit with a photon index of 2.9±0.8 (90% confidence level) is slightly better than the other two. The 0.3-2 keV absorbed flux for the power-law model is 8.0E-13 ergs/cm^2/s. During the analysis, the X-ray column density is fixed at N_H=5.6E20 cm^-2. Compared with the historic ROSAT all-sky survey non-detection, the current X-ray flux of J2354 measured by XRT appears to have increased by a factor of about 1.5. The soft X-ray emission might be arise from the compact object or the visible K7 star's corona activity. Additional Swift follow-up observations have been awarded to monitor J2354. We thank the Swift operations team for kindly scheduling this ToO observation.
