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Orbital ephemeris of Swift 0243.6+6124 estimated jointly with Insight-HXMT and Fermi/GBM

ATel #10907; Mingyu Ge (IHEP), ShuangNan Zhang (IHEP), FangJun Lu (IHEP), Shu Zhang (IHEP), Shanshan Weng (NJNU), Shaolin Xiong (IHEP), Yuan Liu(IHEP), Liming Song (IHEP) and HXMT-collaboration
on 30 Oct 2017; 15:36 UT
Credential Certification: Shan-Shan Weng (wengss@ihep.ac.cn)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Since the discovery of the Be/X-ray binary Swift J0243.6+6124 in an outburst showing up around 2017 October 3, a broad band observation campaign has been coordinated to monitor this source, which includes but is not limited to the joint observations from Insight-HXMT and Fermi/GBM. Accordingly, these observations resulted in the detection of a spin period of 9.86 seconds, and hence identified this system as a Neutron Star (NS) X-ray binary (ATEL #10809, #10811, #10812, #10813, #10815, #10822 and #10866). So far the ongoing monitoring from MAXI and BAT shows that the source flux is still increasing, accompanied with an obvious turn over in spin frequency, which denotes a canonical Doppler effect endorsed by the orbital motion in a NS binary system. We therefore estimate the orbital ephemeris of Swift 0243.6+6124 by using the long term observations from Insight-HXMT and Fermi/GBM. We obtained the following orbital parameters: a period ~ (26.5 +/- 1.4) days, a semi-major axis ~ (109.7 +/- 16.4) light-seconds, an eccentricity ~0.107 +/- 0.004, a periastron passage ~ 120 +/- 9 degrees and T0 ~ 58022 +/- 2(MJD).