A possible superorbital modulation of the pulsating ultraluminous X-ray source NGC 7793 P13
ATel #13753; Chin-Ping Hu (Kyoto U/RIKEN, Japan), A. K. H. Kong (NTHU, Taiwan), Kwan Lok Li (NTHU, Taiwan), Lupin C. C. Lin (CHEA/UNIST, Korea)
on 23 May 2020; 05:15 UT
Credential Certification: Chin-Ping Hu (m929011@astro.ncu.edu.tw)
Subjects: Optical, Ultra-Violet, X-ray, Binary, Neutron Star, Pulsar
Recently, the pulsating ultraluminous X-ray source NGC 7793 P13 has entered an X-ray faint state according to Swift X-ray observations (Soria et al. 2020, ATel #13751). The last X-ray faint state was between late-2011 and 2012. Therefore, the separation between the two faint states is about 3200 days. Here, we propose that the 3200-day separation is the superorbital period of the system.
P13 shows modulations with a period of ~65 days in both the X-ray and the optical/UV bands. Either the optical modulation period (P_opt ~ 64 days) or X-ray period (P_X ~ 65 - 67 days) can be the orbital period and the other is caused by the beating or phase drift due to geometrical effect (Hu et al, 2017, Furst et al. 2018). Both scenarios require a superorbital period likely associated with a precessing accretion disk or orbital resonance. The separation of the two faint X-ray states based on the new Swift observations is about 3200 days which is fully consistent with our predictions of 3200+/-350 days or 3400 +/- 400 days in Hu et al (2017).
A full analysis is ongoing and we encourage further optical monitoring observations to compare with the X-ray period and to investigate the stability of the orbital/superorbital periods.
