Understanding the orbital period and accretion torque in the ultra-luminous X-ray pulsar NGC 7793 P13
Abstract
Ultra-luminous X-ray pulsars (ULPs) are a new class of accreting neutron stars, emitting X-rays orders of magnitude above their Eddington limit. Currenlty only five ULPs are confirmed. Among them, NGC 7793 P13 stands out among as being relatively easily observable and as the only one with a clearly identified optical companion, a B9I supergiant. This allows us to study the binary properties in detail and shed light on the accretion mechanism in this rare source type. The pulsations of the neutron star have a period around 415ms and have been detected in all X-ray observations so far, allowing us to study the long-term spin evolution. I will present results from our 2017-2018 XMM-Newton and NuSTAR campaign on this source. Through X-ray timing, we follow the pulse period evolution and we can unambiguously measure the orbital period to be ~64d. Together with the properties of the mass donor, we describe, for the first time in a ULP, the ephemeris of the system fully. The orbital period is consistent with the optical photometric period, but significantly shorter than the periodicity of the X-ray flux (P_X ~ 66.5d). I will discuss possible explanations for this difference, like a very long super-orbital or a super-hump period. Adding new data from the end of 2018 we see that the spin-up of the source has accelerated, likely due to an increased accretion torque. I will put these findings into context with the other known ULPs and galactic sources.
- Publication:
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AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1710002F