Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 511, Issue 4, Pages 5346-5362Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stac402
Keywords
Accretion discs; stars: neutron; X-rays: binaries
Categories
Funding
- STFC [ST/N50404X/1, ST/K000861/1]
- NASA
- NASA [NAS 5-26555]
- Italian Space Agency, contract ASI/INAF [I/004/11/4]
- Italian Space Agency (ASI)
- National Institute for Astrophysics (INAF) [ASI-INAF I/037/12/0, 2017-14-H.0]
- INAF [43/2018]
- National Science Foundation of China [12073029]
- [14057]
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NGC 1313 X-2 is a known pulsating ultraluminous X-ray source suspected to contain a neutron star accreting at high rates. Two simultaneous observations using XMM-Newton and HST revealed different X-ray behaviors and no accompanying changes in optical/UV fluxes. The results suggest that the optical light is dominated by a companion star, with hints of a possible jet.
NGC 1313 X-2 is one of the few known pulsating ultraluminous X-ray sources (PULXs), and so is thought to contain a neutron star that accretes at highly super-Eddington rates. However, the physics of this accretion remains to be determined. Here, we report the results of two simultaneous XMM-Newton and HST observations of this PULX taken to observe two distinct X-ray behaviours as defined from its Swift light curve. We find that the X-ray spectrum of the PULX is best described by the hard ultraluminous regime during the observation taken in the lower flux, lower variability amplitude behaviour; its spectrum changes to a broadened disc during the higher flux, higher variability amplitude epoch. However, we see no accompanying changes in the optical/UV fluxes, with the only difference being a reduction in flux in the near-infrared (NIR) as the X-ray flux increased. We attempt to fit irradiation models to explain the UV/optical/IR fluxes but they fail to provide meaningful constraints. Instead, a physical model for the system leads us to conclude that the optical light is dominated by a companion O/B star, albeit with an IR excess that may be indicative of a jet. We discuss how these results may be consistent with the precession of the inner regions of the accretion disc leading to changes in the observed X-ray properties, but not the optical, and whether we should expect to observe reprocessed emission from ULXs.
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