2013MNRAS.436.3380E

We report on the discovery with Chandra of a strong modulation ( ∼ 90 per cent pulsed fraction) at ∼ 6.4h from the source CXOU J123030.3+413853 in the star-forming, low-metallicity spiral galaxy NGC 4490, which is interacting with the irregular companion NGC 4485. This modulation, confirmed also by XMM-Newton observations, is interpreted as the orbital period of a binary system. The spectra from the Chandra and XMM-Newton observations can be described by a power-law model with photon index Γ ∼ 1.5. During these observations, which span from 2000 November to 2008 May, the source showed a long-term luminosity variability by a factor of ∼ 5, between ∼ 2x10³⁸ and 1.1x10³⁹erg/s (for a distance of 8Mpc). The maximum X-ray luminosity, exceeding by far the Eddington limit of a neutron star, indicates that the accretor is a black hole. Given the high X-ray luminosity, the short orbital period and the morphology of the orbital light curve, we favour an interpretation of CXOU J123030.3+413853 as a rare high-mass X-ray binary system with a Wolf-Rayet star as a donor, similar to Cyg X-3. This would be the fourth system of this kind known in the local Universe. CXOU J123030.3+413853 can also be considered as a transitional object between high-mass X-ray binaries and ultraluminous X-ray sources (ULXs), the study of which may reveal how the properties of persistent black hole binaries evolve entering the ULX regime.