Optical counterparts of two ULXs in NGC5474 and NGC3627 (M66)

• Published in: arXiv.org 2015-10
• Main Authors: Avdan, S, Vinokurov, A, Fabrika, S, Atapin, K, Avdan, H, Akyuz, A, Sholukhova, O, Aksaker, N, Valeev, A
• Format: Article
• Language: English
• Subjects: Accretion disks; Black holes; Deposition; Galactic clusters; Galaxies; Hubble Space Telescope; Luminosity; Optical counterparts (astronomy); Space telescopes; Spectrum analysis; X ray sources; X-rays
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1510.07915

We identified two optical counterparts of brightest ultraluminous X-ray Sources (ULXs) in galaxies NGC5474 and NGC3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 (\(M_V \approx -4.5\)) and 25.9 (\(M_V \approx -4.2\)), respectively. NGC5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has \(L_X \approx 1.6 \times 10^{40}\) erg/s, the spectrum is best fitted by an absorbed power-law model with a photon index \(\Gamma \approx 0.94\). M66 X-1 varies in X-rays with a factor of ~2.5, its maximal luminosity being \(2.0 \times 10^{40}\) erg/s with \(\Gamma \approx 1.7\). Optical spectroscopy of the NGC5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z=0.359) and a new young cluster of NGC5474. We find that these two ULXs have largest X-ray-to-optical ratios of \(L_X/L_{opt}\) ~ 7000 for NGC5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC1313 X-1.