Mon. Not. R. Astron. Soc., 455, L91-L95 (2016/January-1)
Optical counterparts of two ULXs in NGC 5474 and NGC 3627 (M 66).
AVDAN S., VINOKUROV A., FABRIKA S., ATAPIN K., AVDAN H., AKYUZ A., SHOLUKHOVA O., AKSAKER N. and VALEEV A.
Abstract (from CDS):
We identified two optical counterparts of brightest ultraluminous X-ray sources (ULXs) in galaxies NGC 5474 and NGC 3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 (MV ~ -4.5) and 25.9 (MV ~ -4.2), respectively. NGC 5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has LX ~ 1.6x1040 erg/s, the spectrum is best fitted by an absorbed power law model with a photon index Γ ~ 0.94. M66 X-1 varies in X-rays with a factor of ∼ 2.5, its maximal luminosity being 2.0x1040 erg/s with Γ ~ 1.7. Optical spectroscopy of the NGC 5474 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 NGC 5474. We find that these two ULXs have largest X-ray-to-optical ratios of LX/Lopt ∼ 7000 for NGC 5474 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 NGC 1313 X-1.
© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society (2015)
galaxies: individual: NGC 5474, NGC 3627 - X-rays: general
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