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

TL;DR

This study identifies optical counterparts of two ultraluminous X-ray sources in NGC5474 and NGC3627, revealing their faint optical magnitudes, significant X-ray variability, and high X-ray-to-optical ratios, suggesting stellar-mass black holes with super-Eddington accretion.

Contribution

First optical counterparts of ULXs in NGC5474 and NGC3627 are characterized, providing insights into their optical faintness, variability, and high X-ray-to-optical ratios.

Findings

Optical counterparts are very faint with V magnitudes around 25.

X-ray flux varies by over two orders of magnitude for NGC5474 X-1.

High X-ray-to-optical ratios (~7000-8000) suggest stellar-mass black holes with super-Eddington accretion.

Abstract

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.