Optical Counterparts of Ultraluminous X-Ray Sources NGC 4559 X-10 and NGC 4395 ULX-1

Abstract

We study the optical counterparts of ultraluminous X-ray sources NGC 4559 X-10 and NGC 4395 ULX-1. Their absolute magnitudes, after taking the reddening into account, are M V ≈ −5.3 and M V ≈ −6.2, respectively. The spectral energy distribution of the NGC 4559 X-10 counterpart is well fitted by a spectrum of an F-type star, whereas NGC 4395 ULX-1 has a blue power-law spectrum. Optical spectroscopy of NGC 4395 ULX-1 has shown a broad and variable HeII λ4686 emission, which puts this object in line with all the other spectrally-studied ULXs. Using the Swift archival X-ray data for NGC 4395 ULX-1, we have found a period of 62.8 ± 2.3 days. The X-ray phase curve of the source is very similar to the precession curve of SS 433. The optical variation of the counterpart (between two accurate measurements) amounts to 0.10 mag. Analyzing the absolute magnitudes of 16 well-studied ULX counterparts one may suggest that as the original accretion rate decreases (but nevertheless remains supercritical), the optical luminosity of the wind becomes dimmer and the donor star dominates. However, an observational bias may also influence the distribution.

Discovery and analysis of a ULX nebula in NGC 3521

“…tions, astronomers have focused on detecting the donor star (e.g. Patruno & Zampieri 2008;Heida et al 2016;Vinokurov et al 2018;Qiu et al 2019). If detected, spectroscopic observations can be used to constrain the radial velocity amplitude, and hence, provided the orbital period is determined as well from those data, the black hole mass function (e.g.…”

NIR counterparts to ULXs (III): completing the photometric survey and selected spectroscopic results★

“…Detecting the donor star is the first step (e.g. Patruno & Zampieri 2008;Heida et al 2016;Vinokurov, Fabrika & Atapin 2018;Qiu et al 2019), as subsequent spectroscopic observations can be used to constrain the radial velocity amplitude of the donor star, and hence, the compact object mass function (e.g. Motch et al 2014).…”

Ultraluminous X-ray Sources

“…About 10 optical counterparts have been studied spectrally [79,80,81,76]. All of them show similar optical spectra resembling the spectra of WNL or LBV stars and also SS 433 [81].…”

“…The brightest sources NGC 6946 X-1 and NGC 7793 P-13 (the last one is ultraluminous pulsar, see below) have magnitude M V ≈ −7.5 [77,78] (SS 433 even brighter, M v ≈ −8 [26]). For the magnitudes M V −5 the number of objects rapidly decreases which can be related both to the effects of observational selection and to the physical reasons [76].…”

“…All of them are relatively faint sources with m V 21. The distribution of the absolute magnitudes has a sharp maximum at M V ≈ −6 [76]. The brightest sources NGC 6946 X-1 and NGC 7793 P-13 (the last one is ultraluminous pulsar, see below) have magnitude M V ≈ −7.5 [77,78] (SS 433 even brighter, M v ≈ −8 [26]).…”