X-Ray Spectral and Optical Properties of a Ulx in NGC 4258 (M106)

Abstract

We study the X-ray and optical properties of the ultraluminous X-ray source (ULX) X-6 in the nearby galaxy NGC 4258 (M106) based on the archival XMM-Newton, Chandra, Swift, and Hubble Space Telescope (HST) observations. The source has a peak luminosity of L X ∼ 2 × 10 39 erg sin the XMM-Newton observation of 2004 June. Consideration of the hardness ratios and spectral model parameters shows that the source seems to exhibit possible spectral variations throughout the X-ray observations. In the images from the HST/Advanced Camera for Surveys (ACS), three optical sources have been identified as counterpart candidates within the 1σ error radius of 0 .3. The brightest one has an absolute magnitude of M V ≈ −7.0 and shows extended structure. The remaining two sources have absolute magnitudes of M V ≈ −5.8 and −5.3 mag. The possible spectral types of the candidates from brightest to dimmest were determined as B6−A5, B0−A7, and B2−A3, respectively. The counterparts of the X-ray source possibly belong to a young star cluster. Neither the standard disk model nor the slim disk model provides firm evidence to determine the spectral characteristics of ULX X-6. We argue that the mass of the compact object lies in the range 10 − 15M indicating that the compact source is most likely a stellar-mass black hole.

Optical Counterparts of ULXs in Two Dwarf Galaxies: NGC 4861 and NGC 4449

“…The F 𝑥 / F 𝑜 𝑝𝑡 ratio was calculated in the range of 1.96 − 2.30 for three optical candidates of X7. These rates quite similar to other ULX's optical candidates (Feng & Kaaret 2008;Tao et al 2011;Yang et al 2011;Avdan et al 2016;Aksaker et al 2019).…”

“…The studies in the optical band showed that majority of the ULXs are found to be associated with star forming regions or star clusters (Patruno & Zampieri 2010;Grisé et al 2011;Poutanen et al 2013;Avdan et al 2016Avdan et al , 2019Egorov et al 2017;Urquhart et al 2018). These associations were supported by the counterparts of ULXs as early type OB stars due to their observed blue color (Liu et al 2004;Grisé et al 2005;Soria et al 2005;Roberts et al 2008;Poutanen et al 2013).…”

Optical counterparts of an ultraluminous X-Ray Source X-1 in NGC 2500

“…By making use of large and up to-date ground based and Hubble Space Telescope (HST), optical counterparts of about 20 ULXs have been identified (e.g. Grisé et al 2008;Yang et al 2011;Gladstone et al 2013;Motch et al 2014;Avdan et al 2016b;Urquhart & Soria 2016;Vinokurov et al 2018). Such studies give important hints to estimate the origin of optical emission, which has strong contribution from the accretion disk as well as donor star.…”

Optical Counterparts of ULXs and Their Host Environments in NGC 4490/4485

“…Furthermore, we have studied the environments around these five ULXs in the NGC4490/4485 galaxies. The correlation between ULX source positions and stellar clusters has been already noticed in the literature (see Kaaret et al 2004;Abolmasov et al 2007a;Grise et al 2012;Avdan et al 2016a). Several authors have suggested that some ULXs might have their clusters already dispersed due to their ages (Liu et al 2007;Poutanen et al 2013).…”