X-ray properties of two transient ULX candidates in galaxy NGC 7090

Abstract

We report the X-ray data analysis of two transient ultraluminous X-ray sources (ULXs, hereafter X1 and X2) located in the nearby galaxy NGC 7090. While they were not detected in the 2004 XMM-Newton and 2005 Chandra observations, their 0.3-10 keV X-ray luminosities reached $>3\times 10^{39}\mathrm{erg}{\mathrm{s}}^{-1}$ in later XMM-Newton or Swift observations, showing increases in flux by a factor of $>80$ and $>300$ for X1 and X2, respectively. X1 showed indications of spectral variability: at the highest luminosity, its X-ray spectra can be fitted with a powerlaw ($\Gamma =1.55\pm 0.15$), or a multicolour disc model with $T_{\mathrm{in}}=2.07_{-0.23}^{+0.30}$ keV; the X-ray spectrum became softer ($\Gamma =2.67_{-0.64}^{+0.69}$), or cooler ($T_{\mathrm{in}}=0.64_{-0.17}^{+0.28}$ keV) at lower luminosity. No strong evidence for spectral variability was found for X2. Its X-ray spectra can be fitted with a simple powerlaw model ($\Gamma =1.61_{-0.50}^{+0.55}$), or a multicolour disc model ($1.69_{-0.48}^{+1.17}$ keV). A possible optical counterpart for X1 is revealed in HST imaging. No optical variability is found, indicating that the optical radiation may be dominated by the companion star. Future X-ray and optical observations are necessary to determine the true nature of the compact object.

Accepted for publication in MNRAS, 7 pages, 3 figures

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