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

Abstract

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 × 1039 erg s−1in 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 power law (Γ = 1.55 ± 0.15), or a multicolour disc model with $T_{\mathrm{in}}=2.07^{+0.30}_{-0.23}$ keV; the X-ray spectrum became softer ($\Gamma =2.67^{+0.69}_{-0.64}$), or cooler ($T_\mathrm{in}=0.64^{+0.28}_{-0.17}$ keV) at lower luminosity. No strong evidence for spectral variability was found for X2. Its X-ray spectra can be fitted with a simple power-law model ($\Gamma =1.61^{+0.55}_{-0.50}$), or a multicolour disc model ($1.69^{+1.17}_{-0.48}$ 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.