2019MNRAS.486.5709L

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 x 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_in=2.07^+0.30_–0.23 keV; the X-ray spectrum became softer (Γ=2.67^+0.69_–0.64), or cooler (T_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 (Γ=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.