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.
Highlights
Ultraluminous X-ray sources (ULXs) are point-like off-nuclear extragalactic sources with X-ray luminosity higher than ∼ 1039 erg s−1(Fabbiano 1989; Feng & Soria 2011)
The position of X1 was obtained from the 2006 Chandra observation using the wavdetect task, which gives RA = 21h36m31s.81 and Dec. = −54◦33 57.82, within the error circle of the position measured from the XMM-Newton 2007 observation
We report the X-ray properties of two highly variable ULXs in the nearby star-forming galaxy NGC 7090
Summary
Ultraluminous X-ray sources (ULXs) are point-like off-nuclear extragalactic sources with X-ray luminosity higher than ∼ 1039 erg s−1(Fabbiano 1989; Feng & Soria 2011). 49 HLX1 (Farrell et al 2009) and M82 X-1 (Feng & Kaaret 2010; Pasham et al 2014), both with relatively high peak X-ray luminosity (LX ≥ 1041 erg s−1), are promising IMBH candidates. 300 ULX-1: Carpano et al 2018) show clear evidence that the accretors in those systems are neutron stars (NS), indicating that the apparent X-ray luminosities in those ULXs are at least ≥ 10 times the Eddington limit for a standard NS of mass 1.4M. Some ULXs, similar to the Galactic X-ray binaries (XRBs), can change their spectral state dramatically (Sutton et al 2013; Marlowe et al 2014), e.g. Holmberg IX X-1 showed a two component disc plus power-law spectrum at lower luminosity, while the spectral shape changed. We adopted a distance to NGC 7090 of 6.6 Mpc (Tully et al 1992) throughout this work
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