Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: X-ray properties of two transient ULX candidates in galaxy NGC 7090   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
$>3\times10^{39}\,\mathrm{erg\,s^{-1}}$ 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
($\Gamma=1.55\pm0.15$), 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 powerlaw 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.

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The categories of this paper are: hep-th
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The categories of this paper are: funct-an
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The categories of this paper are: cs
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The categories of this paper are: dg-ga
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The categories of this paper are: solv-int
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The categories of this paper are: chem-ph
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The categories of this paper are: q-bio
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The categories of this paper are: patt-sol
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: nlin
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The categories of this paper are: nucl-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: eess
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The categories of this paper are: hep-ex