Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An 8.56 keV absorption line in the hyperluminous X-ray source in NGC
  4045: ultra-fast outflow or cyclotron line?   We report on the discovery of an absorption line at $E=8.56^{+0.05}_{-0.11}$
keV detected with a significance of $>3.3\sigma$ in the NuSTAR and XMM-Newton
spectra of a newly discovered hyperluminous X-ray source (HLX, $L_{\rm
X}>10^{41}$ ergs$^{-1}$) in the galaxy NGC 4045 at a distance of 32 Mpc. The
source was first discovered serendipitously in a Swift/XRT observation of the
galaxy, and Swift monitoring reveals a highly variable source changing by over
an order of magnitude from maximum to minimum. The origin of the absorption
line appears likely to be by highly ionized iron with a blue shift of 0.19$c$,
indicating an ultrafast outflow (UFO). However, the large equivalent width of
the line (EW$=-0.22^{+0.08}_{-0.09}$ keV) paired with the lack of other
absorption lines detected are difficult to reconcile with models. An
alternative explanation is that the line is due to a cyclotron resonance
scattering feature (CRSF), produced by the interaction of X-ray photons with
the powerful magnetic field of a neutron star.

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The categories of this paper are: hep-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: solv-int
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The categories of this paper are: econ
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The categories of this paper are: q-alg
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The categories of this paper are: gr-qc
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The categories of this paper are: nucl-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-th
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The categories of this paper are: eess
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The categories of this paper are: cond-mat
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The categories of this paper are: patt-sol
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The categories of this paper are: stat