Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Intrinsic Curvature in the X-ray Spectra of BL Lacertae Objects   We report results from {\it XMM-Newton} observations of thirteen X-ray bright
BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The
survey was designed to look for evidence of departures of the X-ray spectra
from a simple power law shape (i.e., curvature and/or line features), and to
find objects worthy of deeper study. Our data are generally well fit by
power-law models, with three cases having hard ($\Gamma<2; dN/dE \propto
E^{-\Gamma}$) spectra that indicate synchrotron peaks at $E \gsim 5$ keV.
Previous data had suggested a presence of absorption features in the X-ray
spectra of some BL Lacs. In contrast, none of these spectra show convincing
examples of line features, either in absorption or emission, suggesting that
such features are rare amongst BL Lacs, or, more likely, artifacts caused by
instrumental effects. We find significant evidence for intrinsic curvature
(steepening by $d\Gamma / d({\rm log} E) = 0.4 \pm 0.15$) in fourteen of the
seventeen X-ray spectra. This cannot be explained satisfactorily via excess
absorption, since the curvature is essentially constant from $0.5-6$ keV, an
observation which is inconsistent with the modest amounts of absorption that
would be required. We use the {\it XMM-Newton} Optical Monitor data with
concurrent radio monitoring to derive broadband spectral energy distributions
and peak frequency estimates. From these we examine models of synchrotron
emission and model the spectral curvature we see as the result of episodic
particle acceleration.

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The categories of this paper are: hep-ex
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The categories of this paper are: stat
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The categories of this paper are: nlin
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The categories of this paper are: cs
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The categories of this paper are: quant-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: alg-geom
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The categories of this paper are: adap-org
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The categories of this paper are: solv-int
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The categories of this paper are: chao-dyn
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The categories of this paper are: patt-sol
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The categories of this paper are: q-alg
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The categories of this paper are: chem-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: hep-ph