Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: X-ray outbursts from nearby `normal' and active galaxies. A review, new
  radio observations, and an X-ray search for further tidal disruption flares   In the last few years, giant-amplitude, non-recurrent X-ray flares have been
observed from several non-active galaxies (NGC 5905, RXJ1242-11, RXJ1624+75,
RXJ1420+53, RXJ1331-32). All of them share similar properties, namely: extreme
X-ray softness in outburst, huge peak luminosity (up to ~10^{44} erg/s), and
the absence of optical signs of Seyfert activity. Tidal disruption of a star by
a supermassive black hole is the favored explanation of these unusual events.
  We present a review of the previous results, a search for radio emission from
all outbursters, based on the NVSS database, and dedicated radio observations
of NGC 5905 carried out with the VLA. These provide important constraints on
the presence of an (obscured) active nucleus (AGN) at the center of each
flaring galaxy. We rigorously explore AGN scenarios to account for the unusual
X-ray outbursts from the optically `normal' galaxies and find AGN-related
models highly unlikely. We conclude that the previously favored scenario -
tidal disruption of a star by a supermassive black hole at the center of each
of the outbursters - provides the best explanation for the X-ray observations.
  Finally, we present results from our on-going search for further X-ray flares
from a sample of ~140 nearby active and non-active galaxies, using the ROSAT
data base. While we do not find another X-ray flaring normal galaxy among this
sample - entirely consistent with the predictions of the tidal disruption
scenario - several highly variable active galaxies are detected. Their
variability is not linked to tidal disruption, but best explained in terms of
absorption or accretion-disk-related models.

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The categories of this paper are: gr-qc
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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: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: chem-ph
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The categories of this paper are: supr-con
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The categories of this paper are: cmp-lg
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The categories of this paper are: nlin
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The categories of this paper are: solv-int
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The categories of this paper are: mtrl-th
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-lat
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The categories of this paper are: q-bio
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The categories of this paper are: hep-th
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The categories of this paper are: nucl-ex