Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Multi-band optical micro-variability observations of the BL Lac object
  S4 0954+658   We have observed S4 0954+658 in the BVRI bands in 2001, and in the BI bands
in 2002. The observations resulted in almost evenly sampled light curves, 3-8
hours long, with an average sampling interval of ~5-15 min. Because of the
dense sampling and the availability of light curves in more than one optical
bands we are able to study the intra-night flux and spectral variability of the
source in detail. Significant observations were observed in all but one cases.
On average, the flux variability amplitude, on time scales of minutes/hours,
increases from ~2-3% in the I, to ~3.5-5.5% in the B band light curves. We do
not detect any flares within the individual light curves. However, there is a
possibility that the April 2001 and late May 2002 observations sample two
flares which lasted longer than ~1-3 days. The evidence is only suggestive
though, due to the limited length of the present light curves with regard to
the duration of the assumed flares. No spectral variations are detected during
the April 2001 observations. The source flux rises and decays with the same
rate, in all bands. This variability behaviour is typical of S4 0954+658, and
is attributed to geometrical effects. However, significant spectral variations
are observed in May 2002. We find that the spectrum hardens/softens as the flux
increases/decreases, respectively. Furthermore, the "hardening" rate of the
energy spectrum is faster than the rate with which the spectrum becomes
"redder" as the flux decays. We also find evidence (although of low statistical
significance) that the I band variations are delayed with respect to the B band
variations. If the May 2002 observations sample a flaring event, these results
suggest that the variations are caused by energetic processes which are
associated with the particle cooling and the source light travel time scales.

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The categories of this paper are: supr-con
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The categories of this paper are: patt-sol
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The categories of this paper are: gr-qc
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The categories of this paper are: ao-sci
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The categories of this paper are: nlin
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
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The categories of this paper are: physics
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The categories of this paper are: q-alg
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The categories of this paper are: dg-ga
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The categories of this paper are: acc-phys
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The categories of this paper are: q-fin
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The categories of this paper are: nucl-ex
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The categories of this paper are: funct-an
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph