Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The nature of optical and near-infrared variability of BL Lacertae   Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the analysis of this huge dataset has been the study of
colour variability. The massive amount of optical and near-infrared data
collected during the campaigns enables us to perform a deep analysis of
multiband data, with the aim of understanding the flux variability mechanisms.
We use a new approach for the analysis of these data, focusing on the source
spectral evolution. We show that the overall behaviour of the BL Lacertae light
and colour curves can be explained in terms of changing viewing angle of a
moving, discrete emitting region, which causes variable Doppler boosting of the
corresponding radiation. A fractal helical structure is suggested to be at the
origin of the different time scales of variability.

31
The categories of this paper are: ao-sci
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org
22
The categories of this paper are: chao-dyn
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th
5
The categories of this paper are: gr-qc
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
3
The categories of this paper are: physics
16
The categories of this paper are: stat