Issue |
A&A
Volume 464, Number 3, March IV 2007
|
|
---|---|---|
Page(s) | 871 - 878 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20066599 | |
Published online | 11 January 2007 |
Spectroscopic monitoring of the BL Lac object AO 0235+164*
1
INAF, Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy e-mail: raiteri@to.astro.it
2
Landessternwarte Heidelberg-Königstuhl, Königstuhl 69117 Heidelberg, Germany
3
INAF-Telescopio Nazionale Galileo, Apartado 565, 38700 Santa Cruz de La Palma, Spain
Received:
19
October
2006
Accepted:
11
December
2006
Aims.Spectroscopic monitoring of BL Lac objects is a difficult task that nonetheless can provide important information on the different components of the active galactic nucleus.
Methods.We performed optical spectroscopic monitoring of the BL Lac object
AO 0235+164 () with the VLT and TNG telescopes from Aug. 2003
to Dec. 2004, during an extended WEBT campaign. The flux of this source
is both contaminated and absorbed by a foreground galactic
system at
, the stars of which can act as gravitational
micro-lenses.
Results.In this period the object was in an optically faint, though variable state, and
a broad emission line was visible at all epochs.
The spectroscopic analysis reveals an overall variation in the
line flux
of a factor 1.9, while the corresponding continuum flux density changed by a factor 4.3.
Most likely, the photoionising radiation can be identified with the emission
component that was earlier recognised to be present as a UV-soft-X-ray bump in the source
spectral energy distribution and that is visible in the optical domain
only in very faint optical states.
We estimate an upper limit to the broad line region (BLR) size of a few light months
from the historical minimum brightness level; from this we infer the maximum amplification
of the
line predicted by the microlensing scenario.
Conclusions.Unless we have strongly overestimated the size of the BLR, only very massive stars could
significantly magnify the broad emission line, but the time scale of variations
due to these (rare) events would be of several years. In contrast, the continuum flux,
coming from much smaller emission regions in the jet, could be affected by microlensing
from the more plausible MACHO deflectors, with variability time scales of the order of
some months.
Key words: galaxies: active / galaxies: BL lacertae objects: general / galaxies: BL lacertae objects: individual: AO 0235+164 / gravitational lensing
Based on observations collected at the European Southern Observatory, Chile (ESO Programme 71.A-0174), and on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
© ESO, 2007
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