Spectroscopic monitoring of the BL Lac object AO 0235+164

Abstract

We performed optical spectroscopic monitoring of the BL Lac object AO 0235+164 (z=0.94) 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 z=0.524, the stars of which can act as gravitational micro-lenses. In this period the object was in an optically faint, though variable state, and a broad Mg II emission line was visible at all epochs. The spectroscopic analysis reveals an overall variation in the Mg II 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 SED and that is visible in the optical domain only in very faint optical states. We estimate an upper limit to the BLR size of a few light months from the historical minimum brightness level; from this we infer the maximum amplification of the Mg II line predicted by the microlensing scenario. Unless we have strongly overestimated the size of the BLR, only very massive stars could significantly magnify the broad Mg II 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 some months.

arXiv comment10 pages, 6 figures (1 jpg, 5 eps included), in press for A&A

References in corpus (18)

• Reverberation Measurements for 17 Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei
• Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei: I. Comparing the Photoionization and Reverberation Techniques
• The cosmological evolution of quasar black-hole masses
• Multi-Epoch VLBA Observations of EGRET-Detected Quasars and BL Lac Objects: Superluminal Motion of Gamma-Ray Bright Blazars
• Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity
• The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005
• The WEBT BL Lacertae Campaign 2001 and its extension
• Dust and Diffuse Interstellar Bands in the z_a = 0.524 Absorption System toward AO 0235+164
• The Influence of Gravitational Microlensing on the Broad Emission Lines of Quasars
• Spectroscopic Gravitational Lensing and Limits on the Dark Matter Substructure in Q2237+0305
• Differential Microlensing of the Continuum and Broad Emission Lines in SDSS J0924+0219, the Most Anomalous Lensed Quasar
• Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis
• A microlensing measurement of the size of the broad emission line region in the lensed QSO 2237+0305
• Harmonic QPOs and Thick Accretion Disk Oscillations in BL Lac Object AO 0235+164
• Dual-Frequency VSOP Observations of AO 0235+164
• X-ray emission from the blazar AO 0235+16: the XMM-Newton and Chandra point of view
• Gravitational microlensing of gamma-ray blazars
• VSOP monitoring of the compact BL Lac object AO 0235+164

Cited by in corpus (27)

• The transition between BL Lac objects and Flat Spectrum Radio Quasars
• The Fermi blazars divide
• From the Blazar Sequence to the Blazar Envelope: Revisiting the Relativistic Jet Dichotomy in Radio-loud AGN
• Similarity of the Optical-IR and Gamma-Ray Time Variability of Fermi Blazars
• An Optical View of BL Lacertae Objects
• Jet and accretion power in the most powerful Fermi blazars
• Gamma-Ray and Parsec-Scale Jet Properties of a Complete Sample of Blazars From the MOJAVE Program
• The Central Engines of Fermi Blazars
• A new activity phase of the blazar 3C 454.3. Multifrequency observations by the WEBT and XMM-Newton in 2007-2008
• Multi-wavelength observations of blazar AO 0235+164 in the 2008-2009 flaring state
• Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006--2007 outburst
• A Time Resolved Study of the Broad Line Region in Blazar 3C 454.3
• Multiwavelength behaviour of the blazar OJ 248 from radio to γ-rays
• Is BL Lacertae an "orphan" AGN? Multiband and spectroscopic constraints on the parent population
• Infrared properties of blazars: putting the GASP-WEBT sources into context
• X-ray Intra-day Variability of the TeV Blazar Mrk 421 with Suzaku
• Fermi Monitoring of Radio-loud Narrow Line Seyfert 1 Galaxies
• The Variability and Period Analysis for the BL Lac AO 0235+164
• A comprehensive study on the variation phenomena of AO 0235+164
• Optical variability power spectrum analysis of blazar sources on intranight timescales
• Variability and evolution of the optical polarization of a sample of gamma-ray blazars
• Are many radio-selected BL Lacs radio quasars in disguise?
• Jet Radiation Properties of 4C +49.22: from the Core to Large-Scale Knots
• Multi-band Optical Variability of the TeV Blazar PG 1553+113 in 2019
• Study of variability in long-term multiwavelength optical lightcurves of blazar AO 0235+164
• Unveiling the monster heart: unbeamed properties of blazar 4C 71.07
• Constraints on VHE gamma-ray emission of Flat Spectrum Radio Quasars with the MAGIC telescopes