Edited by: Mauro D'Onofrio, Università degli Studi di Padova, Italy
Reviewed by: Giovanna Maria Stirpe, Osservatorio Astronomico di Bologna (INAF), Italy; Milan S. Dimitrijevic, Astronomical Observatory, Serbia
*Correspondence: Damien Hutsemékers
This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences
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Gravitational microlensing is a powerful tool allowing one to probe the structure of quasars on sub-parsec scale. We report recent results, focusing on the broad absorption and emission line regions. In particular microlensing reveals the intrinsic absorption hidden in the P Cygni-type line profiles observed in the broad absorption line quasar H1413+117, as well as the existence of an extended continuum source. In addition, polarization microlensing provides constraints on the scattering region. In the quasar Q2237+030, microlensing differently distorts the Hα and CIV broad emission line profiles, indicating that the low- and high-ionization broad emission lines must originate from regions with distinct kinematical properties. We also present simulations of the effect of microlensing on line profiles considering simple but representative models of the broad emission line region. Comparison of observations to simulations allows us to conclude that the Hα emitting region in Q2237+030 is best represented by a Keplerian disk.
When the light from a distant quasar passes through the gravitational field of a galaxy, it is deflected and multiple magnified images of the source are observed. In addition, stars in the lensing galaxy can act as microlenses and produce an extra magnification of some images. The collective effect of these stars generates a complex magnification pattern in the source plane that takes the form of a caustic network. Microlensing magnification varies in time due to the relative motions of the source, lens and observer, on timescales of weeks to years. High magnification events occur when caustics are close to the line of sight (e.g., Schmidt and Wambsganss,
Gravitational lensing magnification strongly depends on the Einstein radius of the system, in the sense that only sources smaller than this radius can be significantly magnified. For a typical lensed system with a lensing galaxy at redshift
Therefore, microlensing differently magnifies the various components of the quasar spectrum. By comparing the spectra of two images of a lensed quasar, one affected by microlensing and the other not, one can separate the part of the spectrum which is microlensed, that is the part coming from the most compact source, from the part of the spectrum which is not affected by microlensing and originates from a more extended region. Information on the geometry and kinematics of the quasar inner regions can thus be obtained. This is illustrated in the following sections, focusing on the broad absorption and emission line regions (respectively, BALR and BELR).
H1413+117, the cloverleaf, is a quadruply lensed broad absorption line (BAL) quasar in which a slowly varying microlensing effect lasting for ~20 years magnifies the continuum of image D, leaving the emission lines essentially unchanged (Angonin et al.,
Figure
Microlensing in the CIV line of the BAL quasar H1413+117.
The light from H1413+117 is linearly polarized and Chae et al. (
Though larger than the source of continuum, the BELR can be partly magnified by microlensing, which results in line profile deformations. Such microlensing-induced line profile deformations have been observed in several lensed quasar spectra, exhibiting various symmetric and asymmetric distortions in both low- and high-ionization lines (Richards et al.,
Making use of the MmD line profile disentangling technique, Braibant et al. (
Microlensing in the CIV and Hα lines of the quasar Q2237+030.
As illustrated in Figure
Microlensing of different BELR structures and the resulting line profile distortions. The left panel sketches two models: a rotating Keplerian disk and a biconical outflow, both seen at intermediate inclination. The location of the approaching and receding gas is indicated in blue and red, respectively. The middle panel shows the BELR superimposed on a typical caustic. At the different positions indicated by colored losanges, different subregions of the BELR are magnified by the caustic. The right panel illustrates the line profiles corresponding to the different positions of the BELR onto the caustic pattern. Microlensing of the Keplerian disk is characterized by asymmetric red/blue line profile distortions while microlensing of the biconical outflow is characterized by symmetric wings/core distortions.
Possible effects of microlensing on broad emission lines have been theoretically investigated by several authors (Nemiroff,
In Braibant et al. (
The simulations show that asymmetric distortions of broad line profiles like those reported in Braibant et al. (
The four indices μ
Examples of models of the CIV and Hα BELRs and their location on the caustic network that simultaneously fit the microlensing observables μ
These results demonstrate the potential of microlensing to probe the geometry and kinematics of the broad line regions and outflows in quasars. Constraints on the location of the scattering regions at the origin of the polarization can also be obtained. Simulations of line profile distortions show that only strong microlensing effects can produce line profile distortions allowing us to discriminate between various BELR models using single epoch data. To benefit from weaker microlensing effects, statistical analyses of larger data sets would be needed. In particular, a long-term spectrophotometric monitoring of the different images of lensed quasars would provide a real scan of their line emitting regions, with the possibility to constrain more complex models than those considered here. Microlensing can thus be a powerful and alternative approach to reverberation mapping, especially since it can be applied to high redshift quasars, with little dependence on their luminosity, and to the study of both the low- and high-ionization BLRs.
DH and DS initiated the project. LB, DH, DS, and TA contributed to the observations, data reduction and analysis. LB, DH, DS, and RG contributed to the BELR microlensing modeling. All authors contributed to the discussions. DH wrote the paper with contributions from DS. LB made the artwork in Figure
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
DH and LB acknowledge support of Belgian F.R.S.-FNRS. Support for TA is provided by project FONDECYT 11130630 and the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS.