Wednesday, 3 August 2011

Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

PhD student, Hugh Garsden, postdoc, Nick Bate, and I just had a paper accepted for publication in MNRAS. As you can see by the title, we combine two separate astrophysical techniques to probe the inner regions of quasars, namely gravitational microlesing.

I don't have much time this week (too much teaching and paperwork to do), but will expand on these in the near future. In summary, microlensing accounts for the gravitational lensing due to the myriad of compact objects, be they stars, planets or black holes, as they pass through the line of sight. Unlike the relatively boring microlensing within the galactic halo, this microlensing results on beautiful and complex magnification maps;
Reverberation mapping happens when a flare from the central engine of a quasar, typically thought to be an accretion disk orbiting a supermassive black hole, propagates through surrounding clouds of high velocity material, the Broad Line Region. I pinched the below picture from Brendon Brewer's recent paper on using Bayesian methods to measure the black hole mass from reverberation mapping;
In combining these two effects, we now have a source which varies its structure over time, with the image of the broad line region changing as the flare travel through, and various regions begin selectively magnified by the microlensing. Here's an example of the BLR sources superimposed on the magnification map;
You should look at the paper to see what the result is when they are combined; it's pretty interesting! But for now, I just want to say, Well Done Hugh and Nick!!

 Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

H. Garsden, N. F. Bate, G. F. Lewis
The kinematics and morphology of the broad emission line region (BELR) of quasars are the subject of significant debate. The two leading methods for constraining BELR properties are microlensing and reverberation mapping. Here we combine these two methods with a study of the microlensing behaviour of the BELR in Q2237+0305, as a change in continuum emission (a "flare") passes through it. Beginning with some generic models of the BELR - sphere, bicones, disk - we slice in velocity and time to produce brightness profiles of the BELR over the duration of the flare. These are numerically microlensed to determine whether microlensing of reverberation mapping provides new information about the properties of BELRs. We describe our method and show images of the models as they are flaring, and the unlensed and lensed spectra that are produced. Qualitative results and a discussion of the spectra are given in this paper, highlighting some effects that could be observed. Our conclusion is that the influence of microlensing, while not strong, can produce significant observable effects that will help in differentiating the properties of BELRs.


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  2. Hi~!

    I just like to mention that regarding such a torus type's accretion disk model in microlensing simulations, please refer to "Astro-ph/0503018".

    That paper originally predicted such a "double peak event" as a prominet signature of the caustic crossing of trous source! though the details may be different from your BLR-models.

    Such a double-peak event was already observed in "OGLE+GLITP data" for QSO2237, if you refer to Astro-ph/0503018 you can see the same feature.
    Thank you^^

    Best wishes,