AAS Nova

Supermassive Black Hole Through a Magnifying Glass

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Q2237+0305
The distant quasar Q2237+0305 is gravitationally lensed by a foreground galaxy. This image, taken with the Nordic Optical Telescope, contains both the lensing foreground galaxy and the four images it produces of the quasar (clustered at the center). Angular separation between the upper and lower images of the quasar is 1.6". [IAC Gravitational Lenses Group]

What happens when light from a distant quasar — powered by a supermassive black hole — is bent not only by a foreground galaxy, but also by individual stars within that galaxy’s nucleus? The neighborhood of the central black hole can be magnified, and we get a close look at the inner regions of its accretion disk!

What is Microlensing?

Our view of Q2237+0305 is heavily affected by a process called gravitational lensing. As evidenced by the four copies of the quasar in the image above, Q2237+0305 undergoes macrolensing, wherein the gravity of a massive foreground galaxy pulls on the light of a background object, distorting the image into arcs or multiple copies.

But Q2237+0305 also undergoes an effect called microlensing. Due to the fortuitous alignment of Q2237+0305 with the nucleus of the foreground galaxy lensing it, stars within the foreground galaxy pass in front of the quasar images. As a star passes, its own gravitational pull also affects the light of the image, causing the image to brighten and/or magnify.

How can we tell the difference between intrinsic brightening of Q2237+0305 and brightening due to microlensing? Brightening that occurs in all four images of the quasar is intrinsic. But if the brightening occurs in only one image, it must be caused by microlensing of that image. The timescale of this effect, which depends on how quickly the foreground galaxy moves relative to the background quasar, is on the order of a few hundred days for Q2237+0305.

Resolving Structure

The light curve of a microlensed image can reveal information about the structure of the distant object. For this reason, a team of scientists led by Evencio Mediavilla (Institute of Astrophysics of the Canaries, University of La Laguna) has studied the light curves of three independent microlensing events of Q2237+0305 images.

double-peaked light curve

Average light curve of the three microlensing events near the peak brightness. The double-peaked structure may be due to light from the innermost region of the quasar’s accretion disk. [Mediavilla et al. 2015]

Mediavilla and collaborators find a two-peaked structure in the light curves. Modeling the data as a standard thin disk that has been lensed, the team shows that the light curve features are consistent with fine structure relatable to the region near the quasar’s central supermassive black hole. The authors find that the diameter of the fine structure is ~6 Schwarzschild radii, leading them to believe that this structure actually represents the innermost region of the accretion disk.

If the team’s models are correct, this represents the first direct measurement of the size of the innermost region of a quasar’s accretion disk. The authors encourage further monitoring of Q2237+0305: as stars within the dense foreground nucleus continue to pass in front of the quasar, many more microlensing events should be observable, allowing further analysis.

Citation

E. Mediavilla et al 2015 ApJ 814 L26. doi:10.1088/2041-8205/814/2/L26