Quasar accretion disks: is the standard model valid?
Understanding quasar accretion is both at the frontier of new physics, and essential in understanding the driving force behind the great power of quasars and their energetic feedback onto their galactic environments. However, the accretion disks are at micro-to-nano arcsecond scales, unresolvable from Earth.
Gravitational microlensing of quasars (by stars in a foreground lensing galaxy) provides statistical information on the microarcsecond structure of the lensed quasar. By measuring the flux ratio in two of the lensed images and comparing to a lens models for the intervening galaxy, we can establish upper limits on the size of the emission region. The principle is now well-established, with variability (due to caustic-crossing events) and statistical modelling of the microlens population giving consistent results. Both techniques have shown tantalising evidence that the emission region does not fit the standard accretion disk model, being larger, and with flatter profiles (change of source size with wavelength), than expected.
However, existing studies have used ground-based data (in which separation of the quasar images from each other and from the host galaxy arc is difficult), and have been unable to probe the UV in detail. Existing multi-wavelength HST observations do not cover the UV and are split across multiple epochs, so it is impossible to eliminate variability as a cause of the observed changes in flux.
We propose contemporaneous medium-band WFC3/UVIS and broad-band WFC3/IR imaging of all known ``anomalous'' lensed quasars, in which the effects of variability are eliminated.
Our sample contains all known quadruply-imaged lensed quasars with a close image pair (a minimum and saddle point image within 0.5 arcsecond), and is selected from the CASTLES survey: http://www.cfa.harvard.edu/glensdata/. In quasars with close image pairs, the light travel time of the two images differs by much less than a day, and variability cannot cause the relative demagnification. The difference in magnification of the close image pair gives us a measurement of the size of the emitting region. We observe each quasar through 6-8 filters (in 1-2 orbits) on the WFC3 UVIS and IR channels.
Criteria for the HST observations:
- Spatial resolution to separate the close image pair (between 0.2 and 0.5 arcsec apart).
- Sensitivity to detect the dim member of the image pair.
- Spatial resolution and depth required to separate out any lensing arc from the quasar host galaxy (superceded by the above in general).
- Same epoch observations of a given quasar covering IR to UV while avoiding strong quasar emission lines.
The observations must be sufficiently deep to measure the faintest lensed quasar image. Typical exposure times are ~10-100 second. We employ a 4-point dither to return critical sampling of the PSF. Each point in our dither attains at least the minimal required depth, so we can fulfil our science objectives even allowing for problems or unusually high backgrounds, CRE's etc. in some of our images. Our fields of interest are small (a few arcseconds) and the total number of images is also small. We employ subarrays in both channels to eliminate the need for buffer dumps until the end of each visit.
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