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The limbs of hot stars are polarized due to electron scattering, but this polarization cancels out due to rotational symmetry. During microlensing, the star is amplified by different amounts across its surface so that the limb polarization no longer cancels out, but can be observable. Polarization can be much higher for a caustic crossing during microlensing by a binary lens than for a single lens.
We show and discuss results and prospects of high-resolution imaging of the supermassive black hole candidate Sgr A*. We also briefly review the latest observational and theoretical progress for this source. The latest millimeter-VLBI observations show compact radio emission from within a region of about 15 Schwarzschild radii. This compact component is most likely responsible for the so-called sub-mm bump in the spectrum and perhaps even for the recently discovered circular polarization discovered up to 43 GHz and some X-ray emission through synchrotron self-Compton emission. Most importantly, however, the sub-mm emission from Sgr A* opens the door to observe, for the first time, the event horizon of a black hole directly with VLBI at sub-mm wavelengths.
We show how the effects of Faraday rotation and a detailed treatment of the radiative transfer can address various aspects of the polarization problem of AGN accretion disks. In particular, we show how accretion disks might explain the rises in polarization observed blueward of the Lyman edge in several quasars.
Observations of the Einstein Cross QSO reveal that the mid-infrared region is unaffected by microlensing. Thus, the infrared emission region must be extended on a scale >1017 cm, ruling out synchrotron models, but consistent with dust emission models. Other constraints rule out starburst models. The flux ratios greatly constrain the lens model.
We present limits on transit timing variations and secondary eclipse depth variations at 8 microns with the Spitzer Space Telescope IRAC camera. Due to the weak limb darkening in the infrared and uninterrupted observing, Spitzer provides the highest accuracy transit times for this bright system, in principle providing sensitivity to secondary planets of Mars mass in resonant orbits. Finally, the transit data provides tighter constraints on the wavelength-dependent atmospheric absorption by the planet.
One of the most exciting results of the Spitzer era has been the ability to construct longitudinal brightness maps from the infrared phase variations of hot Jupiters. We presented the first such map in Knutson et al. (2007), described the mapping theory and some important consequences in Cowan & Agol (2008) and presented the first multi waveband map in Knutson et al. (2008). In these proceedings, we begin by putting these maps in historical context, then briefly describe the mapping formalism. We then summarize the differences between the complementary N-Slice and Sinusoidal models and end with some of the more important and surprising lessons to be learned from a careful analytic study of the mapping problem.
We present Spitzer 8 μm transit observations of the extrasolar planet system HD 149026b. At this wavelength, transit light curves are weakly affected by stellar limb-darkening, allowing for a simpler and more accurate determination of planetary parameters. We measure a planet-star radius ratio of Rp/R∗=0.05158±0.00077, and in combination with ground-based data and independent constraints on the stellar mass and radius, we derive an orbital inclination of i = 85°.4+0°.9−0°.8 and a planet radius of 0.755 ± 0.040 RJ. These measurements further support models in which the planet is greatly enriched in heavy elements.
Roger Blandford, Theoretical Astrophysics, Caltech, Pasadena, CA 91125, USA,
Eric Agol, Theoretical Astrophysics, Caltech, Pasadena, CA 91125, USA,
Avery Broderick, Theoretical Astrophysics, Caltech, Pasadena, CA 91125, USA,
Jeremy Heyl, Center for Astrophysics, 60 Garden St., Cambridge, MA 02173, USA,
Leon Koopmans, Theoretical Astrophysics, Caltech, Pasadena, CA 91125, USA,
Hee-Won Lee, Yonsei University, Seoul, Korea
Recent developments in the spectropolarimetric study of compact objects, specifically black holes (stellar and massive) and neutron stars are reviewed. The lectures are organized around five topics: disks, jets, outflows, neutron stars and black holes. They emphasize physical mechanisms and are intended to bridge the gap between the fundamentals of polarimetry and the phenomenology of observed cosmic sources of polarized radiation, as covered by the other lecturers. There has been considerable recent progress in spectropolarimetry from radio through optical frequencies and this is producing some unique diagnostics of the physical conditions around compact objects. It is argued that there is a great need to develop a correspondingly sensitive polarimetric capability at ultraviolet through γ-ray energies.
Spectropolarimetric observations, particularly those at radio and optical wavelengths, have played an important role in high energy astrophysics. From the discovery of synchrotron radiation to the first good evidence for AGN unification, from the polarization patterns in the coherent emission of radio pulsars to the discovery of variable, linear polarization in the absorption troughs of broad absorption line quasars, polarization studies often provide the best and sometimes the only clue we have as to the geometric disposition of the emitting elements in these diverse sources when we cannot resolve them directly.
These notes summarize lectures delivered by Roger Blandford at the XII Canary Islands Winter School on Astrophysical Spectropolarimetry.
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