Sgr A* Polarization: No ADAF, Low Accretion Rate, and Non-Thermal Synchrotron Emission

The recent detection of polarized radiation from Sgr A* requires a non-thermal electron distribution for the emitting plasma. The Faraday rotation measure must be small, placing strong limits on the density and magnetic field strength. We show that these constraints rule out advection-dominated accretion flow models. We construct a simple two-component model which can reproduce both the radio to mm spectrum and the polarization. This model predicts that the polarization should rise to nearly 100% at shorter wavelengths. The first component, possibly a black-hole powered jet, is compact, low density, and self-absorbed near 1 mm with ordered magnetic field, relativistic Alfven speed, and a non-thermal electron distribution. The second component is poorly constrained, but may be a convection-dominated accretion flow with dM/dt~10^-9 M_Sun/yr, in which feedback from accretion onto the black hole suppresses the accretion rate at large radii. The black hole shadow should be detectable with sub-mm VLBI.Comment: 4 pages, 1 figure, accepted by ApJL, several changes from submitted versio

Rounding up the wanderers: optimizing coronagraphic searches for extrasolar planets

I derive analytic scalings for coronagraphic imaging searches for extrasolar planets. I compute the efficiency of detecting planets about any given star, and from this compute dimensionless distribution functions for the detected planets as a function of planet-star distance and distance to the host stars. I find the following for blind planet surveys: (1) the optimum wavelength is between 4000-5000 Angstroms for Earth-like planets and 4200-5800 Angstroms for Jovian planets; (2) between 21-32% of the number of planets per decade of radius can be detected with an optimized survey; (3) target stars should be ranked from greatest to least by their luminosity divided by distance to the sixth or eighth power, depending on the dominant source of noise for the survey; (4) surveys targeting all main sequence stars will detect ~3 times as many planets as surveys only targeting G-type stars; and (5) stellar populations with different metallicities should have exposure times that vary with the cube of the metallicity. I apply these results to the current suite of proposed coronagraphic satellite telescopes, of which TPF-C is the most powerful, but a much smaller telescope, TOPS, may have a significant chance of detecting Earth-sized planets due to its small inner working angle and high throughput. The most significant uncertainty in these results is the noise contribution of Exo-zodiacal light. These results can be applied to designing coronagraphs, comparing proposed telescope designs, optimizing the observing strategies, determining the properties of detected planet populations, and selecting target stars.Comment: 21 pages, 7 figures, accepted for publication in MNRA

Analytic Lightcurves for Planetary Transit Searches

We present exact analytic formulae for the eclipse of a star described by quadratic or nonlinear limb darkening. In the limit that the planet radius is less than a tenth of the stellar radius, we show that the exact lightcurve can be well approximated by assuming the region of the star blocked by the planet has constant surface brightness. We apply these results to the HST observations of HD 209458, showing that the ratio of the planetary to stellar radii is 0.1207+-0.0003. These formulae give a fast and accurate means of computing lightcurves using limb-darkening coefficients from model atmospheres which should aid in the detection, simulation, and parameter fitting of planetary transits

Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution

Magnetic connections to the plunging region can exert stresses on the inner edge of an accretion disk around a black hole. We recompute the relativistic corrections to the thin-disk dynamics equations when these stresses take the form of a time-steady torque on the inner edge of the disk. The additional dissipation associated with these stresses is concentrated relatively close outside the marginally stable orbit, scaling as r to the -7/2 at large radius. As a result of these additional stresses: spin-up of the central black hole is retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs at a/M=0.94; the disk spectrum is extended toward higher frequencies; line profiles (such as Fe K-alpha) are broadened if the line emissivity scales with local flux; limb-brightening, especially at the higher frequencies, is enhanced; and the returning radiation fraction is substantially increased, up to 58%. This last effect creates possible explanations for both synchronized continuum fluctuations in AGN, and polarization rises shortward of the Lyman edge in quasars. We show that no matter what additional stresses occur, when a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical Journa

Predicting caustic crossing high magnification events in Q2237+0305

The central regions of the gravitationally lensed quasar Q2237+0305 can be indirectly resolved on nano-arcsecond scales if viewed spectrophotometricly during a microlensing high magnification event (HME). Q2237+0305 is currently being monitored from the ground (eg. OGLE collaboration, Apache Point Observatory), with the goal, among others, of triggering ground and spacecraft based target of opportunity (TOO) observations of an HME. In this work we investigate the rate of change (trigger) in image brightness that signals an imminent HME and importantly, the separation between the trigger and the event peak. In addition, we produce colour dependent model light-curves by combining high-resolution microlensing simulations with a realistic model for a thermal accretion disc source. We make hypothetical target of opportunity spectroscopic observations using our determination of the appropriate trigger as a guide. We find that if the source spectrum varies with source radius, a 3 observation TOO program should be able to observe a microlensing change in the continuum slope following a light-curve trigger with a success rate of >80%.Comment: 17 pages, 16 figures, accepted for publication in M.N.R.A.

Kepler-210: An active star with at least two planets

We report the detection and characterization of two short-period, Neptune-sized planets around the active host star Kepler-210. The host star's parameters derived from those planets are (a) mutually inconsistent and (b) do not conform to the expected host star parameters. We furthermore report the detection of transit timing variations (TTVs) in the O-C diagrams for both planets. We explore various scenarios that explain and resolve those discrepancies. A simple scenario consistent with all data appears to be one that attributes substantial eccentricities to the inner short-period planets and that interprets the TTVs as due to the action of another, somewhat longer period planet. To substantiate our suggestions, we present the results of N-body simulations that modeled the TTVs and that checked the stability of the Kepler-210 system.Comment: 8 pages, 8 Encapsulated Postscript figure

A limit on the presence of Earth-mass planets around a Sun-like star

We present a combined analysis of all publicly available, visible HST observations of transits of the planet HD 209458b. We derive the times of transit, planet radius, inclination, period, and ephemeris. The transit times are then used to constrain the existence of secondary planets in the system. We show that planets near an Earth mass can be ruled out in low-order mean-motion resonance, while planets less than an Earth mass are ruled out in interior, 2:1 resonance. We also present a combined analysis of the transit times and 68 high precision radial velocity measurements of the system. These results are compared to theoretical predictions for the constraints that can be placed on secondary planets.Comment: Accepted for publication in MNRAS. 16 pages, 9 figure