The Microlensing Properties of a Sample of 87 Lensed Quasars

Gravitational microlensing is a powerful tool for probing the physical properties of quasar accretion disks and properties of the lens galaxy such as its dark matter fraction and mean stellar mass. Unfortunately the number of lensed quasars ($\sim 90$) exceeds our monitoring capabilities. Thus, estimating their microlensing properties is important for identifying good microlensing candidates as well as for the expectations of future surveys. In this work we estimate the microlensing properties of a sample of 87 lensed quasars. While the median Einstein radius crossing time scale is 20.6 years, the median source crossing time scale is 7.3 months. Broadly speaking, this means that on $\sim 10$ year timescales roughly half the lenses will be quiescent, with the source in a broad demagnified valley, and roughly half will be active with the source lying in the caustic ridges. We also found that the location of the lens system relative to the CMB dipole has a modest effect on microlensing timescales, and in theory microlensing could be used to confirm the kinematic origin of the dipole. As a corollary of our study we analyzed the accretion rate parameters in a sub-sample of 32 lensed quasars. At fixed black hole mass, it is possible to sample a broad range of luminosities (i.e., Eddington factors) if it becomes feasible to monitor fainter lenses.Comment: 31 pages, 7 figures, 2 tables, corrected typos in Table 2, revised version accepted for publication in Ap

Proper Motions Of VLBI Lenses, Inertial Frames and The Evolution of Peculiar Velocities

Precise determinations of the image positions in quad gravitational lenses using VLBI can be used to measure the transverse velocity of the lens galaxy and the observer. The typical proper motions are $\mu$as yr$^{-1}$, so the time scale to measure the motion is ten years. By measuring the dipole of the proper motions in an ensemble of lenses we can set limits on the deviation of the inertial frame defined by the lenses from that defined by the CMB dipole and estimate the Hubble constant. The residual proper motions after subtracting the dipole probe the evolution of peculiar velocities with redshift and can be used to estimate the density parameter $\Omega_0$. For $N$ lenses, VLBI measurement accuracies of $\sigma_\theta$, and a baseline of $T$ years, we estimate that the 2$\sigma$ limit on the rms peculiar velocity of the lens galaxies is 3100 (\sigma_\theta/10\mu\{as})({yrs}/T)/N^{1/2} \kms, and that the time required for the 2--$\sigma$ limit to reach the level of the local rms peculiar velocity $v_{0,rms}$ is approximately 10 N^{-1/2} (v_{0,rms}/600\kms)(\sigma_\theta/10\mu as) years. For a ten year baseline and $N=10$ lenses we expect the 1$\sigma$ limit on the misalignment with the CMB dipole to be $\Delta \theta=20^{\circ}$ or equivalently to obtain an upper limit of $\Delta H_0 /H_0 < 0.34$.Comment: 23 pages, figures included uuencoded gzipped ps-file, submitted to the ApJ. One correction made from the original versio

Stellar Mergers Are Common

The observed Galactic rate of stellar mergers or the initiation of common envelope phases brighter than M_V=-3 (M_I=-4) is of order 0.5 (0.3)/year with 90% confidence statistical uncertainties of 0.24-1.1 (0.14-0.65) and factor of 2 systematic uncertainties. The (peak) luminosity function is roughly dN/dL L^(-1.4+/-0.3), so the rates for events more luminous than V1309 Sco (M_V=-7 mag) or V838Mon (M_V=-10 mag) are lower at r~0.1/year and 0.03/year, respectively. The peak luminosity is a steep function of progenitor mass, L M^(2-3). This very roughly parallels the scaling of luminosity with mass on the main sequence, but the transients are ~2000-4000 times more luminous at peak. Combining these, the mass function of the progenitors, dN/dM M^(-2.0+/-0.8), is consistent with the initial mass function, albeit with broad uncertainties. These observational results are also broadly consistent with the estimates of binary population synthesis models. While extragalactic variability surveys can better define the rates and properties of the high luminosity events, systematic, moderate depth (I>16 mag) surveys of the Galactic plane are needed to characterize the low luminosity events. The existing Galactic samples are only ~20% complete and Galactic surveys are (at best) reaching a typical magnitude limit of <13 mag.Comment: Submitted to MNRAS (13 pages, 6 figures, 3 tables

Gravitational Lens Time Delays in CDM

In standard CDM halo models, the time delay of a gravitational lens is determined by the cold baryon mass fraction, f, of the visible galaxy relative to the overall halo. The observed time delays in PG1115+080, SBS1520+530, B1600+434 and HE2149-2745 give Hubble constants consistent with the HST Key Project value of H0=72+/-8 km/s Mpc only if f>0.2 (1-sided 68% confidence), which is larger than the upper bound of fmax=Omega_b/Omega_0=0.15+/-0.05 estimated from the CMB. If all available baryons cool and f=fmax then the time delays imply H0=65+/-6 km/s Mpc (95% confidence). If local inventories of cold baryons, f=0.013/h70, are correct, then H0=52+/-6 km/s Mpc and the halo parameters closely match isothermal mass models. Isothermal models are also consistent with strong and weak lens studies, stellar dynamics and X-ray observations on these scales, while significantly more centrally concentrated models are not. There is a a conflict between gravitational lens time delays, the local distance scale and standard CDM halo models.Comment: Submitted to ApJ. 22 pages, 7 figure

The quiescent progenitors of four Type II-P/L supernovae

We present Large Binocular Telescope difference imaging data for the final years of four Type II-P/L supernovae progenitors. For all four, we find no significant evidence for stochastic or steady variability in the U, B, V, or R-bands. Our limits constrain variability to no more than roughly 5-10% of the expected R-band luminosities of the progenitors. These limits are comparable to the observed variability of red supergiants in the Magellanic Clouds. Based on these four events, the probability of a Type II-P/L progenitor having an extended outburst after Oxygen ignition is <37% at 90% confidence. Our observations cannot exclude short outbursts in which the progenitor returns to within ~10% of its quiescent flux on the time scale of months with no dust formation.Comment: 9 pages, 8 figures, 1 table. Accepted to MNRA