Lawrence Weintraub

BLAZARS IN THE FERMI ERA: THE OVRO 40 m TELESCOPE MONITORING PROGRAM

The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope provides an unprecedented opportunity to study gamma-ray blazars. To capitalize on this opportunity, beginning in late 2007, about a year before the start of LAT science operations, we began a large-scale, fast-cadence 15 GHz radio monitoring program with the 40 m telescope at the Owens Valley Radio Observatory. This program began with the 1158 northern (δ > –20°) sources from the Candidate Gamma-ray Blazar Survey and now encompasses over 1500 sources, each observed twice per week with about 4 mJy (minimum) and 3% (typical) uncertainty. Here, we describe this monitoring program and our methods, and present radio light curves from the first two years (2008 and 2009). As a first application, we combine these data with a novel measure of light curve variability amplitude, the intrinsic modulation index, through a likelihood analysis to examine the variability properties of subpopulations of our sample. We demonstrate that, with high significance (6σ), gamma-ray-loud blazars detected by the LAT during its first 11 months of operation vary with almost a factor of two greater amplitude than do the gamma-ray-quiet blazars in our sample. We also find a significant (3σ) difference between variability amplitude in BL Lacertae objects and flat-spectrum radio quasars (FSRQs), with the former exhibiting larger variability amplitudes. Finally, low-redshift (z < 1) FSRQs are found to vary more strongly than high-redshift FSRQs, with 3σ significance. These findings represent an important step toward understanding why some blazars emit gamma-rays while others, with apparently similar properties, remain silent.

CGRaBS: An All-Sky Survey of Gamma-Ray Blazar Candidates

We describe a uniform all-sky survey of bright blazars, selected primarily by their flat radio spectra, that is designed to provide a large catalog of likely γ-ray active galactic nuclei (AGNs). The defined sample has 1625 targets with radio and X-ray properties similar to those of the EGRET blazars, spread uniformly across the |b| > 10° sky. We also report progress toward optical characterization of the sample; of objects with known R < 23, 85% have been classified and 81% have measured redshifts. One goal of this program is to focus attention on the most interesting (e.g., high-redshift, high-luminosity,...) sources for intensive multiwavelength study during the observations by the Large Area Telescope (LAT) on GLAST.

The VLBA Imaging and Polarimetry Survey at 5 GHz

We present the first results of the VLBA Imaging and Polarimetry Survey (VIPS), a 5 GHz VLBI survey of 1,127 sources with flat radio spectra. Through automated data reduction and imaging routines, we have produced publicly available I, Q, and U images and have detected polarized flux density from 37% of the sources. We have also developed an algorithm to use each sources I image to automatically classify it as a point-like source, a core-jet, a compact symmetric object (CSO) candidate, or a complex source. Using data from the Sloan Digital Sky Survey (SDSS), we have found no significant trend between optical flux and 5 GHz flux density for any of the source categories. Using the velocity width of the H{beta} emission line and the monochromatic luminosity at 5100 to estimate the central black hole mass, M{sub BH}, we have found a weak trend between M{sub BH} and 5 GHz luminosity density for objects with SDSS spectra. Ongoing optical follow-up for all VIPS sources will allow for more detailed explorations of these issues. The mean ratio of the polarized to total 5 GHz flux density for VIPS sources with detected polarized flux density ranges from 1% to 20% with a median value of about 5%. This ratio is a factor of {approx}3 larger if only the jet components of core-jet systems are considered and is noticeably higher for relatively large core-jet systems than for other source types, regardless of which components (i.e., core, jet, or both) are considered. We have also found significant evidence that the directions of the jets in core-jet systems tend to be perpendicular to the electric vector position angles (EVPAs). The data is consistent with a scenario in which {approx}24% of the polarized core-jets have EVPAs that are anti-aligned with the directions of their jet components and which have a substantial amount of Faraday rotation. Follow-up observations at multiple frequencies will address this issue in more detail. In addition to these initial results, plans for future follow-up observations are discussed.

CHARACTERISTICS OF EGRET BLAZARS IN THE VLBA IMAGING AND POLARIMETRY SURVEY (VIPS)

We examine the radio properties of EGRET-detected blazars observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly an order of magnitude below the MOJAVE survey and most other samples that have been used to study the properties of EGRET blazars. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux density. We do find that the EGRET-detected blazars tend to have higher brightness temperatures, greater core fractions, and possibly larger than average jet opening angles. A weak correlation is also found with jet length and with polarization. All of the well-established trends can be explained by systematically larger Doppler factors in the gamma-ray-loud blazars, consistent with the measurements of higher apparent velocities found in monitoring programs carried out at radio frequencies above 10 GHz.

A 31 GHz Survey of Low-Frequency Selected Radio Sources

The 100 m Robert C. Byrd Green Bank Telescope and the 40 m Owens Valley Radio Observatory telescope have been used to conduct a 31 GHz survey of 3165 known extragalactic radio sources over 143 deg^2 of the sky. Target sources were selected from the NRAO VLA Sky Survey in fields observed by the Cosmic Background Imager (CBI); most are extragalactic active galactic nuclei (AGNs) with 1.4 GHz flux densities of 3-10 mJy. The resulting 31 GHz catalogs are presented in full online. Using a maximum-likelihood analysis to obtain an unbiased estimate of the distribution of the 1.4-31 GHz spectral indices of these sources, we find a mean 31-1.4 GHz flux ratio of 0.110 ± 0.003 corresponding to a spectral index of α = –0.71 ± 0.01 (S_ν ∝ ν^α); 9.0% ± 0.8% of sources have α > –0.5 and 1.2% ± 0.2% have α > 0. By combining this spectral-index distribution with 1.4 GHz source counts, we predict 31 GHz source counts in the range 1 mJy S_(31)) = (16.7 ± 1.7) deg^(-2)(S_(31)/1 mJy)^(–0.80±0.07). We also assess the contribution of mJy-level (S_(1.4 GHz) < 3.4 mJy) radio sources to the 31 GHz cosmic microwave background power spectrum, finding a mean power of l(l + 1)C^(src)_l /(2π) = 44 ± 14 μK^2 and a 95% upper limit of 80 μK^2 at l = 2500. Including an estimated contribution of 12 μK^2 from the population of sources responsible for the turn-up in counts below S_(1.4 GHz) = 1 mJy, this amounts to 21% ± 7% of what is needed to explain the CBI high-l excess signal, 275 ± 63 μK^2. These results are consistent with other measurements of the 31 GHz point-source foreground.