High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)

We present multi-frequency, high resolution VLBA circular polarization images of the radio source 3C 84 in the center of NGC 1275. Our images reveal a complex distribution of circular polarization in the inner parsec of the radio jet, with local levels exceeding 3% polarization, the highest yet detected with VLBI techniques. The circular polarization changes sign along the jet, making 3C 84 also the first radio jet to show both signs of circular polarization simultaneously. The spectrum and changing sign of the circular polarization indicate that it is unlikely to be purely intrinsic to the emitted synchrotron radiation. The Faraday conversion process makes a significant and perhaps dominant contribution to the circular polarization, and the observed spectrum suggests the conversion process is near saturation. The sign change in the circular polarization along the jet may result from this saturation or may be due to a change in magnetic field order after an apparent bend in the jet. From the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels of circular polarization inferred for the intra-day variable source PKS 1519-273, we suggest a connection between small spatial scales and efficient production of circular polarization.Comment: 4 pages, accepted in ApJ Letter

An Investigation of Gravitational Lensing in the Southern BL Lac PKS 0537-441

The BL-Lac family of active galaxies possess almost featureless spectra and exhibit rapid variability over their entire spectral range. A number of models have been developed to explain these extreme properties, several of which have invoked the action of microlensing by sub-stellar mass objects in a foreground galaxy; this not only introduces variability, but also amplifies an otherwise normal quasar source. Here we present recent spectroscopy and photometry of the southern BL Lac PKS 0537-441; with an inferred redshift of z~0.9 it represents one of the most distant and most luminous members of the BL Lac family. The goal of the observations was not only to confirm the redshift of PKS~0537-441, but also to determine the redshift of a putative galaxy along the line of sight to the BL-Lac; it has been proposed that this galaxy is the host of microlensing stars that account for PKS 0537-441's extreme properties. While several observations have failed to detect any extended emission in PKS 0537-441, the HST imaging data presented here indicate the presence of a galactic component, although we fail to identify any absorption features that reveal the redshift of the emission. It is also noted that PKS 0537-441 is accompanied by several small, but extended companions, located a few arcseconds from the point-like BL-Lac source. Two possibilities present themselves; either they represent true companions of PKS 0537-441, or are themselves gravitationally lensed images of more distant sources.Comment: 13 Pages with 4 Figures; Accepted for Publication by the Astrophysical Journa

Peptic and Tryptic Digestion Products as Inexpensive Culture Mediums for Routine Bacteriologic Work

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The Properties of the Radio-Selected 1Jy Sample of BL Lacertae Objects

We present new optical and near-IR spectroscopy as well as new high dynamic range, arcsecond-resolution VLA radio maps of BL Lacs from the complete radio-selected "1 Jansky" (1Jy) sample (RBLs) for which such data were not previously available. Unlike BL Lacs from the complete X-ray-selected Einstein Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but moderately luminous emission lines. And whereas nearly all XBLs have extended power levels consistent with FR-1s, more than half of the RBLs have extended radio power levels too luminous to be beamed FR-1 radio galaxies. In fact, we find evidence for and examples of three distinct mechanisms for creating the BL Lac phenomenon in the 1Jy sample: beamed FR-1s, beamed FR-2s and possibly a few gravitationally-lensed quasars. The v/v_max determined for the 1Jy sample is 0.614+/-0.047, which is markedly different from the negative evolution seen in the EMSS and other XBL samples. A correlation between logarithmic X-ray to radio flux ratio and v/v_max is observed across the EMSS and 1Jy samples, from negative evolution in the more extreme XBLs to positive evolution in the more extreme RBLs. There is evidence that the selection criteria chosen by Stickel et al. eliminates some BL Lac objects from the 1Jy sample, although how many is unknown. And several objects currently in the sample have exhibited strong emission lines in one or more epochs, suggesting they should be reclassified as FSRQs. However these selection effects cannot account for the observed discrepancy in XBL and RBL properties. From these observational properties we conclude that RBLs and XBLs cannot be related by viewing angle alone, and that RBLs are more closely related to FSRQs.Comment: 29 pages, 47 figures, accepted A

Long-term variability of radio-bright BL Lacertae objects

Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit prominent flaring. We use a sample of 24 BLOs to get a clear idea of their flaring behavior and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational time scales and the observed variability parameters determined directly from the flux curves. Also, we wanted to find out if the BLO flares adhere to the generalized shock model. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90 and 230 GHz. The structure function, discrete correlation function and Lomb-Scargle periodogram time scales, calculated in a previous study, are analyzed in more detail. We determine flare durations, rise and decay times, absolute and relative peak fluxes from the monitoring data. We find that BLOs demonstrate a wide range of variability behavior. BLOs include sources with fast and strong variability, such as OJ 287, PKS 1749+096 and BL Lac, but also sources with more rolling fluctuations like PKS 0735+178. The most extreme flares can last for up to 13 years or have peak fluxes of approximately 12 Jy in the observer's frame. When the Doppler boosting effect is taken into account, the peak flux of a flare does not depend on the duration of the flare. A rough analysis of the time lags and peak flux evolution indicates that BLO flares in the mm - cm wavelengths are high-peaking, i.e., are in the adiabatic stage. Thus, the results concur with the generalized shock model.Comment: Published in the Astronomical Journa

Evidence for the evolutionary sequence of blazars: different types of accretion flows in BL Lac objects

The limits on the mass of the black hole in 23 BL Lac objects are obtained from their luminosities of the broad emission line H\beta on the assumption that broad emission lines are emitted from clouds ionized by the radiation of the accretion disk surrounding a black hole. The distribution of line luminosity L_{H\beta} of all these BL Lac objects suggests a bimodal nature, although this cannot be statistically proven on the basis of the present, rather small sample. We found that standard thin disks are probably in the sources with L_{H\beta}>10^{41} erg s^{-1}. The central black holes in these sources have masses of 10^{8-10} M_\odot, if the matter is accreting at the rate of 0.025 {\dot M}_{\rm Edd}. For the sources with L_{H\beta}<10^{41} erg s^{-1}, the accretion flows have transited from standard thin disk type to the ADAF type. The lower limits on the mass of the black hole in these sources are in the range of 1.66-24.5\times 10^{8} M_\odot. The results support the evolutionary sequence of blazars: FSRQ\to LBL\to HBL.Comment: 5 pages, accepted for publication in ApJ Letter

Jet formation in BL Lacertae objects with different accretion modes

We estimate the masses of massive black holes in BL Lac objects from their host galaxy luminosity. The power of jets and central optical ionizing luminosity for a sample of BL Lac objects are derived from their extended radio emission and the narrow-line emission, respectively. The maximal jet power can be extracted from a standard thin accretion disk/spinning black hole is calculated as a function of dimensionless accretion rate $\dot{m}$ ($\dot{m}=\dot{M}/\dot{M}_{\rm Edd}$). Comparing with the derived jet power, we find that the accretion disks in most BL Lac objects should not be standard accretion disks. For a pure advection dominated accretion flow (ADAF), there is an upper limit on its optical continuum luminosity due to the existence of an upper limit $\dot{m}_{\rm crit}$ on the accretion rate. It is found that a pure ADAF is too faint to produce the optical ionizing luminosity of BL Lac objects derived from their narrow-line luminosity. We propose that an ADAF is present in the inner region of the disk and it becomes a standard thin disk in the outer region in most BL Lac objects, i.e., ADAF+SD(standard disk) scenario. This ADAF+SD scenario can explain both the jet power and optical ionizing continuum emission of these BL Lac objects. The inferred transition radii between the inner ADAF and outer SD are in the range of $40-150 GM_{bh}/c^2$, if the disks are accreting at the rate $\dot{m}=0.01$.Comment: accepted by Ap

The HST Survey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies

We report on a large HST imaging survey of BL Lac objects, at spatial resolution ~10 times better than previous ground-based surveys. We focus on data reduction and analysis, describing the procedures used to model the host galaxy surface brightness radial profiles. A total of 69 host galaxies were resolved out of 110 objects observed, including almost all sources at z < 0.5. We classify them morphologically by fitting with either an exponential disk or a de~Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69 cases, it is invariably the elliptical morphology. This is a very strong result given the large number of BL Lac objects, the unprecedented spatial resolution, and the homogeneity of the data set. With the present reclassification of the host galaxy of 1418+546 as an elliptical, there remain no undisputed examples of a disk galaxy hosting a BL Lac nucleus. This implies that, at 99% confidence, fewer than 7% of BL Lacs can be in disk galaxies. The apparent magnitude of the host galaxies varies with distance as expected if the absolute magnitudes are approximately the same, with a spread of +-1 mag, out to redshift z < 0.5. At larger redshifts, only 6 of 23 BL Lacs are resolved so the present data do not constrain possible luminosity evolution of the host galaxies. The collective Hubble diagram for BL Lac host galaxies and radio galaxies strongly supports their unification.Comment: Accepted for publication on ApJS. 43 pages. 10 figures. Figure 1 can also be downloaded from http://icarus.stsci.edu/~scarpa/tmp/hst_figure1.ta

Neutral Hydrogen 21cm Absorption at Redshift 0.673 towards 1504+377

We detect the 21 cm line of neutral hydrogen in absorption at a redshift of 0.673 towards the 1 Jy radio source 1504+377. The 1504+377 radio source is located toward the center of what appears to be an inclined disk galaxy at z = 0.674. The 21 cm absorption line shows multiple velocity components over a velocity range of about 100 km sec$^{-1}$, with a total HI column density: N(HI) = $3.8\times10^{19}\times({{T_s}\over{f}})$ cm$^{-2}$. The velocity-integrated optical depth of this system is the largest yet seen for redshifted HI 21 cm absorption line systems (Carilli 1995). The 21 cm absorption line is coincident in redshift with a previously detected broad molecular absorption line system (Wiklind and Combes 1996). We do not detect HI 21 cm absorption associated with the narrow molecular absorption line system at z = 0.67150, nor do we detect absorption at these redshifts by the 18 cm lines of OH, nor by the 2 cm transition of H$_2$CO. There is no evidence for a bright optical AGN in 1504+377, suggesting significant obscuration through the disk -- a hypothesis supported by the strong absorption observed. The 1504+377 system resembles the ``red quasar'' PKS 1413+135, which has been modeled as a optically obscured AGN with a very young radio jet in the center of a gas rich disk galaxy (Perlman et al. 1996). The presence of very bright radio jets at the centers of these two disk galaxies presents a challenge to unification schemes for extragalactic radio sources and to models for the formation of radio loud AGN.Comment: 17 pages, postscrip