J. R. Forster

Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Jet Kinematics from Bimonthly Monitoring with the Very Long Baseline Array

We present total and polarized intensity images of 15 active galactic nuclei obtained with the Very Long Baseline Array at 7 mm wavelength at 17 epochs from 1998 March to 2001 April. At some epochs the images are accompanied by nearly simultaneous polarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. Here we analyze the 7 mm images to define the properties of the jets of two radio galaxies, five BL Lac objects, and eight quasars on angular scales 0.1 mas. We determine the apparent velocities of 106 features in the jets. For many of the features we derive Doppler factors using a new method based on a comparison of the timescale of decline in flux density with the light-travel time across the emitting region. This allows us to estimate the Lorentz factors (Γ), intrinsic brightness temperatures, and viewing angles of 73 superluminal knots, as well as the opening angle of the jet for each source. The Lorentz factors of the jet flows in the different blazars range from Γ ~ 5 to 40 with the majority of the quasar components having Γ ~ 16–18, while the values in the BL Lac objects are more uniformly distributed. The brightest knots in the quasars have the highest apparent speeds, while the more slowly moving components are pronounced in the BL Lac objects. The quasars in our sample have similar opening angles and marginally smaller viewing angles than the BL Lacs. The two radio galaxies have lower Lorentz factors and wider viewing angles than the blazars. Opening angle and Lorentz factor are inversely proportional, as predicted by gasdynamical models. The brightness temperature drops more abruptly with distance from the core in the BL Lac objects than in the quasars and radio galaxies, perhaps owing to stronger magnetic fields in the former resulting in more severe synchrotron losses of the highest energy electrons. In nine sources we detect statistically meaningful deviations from ballistic motion, with the majority of components accelerating with distance from the core. In six sources we identify jet features with characteristics of trailing shocks that form behind the primary strong perturbations in jet simulations. The apparent speeds of these components increase with distance from the core, suggestive of acceleration of the underlying jet.

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.

Multiwaveband polarimetric observations of 15 active galactic nuclei at high frequencies: correlated polarization behavior

We report on multifrequency linear polarization monitoring of 15 active galactic nuclei containing highly relativistic jets with apparent speeds from ~4c to >40c. The measurements were obtained at optical, 1 mm, and 3 mm wavelengths, and at 7 mm with the Very Long Baseline Array. The data show a wide range in degree of linear polarization among the sources, from 30%, and interday polarization variability in individual sources. The polarization properties suggest separation of the sample into three groups with low, intermediate, and high variability of polarization in the core at 7 mm (LVP, IVP, and HVP, respectively). The groups are partially associated with the common classification of active galactic nuclei as radio galaxies and quasars with low optical polarization (LVP), BL Lacertae objects (IVP), and highly optically polarized quasars (HVP). Our study investigates correlations between total flux, fractional polarization, and polarization position angle at the different wavelengths. We interpret the polarization properties of the sources in the sample through models in which weak shocks compress turbulent plasma in the jet. The differences in the orientation of sources with respect to the observer, jet kinematics, and abundance of thermal matter external to the jet near the core can account for the diversity in the polarization properties. The results provide strong evidence that the optical polarized emission originates in shocks, most likely situated between the 3 and 7 mm VLBI cores. They also support the idea that the 1 mm core lies at the edge of the transition zone between electromagnetically dominated and turbulent hydrodynamic sections of the jet.

Imaging the HL Tauri Disk at λ = 2.7 Millimeters with the BIMA Array

We have obtained a subarcsecond image of the disk associated with the T Tauri star HL Tau at a wavelength of 2.7 mm using the new high-resolution capability of the BIMA Array. The disk is elongated with a deconvolved Gaussian source size of 10 ± 02 × 05 ± 02, implying a semimajor axis of 70 ± 15 AU for a distance of 140 pc; the minor axis may be unresolved. The position angle of the major axis (125° ± 10°) is orthogonal to the axis of the optical jet. The disk centroid is coincident with the VLA λ = 3.6 cm source position and nearly coincident with recent measurements of the near-infrared emission peak. The λ = 2.7 mm images, along with previous interferometric measurements at λ = 0.87 mm and flux measurements from 10 μm to 1.3 cm, are well fitted by a simple power-law disk model with a shallow radial dependence to the surface density [Σ(r) ∝ r0 to r-1], an outer radius between 90 and 160 AU, and a dust opacity law proportional to ν1.

A Dramatic Millimeter Wavelength Flare in the Gamma-Ray Blazar NRAO 530

We present 3 mm wavelength VLBI observations and up to 30 yr of monitoring at radio through submillimeter wavelengths of the gamma-ray blazar NRAO 530. We discuss the dramatic flare that tripled the millimeter wavelength flux in 1994 and 1995 following three decades of increasing activity. VLBI observations during the flare show the creation of new components in a bent jet on subparsec scales. The components move at an apparent velocity of 7c. The observations also imply expansion in a separate component with a speed greater than 26c. We apply synchrotron self-Compton models to the core and jet to determine magnetic field strengths, particle densities, and Doppler boosting factors. We are unable to provide a satisfactory physical explanation for the high apparent velocity component. The increase in millimeter and radio wavelength fluxes is correlated with a rise in the gamma-ray flux detected by EGRET. We show that gamma-ray detections do not always occur during or immediately prior to a millimeter wavelength flare, contrary to assertions in the literature. We associate the VLBI component creation with the gamma-ray activity, although not with a specific flare. We consider what may distinguish gamma-ray blazars from other variable, flat-spectrum radio objects. Our data strengthen the correlation between superluminal motion and gamma-ray emission in blazars. The distribution of jet misalignment angle for 28 EGRET sources, determined from a survey of the literature, does not differ significantly from that of other blazars. This suggests that the geometries of gamma-ray blazars do not differ from those of gamma-ray-quiet blazars on the scales sampled by VLBI.

Formaldehyde in Comets C/1995 O1 (Hale-Bopp), C/2002 T7 (LINEAR), and C/2001 Q4 (NEAT): Investigating the Cometary Origin of H2CO

Observations offormaldehyde (H2CO) have been conducted toward comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12 m telescope at 1.2 and 2 mm. Aperture synthesis maps of H2CO at 3 mm were made using the Berkeley-Illinois-Maryland Association (BIMA) interferometer toward comet Hale-Bopp. These data indicate that the production rate of H2CO is � 3:7 ; 10 28 s � 1 at � 1 AU in comet Hale-Bopp, using a simple Monte Carlo model, if a nuclear origin for the molecule is assumed. However, maps of H2CO in Hale-Bopp, in comparison with CO, show an extended distribution (rs � 15,000 km) with small-scale structure oriented roughly along the comet-Sun direction. This result suggests a source of H2CO other than the comet nucleus. The extended source of formaldehyde is probably grains composed of a mixture of silicates and organicmaterial.Theproductionrate forH2CO increasestoQ � 1:4 ; 10 29 s � 1 assuming suchanextended grain source. This value implies a Q/Q(H2O) � 1:4%, which is similar to the production rate ratio of Q/Q(H2O) � 4% derived from in situ measurements of H2CO in comet Halley. Production rates for H2CO toward comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1:4 ; 10 27 and 5:6 ; 10 26 s � 1 , respectively, modeled using the extended grain source. The spectra of H2CO measured toward comet C/2002 T7 (LINEAR) show evidence for a second velocity component, most likely arising from comet fragmentation. Subject headingg astrobiology — comets: individual (Hale-Bopp (C/1995 O1), NEAT (C/2001 Q4), LINEAR (C/2002 T7)) — radio lines: solar system — techniques: interferometric

Bima array photodissociation measurements of HCN and CS in comet Hale-Bopp (C/1995 O1)

We present single-field images, cross-correlation spectra, and autocorrelation spectra of HCN and CS from comet Hale-Bopp (C/1995 O1) observed with the BIMA array. Haser modeling of our HCN images yields scale length values that are in excellent agreement with both the theoretical prediction and our previous mosaicked-image value. On the other hand, the same method applied to CS yields a scale length that is ~10 times smaller than the commonly accepted value. Consequently, the CS fraction in comets would be increased with respect to some earlier determinations.

R Aquarii: Evidence for Differential Rotation of the SiO Maser Shell

We previously reported Very Large Array and Berkeley-Illinois-Maryland Association (BIMA) array observations that suggested rotation of the SiO maser shell surrounding the long-period variable (LPV) in the R Aquarii binary system. In the present Very Long Baseline Array (VLBA) work, we report high spatial and spectral resolution observations of the v = 1, J = 1-0, SiO maser line that confirm our previous result and further suggest that the LPV maser shell is undergoing differential rotation. The 8-34 yr range of rotational periods resulting from differential rotation of the maser shell contains the ~18 yr period reported previously. The velocity structure of the VLBA data suggests a rotation symmetry axis oriented at a position angle of ~150°. The differential rotation model can be envisioned as a series of nested thin spherical shells that have a common rotation axis; each thin shell is characterized by its radius, r, with the innermost shell rotating fastest and the outermost shell slowest, in accordance with an equatorial plane velocity law of the form v ∝ 1/2. We find that q ≈ 1.09 is necessary to approximate the VLBA data, suggesting that the differential rotation is approximately Keplerian.

BIMA array detections of HCN in comets linear (C/2002 T7) and NEAT (C/2001 Q4)

We present interferometric detections of HCN in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) array. With a 254 × 203 synthesized beam around comet LINEAR and using a variable temperature and outflow velocity (VTOV) model, we found an HCN column density of NT = (6.4 ± 2.1) × 1012 cm-2 and a production rate of Q(HCN) = (6.5 ± 2.2) × 1026 s-1, giving a production rate ratio of HCN relative to H2O of ~(3.3 ± 1.1) × 10-3 and relative to CN of ~4.6 ± 1.5. With a 213 × 175 synthesized beam around comet NEAT and using a VTOV model, we found an HCN column density of = (8.5 ± 4.5) × 1011 cm-2 and a production rate of Q(HCN) = (8.9 ± 4.7) × 1025 s-1, giving a production rate ratio of HCN relative to H2O of ~(7.4 ± 3.9) × 10-4 and relative to CN of ~0.3 ± 0.2. For both comets, the production rates relative to H2O are similar to those found in previous comet observations. For comet LINEAR, the production rate relative to CN is consistent with HCN being the primary parent species of CN, while for comet NEAT it is too low for this to be the case.

Primary Beam and Dish Surface Characterization at the Allen Telescope Array by Radio Holography

The Allen Telescope Array (ATA) is a cm-wave interferometer in California, comprising 42 antenna elements with 6-m diameter dishes. We characterize the antenna optical accuracy using two-antenna interferometry and radio holography. The distortion of each telescope relative to the average is small, with RMS differences of 1% of beam peak value. Holography provides images of dish illumination, characterizing as-built mirror surfaces. Maximal distortions across ~ 2 meter lengths appear to result from mounting stresses or solar radiation. Experimental RMS errors are 0.7 mm at night and 3 mm under worst-case solar illumination. For frequencies 4, 10, and 15 GHz, the nighttime values indicate sensitivity losses of 1, 10 and 20%, respectively. ATAs wide-bandwidth receiver permits observations over a continuous range 0.5-11.2 GHz. We probe the antenna optical gain and beam pattern stability as a function of focus position and observation frequency, concluding that ATA can produce high fidelity images over a decade of simultaneous observation frequencies. We quantify solar heating effects on antenna sensitivity and pointing accuracy. We find that during the day, observations >;5 GHz will suffer some sensitivity loss and it may be necessary to make antenna pointing corrections on a 1-2 hourly basis.