Stephen C. Unwin

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

Interferometer Observations of Subparsec-Scale Infrared Emission in the Nucleus of NGC 4151

We report novel, high angular resolution interferometric measurements that imply that the near-infrared nuclear emission in NGC 4151 is unexpectedly compact. We have observed the nucleus of NGC 4151 at 2.2 μm using the two 10 m Keck telescopes as an interferometer and find a marginally resolved source ≤0.1 pc in diameter. Our measurements rule out models in which a majority of the K-band nuclear emission is produced on scales larger than this size. The interpretation of our measurement most consistent with other observations is that the emission mainly originates directly in the central accretion disk. This implies that active galactic nucleus unification models invoking hot, optically thick dust may not be applicable to NGC 4151.

Kinematics of the Parsec-Scale Relativistic Jet in Quasar 3C 279: 1991-1997

We present results of long-term high-frequency VLBI monitoring of the relativistic jet in 3C 279, consisting of 18 epochs at 22 GHz from 1991 to 1997 and 10 epochs at 43 GHz from 1995 to 1997. Three major results of this study are apparent speeds measured for six superluminal components range from 4.8c to 7.5c (H0 = 70 km s-1 Mpc-1, q0 = 0.1), variations in the total radio flux are due primarily to changes in the VLBI core flux, and the uniform-sphere brightness temperature of the VLBI core is ~1 × 1013 K at 22 GHz after 1995, one of the highest direct estimates of a brightness temperature. If the variability brightness temperature measured for 3C 279 by Lahteenmaki & Valtaoja is an actual value and not a lower limit, then the rest-frame brightness temperature of 3C 279 is quite high and limited by inverse Compton effects rather than equipartition. The parsec-scale morphology of 3C 279 consists of a bright, compact VLBI core, a jet component (C4) that moved from ~2 to ~3.5 mas from the core during the course of our monitoring, and an inner jet that extends from the core to a stationary component, C5, at ~1 mas from the core. Component C4 followed a curved path, and we reconstruct its three-dimensional trajectory using polynomial fits to its position versus time. Component C5 faded with time, possibly due to a previous interaction with C4 similar to interactions seen in simulations by Gomez et al. Components in the inner jet are relatively short lived and fade by the time they reach ~1 mas from the core. The components have different speeds and position angles from each other, but these differences do not match the differences predicted by the precession model of Abraham & Carrara. Although VLBI components were born about six months prior to each of the two observed γ-ray high states, the sparseness of the γ-ray data prevents a statistical analysis of possible correlations.

The milliarcsecond structure of highly variable radio sources

We define a new sample of core-dominated extragalactic sources: those whose flux density has exceeded 4.5 Jy at 8 GHz at any epoch. This Variable Source Sample has 41 members. We have begun a program of VLBI studies of those members of the Variable Source Sample which are not otherwise being observed. In this paper we show first-epoch maps 12 of the 13 new objects. We discuss the morphologies and «scale lengths» (analogous to e-folding lengths) of sources in the complete Variable Source Sample to the extent these quantities are known, and compare them to the sources in the complete flux-limited sample of Pearson & Readhead (1988, ApJ, 328)

VSOP and Ground-Based VLBI Imaging of the TEV Blazar Markarian 421 at Multiple Epochs

We present 30 VLBI images of the TeV blazar Markarian 421 (1101+384) at 15 epochs spanning the time range 1994-1997 and at six different frequencies from 2.3 to 43 GHz. The imaged observations include a high-resolution 5 GHz VLBI Space Observatory Programme (VSOP) observation with the HALCA satellite on 1997 November 14; full-track Very Long Base Array (VLBA) observations from 1994 April, 1996 November, and 1997 May at frequencies between 5 and 43 GHz; six epochs of VLBA snapshot observations at frequencies between 2 and 15 GHz from Radio Reference Frame studies; and five geodetic VLBI observations at 2 and 8 GHz from the archive of the Washington VLBI Correlator Facility located at the US Naval Observatory. The dense time coverage of the images allows us to track components unambiguously in the parsec-scale jet over the observed time range. We measure the speeds of three inner jet components located between 0.5 and 5 mas from the core (0.3-3 pc projected linear distance) to be 0.19 ± 0.27c, 0.30 ± 0.07c, and -0.07 ± 0.07c (H0 = 65 km s-1 Mpc-1). If the sole 43 GHz image is excluded, all measured speeds are consistent with no motion. These speeds differ from tentative superluminal speeds measured by Zhang & Baath from three epochs of data from the early 1980s. Possible interpretations of these subluminal speeds in terms of the high Doppler factor demanded by the TeV variability of this source are discussed.

Astrometric Detection of Terrestrial Planets in the Habitable Zones of Nearby Stars with SIM PlanetQuest

ABSTRACT SIM PlanetQuest (formerly the Space Interferometry Mission) is a space‐borne Michelson interferometer for precision stellar astrometry, with a 9 m baseline, currently slated for launch in 2016. One of the principal science goals is the astrometric detection and orbital characterization of terrestrial planets in the habitable zones of nearby stars. Differential astrometry of the target star against a set of reference stars lying within 1° will allow measurement of the target star’s reflex motion with astrometric accuracy of 1 μas in a single measurement. The purpose of the present paper is to quantitatively assess SIM’s capability for detection (as opposed to characterization by orbital determination) of terrestrial planets in the habitable zones of nearby stars. Note that the orbital periods of these planets are generally shorter than the 5 year SIM mission. We formulate a “joint periodogram” as a tool for planet detection from astrometric data. For adequately sampled orbits (i.e., five or more o...

Keck Interferometer Observations of FU Orionis Objects

We present new K-band long-baseline interferometer observations of three young stellar objects of the FU Orionis class, namely, V1057 Cyg, V1515 Cyg, and Z CMa-SE, obtained at the Keck Interferometer during its commissioning science period. The interferometer clearly resolves the source of near-infrared emission in all three objects. Using simple geometric models, we derive size scales (0.5-4.5 AU) for this emission. All three objects appear significantly more resolved than expected from simple models of accretion disks tuned to fit the broadband optical and infrared spectrophotometry. We explore variations in the key parameters that are able to lower the predicted visibility amplitudes to the measured levels and conclude that accretion disks alone do not reproduce the spectral energy distributions and K-band visibilities simultaneously. We conclude that either disk models are inadequate to describe the near-infrared emission or additional source components are needed. We hypothesize that large-scale emission (tens of AU) in the interferometer field of view is responsible for the surprisingly low visibilities. This emission may arise in scattering by large envelopes believed to surround these objects.

Space Interferometry Mission

The Space Interferometry Mission (SIM) will be a 10-m Michelson interferometer in Earth-trailing solar orbit. As a part of NASAs Origins program, it will be the first space-based optical interferometer designed for precision astrometry. As well as enabling forefront astronomical science, SIM will serve as a technology pathfinder for future Origins missions, such as the Terrestrial Planet Finder (TPF) SIM is being designed for 4 microarcsecond (μas) precision absolute position measurements of stars down to 20 magnitude. Launch is currently planned for mid-2006, and the mission duration is 5 years. By searching for the astrometric `wobble signature caused by a companion to a target star, SIM will be very sensitive to sub-stellar and planetary companions. By observing stars relative to nearby reference stars, an accuracy of 1 microarcsecond (μas) in a 1-hour measurement is expected, which is sufficient to detect planets of less than about three Earth masses around the nearest stars. With precision global astrometry, SIM allows parallax distance measurements to 10% accuracy, even on the far side of the Galaxy. During its 5-year mission, SIM will address a important variety of science questions relating to the formation and dynamics of our Galaxy. In addition to astrometry, SIM will demonstrate the technique of imaging using aperture synthesis. While this is routinely used by ground-based radio telescopes, a space-based optical synthesis imaging demonstration is the first step toward a later generation of large imaging telescopes SIM will have resolution of 10 milliarcsec in the V band. It will also perform a demonstration of interferometric nulling. in which the on-axis starlight is suppressed to a level of 10 -4 , a key requirement for TPF. In this paper we outline the design of the SIM instrument and how it will be used in the search for extrasolar planets. We also briefly describe some selected topics from the SIM astrometric science program.

THE SPEED AND ORIENTATION OF THE PARSEC-SCALE JET IN 3C 279

A high degree of relativistic beaming is inferred for the jets of blazars on the basis of several lines of evidence, but the intrinsic speed and angle of the jet to the line of sight for individual sources are difficult to measure. We have calculated inverse Compton Doppler factors for 3C 279 using the collection of VLBI data (including high-resolution space VLBI data at low frequencies) recently published by us (as Wehrle et al. and Piner et al.) and the collection of multiwavelength spectra recently published by Hartman et al. From the Doppler factor and superluminal apparent speed, we then calculate the Lorentz factor and angle to the line of sight of the parsec-scale relativistic jet. We follow the method previously used by Unwin et al. for 3C 345 to model the jet components as homogeneous spheres and the VLBI core as an unresolved inhomogeneous conical jet, using K?nigls formalism. The conical jet model can be made to match both the observed X-ray emission and the VLBI properties of the core with a suitable choice of Doppler factor, implying that the core makes a significant contribution to the X-ray emission, in contrast to the situation for 3C 345, where the jet components dominated the X-ray emission. The parameters of the K?nigl models indicate that the jet is particle dominated at the radii that produce significant emission (from ~5 to 20 pc from the apex of the jet for most models) and is not in equipartition. At the inner radius of the K?nigl jet the magnetic field is of order 0.1 G and the relativistic-particle number density is of order 10 cm-3. The kinetic energy flux in the jet is of order 1046(1 + k) ergs s-1, where k is the ratio of proton to electron energy, which implies a mass accretion rate of order 0.1(1 + k)/? M? yr-1, where ? is the efficiency of conversion of mass to kinetic energy. When all components are included in the calculation, then on average the core produces about half of the X-rays, with the other half being split between the long-lived component C4 and the brightest inner-jet component. We calculate an average speed and angle to the line of sight for the region of the jet interior to 1 mas of v = 0.992c (? = 8) and ? = 4? and an average speed and angle to the line of sight for C4 (at r ? 3 mas) of v = 0.997c (? = 13) and ? = 2?. These values imply average Doppler factors of ? = 12 for the inner jet and ? = 21 for C4.

Kinematics of the parsec-scale relativistic jet in 3C 345

The paper presents two 5 GHz most recently obtained VLBI images of the nucleus of the quasar 3C 345 (being observed since 1977), which were obtained in 1989 and 1990 with 4 pc resolution. The images show a variety of morphological features in the jet, which can help to understand the dynamics of the central energy source and the transport of energy along the jet. Also presented are the analysis of the images, temporal variations, and the jet profile. The jet speed and the orientation of 3C 345 were determined from various observational constraints, within the framework of the relativistic twin-jet model. 46 refs.