529 research outputs found
VLBA Imaging of NGC 4261: Symmetric Parsec-scale Jets and the Inner Accretion Region
We observed the nuclear region of NGC 4261 (3C270) with the VLBA at two
frequencies (1.6 and 8.4 GHz) simultaneously. We find that the position angle
of the parsec-scale radio axis agrees, within the errors, with the position
angle of the VLA-scale jet. Our observations also reveal basically symmetric
radio structures at both 1.6 and 8.4 GHz. Analysis of these images shows that
most of the central 10 pc of this source is not significantly affected by
free-free absorption, even though HST images show that the nucleus contains a
nearly edge-on disk of gas and dust on larger scales. Our highest angular
resolution image shows a very narrow gap in emission, which we interpret as an
absorption feature, just east of the radio core. This suggests that there may
be a small, dense inner accretion disk whose width is less than 0.1 pc. If the
inclination of this inner disk is close to that of the larger-scale HST disk it
becomes optically thin to 8.4 GHz radiation at a deprojected radius of about
0.8 pc. The brightness of the pc-scale jets falls off very rapidly on both
sides of the core, suggesting that the jets are rapidly expanding during the
the first several pc of their travel. It appears that there is a small dense
inner disk centered on the radio core (the base of the jets; less than 1 pc), a
low density bubble filling most of the the inner several pc of the nucleus
(within which the radio jets expand rapidly; ~10 pc), and a surrounding cool,
higher density region (of which the HST absorption disk is part; tens to
hundreds of pc) within which the transverse expansion of the radio jets, as
implied by the rate of decrease in jet brightness, is nearly halted.Comment: Accepted by the Astrophysical Journa
Space VLBI Observations of 3C 279 at 1.6 and 5 GHz
We present the first VLBI Space Observatory Programme (VSOP) observations of
the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and
VLBA-only images at these two frequencies maps the jet structure on scales from
1 to 100 mas. On small angular scales the structure is dominated by the quasar
core and the bright secondary component `C4' located 3 milliarcseconds from the
core (at this epoch). On larger angular scales the structure is dominated by a
jet extending to the southwest, which at the largest scale seen in these images
connects with the smallest scale structure seen in VLA images. We have
exploited two of the main strengths of VSOP: the ability to obtain
matched-resolution images to ground-based images at higher frequencies and the
ability to measure high brightness temperatures. A spectral index map was made
by combining the VSOP 1.6 GHz image with a matched-resolution VLBA-only image
at 5 GHz from our VSOP observation on the following day. The spectral index map
shows the core to have a highly inverted spectrum, with some areas having a
spectral index approaching the limiting value for synchrotron self-absorbed
radiation of 2.5. Gaussian model fits to the VSOP visibilities revealed high
brightness temperatures (>10^{12} K) that are difficult to measure with
ground-only arrays. An extensive error analysis was performed on the brightness
temperature measurements. Most components did not have measurable brightness
temperature upper limits, but lower limits were measured as high as 5x10^{12}
K. This lower limit is significantly above both the nominal inverse Compton and
equipartition brightness temperature limits. The derived Doppler factor,
Lorentz factor, and angle to the line-of-sight in the case of the equipartition
limit are at the upper end of the range of expected values for EGRET blazars.Comment: 11 pages, 6 figures, emulateapj.sty, To be published in The
Astrophysical Journal, v537, Jul 1, 200
VLBI Observations of a Complete Sample of Radio Galaxies V. 3C346 and 4C31.04: two Unusual CSS Sources
We present observations at 1.7 and 8.4 GHz of two Compact Steep Spectrum
(CSS) sources from a complete sample of low-intermediate power radio galaxies.
3C346 shows an asymmetric structure with a one-sided ``jet'' and ``hot spot''.
Present observations suggest that the classification of this source as a CSS is
inappropriate, and that it is a common radio galaxy at a small angle to the
line of sight. Its properties are in agreement with the predictions of unified
schemes models. 4C31.04 shows more complex structure with the possibility of a
centrally located flat spectrum core in between two close lobes. We suggest
that this source could be a low redshift Compact Symmetric Object.Comment: 15 pages, LATEX, uuenconde ps figures To be published in the
Astrophysical Journal, October 20th issu
Variability in the X-ray Flux of Quasar 3C345: Inverse-Compton Emission from the Parsec-Scale Jet?
We present the results of the first systematic study of variability in the X-ray emission from the 'superluminal' quasar 3C 345. Its power-law 1-keV X-ray emission varies by a factor of two on a timescale of years, but with no change in spectral index, closely following the high-frequency
radio flux. Using VLBI images, we show that one of the superluminal 'knots' in the jet (at a distance of â 15 pc from the nucleus), rather than the nucleus, produces most of the observed X-rays, via the synchrotron self-Compton process. We show that this knot accelerates as it moves away from the nucleus, along along a path at â 10° from the line of sight
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-1Mpc-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 ~1Ă1013 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 LĂ€hteenmĂ€ki & 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 GĂłmez 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 Îł-ray high states, the sparseness of the Îł-ray data prevents a statistical analysis of possible correlations
The Speed and Orientation of the Parsec-Scale Jet in 3C 279
We have calculated inverse-Compton Doppler factors for 3C 279 using the collection of VLBI data recently published by us, 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 model the jet components as homogeneous spheres and the VLBI core as an unresolved inhomogeneous conical jet. 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 the core makes a significant contribution to the X-ray emission. The parameters of the conical models indicate the jet is particle dominated at the radii that produce significant emission, and is not in equipartition. At the inner radius of the conical jet the magnetic field is of order 0.1 G and the relativistic-particle number density is of order 10 cm^{-3}. 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 (gamma=8) and 4 degrees, and an average speed and angle to the line-of-sight for C4 (at a distance from the core of 3 mas) of v=0.997c (gamma=13) and 2 degrees. These values imply average Doppler factors of delta=12 for the inner jet, and delta=21 for C4
Quasar Astrophysics with the Space Interferometry Mission
recision optical astrometry of quasars and active galaxies can provide important insight into the spatial distribution and variability of emission in compact nuclei. SIM - the Space Interferometry Mission - will be the first optical interferometer capable of precision astrometry on quasars. Although it is not expected to resolve the emission, it will be very sensitive to astrometric shifts, for objects as faint as R magnitude 20. In its wide-angle mode, SIM will yield 4 microarcsecond absolute positions, and proper motions to about 2 microarcsecond/yr. A variety of AGN phenomena are expected to be visible to SIM on these scales, including time and spectral dependence in position offsets between accretion disk and jet emission. SIM should be able to answer the following questions. Does the most compact optical emission from an AGN come from an accretion disk or from a relativistic jet? Do the relative positions of the radio core and optical photocentre of quasars used for the reference frame tie change on the timescales of their photometric variability? Do the cores of galaxies harbour binary supermassive black holes remaining from galaxy mergers? In this paper we briefly describe the operation of SIM and the quasar measurements it will make. We estimate the size of the astrometric signatures which may be expected, and we discuss prospects for using astrometry as a fundamental tool for understanding quasar nuclei
Relation between millimeter wavelengths emission and high-energy emission for active galactic nuclei
After comparing the flux densities of a sample of active galactic nuclei
detected by energetic gamma-ray experiment telescope at 90 and 230 GHz with the
-ray emissions detected by Compton Gamma Ray Observatory and x-ray
emission, a strong correlation between the emissions at the millimeter
wavelength and the -ray emission is found. The average flux density of
x-ray is almost proportional to the average flux density at the millimeter
wavelength for quasars detected by energetic gamma-ray experiment telescope,
which strongly supports the previous idea that the x-ray emissions of this kind
sources are mainly produced by Synchrotron Self-Compton process.Comment: 6 pages, Chinese Physics Letters in pres
Blazar 3C 454.3 in Outburst and Quiescence During 2005-2007: Two Variable Synchrotron Emission Peaks
We monitored the flaring blazar 3C 454.3 during 2005 June-July with the
Spitzer Infrared Spectrograph (IRS: 15 epochs), Infrared Array Camera (IRAC: 12
epochs) and Multiband Imaging Photometer (MIPS: 2 epochs). We also made Spitzer
IRS, IRAC, and MIPS observations from 2006 December-2007 January when the
source was in a low state, the latter simultaneous with a single Chandra X-ray
observation. In addition, we present optical and sub-mm monitoring data. The
2005-2007 period saw 3 major outbursts. We present evidence that the
radio-optical SED actually consists of two variable synchrotron peaks, the
primary at IR and the secondary at sub-mm wavelengths. The lag between the
optical and sub-mm outbursts may indicate that these two peaks arise from two
distinct regions along the jet separated by a distance of 0.07-5 pc. The flux
at 5-35 microns varied by a factor of 40 and the IR peak varied in frequency
from <1E13 Hz to 4E13 Hz between the highest and lowest states in 2005 and
2006, respectively. Variability was well correlated across the mid-IR band,
with no measurable lag. Flares that doubled in flux occurred on a time scale of
3 days. The IR SED peak moved to higher frequency as a flare brightened, then
returned to lower frequency as it decayed. The fractional variability amplitude
increased with frequency, which we attribute to decreasing synchrotron-self
absorption optical depth. Mid-IR flares may signal the re-energization of a
shock that runs into inhomogeneities along the pre-existing jet or in the
external medium. The synchrotron peak frequencies during each major outburst
may depend upon both the distance from the jet apex and the physical conditions
in the shocks. Variation of the Doppler parameter along a curved or helical jet
is another possibility. Frequency variability of the IR synchrotron peak may
have important consequences for the interpretation of the blazar sequence, and
the presence of a secondary peak may give insight into jet structure.Comment: 38 pages, 15 figures, submitted to ApJS, comments welcom
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