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Register a new userThe Central Parsecs of the Bright Quasar PKS 1921 - 293
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We report on a VLBA imaging study of the nearby bright southern blazar PKS 1921 - 293 (OV - 236). High resolution VLBA observations, made at four frequencies (5, 12, 15, and 43 GHz) over the period 1994 - 2000, have revealed a strongly curved jet extending out to about 50 parsecs from the presumed central engine. Two epoch VLBA observations, each simultaneously carried out at both 5 and 43 GHz, show a large position angle difference of 51 - 67 degrees between the jet emission at 5 and 43 GHz. Although the core of PKS 1921 - 293 has one of the highest brightness temperatures measured in any compact radio source, unlike other bright blazars it is not a source of gamma-ray emission. However, there is evidence in these images for superluminal motion within the central region (a few parsecs from the core) and within the north-east diffuse emission region. In all six-epoch 43 GHz images, two equally compact bright components within the central parsec are seen.
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We present a high resolution 1.6 GHz VSOP image of the southern blazar PKS 1921-293. The image shows a typical core-jet morphology, consistent with ground-based VLBI images. However, the addition of data from the space antenna has greatly improved the angular resolution (especially along the north-south direction for this source), and thus allowed us to clearly identify the core. Model fitting reveals an inner jet component ~1.5 mas north of the core. This jet feature may be moving on a common curved path connecting the jet within a few parsecs to the 10-parsec-scale jet. The compact core has a brightness temperature of 2.6*10**12 K (in the rest frame of the quasar), an indication of relativistic beaming. We analyzed the source in terms of three models, involving the inverse Compton catastrophe, an inhomogeneous relativistic jet, and the equipartition of energy between the radiating particles and the magnetic field. Our analysis of this gamma-ray-quiet blazar shows no preference to any particular one of these models.
Spectral energy distributions (SEDs) of the central few tens of parsec region of some of the nearest, most well studied, active galactic nuclei (AGN) are presented. These genuine AGN-core SEDs, mostly from Seyfert galaxies, are characterised by two main features: an IR bump with the maximum in the 2-10 micron range, and an increasing X-ray spectrum in the 1 to ~200 keV region. These dominant features are common to Seyfert type 1 and 2 objects alike. Type 2 AGN exhibit a sharp drop shortward of 2 micron, with the optical to UV region being fully absorbed, while type 1s show instead a gentle 2 micron drop ensued by a secondary, partially-absorbed optical to UV emission bump. Assuming the bulk of optical to UV photons generated in these AGN are reprocessed by dust and re-emitted in the IR in an isotropic manner, the IR bump luminosity represents >70% of the total energy output in these objects while the high energies above 20 keV are the second energetically important contribution. Galaxies selected by their warm IR colours, i.e. presenting a relatively-flat flux distribution in the 12 to 60 micron range have often being classified as AGN. The results from these high spatial resolution SEDs question this criterion as a general rule. It is found that the intrinsic shape of the IR SED of an AGN and inferred bolometric luminosity largely depart from those derived from large aperture data. AGN luminosities can be overestimated by up to two orders of magnitude if relying on IR satellite data. We find these differences to be critical for AGN luminosities below or about 10^{44} erg/s. Above this limit, AGNs tend to dominate the light of their host galaxy regardless of the aperture size used. We tentatively mark this luminosity as a threshold to identify galaxy-light- vs AGN- dominated objects.
We report new observations of the H_2O megamaser in the Seyfert 2 galaxy Mrk348. The line is redshifted by about 130 km s^-1 with respect to the systemic velocity, is extremely broad, with a FWHM of 130 km s^-1, and has no detectable high velocity components within 1500 km s^-1 on either side of the observed line. The unusual line profile led us to suspect that this source, might belong to a class of megamaser galaxies in which the amplified emission is the result of an interaction between the radio jet and an encroaching molecular cloud, rather than occurring in a circumnuclear disk. Our initial VLBA observations show that the maser emission emanates entirely from a region <0.25 pc in extent, located toward a continuum component thought to be associated with the receding jet. The very high linewidth occurring on such small spatial scales and the rapid variability indicate that the H_2O emission is likely to arise from a shocked region at the interface between the energetic jet material and the molecular gas in the cloud where the jet is boring through. The orientation of the radio jets close to the plane of the sky also results in shocks with the preferred orientation for strong masers from our vantage point. Single-dish monitoring with the Effelsberg 100m telescope showed that the line and continuum emission flared on very similar timescales. The close temporal correlation between this activity in the maser emission and the continuum flare further suggest that the masing region and the continuum hotspots are nearly equidistant from the central engine and may be different manifestations of the same dynamical events. (abridged abstract)
We report the results from observations of H30$alpha$ line emission in Sgr A West with the Submillimeter Array at a resolution of 2arcsec and a field of view of about 40arcsec. The H30$alpha$ line is sensitive to the high-density ionized gas in the minispiral structure. We compare the velocity field obtained from H30$alpha$ line emission to a Keplerian model, and our results suggest that the supermassive black hole at Sgr A* dominates the dynamics of the ionized gas. However, we also detect significant deviations from the Keplerian motion, which show that the impact of strong stellar winds from the massive stars along the ionized flows and the interaction between Northern and Eastern arms play significant roles in the local gas dynamics.
We have observed profound variability in the radio flux density of the quasar PKS 0405-385 on timescales of less than an hour; this is unprecedented amongst extragalactic sources. If intrinsic to the source, these variations would imply a brightness temperature 10^21 K, some nine orders of magnitude larger than the inverse Compton limit for a static synchrotron source, and still a million times greater than can be accommodated with bulk relativistic motion at a Lorentz factor equal to 10. The variability is intermittent with episodes lasting a few weeks to months. Our data can be explained most sensibly as interstellar scintillation of a source component which is < 5 microarcsec in size - a source size which implies a brightness temperature > 5 times 10^14 K, still far above the inverse Compton limit. Simply interpreted as a steady, relativistically beamed synchrotron source, this would imply a bulk Lorentz factor 1000.
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