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

The Radio Jets and Accretion Disk in NGC 4261

The structure of AGN accretion disks on sub-parsec scales can be probed through free-free absorption of synchrotron emission from the base of symmetric radio jets. We present new VLBA observations of the nearby FR-I radio galaxy NGC 4261 at 22 and 43 GHz, and combine these with previous VLBA observations at 1.6 and 8.4 GHz to map absorption caused by an inner accretion disk. Assuming the disk is geometrically and optically thin and composed of a uniform 10^4 K plasma, the average electron density in the inner 0.1 pc is 10^3 - 10^8 cm^-3. Equating thermal gas pressure and magnetic field strength gives a disk magnetic field of 10^-4 - 10^-2 Gauss at 0.1 pc. The jet opening angle is between 0.3 and 20 degrees during the first 0.2 pc of the jet, and must be less than 5 degrees during the first 0.8 pc. We include an appendix containing expressions for a simple, optically thin, gas pressure dominated accretion disk model which may be applicable to other galaxies in addition to NGC 4261.Comment: 15 pages plus 6 postscript figures, accepted by Ap

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

Orientation and Speed of the Parsec-Scale Jet in NGC4261 (3C270)

NGC 4261 (3C 270) is an elliptical galaxy containing a 300 pc–scale nuclear disk of gas and dust imaged by the Hubble Space Telescope (HST), around a central supermassive black hole. Previous VLBI observations of NGC 4261 revealed a gap in emission in the radio counterjet, presumably due to free-free absorption in the inner parsec of the accretion disk. Here we present three 8 GHz VLBA observations of NGC 4261 that allow us to monitor the location and depth of the gap and check for motions in the jet and counterjet. The separation between the brightest peak and the gap is stable, with an upper limit to its motion of 0.01c, supporting the interpretation of the gap as absorption by an accretion disk rather than as an intrinsic jet feature. These observations span a time of order that required for orbiting material in the disk to transit the counterjet, so we are able to search for density changes (clumps) in the disk by monitoring the optical depth of the gap. The optical depth of the gap is stable to within 20% over 5 years at τ = 1.1 ± 0.1, corresponding to an electron density in the disk that is constant to within 10%. We measure an apparent speed in the jet of (0.52 ± 0.07)c. An apparent speed could not be measured for the counterjet because of a lack of identifiable features. From the apparent jet speed and the jet-to-counterjet brightness ratio, we calculate the viewing angle of the jet to be 63° ± 3° and its intrinsic speed to be (0.46 ± 0.02)c. From the inclination and position angles of the parsec-scale radio jet and outer HST disk rotation axis we calculate a difference between the parsec-scale radio jet and outer HST disk rotation axis of 12° ± 2°. Because of its well-defined HST disk and bright parsec-scale radio jet and counterjet, NGC 4261 is ideal for studying the combined disk-jet system, and this is the first case known to us in which both the inclination and position angles of both the disk and jet have been determined