Possible Detection of Apparent Superluminal inward motion in Markarian 421 after the Giant X-ray flare in February, 2010

We report on the VLBI follow-up observations using the Japanese VLBI Network (JVN) array at 22 GHz for the largest X-ray flare of TeV blazar Mrk 421 that occurred in mid-February, 2010. The total of five epochs of observations were performed at intervals of about 20 days between March 7 and May 31, 2010. No new-born component associated with the flare was seen directly in the total intensity images obtained by our multi-epoch VLBI observations. However, one jet component located at ~1 mas north-west from the core was able to be identified, and its proper motion can be measured as -1.66+/-0.46 mas yr^-1, which corresponds to an apparent velocity of -3.48+/-0.97 c. Here, this negative velocity indicates that the jet component was apparently moving toward the core. As the most plausible explanation, we discuss that the apparent negative velocity was possibly caused by the ejection of a new component, which could not be resolved with our observations. In this case, the obtained Doppler factor of the new component is around 10 to 20, which is consistent with the ones typically estimated by model fittings of spectral energy distribution for this source.Comment: 9 pages, 6 figures, 3 tables, accepted for publication in Ap

Observations of OJ 287 from the Geodetic VLBI Archive of the Washington Correlator

We present 27 geodetic VLBI maps of OJ 287 obtained from the archive of the Washington correlator. The observations presented here were made between 1990 October and 1996 December. During this period a sequence of six superluminal components has been identified. We measured the proper motion of these components to be approximately 0.5 mas/yr, which is about twice as high as that seen in previous VLBI observations. These results imply a higher component ejection rate than previously observed, in good agreement with the observed occurrences of radio outbursts. We have examined a possible connection between VLBI components and optical flares in the framework of a binary black hole system.Comment: 9 pages, 5 included figures, emulateapj.sty, accepted to The Astrophysical Journa

The blazar-like radio structure of the TeV source IC310

Context. The radio galaxy IC310 in the Perseus cluster has recently been detected in the gamma-ray regime at GeV and TeV energies. The TeV emission shows time variability and an extraordinarily hard spectrum, even harder than the spectrum of the similar nearby gamma-ray emitting radio galaxy M87. Aims. High-resolution studies of the radio morphology help to constrain the geometry of the jet on sub-pc scales and to find out where the high-energy emission might come from. Methods. We analyzed May 2011 VLBA data of IC310 at a wavelength of 3.6 cm, revealing the parsec-scale radio structure of this source. We compared our findings with more information available from contemporary single-dish flux density measurements with the 100-m Effelsberg radio telescope. Results. We have detected a one-sided core-jet structure with blazar-like, beamed radio emission oriented along the same position angle as the kiloparsec scale radio structure observed in the past by connected interferometers. Doppler-boosting favoritism is consistent with an angle of theta < 38 degrees between the jet axis and the line-of-sight, i.e., very likely within the boundary dividing low-luminosity radio galaxies and BL Lac objects in unified schemes. Conclusions. The stability of the jet orientation from parsec to kiloparsec scales in IC310 argues against its classification as a headtail radio galaxy; i.e., there is no indication of an interaction with the intracluster medium that would determine the direction of the tail. IC310 seems to represent a low-luminosity FRI radio galaxy at a borderline angle to reveal its BL Lac-type central engine.Comment: 5 pages, 3 figures (1 color); A&A, accepte

MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration

We discuss acceleration measurements for a large sample of extragalactic radio jets from the MOJAVE program which studies the parsec-scale jet structure and kinematics of a complete, flux-density-limited sample of Active Galactic Nuclei (AGN). Accelerations are measured from the apparent motion of individual jet features or "components" which may represent patterns in the jet flow. We find that significant accelerations are common both parallel and perpendicular to the observed component velocities. Parallel accelerations, representing changes in apparent speed, are generally larger than perpendicular acceleration that represent changes in apparent direction. The trend for larger parallel accelerations indicates that a significant fraction of these changes in apparent speed are due to changes in intrinsic speed of the component rather than changes in direction to the line of sight. We find an overall tendency for components with increasing apparent speed to be closer to the base of their jets than components with decreasing apparent speed. This suggests a link between the observed pattern motions and the underlying flow which, in some cases, may increase in speed close to the base and decrease in speed further out; however, common hydro-dynamical processes for propagating shocks may also play a role. About half of the components show "non-radial" motion, or a misalignment between the component's structural position angle and its velocity direction, and these misalignments generally better align the component motion with the downstream emission. Perpendicular accelerations are closely linked with non-radial motion. When observed together, perpendicular accelerations are usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa

In the Shadow of the Accretion Disk: Higher Resolution Imaging of the Central Parsec in NGC 4261

The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption (by ionized gas in the disk) of background emission from a radio counterjet. We report high angular resolution VLBI observations of the nearby (about 40 Mpc) radio galaxy NGC 4261 that confirm free-free absorption of radio emission from a counterjet by a geometrically thin, nearly edge-on disk at 1.6, 4.8, and 8.4 GHz. The angular width and depth of the absorption appears to increase with decreasing frequency, as expected. We derive an average electron density of ~10E4 per cc at a disk radius of about 0.2 pc, assuming that the inner disk inclination and opening angles are the same as at larger radii. Pressure balance between the thermal gas and the magnetic field in the disk implies an average field strength of 0.1 milligauss at a radius of 0.2 pc. These are the closest-in free-free absorption measurements to date of the conditions in an extragalactic accretion disk orbiting a black hole with a well-determined mass. If a standard advection-dominated accretion flow exists in the disk center, then the transition between thin and thick disk regions must occur at a radius less than 0.2 pc (4000 Schwarzschild radii).Comment: 20 pages including 12 figures. Accepted for publication in Ap

Observations of BL Lacertae from the Geodetic VLBI Archive of the Washington Correlator

We present maps of BL Lac obtained from geodetic VLBI data from the archive of the Washington correlator. The observations were made from 1996 March to 1996 November, with periods from one experiment to another as short as 1 month. The dominant structure of the maps is given by a superluminal component (C2) moving predominantly at position angle ~190°. At the later epochs a new superluminal component (C3) emerges from the core and moves along a trajectory at a higher position angle (200°). We also include maps from observations made in 1995 June and August showing the main component (C2) in the early stages of its evolution, as well as an older component (C1). The position angle of the component C1 is nearly the same as that of the new component C3. The component C2 shows indications of nonradial motion, which is discussed in terms of a helically distorted jet. The best fit was obtained for a half-opening angle of the jet of 2.6d and an angle of the helical axis to the line of sight of 17°

Quantitative Nanofriction Characterization of Corrugated Surfaces by Atomic Force Microscopy

Atomic Force Microscopy (AFM) is a suitable tool to perform tribological characterization of materials down to the nanometer scale. An important aspect in nanofriction measurements of corrugated samples is the local tilt of the surface, which affects the lateral force maps acquired with the AFM. This is one of the most important problems of state-of-the-art nanotribology, making difficult a reliable and quantitative characterization of real corrugated surfaces. A correction of topographic spurious contributions to lateral force maps is thus needed for corrugated samples. In this paper we present a general approach to the topographic correction of AFM lateral force maps and we apply it in the case of multi-asperity adhesive contact. We describe a complete protocol for the quantitative characterization of the frictional properties of corrugated systems in the presence of surface adhesion using the AFM.Comment: 33 pages, 9 figures, RevTex 4, submitted to Journal of Applied Physic

Observations of OJ 287 from the Geodetic-VLBI Archive of the Washington Correlator

We present 27 geodetic VLBI maps of OJ 287 obtained from the archive of the Washington correlator. The observations presented here were made between 1990 October and 1996 December. During this period a sequence of six superluminal components has been identified. We measured the proper motion of these components to be approximately 0.5 mas yr-1, which is about twice as high as that seen in previous VLBI observations. These results imply a higher component ejection rate than previously observed, in good agreement with the observed occurrences of radio outbursts. We have examined a possible connection between VLBI components and optical flares in the framework of a binary black hole system

Chandra Discovery of a 100 kpc X-ray Jet in PKS 0637--752

The quasar PKS 0637-753, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous (~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extra-nuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.Comment: 5 Pages, 2 Figures. Submitted to Ap. J. Letter