arXiv subjects

Eduardo Ros

Publications and source records attributed to Eduardo Ros.

77 records · Page 5Linked to original sources

arXiv2007

The Flaring Corona of UX Arietis

Here we present observational results on the RS CVn star UX Arietis from four very-long-baseline interferometry observations distributed in time to cover the rotational period of 6.44 days. The data are better fit by two Gaussian components than by the usual core-plus-halo model. In the first three days the sizes of the two components did not change much from hour to hour but their relative position and orientation changed from day to day. The origin of this evolution can be explained by geometrical factors (i.e., star rotation). The fourth day a large flare occurred and dramatic changes in the sizes of the Gaussian components were seen.

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arXiv2006

The innermost regions of Active Galactic Nuclei - from radio to X-rays

Active Galactic Nuclei (AGN) can be probed by at different regions of the electromagnetic spectrum: e.g., radio observations reveal the nature of their relativistic jets and their magnetic fields, and complementarily, X-ray observations give insight into the changes in the accretion disk flows. Here we present an overview over the AGN research and results from an ongoing multi-band campaign on the active galaxy NGC1052. Beyond these studies, we address the latest technical developments and its impact in the AGN field: the Square Kilometre Array (SKA), a new radio interferometer planned for the next decade, and the oncoming X-ray and gamma-ray missions.

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arXiv2006

Cosmological evolution of compact AGN at 15 GHz

We study the uniformity of the distribution of compact flat-spectrum AGN on the sky and the evolution of their relativistic jets with cosmic epoch. A complete sample of compact extragalactic radio sources at 15 GHz was recently compiled to conduct the MOJAVE program. The MOJAVE sample comprises 133 radio-loud flat-spectrum AGN with compact relativistic outflows detected at parsec scales. The source counts of compact AGN shows that the MOJAVE sample represents a flux-limited complete sample. Analysis of the population of flat-spectrum quasars of the sample reveals that the pc-scale jets of quasars have intrinsic luminosities in the range between ~10^24 W/Hz and ~10^27 W/Hz and Lorentz factors distributed between 3 and 30. We find that the apparent speed (or Lorentz factor) of jets evolves with redshift, increasing from z~0 to z~1 and then falling at higher redshifts (z~2.5) by a factor of 2.5. The evolution of apparent speeds does not affect significantly the evolution of the beamed luminosity function of quasars, which is most likely to be dependent on the evolution of radio luminosity. Furthermore, the beamed radio luminosity function suggests that the intrinsic luminosity function of quasars has a double power-law form: it is flat at low luminosities and steep at high luminosities. There is a positive evolution of quasars at low redshifts (z<0.5) and strong negative evolution at redshifts >1.7 with space density decline up to z~2.5. This implies that the powerful jets were more populous at redshifts between 0.5 and 1.7. We show that the evolution of compact quasars is luminosity dependent and it depends strongly on the speed of the jet suggesting that there are two distinct populations of quasars with slow and fast jets which evolve differently with redshift.

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arXiv2006

An Extremely Curved Relativistic Jet in PKS 2136+141

We report the discovery of an extremely curved jet in the radio-loud quasar PKS2136+141. Multi-frequency Very Long Baseline Array (VLBA) images show a bending jet making a turn-around of 210 degrees in the plane of the sky, which is, to our knowledge, the largest ever observed change in the position angle of an astrophysical jet. Images taken at six different frequencies, from 2.3 to 43 GHz, reveal a spiral-like trajectory, which is likely a sign of an intrinsic helical geometry. A space-VLBI image, taken with the HALCA satellite at 5 GHz and having comparable resolution to our ground-based 15 GHz data, confirms that the bend is a frequency-independent structure. VLBA monitoring data at 15 GHz, covering eight years of observations, show knots in the jet clearly deviating from ballistic motion, which suggests that the bending may be caused by a growing helical Kelvin-Helmholtz normal mode. The jet appearance suggests a helical wave at a frequency well below the "resonant" frequency of the jet, which indicates that the wave is driven by a periodic perturbation at the base of the jet. We fit the observed structure in the source with a helical twist, and we find that a simple isothermal model with a constant wave speed and wavelength gives a good fit. The measured apparent velocities indicate some degree of acceleration along the jet, which together with an observed change in the apparent half-opening angle of the jet allow us to estimate the changes in the angle between the local jet direction and our line of sight. We suggest that the jet in PKS2136+141 is distorted by a helical Kelvin-Helmholtz normal mode externally driven into the jet (e.g. by precession), and that our line of sight falls within the opening angle of the helix cone.

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arXiv2003

High Precision Differential Astrometry

While conventional imaging in VLBI provides information only about the relative position between different features of a given source, phase-referenced observations can provide precise positional information with respect to an external reference. The use of differential techniques relies on the fact that most of the instrumental and propagation effects are common for radio sources sufficiently close together in the sky. These effects cancel by subtracting the visibility phases of the reference source from that of the neighboring objects. Differential VLBI astrometry yields relative positions of radio sources with sub-milliarcsecond accuracy. These high precisions can be applied to solve various astrophysical problems. In this paper I will review the progress made recently in this field, both in technique and science.

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