arXiv subjects

E. Ros

Publications and source records attributed to E. Ros.

At least 199 records · Page 11Linked to original sources

arXiv2002

Superluminal Motion and Relativistic Beaming in Blazar Jets

High resolution radio observations remain the most direct way to study the formation and evolution of radio jets associated with the accretion onto massive black holes. We report preliminary results of our seven year VLBA observational program to understand the nature of relativistic beaming in blazars and the surrounding environment of massive black holes. Most blazars show an apparent outward flow away from an active core. However, in a few sources the motion appears inward, most likely the result of projection of a curved trajectory which bends back toward along the line of sight. The apparent motion of jet features is not always oriented along the direction separating the feature from the core, and in a few cases we have observed a clear change in the direction and velocity of a feature as it flows along the jet. In other sources, the motion appears to follow a simple ballistic trajectory. We find no simple relation between the time scales of flux density changes and apparent component velocities.

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arXiv2002

The VLBA 2cm Survey: Kinematics of pc-Scale Structures in Active Galactic Nuclei

The kinematics of jets in active galactic nuclei (QSOs, BL Lacs, Radio Galaxies and Empty Field objects) on parsec scales is being studied with Very Long Baseline Array observations at 15 GHz of a sample of more than 170 radio sources. More than 1000 images have been taken since 1994. Here we present an overview of the results of our study, including the proper motions of components in the jets, and their relationship with other source properties.

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arXiv2002

A Multi-Frequency Study of 3C309.1

Here we summarize our results from a detailed multi-frequency study of the QSO 3C309.1 based on the Very Long Baseline Array (VLBA) observations made in mid 1998. From our images, we find a curved jet extending up to 100 milliarcseconds (mas) to the east at low frequencies with two main components, A and B. A preliminary astrometric analysis (Ros and Lobanov 2001) provides a determination of the core position at different frequencies by phase-referencing to a nearby radio source, QSO S5 1448+76. The changes of the core position with frequency suggest high opacity close to the core caused by synchrotron self-absorption. Due to the large astrometric uncertainties we cannot draw any conclusions about the values of the opacity gradients at high frequencies. We believe that a detailed analysis of the frequency depedence of the core position will reveal the profile of the matter distribution in the broad line region, as was initially suggested by Lobanov (1998).

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arXiv2002

Confirmation of persistent radio jets in the microquasar LS 5039

We present here new observations conducted with the EVN and MERLIN of the persistent microquasar LS 5039 discovered by Paredes et al. (2000) with the VLBA. The new observations confirm the presence of an asymmetric two-sided jet reaching up to 1000 AU on the longest jet arm. The results suggest a bending of the jets with increasing distance from the core and/or precession. The origin and location of the high-energy gamma-ray emission associated with the system is discussed and an estimate of the magnetic field at the base of the jet given. Our results suggest a well collimated radio jet. We also comment on new observing strategies to be used with satellites and forthcoming detectors, since this persistent source appears to be a rather good laboratory to explore the accretion/ejection processes taking place near compact objects.

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arXiv2002

4C +12.50: A Superluminal Precessing Jet in the Recent Merger System IRAS 13451+1232

We present the results of a multi-epoch VLBA study of the powerful radio source 4C +12.50 (PKS 1345+125) at a wavelength of 2 cm. This compact radio source is associated with a hidden quasar whose host galaxy shows signs of a recent merger. It has been classified as a compact symmetric source (CSO) due to its small overall size (~220 pc) and twin-jet morphology, although it also has faint extended emission that may be a relic of previous activity. We report the detection of exceedingly high linear fractional polarization in isolated features of the southern jet (up to 60%), which is highly unusual for a CSO. Given the large amount of gas present in the host galaxy, we would expect significant Faraday depolarization across the whole source, unless the depolarizing gas is fairly clumpy. The southern jet also contains two features that are moving outward from the core at apparent speeds of v/c = 1.0 +/- 0.3 and 1.2 +/- 0.3. These represent the first positive detections of superluminal motion in a CSO, and taken together with the jet to counter-jet flux ratio, suggest an intrinsic flow speed of v/c = 0.84 +/- 0.12. The apparent ridgeline of the jet and counter-jet are consistent with a conical helix of wavelength 280 pc that is the result of Kelvin-Helmholtz instabilities driven by a slow precession of the jet nozzle. A fit to the data implies that the nozzle is precessing around a cone with half-angle 23 degrees, whose axis lies at an angle of 82 degrees to the line of sight. We suggest that the "S"-shaped jet morphologies commonly seen in recent AGN outflows such as 4C +12.50 may simply reflect the fact that their black hole spin axes are still precessing, and have not had sufficient time to align with their accretion disks.

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arXiv2002

The shroud around the "compact, symmetric" radio jets in NGC 1052

This is a paper on young jet material in a frustratingly complex environment. NGC 1052 has a compact, flat- or GHz-peaked-spectrum radio nucleus consisting of bi-symmetric jets, oriented close to the plane of the sky. Many features on both sides move away at v_app~0.26c (H_0=65 km/s/Mpc). VLBI at seven frequencies shows a wide range of spectral shapes and brightness temperatures; there is clearly free-free absorption, probably together with synchrotron self-absorption, on both sides of the core. The absorbing structure is likely to be geometrically thick and oriented roughly orthogonal to the jets, but it is patchy. HI VLBI shows atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems, at least two of which are local to the AGN environment. The "high velocity system", 125 to 200 km/s redward of systemic, seems restricted to a shell 1 to 2 pc away from the core. Closer to the centre, this gas might be largely ionised; it could cause the free-free absorption. WSRT spectroscopy shows 1667 and 1665 MHz OH absorption over a wide velocity range. OH and HI profile similarity suggests co-location of molecular and atomic ``high velocity'' gas; the connection to H2O masing gas is unclear. Further, at ``high velocity'' we detected the OH 1612 MHz satellite line in absorption and the 1720 MHz line in emission, with complementary strengths.

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arXiv2002

A distorted radio shell in the young supernova SN1986J

We report here on 5 GHz global very-long-baseline interferometry (VLBI) observations of SN 1986J, 16 yr after its explosion. We obtained a high-resolution image of the supernova, which shows a distorted shell of radio emission, indicative of a deformation of the shock front. The angular size of the shell is $\sim4.7 {\rm mas}$, corresponding to a linear size of $\sim6.8 \times 10^{17} {\rm cm}$ for a distance of 9.6 Mpc to NGC 891. The average speed of the shell has decreased from $\sim$7400 \kms in 1988.74 down to about $6300 {\rm km s^{-1}}$ in 1999.14, indicative of a mild deceleration in the expansion of SN 1986J. Assuming a standard density profile for the progenitor wind ($ρ_{\rm cs} \propto r^{-s}, s=2$), the swept-up mass by the shock front is $\sim$2.2\Ms. This large swept-up mass, coupled with the mild deceleration suffered by the supernova, suggests that the mass of the hydrogen-rich envelope ejected at explosion was $\ga12$ \Ms. Thus, the supernova progenitor must have kept intact most of its hydrogen-rich envelope by the time of explosion, which favours a single, massive star progenitor scenario. We find a flux density for SN 1986J of $\sim$7.2 mJy at the observing frequency of 5 GHz, which results in a radio luminosity of $\sim1.4 \times 10^{37} {\rm erg s^{-1}}$ for the frequency range $10^7$--$10^{10}$ Hz ($α=-0.69; S_ν\propto ν^α)$. We detect four bright knots that delineate the shell structure, and an absolute minimum of emission, which we tentatively identify with the centre of the supernova explosion. If this is the case, SN 1986J has then suffered an asymmetric expansion. We suggest that this asymmetry is due to the collision of the supernova ejecta with an anisotropic, clumpy (or filamentary) medium.

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arXiv2002

EVN and MERLIN confirmation of the LS5039 jets

The microquasar nature of LS5039 was revealed by May 1999 VLBA+VLA observations showing a two-sided jet at milliarcsecond scales. Here we present follow-up interferometric observations carried out with the EVN and MERLIN at 5 GHz in March 2000. The obtained maps with both the EVN and MERLIN show a two-sided jet with a similar position angle to the previous VLBA+VLA map. The total length of the jet arms is ~60 mas in the EVN map and ~300 mas in the MERLIN map. A brightness and length asymmetry of the jets, compatible with the earlier observations, is also present in the maps. Overall, these observations confirm the existence of a two-sided jet structure in LS 5039 and seem to indicate their persistent nature.

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arXiv2002

Is 0716+714 a superluminal blazar?

We present an analysis of new and old high frequency VLBI data collected during the last 10 years at 5--22 GHz. For the jet components in the mas-VLBI jet, two component identifications are possible. One of them with quasi-stationary components oscillating about their mean positions. Another identification scheme, which formally gives the better expansion fit, yields motion with $\sim 9$ $c$ for $H_0=65$ km s$^{-1}$ Mpc$^{-1}$ and $q_0=0.5$. This model would be in better agreement with the observed rapid IDV and the expected high Lorentz-factor, deduced from IDV.

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arXiv2002

Kinematics of parsec-scale structures in AGN: the 2cm VLBA Survey

We are investigating the kinematics of jets in active galactic nuclei on parsec scales by studying a representative population of sources. This study is being carried out using the Very Long Baseline Array at 15 GHz, with more than 800 images taken since 1994. In this contribution we present an overview of the diversity of kinematics for a complete sample of sources.

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arXiv2002

How is really decelerating the expansion of SN1993J?

SN1993J is to date the radio supernova whose evolution has been monitored in greatest detail and the one which holds best promise for a comprehensive theoretical-observational analysis. The shell-like radio structure of SN1993J has expanded in general accord with models of shock excited emission, showing almost circular symmetry for over 8 years, except for a bright feature at the south-eastern region of the shell that has been observed at every epoch. The spectrum of SN1993J has flattened from alpha =-1 to alpha =-0.67 (S_(ν) propto nu**(alpha)). The decelerated expansion can be modeled well with a single slope but apparently better with two slopes. There are also intriguing hints of structure in the expansion curve. The results by the two VLBI groups carrying out this research show general agreement, but also some differences. A comparison of the optical and VLBI results about the details of the deceleration show some discrepancies.

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arXiv2002

Investigation of magnetic loop structures in the corona of UX Arietis

Most of the emission coming from the solar corona is confined into closed magnetic structures in the form of arcs (loops). Very little is known about the structure of stellar coronae. The magnetic topology, however, can be inferred by studying the radio emission coming from electrons trapped in the magnetic loops. Evident morphological changes are produced in fact by stellar rotation. We have performed 4 VLBA+Effelsberg runs distributed in time so as to cover well the rotational period of 6.44 days of the active star UX Arietis. We present here some preliminary results, from those observations.

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arXiv2002

Searching for low mass objects around nearby dMe radio stars

Nearby M-dwarfs are best suited for searches of low mass companions. VLBI phase-referencing observations with sensitive telescopes are able to detect radio star flux-densities of tenths of mJy as well as to position the star on the sky with submilliarcsecond precision. We have initiated a long-term observational program, using EVN telescopes in combination with NASA DSN dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of the astrometry allows us to search for possible companions with masses down to 1 Jupiter mass. In this contribution we report preliminary results of the first observation epochs, in which we could detect some of the radio stars included in our program.

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arXiv2002

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. II. Additional Sources

We report further results from our imaging survey at 15GHz (lambda=2 cm) with the Very Long Baseline Array. This paper presents single epoch images for 39 sources, bringing the total number of objects in the sample to 171. Our sample is representative of a complete unbiased sample and it will be used for statistical analysis of source properties. We compare the observed brightness temperatures derived from our VLBA observations to those derived from total intensity variations at 22 and 37 GHz. These are consistent with intrinsic brightness temperatures in the range 10^10 to 10^12 K. We also present three new spectroscopic redshift values: z=0.517+/-0.001 for 0026+346, z=1.591+/-0.003 for 0727-115, and z=0.2016+/-0.0004 for 1155+251. Images from this VLBA 2 cm survey are available on the Internet under http://www.cv.nrao.edu/2cmsurvey.

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arXiv2002

NGC1052 - A study of the pc-scale twin jet

We present results of a VLBA multi-frequency study of the pc-scale twin jet in NGC1052. We observed this object at epoch 1998.99 with the VLBA at 5, 8.4, 22 and 43 GHz both in total and linearly polarized intensity. The spectral analysis confirms the necessity of a free-free absorbing medium, obscuring the innermost part of both jets. At 5 GHz we found a compact linearly polarized emission region at the base of the eastern jet with a degree of polarization of 1.5%. At higher frequencies there is no evidence for polarization in our data. A core shift analysis constrains the position of the central engine to ~0.03 pc. The shift rates of the apparent core position with frequency confirm the strong influence of free-free absorption in conjunction with steep pressure gradients at the bases of both jets.

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arXiv2002

Strongly decelerated expansion of SN1979C

We observed SN1979C in M100 on 4 June 1999, about twenty years after explosion, with a very sensitive four-antenna VLBI array at the wavelength of 18cm. The distance to M100 and the expansion velocities are such that the supernova cannot be fully resolved by our Earth-wide array. Model-dependent sizes for the source have been determined and compared with previous results. We conclude that the supernova shock was initially in free expansion for 6 +/- 2 yrs and then experienced a very strong deceleration. The onset of deceleration took place a few years before the abrupt trend change in the integrated radio flux density curves. We estimate the shocked swept-up mass to be about 1.6 solar masses, assuming a standard density profile for the CSM. Such a swept-up mass for SN1979C suggests a mass of the hydrogen-rich envelope ejected at explosion no larger than $M_{\rm env} \sim$ 0.9 solar masses. If SN1979C originated in a binary star the low value of M_env suggests that the companion of the progenitor star stripped off most of the hydrogen-rich envelope mass of the presupernova star prior to the explosion.

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arXiv2001

Opacity in the jet of 3C309.1

The "core" of a radio source is believed to mark the frequency-dependent location where the optical depth to synchrotron self--absorption tau_s~1. The frequency dependence can be used for derive physical conditions of the radio emitting region and the ambient environment near the central engine of the radio source. In order to test and improve the models to derive this information, we made multi-frequency dual-polarization observations of 3C309.1 in 1998.6, phase-referenced to the QSOs S5 1448+76 and 4C72.20 (S5 1520+72) (4d26' and 1d49' away, respectively). We present here preliminary results from these observations: total and polarized intensity maps, spectral information deduced from these images, and the relative position of 3C309.1 with respect to S5 1448+76 at different frequencies. Finally, we discuss briefly the observed shift of the core position.

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arXiv2001

Absolute kinematics of radio source components in the complete S5 polar cap sample I. First and second epoch maps at 8.4 GHz

We observed the thirteen extragalactic radio sources of the S5 polar cap sample at 8.4 GHz with the Very Long Baseline Array, on 1997.93 and 1999.41. We present the maps from those two epochs and briefly discuss the morphological changes experimented by some of the radio sources in the 1.4 yr elapsed. These results correspond to the first two epochs at 8.4 GHz of a program directed to study the absolute kinematics of the radio source components of the members of the sample by means of phase delay astrometry at 8.4, 15 and 43 GHz.

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