arXiv subjects

I. Agudo

Publications and source records attributed to I. Agudo.

At least 145 records · Page 8Linked to original sources

arXiv2010

The redshift and broad band spectral energy distribution of NRAO 150

Context. NRAO 150 is one of the brightest radio and mm AGN sources on the northern sky. It has been revealed as an interesting source where to study extreme relativistic jet phenomena. However, its cosmological distance has not been reported so far, because of its optical faintness produced by strong Galactic extinction. Aims. Aiming at measuring the redshift of NRAO 150, and hence to start making possible quantitative studies from the source. Methods. We have conducted spectroscopic and photometric observations of the source in the near-IR, as well as in the optical. Results. All such observations have been successful in detecting the source. The near-IR spectroscopic observations reveal strong H$α$ and H$β$ emission lines from which the cosmological redshift of NRAO 150 ($z=1.517\pm0.002$) has been determined for the first time. We classify the source as a flat-spectrum radio-loud quasar, for which we estimate a large super-massive black-hole mass $\sim5\times 10^{9} \mathrm{M_\odot}$. After extinction correction, the new near-IR and optical data have revealed a high-luminosity continuum-emission excess in the optical (peaking at $\sim2000$\,Å, rest frame) that we attribute to thermal emission from the accretion disk for which we estimate a high accretion rate, $\sim30$\,% of the Eddington limit. Conclusions. Comparison of these source properties, and its broad-band spectral-energy distribution, with those of Fermi blazars allow us to predict that NRAO 150 is among the most powerful blazars, and hence a high luminosity -although not detected yet- $γ$-ray emitter.

astro-ph.CO↗

arXiv2010

Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "Crazy Diamond"

We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) carried out in July 2007-January 2009. We show the results of the AGILE campaigns which took place on May-June 2008, July-August 2008, and October 2008-January 2009. During the May 2008-January 2009 period, the source average flux was highly variable, from an average gamma-ray flux F(E>100MeV) > 200E-8 ph/cm2/s in May-June 2008, to F(E>100MeV)~80E-8 ph/cm2/s in October 2008-January 2009. The average gamma-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law (Gamma_GRID ~ 2.0 to 2.2). Only 3-sigma upper limits can be derived in the 20-60 keV energy band with Super-AGILE. During July-August 2007 and May-June 2008, RXTE measured a flux of F(3-20 keV)= 8.4E-11 erg/cm2/s, and F(3-20 keV)=4.5E-11 erg/cm2/s, respectively and a constant photon index Gamma_PCA=1.65. Swift/XRT observations were carried out during all AGILE campaigns, obtaining a F(2-10 keV)=(0.9-7.5)E-11 erg/cm2/s and a photon index Gamma_XRT=1.33-2.04. BAT measured an average flux of ~5 mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period from the radio to the optical. A correlation analysis between the optical and the gamma-ray fluxes shows a time lag of tau=-0.4 days. An analysis of 15 GHz and 43 GHz VLBI core radio flux observations shows an increasing trend of the core radio flux, anti- correlated with the higher frequency data. The modeling SEDs, and the behavior of the long-term light curves in different energy bands, allow us to compare the jet properties during different emission states, and to study the geometrical properties of the jet on a time-span longer than one year.

astro-ph.HE↗

arXiv2010

Non-radial motion in the TeV blazar S5 0716+714 The pc-scale kinematics of a BL Lac Object

For the BL Lac object S5 0716+714 conflicting apparent velocities of jet component motion have been reported in the literature. This object is an intra-day variable source and suited to investigate a possible correlation between kinematic properties and flux-density variability on different timescales. We (re-)analyze 50 VLBI observations obtained with the VLBA at 5 different frequencies (5 to 43 GHz) between 1992.73 and 2006.32. We analyze the jet component motion in detail taking care not only to account for motion in the radial but also in the orthogonal direction. We study the evolution of the jet ridge line and search for correlations between radio band light curves and the kinematic properties of the jet components. We present an alternative kinematic scenario for jet component motion in S5 0716+714 with apparent stationarity of jet components (with regard to their core separation) with time. Jet components, however, do seem to move significantly non-radially with regard to their position angle and in a direction perpendicular to the major axis of the jet. We discuss a possible correlation between the long-term radio flux-density variability and the kinematics. We conclude that a geometric contribution to the origin of the long-term variability might not be negligible. Subluminal motion has been reported for most of the TeV blazars. Our analysis also confirms this for S5 0716+714. This result increases the number of TeV blazars showing apparent subluminal motion to 7.

astro-ph.CO↗

arXiv2009

AGILE detection of a rapid gamma-ray flare from the blazar PKS 1510-089 during the GASP-WEBT monitoring

We report the detection by the AGILE satellite of a rapid gamma-ray flare from the powerful gamma-ray quasar PKS 1510-089, during a pointing centered on the Galactic Center region from 1 March to 30 March 2008. This source has been continuosly monitored in the radio-to-optical bands by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). Moreover, the gamma-ray flaring episode triggered three ToO observations by the Swift satellite in three consecutive days, starting from 20 March 2008. In the period 1-16 March 2008, AGILE detected gamma-ray emission from PKS 1510-089 at a significance level of 6.2-sigma with an average flux over the entire period of (84 +/- 17) x 10^{-8} photons cm^{-2} s^{-1} for photon energies above 100 MeV. After a predefined satellite re-pointing, between 17 and 21 March 2008, AGILE detected the source at a significance level of 7.3-sigma, with an average flux (E > 100 MeV) of (134 +/- 29) x 10^{-8} photons cm^{-2} s^{-1} and a peak level of (281 +/- 68) x 10^{-8} photons cm^{-2} s^{-1} with daily integration. During the observing period January-April 2008, the source also showed an intense and variable optical activity, with several flaring episodes and a significant increase of the flux was observed at millimetric frequencies. Moreover, in the X-ray band the Swift/XRT observations seem to show an harder-when-brighter behaviour of the source spectrum. The spectral energy distribution of mid-March 2008 is modelled with a homogeneous one-zone synchrotron self Compton emission plus contributions from inverse Compton scattering of external photons from both the accretion disc and the broad line region. Indeed, some features in the optical-UV spectrum seem to indicate the presence of Seyfert-like components, such as the little blue bump and the big blue bump.

astro-ph.HE↗

arXiv2009

Spectral evolution of superluminal components in parsec-scale jets

(Abridged) We present numerical simulations of the spectral evolution and emission of radio components in relativistic jets. We have developed an algorithm (SPEV) for the transport of a population of non-thermal electrons including radiative losses. For large values of the ratio of gas pressure to magnetic field energy density, \ab \sim 6\times 10^4, quiescent jet models show substantial spectral evolution, with observational consequences only above radio frequencies. Larger values of the magnetic field (\ab \sim 6\times 10^2), such that synchrotron losses are moderately important at radio frequencies, present a larger ratio of shocked-to-unshocked regions brightness than the models without radiative losses, despite the fact that they correspond to the same underlying hydrodynamic structure. We also show that jets with a positive photon spectral index result if the lower limit γ_min of the non-thermal particle energy distribution is large enough. A temporary increase of the Lorentz factor at the jet inlet produces a traveling perturbation that appears in the synthetic maps as a superluminal component. We show that trailing components can be originated not only in pressure matched jets, but also in over-pressured ones, where the existence of recollimation shocks does not allow for a direct identification of such features as Kelvin-Helmholtz modes, and its observational imprint depends on the observing frequency. If the magnetic field is large (\ab \sim 6\times 10^2), the spectral index in the rarefaction trailing the traveling perturbation does not change much with respect to the same model without any hydrodynamic perturbation. If the synchrotron losses are considered the spectral index displays a smaller value than in the corresponding region of the quiescent jet model.

astro-ph↗

arXiv2009

Synthetic synchrotron emission maps from MHD models for the jet of M87

We present self-consistent global, steady-state MHD models and synthetic optically thin synchrotron emission maps for the jet of M87. The model consist of two distinct zones: an inner relativistic outflow, which we identify with the observed jet, and an outer cold disk-wind. While the former does not self-collimate efficiently due to its high effective inertia, the latter fulfills all the conditions for efficient collimation by the magneto-centrifugal mechanism. Given the right balance between the effective inertia of the inner flow and the collimation efficiency of the outer disk wind, the relativistic flow is magnetically confined into a well collimated beam and matches the measurements of the opening angle of M87 over several orders of magnitude in spatial extent. The synthetic synchrotron maps reproduce the morphological structure of the jet of M87, i.e. center-bright profiles near the core and limb-bright profiles away from the core. At the same time, they also show a local increase of brightness at some distance along the axis associated to a recollimation shock in the MHD model. Its location coincides with the position of the optical knot HST-1. In addition our best fitting model is consistent with a number of observational constraints such as the magnetic field in the knot HST-1, and the jet-to-counterjet brightness ratio.

astro-ph.GA↗

arXiv2008

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71, III. Rapid and correlated flux density variability from radio to sub-mm bands

The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures. The observing campaign involved simultaneous monitoring at radio to IR/optical wavelengths centered around a 500-ks INTEGRAL pointing (November 10-17, 2003). We present the combined analysis and results of the cm- to sub-mm observations including a detailed study of the inter- to intra-day variability and spectral characteristics of 0716+714. We further constrain the variability brightness temperatures (T_B) and Doppler factors (delta) comparing the radio-bands with the hard X-ray emission (3-200 keV). 0716+714 was in an exceptionally high state (outburst) and different (slower) phase of short-term variability. The flux density variability in the cm- to mm-bands is dominated by a correlated, ~4 day time scale amplitude increase of up to ~35% systematically more pronounced towards shorter wavelengths. This contradicts expectations from standard interstellar scintillation (ISS) and suggests a source-intrinsic origin of the variability. The derived lower limits to T_B exceed the 10^12 K IC-limit by up to 3-4 orders of magnitude. Assuming relativistic boosting, we obtain robust and self-consistent lower limits of delta >= 5-33, in good agreement with delta_VLBI obtained from VLBI studies and the IC-Doppler factors delta_IC > 14-16 obtained from the INTEGRAL data. Since a strong contribution from ISS can be excluded and a simultaneous IC catastrophe was not observed, we conclude that relativistic Doppler boosting naturally explains the apparent violation of the theoretical limits within standard synchrotron-self-Compton (SSC) jet models of AGN.

astro-ph↗

arXiv2008

Hydrodynamics of a perturbation in the jet of 3C111

We present results on the modelling of the ejection of a superluminal component in the jet of 3C111. We propose that the component is generated by an injection of dense material followed by a decrease in the injection rate of bulk particles in the jet. Our model is supported by 1D relativistic hydrodynamics and emission simulations, and is capable of reproducing the brightness evolution of two features, as revealed by 15 GHz VLBA observations. We show that other scenarios, such as an increase of the Lorentz factor in the material of the perturbation, fails to reproduce the observed evolution of this flare.

astro-ph↗

arXiv2008

On the nature of an ejection event in the jet of 3C111

We present a possible scenario for the ejection of a superluminal component in the jet of the Broad Line Radio Galaxy 3C111 in early 1996. VLBI observations at 15 GHz discovered the presence of two jet features on scales smaller than one parsec. The first component evolves downstream, whereas the second one fades out after 1 parsec. We propose the injection of a perturbation of dense material followed by a decrease in the injection rate of material in the jet as a plausible explanation. This scenario is supported by 1D relativistic hydrodynamics and emission simulations. The perturbation is modeled as an increase in the jet density, without modifying the original Lorentz factor in the initial conditions. We show that an increase of the Lorentz factor in the material of the perturbation fails to reproduce the observed evolution of this flare. We are able to estimate the lifetime of the ejection event in 3C111 to be 36\pm7 days.

astro-ph↗

arXiv2008

Stratification in polarization and Faraday rotation in the jet of 3C 120

Very long baseline interferometric observations of the radio galaxy 3C 120 show a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. These are revealed by the passage of multiple superluminal components throughout the jet as they move out from the core in a sequence of 12 monthly polarimetric observations taken with the VLBA at 15, 22, and 43 GHz. The degree of polarization has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. Superposed on this structure we find a localized region of high Faraday rotation measure (about 6000 rad/m^2) between approximately 3 and 4 mas from the core. This region of enhanced Faraday rotation may result from the interaction of the jet with the ambient medium, which may also explain the stratification in degree of polarization. The data are also consistent with a helical magnetic field in a two-fluid jet model, consisting of an inner emitting jet and a sheath of nonrelativistic electrons.

astro-ph↗

arXiv2008

Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?

We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA observations of the radio galaxy 3C 120 revealing a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. The degree of polarization increases with distance from the core and toward the jet edges, and has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. We find a localized region of high Faraday rotation measure superposed on this structure between approximately 3 and 4 mas from the core, with a peak of about 6000 rad/m^2. Interaction of the jet with the external medium or a cloud would explain the confined region of enhanced Faraday rotation, as well as the stratification in degree of polarization and the flaring of superluminal knots when crossing this region. The data are also consistent with a helical field in a two-fluid jet model, consisting of an inner, emitting jet and a sheath containing nonrelativistic electrons. However, this helical magnetic field model cannot by itself explain the localized region of enhanced Faraday rotation. The polarization electric vectors, predominantly perpendicular to the jet axis once corrected for Faraday rotation, require a dominant component parallel to the jet axis (in the frame of the emitting plasma) for the magnetic field in the emitting region.

astro-ph↗

arXiv2008

A VLBI Search for the Origin of Wobbling in Blazar Jets

An increasing number of blazars have been reported to show jet wobbling (i.e., non-regular rotations of the structural position angle of their innermost jets in the plane of the sky with amplitudes between 20 deg. and 50 deg., and time scales between 2 yr. and 20 yr.). The physical origin for the observed jet wobbling is still poorly understood, but as this phenomenon is triggered in the innermost regions of the jets, it must be tied to fundamental properties of the inner regions of the accretion system. Thus, jet wobbling may be an interesting potential tool for supermassive black hole, accretion and jet launching studies. As part of a joint theoretical/numerical and observational effort to characterize the observational properties and differences between these three possible scenarios we have started a long-term polarimetric phase-reference 43 GHz VLBA monitoring program to observe the jet structure and the absolute motion of the jet core of four of the blazars which have shown some of the clearest evidence of large amplitude jet wobbling: NRAO 150, OJ 287, 3C 273, and 3C 345 . Here we present this project and we argue about its suitability for future VSOP-2 observations.

astro-ph↗

arXiv2008

The Influence of Helical Magnetic Fields in the Dynamics and Emission of Relativistic Jets

We present numerical relativistic magnetohydrodynamic and emission simulations aimed to study the role played by the magnetic field in the dynamics and emission of relativistic jets in Active Galactic Nuclei. We focus our analysis on the study of the emission from recollimation shocks since they may provide an interpretation for the stationary components seen at parsec-scales in multiple sources. We show that the relative brightness of the knots associated with the recollimation shocks decreases with increasing jet magnetization, suggesting that jets presenting stationary components may have a relatively weak magnetization, with magnetic fields of the order of equipartition or below.

astro-ph↗

arXiv2008

The Trails of Superluminal Jet Components in 3C111

In 1996, a major radio flux-density outburst occured in the broad-line radio galaxy 3C111. It was followed by a particularly bright plasma ejection associated with a superluminal jet component, which has shaped the parsec-scale structure of 3C111 for almost a decade. Here, we present results from 18 epochs of Very Long Baseline Array (VLBA) observations conducted since 1995 as part of the VLBA 2 cm Survey and MOJAVE monitoring programs. This major event allows us to study a variety of processes associated with outbursts of radio-loud AGN in much greater detail than has been possible in other cases: the primary perturbation gives rise to the formation of a leading and a following component, which are interpreted as a forward and a backward-shock. Both components evolve in characteristically different ways and allow us to draw conclusions about the work flow of jet-production events; the expansion, acceleration and recollimation of the ejected jet plasma in an environment with steep pressure and density gradients are revealed; trailing components are formed in the wake of the primary perturbation possibly as a result of coupling to Kelvin-Helmholtz instability pinching modes from the interaction of the jet with the external medium. The interaction of the jet with its ambient medium is further described by the linear-polarization signature of jet components traveling along the jet and passing a region of steep pressure/density gradients.

astro-ph↗

arXiv2007

Superluminal non-ballistic jet swing in the quasar NRAO 150 revealed by mm-VLBI

NRAO 150 -a compact and bright radio to mm source showing core/jet structure- has been recently identified as a quasar at redshift z=1.52 through a near-IR spectral observation. To study the jet kinematics on the smallest accessible scales and to compute the first estimates of its basic physical properties, we have analysed the ultra-high-resolution images from a new monitoring program at 86 GHz and 43 GHz with the GMVA and the VLBA, respectively. An additional archival and calibration VLBA data set, covering from 1997 to 2007, has been used. Our data shows an extreme projected counter-clock-wise jet position angle swing at an angular rate of up to ~11 deg./yr within the inner ~31 pc of the jet, which is associated with a non-ballistic superluminal motion of the jet within this region. The results suggest that the magnetic field could play an important role in the dynamics of the jet in NRAO 150, which is supported by the large values of the magnetic field strength obtained from our first estimates. The extreme characteristics of the jet swing make NRAO 150 a prime source to study the jet wobbling phenomenon.

astro-ph↗

arXiv2006

Rapid and correlated variability of blazar S5 0716+71 from radio- to sub-mm bands

The BL Lac object S5 0716+71 was target of a coordinated and global multi-frequency campaign to search for rapid and correlated variability and signatures of the inverse-Compton catastrophe. Here we present first results obtained from a combined analysis of the cm- to sub-mm observations over a period of seven days aiming at a detailed study of the intra- to inter-day variability characteristics and to obtain constraints on the variability brightness temperatures and Doppler factors comparing the radio data with the high energy emission recorded by INTEGRAL. A more detailed description of the whole cm- to sub-mm observations and our analysis/results will be presented in a forthcoming paper. Our analysis reveals the source to be in a particular short-term variability phase when compared to the past with a correlated >~4 day time scale amplitude increase of up to 35%, which is systematically more pronounced towards higher frequencies. The obtained frequency dependent variability amplitudes and time lags contradict expectations from interstellar scintillation and strongly suggest a source intrinsic origin of this inter-day variability. A 7-day spectral evolution study indicate time-variable synchrotron self-absorption and expansion of the emission region, consistent with standard models. Assuming relativistic boosting, our different estimates of the Doppler factor yield robust lower limits of D_var,IC > 5-22 using the inverse-Compton limit and D_var,eq > 8-33 using the equipartition argument. Although high, these values are in good agreement with Doppler factors obtained from recent VLBI studies and from the inverse-Compton Doppler factors D_IC > 14-16 derived with the X-ray emission seen by INTEGRAL at 3-200 keV.

astro-ph↗

arXiv2006

VLBI at the highest frequencies - AGN studied with micro-arcsecond resolution

Compact galactic and extragalactic radio sources can be imaged with an unsurpassed angular resolution of a few ten micro-arcseconds, adopting the observing technique of global millimeter VLBI. Here we present the Global Millimeter VLBI Array (GMVA) and discuss its present performance. For individual and partially archetypical radio sources with prominent VLBI jets (e.g. 3C120, Cygnus A, M87, 3C454.3, NRAO150),we show and discuss new results obtained with the GMVA. The variety of observed effects range from jet propagation and bending, partial fore-ground absorption in the nucleus, and jet component ejection after major flares to new and very small (15-20 Schwarzschild radii) upper limits to the jet base of M87. We also discuss the future development of mm-VLBI at 3mm and towards shorter wavelengths, and make suggestions for possible improvements.

astro-ph↗

arXiv2006

"Oscillating" components in the BL Lac object 0716+714?

15 VLBA observations of the BL Lac object 0716+714, performed between 1994.67 and 2006.40 at 15 GHz, have been analyzed. Part of the data (12 epochs) were obtained within the Mojave-survey project and reanalyzed by us. We present a new motion scenario for jet model component motion in this source which suggests that no longterm-outward motion but rather an oscillation of components around an average core separation is taking place. Although no significant outward motion in the core separation can be found, motion with regard to the position angle is observed. We give lower limits for the derived apparent motions and Doppler factors and compare them with values published in the literature. We find a relation between the total flux-density evolution at 14.5 GHz (UMRAO data) and the position angle changes observed for the jet component closest to the core. We suggest a significant geometric contribution to the longterm flux-density variability in 0716+714.

astro-ph↗