Radio Very Long Baseline Interferometry Research Articles

The Core Starbursts of the Galaxy NGC 3628: Radio Very Long Baseline Interferometry and X-Ray Studies

We present radio very long baseline interferometry (VLBI) and X-ray studies of the starburst galaxy NGC 3628. The VLBI observation at 1.5 GHz reveals seven compact (0.7−7 pc) radio sources in the central ∼250 pc region of NGC 3628. Based on their morphology, high radio-brightness temperatures (105–107 K), and steep radio spectra, none of these seven sources can be associated with active galactic nuclei (AGNs); instead, they can be identified as supernova remnants (SNRs), with three of them appearing consistent with partial shells. Notably, one of them (W2) is likely a nascent radio supernova (SN) and appears to be consistent with the star formation rate of NGC 3628 when assuming a canonical initial mass function. The VLBI observation provides the first precise measurement of the diameter of the radio sources in NGC 3628, which allow us to fit a well-constrained radio surface brightness—diameter (Σ–D) correlation by including the detected SNRs. Furthermore, future VLBI observations can be conducted to measure the expansion velocity of the detected SNRs. In addition to our radio VLBI study, we analyze Chandra and XMM-Newton spectra of NGC 3628. The spectral fitting indicates that the SNR activities could well account for the observed X-ray emissions. Along with the Chandra X-ray image, it further reveals that the X-ray emission is likely maintained by the galactic-scale outflow triggered by SN activities. These results provide strong evidence that SN-triggered activities play a critical role in generating both radio and X-ray emissions in NGC 3628.

Flare echoes from relaxation shocks in perturbed relativistic jets

Context. One of the main scenarios to account for the multiwavelength flux variability observed in relativistic jets of active galactic nuclei (AGNs) is based on the diffusive shock acceleration of a population of relativistic electrons on internal shocks of various origins. Any complete AGN emission scenario has to be able to explain the wide range of observed variability timescales, which change over several orders of magnitude between the radio and gamma-ray band. In addition to observations of flux variability, constraints are also provided by very-long-baseline interferometry (VLBI), which shows a large variety of moving and standing emission zones with distinct behaviors. Aims. Combining dynamic hydrodynamic jet simulations with radiative transfer, we aim to characterize the evolution of stationary and moving emission zones in the jet and study their multiwavelength signatures through emission maps and light curves. We focus our study on flare events that occur during strong interactions between moving ejecta and stationary recollimation shocks. Such events are shown to lead to a significant perturbation of the stationary jet structure. Methods. We simulate relativistic jets with the magneto-hydrodynamic code MPI-AMRVAC and inject nonthermal particle distributions of electrons into shock regions. We follow the propagation of a moving shock and its interactions with a structure of standing recollimation shocks in the jet. Synchrotron emission and radiative transfer are calculated in the post-processing code RIPTIDE for given observation angles and frequencies, assuming a turbulent magnetic field and taking the light crossing effect into account. Results. In the case of strong shock–shock interactions, we demonstrate the appearance of trailing components behind the leading moving shock. The latter destabilizes the jet, causing the emergence of oscillating standing shocks and relaxation shocks. Emissions from these regions can dominate the overall flux or lead to “flare echoes” in the light curve. Another observational marker for the presence of relaxation shocks appears in time-distance plots of bright VLBI components of the jet. Our scenario provides a plausible explanation for radio VLBI observations of the radio galaxy 3C 111, where trailing components were observed during a radio outburst event in 1997, and may be applicable to other sources with similar features.

Constraining properties of neutron star merger outflows with radio observations

ABSTRACT The jet opening angle and inclination of GW170817 – the first detected binary neutron star merger – were vital to understand its energetics, relation to short gamma-ray bursts, and refinement of the standard siren-based determination of the Hubble constant, H0. These basic quantities were determined through a combination of the radio light curve and Very Long Baseline Interferometry (VLBI) measurements of proper motion. In this paper, we discuss and quantify the prospects for the use of radio VLBI observations and observations of scintillation-induced variability to measure the source size and proper motion of merger afterglows, and thereby infer properties of the merger including inclination angle, opening angle, and energetics. We show that these techniques are complementary as they probe different parts of the circum-merger density/inclination angle parameter space and different periods of the temporal evolution of the afterglow. We also find that while VLBI observations will be limited to the very closest events it will be possible to detect scintillation for a large fraction of events beyond the range of current gravitational wave detectors. Scintillation will also be detectable with next-generation telescopes such as the Square Kilometre Array, 2000 antenna Deep Synoptic Array, and the next-generation Very Large Array, for a large fraction of events detected with third-generation gravitational wave detectors. Finally, we discuss prospects for the measurement of the H0 with VLBI observations of neutron star mergers and compare this technique to other standard siren methods.

Constrained transport and adaptive mesh refinement in the Black Hole Accretion Code

Context. Worldwide very long baseline radio interferometry (VLBI) arrays are expected to obtain horizon-scale images of supermassive black hole candidates and of relativistic jets in several nearby active galactic nuclei. This, together with the expected detection of electromagnetic counterparts of gravitational-wave signals, motivates the development of models for magnetohydrodynamic flows in strong gravitational fields. Aims. The Black Hole Accretion Code (BHAC) is a publicliy available code intended to aid with the modeling of such sources by performing general relativistic magnetohydrodynamical simulations in arbitrary stationary spacetimes. New additions to the code are required in order to guarantee an accurate evolution of the magnetic field when small and large scales are captured simultaneously. Methods. We discuss the adaptive mesh refinement (AMR) techniques employed in BHAC, which are essential to keep several problems computationally tractable, as well as staggered-mesh-based constrained transport (CT) algorithms to preserve the divergence-free constraint of the magnetic field. We also present a general class of prolongation operators for face-allocated variables compatible with them. Results. After presenting several standard tests for the new implementation, we show that the choice of the divergence-control method can produce qualitative differences in the simulation results for scientifically relevant accretion problems. We demonstrate the ability of AMR to decrease the computational costs of black hole accretion simulations while sufficiently resolving turbulence arising from the magnetorotational instability. In particular, we describe a simulation of an accreting Kerr black hole in Cartesian coordinates using AMR to follow the propagation of a relativistic jet while self-consistently including the jet engine, a problem set up for which the new AMR implementation is particularly advantageous. Conclusions. The CT methods and AMR strategies discussed here are currently being used in the simulations performed with BHAC for the generation of theoretical models for the Event Horizon Telescope collaboration.

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

The Fermi Large Area Telescope (LAT) revealed that blazars, representing the most extreme radio-loud active galactic nuclei (AGN) population, dominate the census of the gamma-ray sky, and a significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range. However, the possible connection between radio and very high energy (VHE, E>0.1 TeV) emission still remains elusive, owing to the lack of a homogeneous coverage of the VHE sky. The main goal of this work is to quantify and assess the significance of a possible connection between the radio emission on parsec scale measured by the very long baseline interferometry (VLBI) and GeV-TeV gamma-ray emission in blazars, which is a central issue for understanding the blazar physics and the emission processes. We investigate the radio VLBI and high energy gamma-ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi-LAT catalogs of hard gamma-ray sources detected above 10 GeV (1FHL) and 50 GeV (2FHL). For comparison, we perform the same correlation analysis by using the 0.1-300 GeV gamma-ray energy flux provided by the third Fermi-LAT source catalog. We find that the correlation strength and significance depend on the gamma-ray energy range with a different behavior among the blazar sub-classes. Overall, the radio and gamma-ray emission above 10 GeV turns out to be uncorrelated for the full samples and for all of the blazar sub-classes with the exception of high synchrotron peaked (HSP) objects, which show a strong and significant correlation. On the contrary, when 0.1-300 GeV gamma-ray energies are considered, a strong and significant correlation is found for the full blazar sample as well as for all of the blazar sub-classes. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties.

No AGN evidence in NGC 1614 from deep radio VLBI observations

Abstract We present deep dual-band 5.0- and 8.4-GHz European VLBI Network (EVN) observations of NGC 1614, a local luminous infrared galaxy with a powerful circumnuclear starburst ring, and whose nuclear engine origin is still controversial. We aim at detecting and characterizing compact radio structures both in the nuclear region and in the circumnuclear ring. We do not find any compact source in the central 200 pc region, setting a very tight 5σ upper limit of 3.7 × 1036 and 5.8 × 1036 erg s−1, at 5.0 and 8.4 GHz, respectively. However, we report a clear detection at both frequencies of a compact structure in the circumnuclear ring, 190 pc to the north of the nucleus, whose luminosity and spectral index are compatible with a core-collapse supernova, giving support to the high star formation rate in the ring. Our result favours the pure starburst scenario, even for the nucleus of NGC 1614, and shows the importance of radio VLBI (very long baseline interferometry) observations when dealing with the obscured environments of dusty galaxies.

Comparison of VLBI radio source catalogs

Aims. I checked the consistency of recent astrometric radio source catalogs obtained by geodetic very long baseline radio interferometry (VLBI) with the second realization of the International Celestial Reference Frame (ICRF2), released in 2009, which is the most accurate astrometric catalog currently available.Methods. The catalogs were compared to the ICRF2 in terms of radio source coordinates, global second-degree deformations, and error distribution.Results. All catalogs were found to be consistent with the ICRF2 within 20 μ as. At high observational rates, the formal error is likely limited to the level of ~10 μ as by correlated-noise errors. The comparison of offsets to ICRF2 against formal errors raised noise floors of the differences between 50 μ as and 100 μ as, and hence no improvement with respect to the ICRF2.Conclusions. The inconsistencies between catalogs result in differences significantly larger than the accuracy expected for the future realizations of the celestial reference frame. These inconsistencies have to be clarified in the near future in view of the next ICRF realization and accurate linking to reference frames at other frequencies.

Estimating the apparent motions of extragalactic radio sources

The apparent motions of 573 extragalactic radio sources are estimated. For this purpose, we analyzed the data that were obtained during 3193 series of observations using very-long baseline radio interferometry (VLBI) from 1984 to 2010. To find a solution, the least-squares method with constraints is applied. It is shown that most radio sources (including defining sources) are characterized by significant apparent motions. Since the stability of the celestial reference system is due to the absence of net rotation (no-net-rotation condition) relative to the defining sources, a change in their coordinates leads to the rotation of the celestial coordinate system (CCS). Owing to this, we believe that the apparent motions of radio sources must be given in catalogs.

Observational evidence for the link between the variable optical continuum and the subparsec-scale jet of the radio galaxy 3C 390.3

The mechanism and the region of generation of variable continuum emission are poorly understood for radio-loud AGN because of a complexity of the nuclear region. High-resolution radio VLBI (very long baseline interferometry) observations allow zooming into a subparsec-scale region of the jet in the radio-loud galaxy 3C 390.3. We combined the radio VLBI and the optical data covering the time period of 14 years to look for a link between optical flares and parsec-scale jet. We identify two stationary and nine moving radio features in the innermost subparsec-scale region of the jet. All nine ejections are associated with optical flares. We found a significant correlation (at a confidence level of $>99.99%$) between the ejected jet components and optical continuum flares. The epochs at which the moving knots pass through the location of a stationary radio feature and the optical light curve reaches the maximum are correlated. The radio events follow the maxima of optical flares with the mean time delay of $0.10\pm0.04$ years. This correlation can be understood if the variable optical continuum emission is generated in the innermost part of the jet. A possible mechanism of the energy release is the ejection of knots of high-energy electrons that are accelerated in the jet flow and generate flares of synchrotron continuum emission in the wide range of frequencies from radio to X-ray bands. In this scenario, the beamed optical continuum emission from the jet and counterjet ionizes a gas in a subrelativistic outflow surrounding the jet, which results in a formation of two outflowing conical regions with broad emission lines (in addition to the conventional broad line region around the central nucleus).

High-Precision Algorithms for Astrometry: A Comparison of Two Approaches

This paper discusses the correspondence between two approaches to astrometric observational reductions: the approach based on angular observables used for optical observations, and the approach based on the interferometric delay observable used for very long baseline radio interferometry (VLBI) observations. A procedure is presented by which VLBI algorithms can be used for optical observations. This scheme can help to guarantee consistent treatment of observational results in the two regimes. Differences between angle- and delay-based algorithms in current use are shown to be less than 1 μas. However, the physical models used as the bases for the algorithms must be improved to reach external accuracies at such levels.

Very-long-baseline radio interferometry (VLBI) observations of gamma-ray blazars: results from millimeter-VLBI observations.

VLBI observations of the extremely gamma-bright blazar PKS 0528+134 at 8, 22, 43, and 86 GHz reveal a strongly bent one-sided-core jet structure with at least three moving and two apparently stationary jet components. At the highest observing frequencies the brightest and most compact jet component (the VLBI core) is unresolved with an upper limit to its size of approximately 50 microarcsec corresponding to approximately 0.2 parsec [H0 = 100 km.s-1.Mpc-1 (megaparsec-1), q0 = 0.5, where H0 is Hubble constant and q0 is the deceleration parameter]. Two 86-GHz VLBI observations performed in 1993.3 and 1994.0 reveal a new jet component emerging with superluminal speed from the core. Linear back-extrapolation of its motion yields strong evidence that the ejection of this component is related to an outburst in the millimeter regime and a preceding intense flare of the gamma-flux density observed in early 1993. This and the radio/optical "light curves" and VLBI data for two other sources (S5 0836+710 and 3C 454.3) suggest that the observed gamma-radiation might be Doppler-boosted and perhaps is closely related to the physical processes acting near the "base" of the highly relativistic jets observed in quasars.

Very-long-baseline radio interferometry surveys of the compact structure in active galactic nuclei.

Very-long-baseline radio interferometry (VLBI) imaging surveys have been undertaken since the late 1970s. The sample sizes were initially limited to a few tens of objects but the snapshot technique has now allowed samples containing almost 200 sources to be studied. The overwhelming majority of powerful compact sources are asymmetric corejects of one form or another, most of which exhibit apparent superluminal motion. However 5-10% of powerful flat-spectrum sources are 100-parsec (pc)-scale compact symmetric objects; these appear to form a continuum with the 1-kpc-scale double-lobed compact steep-spectrum sources, which make up 15-20% of lower frequency samples. It is likely that these sub-galactic-size symmetric sources are the precursors to the large-scale classical double sources. There is a surprising peak around 90 degrees in the histogram of misalignments between the dominant source axes on parsec and kiloparsec scales; this seems to be associated with sources exhibiting a high degree of relativistic beaming. VLBI snapshot surveys have great cosmological potential via measurements of both proper motion and angular size vs. redshift as well as searches for gravitational "millilensing."

Probes of the inner jets of blazars.

I review models for the "inner jet" in blazars, the section that connects the central engine with the radio jet. I discuss how the structure and physics of the inner jet can be explored using millimeter-wave VLBI (very-long-baseline radio interferometry) as well as multiwaveband observations of blazars. Flares at radio to gamma-ray frequencies should exhibit time delays at different wavebands that can test models for both the high-energy emission mechanisms and the nature of the inner jet in blazars.

Precise position measurements of jupiter, saturn and uranus systems with the very large array

The discovery of Quasi-Stellar Objects (QSO’s) and the development of Very Long Baseline Interferometry (VLBI) techniques since the early 1960’s have afforded an extremely precise coordinate grid system or reference frame on the sky. Since the QSO’s are both stellar-like objects and strong emitters of radio flux, their positions can be measured with both optical and radio techniques. However, the radio VLBI measurements on simple, compact sources are by far the most precise, the useful unit being micro-radians. The origin of the VLBI reference frame is tied to the stellar reference frame by the measurements of Hazard et al. (1971) who precisely measured the optical position of the QSO 3C273 with lunar occultations, which, when combined with VLBI position measurements of the same source, ‘‘tied’’ the VLBI and stellar position frames together. Their reported accuracy is ±0.15 arc seconds, but there is good reason to believe that the error in that frame tie is significantly smaller. Radio tracking measurements of spacecraft associated with the inner planets, such as the Viking Orbiters around Mars, combined with VLBI measurements indicate that the frames of the inner planets (i.e. their ephemerides), stellar system, and the VLBI reference sources are consistent to better than ±0:’03 (Newhall et al., 1984).

Studying tectonics with quasar VLBI

To study the tectonic deformation of the Earth's crust, geophysicists would like to be able to measure intercontinental distances to within about a centimeter. The long-term movement of tectonic plates is typically a few centimeters per year. Recent developments in very-long-baseline radiointerferometry (VLBI) and laser ranging, using quasars and orbiting satellites as reference markers, appear to be putting these techniques within striking distance of this extraordinary accuracy. By such means one also measures very precisely tidal deformations of the Earth's crust, and variations in the direction and speed of its rotation.

Plans for VLBI with 10 antennas

For more than a decade, astronomers have been using very‐long‐baseline radio interferometry (VLBI) to investigate the structure of distant objects three orders of magnitude smaller than the resolution limit of any optical telescope. A network of seven radio telescopes, from Massachusetts to California, acting as a transcontinental interferometer, has in recent years given us radio images of quasars and galactic nuclei, showing features down to a fraction of a milli‐arcsecond. No other astronomical technique—existing or proposed—can come within two orders of magnitude of such resolving power.