Two-sided Jet Research Articles

First 3 mm-VLBI imaging of the two-sided jet in Cygnus A

First 3 mm-VLBI imaging of the two-sided jet in Cygnus A

The stratified two-sided jet of Cygnus A

High-resolution Very-Long-Baseline Interferometry observations of relativistic jets are essential to constrain fundamental parameters of jet formation models. At a distance of 249 Mpc, Cygnus A is a unique target for such studies, being the only Fanaroff-Riley Class II radio galaxy for which a detailed sub-parsec scale imaging of the base of both jet and counter-jet can be obtained. Observing at millimeter wavelengths unveils those regions which appear self-absorbed at longer wavelengths and enables an extremely sharp view towards the nucleus to be obtained. We performed 7 mm Global VLBI observations, achieving ultra-high resolution imaging on scales down to 90 $\mu$as. This resolution corresponds to a linear scale of only ${\sim}$400 Schwarzschild radii (for $M_{\mathrm{BH}}=2.5 \times 10^9 M_{\odot}$). We studied the kinematic properties of the main emission features of the two-sided flow and probed its transverse structure through a pixel-based analysis. We suggest that a fast and a slow layer, with different acceleration gradients, exist in the flow. The extension of the acceleration region is large (${\sim} 10^4 R_{\mathrm{S}}$), indicating that the jet is magnetically-driven. The limb brightening of both jet and counter-jet and their large opening angles ($\phi_\mathrm{J}{\sim} 10^{\circ}$) strongly favor a spine-sheath structure. In the acceleration zone, the flow has a parabolic shape ($r\propto z^{0.55\pm 0.07}$). The acceleration gradients and the collimation profile are consistent with the expectations for a jet in "equilibrium'' (Lyubarsky 2009), achieved in the presence of a mild gradient of the external pressure ($p\propto z^{-k}, k\leq2$).}

BROADBAND OBSERVATIONS OF THE COMPTON-THICK NUCLEUS OF NGC 3393

We present new NuSTAR and Chandra observations of NGC 3393, a galaxy reported to host the smallest separation dual AGN resolved in the X-rays. While past results suggested a 150 pc separation dual AGN, three times deeper Chandra imaging, combined with adaptive optics and radio imaging suggest a single, heavily obscured, radio-bright AGN. Using VLA and VLBA data, we find an AGN with a two-sided jet rather than a dual AGN and that the hard X-ray, UV, optical, NIR, and radio emission are all from a single point source with a radius <0.2". We find that the previously reported dual AGN is most likely a spurious detection resulting from the low number of X-ray counts (<160) at 6-7 keV and Gaussian smoothing of the data on scales much smaller than the PSF (0.25" vs. 0.80" FWHM). We show that statistical noise in a single Chandra PSF generates spurious dual peaks of the same separation (0.55$\pm$0.07" vs. 0.6") and flux ratio (39$\pm$9% vs. 32% of counts) as the purported dual AGN. With NuSTAR, we measure a Compton-thick source (NH=$2.2\pm0.4\times10^{24}$ cm$^{-2}$) with a large torus half-opening angle, {\theta}=79 which we postulate results from feedback from strong radio jets. This AGN shows a 2-10 keV intrinsic to observed flux ratio of 150. Using simulations, we find that even the deepest Chandra observations would severely underestimate the intrinsic luminosity of NGC 3393 above z>0.2, but would detect an unobscured AGN of this luminosity out to high redshift (z=5).

DETERMINATION OF CENTRAL ENGINE POSITION AND ACCRETION DISK STRUCTURE IN NGC 4261 BY CORE SHIFT MEASUREMENTS

We report multifrequency phase-referenced observations of the nearby radio galaxy NGC 4261, which has prominent two-sided jets, using the Very Long Baseline Array at 1.4-43 GHz. We measured radio core positions showing observing frequency dependences (known as "core shift") in both approaching jets and counter jets. The limit of the core position as the frequency approaches infinity, which suggests a jet base, is separated by 82$\pm$16 ${\mu}$as upstream in projection, corresponding to (310$\pm$60)Rs (Rs: Schwarzschild radius) as a deprojected distance, from the 43 GHz core in the approaching jet. In addition, the innermost component at the counter jet side appeared to approach the same position at infinity of the frequency, indicating that cores on both sides are approaching the same position, suggesting a spatial coincidence with the central engine. Applying a phase referencing technique, we also obtained spectral index maps, which indicate that emission from the counter jet is affected by free-free absorption (FFA). The result of the core shift profile on the counter jet also requires FFA because the core positions at 5-15GHz cannot be explained by a simple core shift model based on synchrotron self-absorption (SSA). Our result is apparently consistent with the SSA core shift with an additional disk-like absorber over the counterjet side. Core shift and opacity profiles at the counter jet side suggest a two-component accretion: a radiatively inefficient accretion flow at the inner region and a truncated thin disk in the outer region. We proposed a possible solution about density and temperature profiles in the outer disk on the basis of the radio observation.

The core shift measurements for two-sided jets affected by Free-Free absorption using VLBA

A radio core represents the peak of intensity in VLBI images and is located at the base of jets. It appears at different positions depending on frequencies. This is known as “core shift”, caused by absorption of the core emission. The position of the central engine in an AGN can be estimated accurately by measuring the core shift with multifrequency and phase-referencing observations. We observed NGC 4261 using the VLBA at seven frequencies. This source is a nearby FR–I type radio galaxy at the distance of 30 Mpc and has prominent two-sided jets. We measured the core shifts in not only approaching side but also counter side of the jets. The positions of core at infinity of frequency in both side indicated to come close asymptotically to the same position, which was separated by 82±16 μas from 43 GHz core position, corresponding to 310±60 R s (Schwarzschild radius). This source also has another feature that there is a region affected by free–free absorption (FFA) in the vicinity of the core and toward the counter jet. Moreover, we also found the same feature in other three sources, 3C 84, Cen A and Cyg A, which are also nearby galaxies with two-sided jets and with an indication of the FFA regions. We will measure the core shifts in these sources by using same technique as NGC 4261 in order to study the structure of circumnuclear plasma, to determine the position of the central engine and to test core shifts due to FFA.

The radio core and jet in the broad absorption-line quasar PG 1700+518

Abstract The blueshifted broad absorption lines (BAL) or troughs are observed in active galactic nuclei (AGNs) when our line of sight is intercepted by a high-speed outflow (wind), likely originating in the accretion disc. The outflow or wind can shed light on the internal structure obscured by the AGN torus. Recently, it has been shown that this outflow is rotating in the BAL quasar PG 1700+518, further supporting the accretion disc origin of the wind. With the purpose of giving independent constraints on the wind geometry, we performed high-resolution European very long baseline interferometry (VLBI) Network (EVN) observations at 1.6 GHz in 2010. Combining the results with the Very Large Array (VLA) archival data at 8.4 GHz, we present its jet structure on scales of parsec (pc) to kiloparsec (kpc) for the first time. The source shows two distinct jet features in east–west direction with a separation of around 4 kpc. The eastern feature, which has so far been assumed to hide the core, is a kpc-scale hotspot, which is completely resolved out in the EVN image. In the western jet feature, we find a compact jet component, which pinpoints the position of the central black hole in the galaxy. Jet components on both sides of the core are additionally detected in the north-west–south-east direction, and they show a symmetric morphology on scale of &amp;lt;1 kpc. This two-sided jet feature is not common in the known BAL quasars and indicates that the jet axis is far away from the line of sight. Furthermore, it is nearly parallel to the scattering plane revealed earlier by optical polarimetry. By analogy to polar-scattered Seyfert 1 galaxies, we conclude that the jet likely has a viewing angle around 45°. The analogy is further supported by the recent report of significant cold absorption in the soft X-rays, a nearly unique feature to polar-scattered Seyfert galaxies. Finally, our observations have confirmed the earlier finding that the majority of radio emission in this galaxy arises from AGN activity rather than star formation.

The asymmetric radio structure and record jet of giant quasar 4C 34.47

Giant double-lobed radio source 4C34.47 displays a straight one-sided jet, measuring a record length of 380kpc, in its double-lobed radio structure. Assuming an intrinsically symmetric two-sided jet structure the radio source jet axis must be at least 33 degrees away from the sky plane, that is within 57 degrees from the line of sight. The radio polarization properties indicate that this giant source has largely outgrown the depolarizing halo generally associated with the host galaxies of powerful radio sources. The measured small depolarization asymmetry is nevertheless in accordance with its inferred orientation. All data for this giant radio source are in agreement with its preferred orientation as predicted within the unification scheme for powerful radio sources. Seen under a small aspect angle the radio source is large but not excessively large. The global properties of 4C34.47 do not differ from other giant (old) FR2 radio sources: it is a slowly expanding low-luminosity radio source.

Powering the jets in NGC 1052

We have studied the inner regions of the LINER galaxy NGC 1052 since the mid 1990s at high resolution with 15 GHz very-long-baseline interferometry observations. A compact, two-sided jet structure is revealed, with multiple sub-parsec scale features moving outward from the central region with typical speeds of 0.26 c. Complementary to this, since early 2005 we are performing a multi-mission campaign of observations of this source, including X-ray spectroscopy, X-ray, and radio flux density monitoring, and VLBA observations at 22 GHz and 43 GHz. X-ray variability is present at time scales of weeks, comparable with the structural changes observed by VLBI. Here we present first results of the high-resolution imaging observations and discuss these findings in the context of the multi-band campaign.

Non-uniform model for the synchrotron radiation of Sgr A* and other low-luminosity galactic nuclei

A model for non-uniform source of synchrotron radiation with a power-law radial distribution of the magnetic field and relativistic-electron density along one-or two-sided jets is described. Non-relativistic jets with both constant cross sections (collimated jets) and cross sections that are proportional to distance (conical jets) are considered. Formulas that can be used to determine source parameters from the spectral index, source size, and index of the relativistic-electron energy spectrum based on multi-frequency observations are obtained. In the case of a conical jet, these formulas coincide with the analogous formulas for a spherical source obtained by A.P. Marscher. Relations that can be used to estimate the magnetic-field strength from the brightness temperature in the self-absorbed region are also obtained. As examples, the inhomogeneous-source model is applied to the compact radio sources at the centers of the Milky Way, Sgr A*, and the low-luminosity galactic nuclei M81* and M87*, which are associated with supermassive black holes. The inner radius of the radiation region is determined. For Sgr A*, this distance turns out to be comparable to the gravitational radius, smaller than the radius of the last stable orbit for a non-rotating black hole, and consistent with the radius of the last stable orbit expected for a rotating black hole. The inner radii in M81* and M87* are ∼15 RS, an order of magnitude larger than for Sgr A*. Estimates of the magnetic field at the inner radius are 400 G for M81*, 0.65–5.3 kG for Sgr A*, and 20–100 kG for M87*. These magnetic fields and the Blandford-Znajek model for the radiation of a rotating black hole are used to estimate the rotational speed of the black holes, which are in agreement with the characteristic variability time scales for these three objects. However, the accuracy of these estimates is modest, and is limited primarily by the accuracy of interferometric measurements at millimeter wavelengths.

Positional coincidence between an H2O maser and a plasma torus in NGC 1052

AbstractWe present VLBA observation towards the nucleus of a nearby radio galaxy NGC 1052. In NGC 1052, two-sided jet structure and a dense plasma circumnuclear torus with a radius of 0.7 pc have been found around the central mass. It emits a H2O megamaser, which is redshifted with respect to the systemic velocity of the galaxy (1491 kms−1) with a large velocity width of 100 kms−1 (FWHM). The maser gas is found at the inner jet components of both the approaching and receding jets. The maser gas is positionally coincident with a plasma torus. The maser gas in NGC 1052 could be explained as a circumnuclear torus or disk, as found for the nucleus of NGC 4258.

A possible EGRET source with a two-sided radio jet structure on a parsec scale - 3EG J0808+5114

We report on the European VLBI Network observations of the radio source 0803+5126 in the field of the Energetic Gamma Ray Experiment Telescope (EGRET) source 3EG J0808+5114. High-resolution images of 0803+5126 at 6 cm showed a core emission with an extremely low brightness temperature plus an asymmetric two-sided jet structure on both parsec and tens of parsec scales, suggesting a relatively weak beaming effect in radio emission. The radio structures on different scales exhibit quite different orientations of extension, indicating the occurrence of jet bending. Based on the positional uncertainty of 3EG J0808+5114, its separations to various possible radio counterparts, and very low gamma-ray variability, it is reasonable to believe that 0803+5126 is the most probable radio counterpart of 3EG J0808+5114, making it the first EGRET-detected active galactic nucleus to be associated with a two-sided jet structure on a parsec scale. The central engine is estimated to have a mass of similar to 5.0 x 10(8) M-circle dot by using Mg II emission line features, for which the Eddington limit is approximately two times lower than the integrated gamma-ray luminosity. The minimum Doppler factor in gamma-ray emission is similar to 2.5, but the true one should not be much larger than this value on account of its quiet stable gamma-ray emission. This is consistent with our very long baseline interferometry results of the two-sided radio jet structure. The relativistic beaming effect and downstream jet bending may play an important role in the paradox between the large gamma-ray luminosity far above its Eddington limit and the observed asymmetric two-sided radio structure.

Thomson-thick AGN — A Radio Emission View

We have investigated the symmetric twin-jets related to innermost region of some AGNs, try to get their Thomson-thick properties. We propose to use radio method to obtain the column densities for 2 sources OQ208 and NGC 4261, the result for OQ208 is consistent with the estimate from X-ray method on average, and the column density for NGC 4261 is a bit less than that from X-ray. Other 2 sources NGC 1052 and Mrk 231 are also discussed. More compact symmetric sources with two-sided jets could be suitable to be used to study Thomson-thick/thin AGN properties in future.

Very Large Array Simultaneous 1.3 cm Continuum and H2O Maser Observations toward IRAS 20126+4104

We have observed simultaneously the 1.3 cm continuum and H2O maser emission toward the high-mass protostar IRAS 20126+4104 with the Very Large Array (VLA) in its A configuration, providing an angular resolution of 01. We have detected an unresolved continuum source [I20126N(1.3 cm)] nearly coinciding with the strongest peak of the northern double continuum source previously detected at 3.6 cm in the field and suggested to be a radio jet. In addition, we have detected 29 water maser spots, 21 of which are tightly grouped in a cluster of 01 size displaced 007 (120 AU) northwest of I20126N(1.3 cm). The relative positions of the masers with respect to I20126N(1.3 cm) have been established with 10 mas of accuracy. The overall spatial distribution and line-of-sight velocity components of the water maser spots associated with I20126N(1.3 cm) are fully consistent with those previously found by Moscadelli and collaborators through VLBI measurements and interpreted as tracing a conical outflow, with the powering source (suggested to be traced by a one-sided radio continuum jet) located at the cone vertex. Within this very reasonable scenario, our observations would indicate that the source I20126N(1.3 cm) is showing the position of the high-mass protostar. Analyzing the spatiokinematic distribution of the VLA water masers, we propose that their motions also show, in addition to proper motions on the order of 100 km s-1 seen in the plane of the sky, a component of rotation with velocities on the order of 20 km s-1. The water masers seem then to be both rotating (as evidenced from their radial velocities) and changing their position in the plane of the sky (from the proper motions). Within this scenario, some of the water maser spots could be within a rotating circumstellar disk of 170 AU size around a protostar of 20 M⊙ if it is located at the center of the water maser cluster rather than located at the position of I20126N(1.3 cm). Then, I20126N(1.3 cm) would trace one of the two peaks of a two-sided thermal jet, with the high-mass protostar located in between. We think that simultaneous high angular resolution and sensitive multifrequency (from centimeter to submillimeter wavelengths) observations are the key studies to knowing both the nature of the continuum emission (one- or two-sided jet?) and the location of the high-mass protostar, which is very relevant for modeling this important object.

Low power compact radio galaxies at high angular resolution

We present sub-arcsecond resolution multi-frequency (8 and 22 GHz) VLA images of five low power compact (LPC) radio sources, and phase referenced VLBA images at 1.6 GHz of their nuclear regions. At the VLA resolution we resolve the structure and identify component positions and flux densities. The phase referenced VLBA data at 1.6 GHz reve als flat-spectrum, compact cores (down to a few milliJansky) in f our of the five sources. The absolute astrometry provided by t he phase referencing allows us to identify the center of activi ty on the VLA images. Moreover, these data reveal rich structures, including two-sided jets and secondary components. On the basis of the arcsecond scale structures and of the nuclear properties, we rule out the presence of strong relativistic effects in our LPCs, which must be intrinsically small (deprojected linear sizes < 10 kpc). Fits of continuous injection models reveal break frequencies in the GHz domain, and ages in the range 10 5 − 10 7 yrs. In LPCs, the outermost edge may be advancing more slowly than in more powerful sources or could even be stationary; some LPCs might also have ceased their activity. In general, the properties of LPCs can be related to a number of reasons, including, but not limited to: youth, frustration, low kinematic power jets, and short-lived activity in the radio.

VLBA Imaging of Central Engines in Radio‐Quiet Quasars

We have used the Very Long Baseline Array (VLBA) to image five radio-quiet quasars (RQQs) at milliarcsecond resolution, at frequencies between 1.4 and 5 GHz. These quasars have typical total flux densities of a few millijanskys at gigahertz frequencies and are compact on arcsecond scales. The VLBA images reveal that four of the quasars are dominated by unresolved radio cores, while the fifth has an apparent two-sided jet. Typical core brightness temperatures range from 108 to at least 109 K. The compact radio morphologies and X-ray luminosities of many objects in the RQQ sample seem to indicate classical accretion onto black holes as massive as 109 M☉, with emission physics in many ways similar to their radio-loud counterparts. Therefore, the relatively small amount of radiative energy emerging at radio wavelengths in the RQQs may simply be due to the presence of less powerful radio jets.

Giant Metre-Wave Radio Telescope 615 MHz observations of microquasar candidates

Observations at 245 and 615 MHz were carried out in July–August 2002 with the Indian low-frequency array Giant Metre-wave Radio Telescope (GMRT). 26 microquasar candidates were included in the source list. The radio structures of these sources have been obtained for many of these. Some sources (MCQC J0014+58 and MCQC J0824-30) demonstrate the two-sided radio jet as a GMRT scale and could be considered as promising microquasar candidates. This paper contains a description of the processing method used for 615 MHz data. This processing has been carried out with the software Astro Space Locator (ASL for Windows).

Altered Luminosity Functions of Relativistically Beamed Jet Populations

The intrinsic luminosity functions of extremely fast jets found in many active galaxies and gamma-ray bursts are difficult to measure since their apparent luminosities are strongly affected by relativistic beaming. Past studies have only provided analytical predictions for the beamed characteristics of populations in which all jets have the same Lorentz factor. However, the jets found in active galaxies are known to span a large range of speeds. Here we derive analytical expressions for the expected Doppler factor distributions and apparent (beamed) luminosity functions of randomly oriented, two-sided jet populations that have bulk Lorentz factors distributed according to a simple power law over a range [γ1, γ2]. We find that if a jet population has a uniform or inverted Lorentz factor distribution, its Doppler factor distribution and beamed luminosity function will be very similar to that of a jet population with Lorentz factors all equal to γ2. In particular, the beamed and intrinsic luminosity functions will have the same slope at high luminosities. In the case of a steep Lorentz factor distribution, the slopes at high luminosities will also be identical, but only up to an apparent luminosity equivalent to the upper cutoff of the intrinsic luminosity function. In addition, at very high apparent luminosities, the slope will be proportional to the power-law slope of the Lorentz factor distribution. At very low luminosities, the form of the apparent luminosity function is very sensitive to the lower cutoff and steepness of the Lorentz factor distribution. We discuss how it should be possible to recover useful information about the intrinsic luminosity functions of relativistic jets by studying flux-limited samples selected on the basis of beamed jet emission.

Time‐sequenced Multi–Radio Frequency Observations of Cygnus X‐3 in Flare

Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over three decades in frequency, over a period of six days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to nu^{-0.6}, after which the spectral index remains at this latter terminal value; a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are approximately 8 milliarcseconds. The light-crossing time within these plasmons is of the same order as the time-scale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons which are initially optically thick, but which after expansion become optically thin, explains the key features of the spectral evolution, for example the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is roughly beta = 0.63 and the precession period is about 5 days, significantly lower than fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero-separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cygnus X-3.

Uses and limitations of relativistic jet proper motions: lessons from Galactic microquasars

It is shown that the two-sided jet proper motions observed from the Galactic microquasars GRS 1915+105 and GRO J1655−40 in practice only allow us to place lower limits on the Lorentz factors of the outflows. As a consequence, it is not possible to rule out the possibility that jets from X-ray binaries are just as relativistic as those from active galactic nuclei (AGN). This results from the fact that distance estimates place the sources, within uncertainties, at the maximum distance dmax which corresponds to an intrinsic velocity v=c. The general case is explored, for a range of intrinsic Lorentz factors and angles to the line of sight, and it is shown that a source of significantly relativistic jets will nearly always be observed close to dmax, and as a result it is unlikely that we will ever be able to measure with any accuracy the Lorentz factor of a jet from two-sided proper motions. We will generally not be able to do more than place a lower limit on the Lorentz factor of the flow, and this limit is naturally even lower in the cases where we only observe the approaching jet. On the other hand, under the assumption that any two-sided jets we see are intrinsically relativistic, we can confidently place the source at a distance d∼dmax. As a result, observations of two-sided proper motions in relativistic jets from AGN would be extremely important for calibration of the cosmological distance scale. While the proper motions do not allow us to measure the Doppler shifts associated with the jets, the ratio of proper motions will correspond to the ratio of frequencies of any emission lines emitted by both jets, which will aid in searching for such lines. Furthermore, it is shown that if the jet is precessing, the product of the proper motions as a function of angle to the line of sight may be used to determine if the jet is only mildly relativistic.

EVN and MERLIN observations of microquasar candidates at low galactic latitudes

In an attempt to increase the number of known microquasars, Paredes et al. (2002) have presented a long-term project focused on the search for new objects of this type. They performed a cross-identification between X-ray and radio catalogs under very restrictive selection criteria for sources with |b|<5 degrees, and obtained a sample of 13 radio-emitting X-ray sources. Follow-up observations of 6 of these sources with the VLA provided accurate coordinates, which were used to discover optical counterparts for all of them. We have observed these six sources with the EVN and MERLIN at 5 GHz. Five of the six objects have been detected and imaged, presenting different morphologies: one source has a two-sided jet, three sources have one-sided jets, and one source is compact. With all the presently available information, we conclude that two of the sources are promising microquasar candidates in our Galaxy.