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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

The fate of dead radio-loud active galactic nuclei: A new prediction of long-lived shell emission

We examine the fate of a dead radio source in which jet injection from the central engine has stopped at an early stage of its evolution (t=t{sub j}≲10{sup 5} years). To this aim, we theoretically evaluate the evolution of the emission from both the lobe and the shell, which are composed of shocked jet matter and a shocked ambient medium (i.e., shell), respectively. Based on a simple dynamical model of expanding lobe and shell, we clarify how the broadband spectrum of each component evolves before and after the cessation of the jet activity. It is shown that the spectrum is strongly dominated by the lobe emission while the jet is active (t⩽t{sub j}). On the other hand, once the jet activity has ceased (t>t{sub j}), the lobe emission fades out rapidly, since fresh electrons are no longer supplied from the jet. Meanwhile, shell emission only shows a gradual decrease, since fresh electrons are continuously supplied from the bow shock that is propagating into the ambient medium. As a result, overall emission from the shell overwhelms that from the lobe at a wide range of frequencies from radio up to gamma-ray soon after the jet activity has ceased. Our result predicts a new class of dead radio sources that are dominated by shell emission. We suggest that the emission from the shell can be probed in particular at radio wavelengths with the Square Kilometer Array phase 1.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

The dynamics of Abell 2634

We have amassed a large sample of velocity data for the cluster of galaxies Abell 2634 which contains the wide-angle tail (WAT) radio source 3C 465. Robust indicators of location and scale and their confidence intervals are used to determine if the cD galaxy, containing the WAT, has a significant peculiar motion. We find a cD peculiar radial velocity of 219 plus or minus 98 km s(exp -1). Further dynamical analyses, including substructure and normality tests, suggest that A 2634 is an unrelaxed cluster whose radio source structure may be bent by the turbulent gas of a recent cluster-subcluster merger.

Pinkney, J.↗
DOE OSTI2019

Optical Detection of Star Formation in a Cold Dust Cloud in the Counterjet Direction of Centaurus A

We have identified a set of optical emission-line features 700″ (12 kpc) to the southwest of the nucleus of Centaurus A, roughly opposite to the radio jet and well-known optical emission filaments associated with the northern radio structure. This location is roughly along the axis of the southwestern radio lobes, although there is no coherent jet at this radius. We use integral-field optical spectroscopy to trace the ratios of strong emission lines, showing changes in excitation across the region, and significant local reddening. The emission regions are spatially associated with far-infrared emission peaks in one of two cold dust clouds identified using Herschel and Spitzer data, and there may be a mismatch between the low temperature of the dust and the expected heating effect of young stars. The strong emission lines have ratios consistent with photoionization in normal H ii regions, requiring only modest numbers of OB stars; these stars and their cooler accompanying populations must be largely obscured along our line of sight. These data fit with a picture of fairly ordinary formation of clusters in a large giant molecular cloud, or network of such clouds. The location, projected near the radio-source axis and within the radius where a starburst wind has been inferred on the other side of the galaxy, raises the question of whether this star-forming episode was enhanced or indeed triggered by an outflow from the central parts of Centaurus A. However, the level of star formation is normal for the associated cold-gas mass and column density, and optical emission-line ratios and line widths limit the role of shocks on the gas, so any interaction with an outflow, associated either with the radio source or star formation in the gas-rich disk of Centaurus A, can at most have compressed the gas weakly. We speculate that the presence of similar star-forming regions on both sides of the galaxy, contrasted with the difference in the character of the emission-line clouds, reflects the presence of a collimated radio jet to the northeast and perhaps anisotropic escape of ionizing radiation from the AGN as well. In this view, the star formation on the southwestern side of Cen A could be enhanced indirectly via compression by a broad outflow (whether originated by a starburst or AGN), distinct from the radio jet and lobes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Gems of the Galaxy Zoos—A Wide-ranging Hubble Space Telescope Gap-filler Program*

We describe the Gems of the Galaxy Zoos (Zoo Gems) project, a gap-filler project using short windows in the Hubble Space Telescope's schedule. As with previous snapshot programs, targets are taken from a pool based on position; we combine objects selected by volunteers in both the Galaxy Zoo and Radio Galaxy Zoo citizen-science projects. Zoo Gems uses exposures with the Advanced Camera for Surveys to address a broad range of topics in galaxy morphology, interstellar-medium content, host galaxies of active galactic nuclei, and galaxy evolution. Science cases include studying galaxy interactions, backlit dust in galaxies, post-starburst systems, rings and peculiar spiral patterns, outliers from the usual color–morphology relation, Green Pea compact starburst systems, double radio sources with spiral host galaxies, and extended emission-line regions around active galactic nuclei. For many of these science categories, final selection of targets from a larger list used public input via a voting process. Highlights to date include the prevalence of tightly wound spiral structure in blue, apparently early-type galaxies, a nearly complete Einstein ring from a group lens, redder components at lower surface brightness surrounding compact Green Pea starbursts, and high-probability examples of spiral galaxies hosting large double radio sources.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1996

VLBI Observations of Southern EGRET Identifications: PKS 0208-512, PKS 0521-365 and PKS 0537-441 - 1

We present high-resolution very long baseline interferometry images of three southern radio sources that the Energetic Gamma-Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, has identified as greater than 100 MeV gamma-ray sources. These are the first results in a continuing program of VLBI observations of southern EGRET identifications. For two of these sources, PKS 0208-512 (at 4.851 GHz) and PKS 0537-441 (at 4.851 and 8.418 GHz), the images represent first-epoch observations. For the remaining lower redshift object, PKS 0521-365, we present images from three epochs at 4.851 GHz and an image from one further epoch at 8.418 GHz, spanning approximately 1 yr. We discuss the need for further extensive VLBI observations of EGRET-identified radio sources.

Tingay, S. J.↗
NASA NTRS1985

Does orientation affect the smoothness of parsec-scale radio jets?

The compact, nonthermal radio sources which are studied with very-long-baseline interferometry (VLBI) are often found to have a linear structure with a bright, nearly unresolved component at one end. This asymmetric structure is usually referred to as a 'core-jet' morphology. In some sources there are bright bumps or knots - regions of enhanced radio emission - along the 'jets', while other sources have relatively smooth, featureless jets. This paper reports the result of a study of the differences between compact radio sources with bumpy and smooth jets. It is concluded that most, if not all, of the difference in jet smoothness can be attributed to the orientation of the radio axis with respect to the line of sight. Thus, the VLBI observations do not necessarily imply that there are intrinsic differences in the smoothness of parsec-scale radio jets.

Jones, D. L.↗
DOE OSTI2013

RADIO TRANSIENTS FROM THE ACCRETION-INDUCED COLLAPSE OF WHITE DWARFS

It has long been expected that in some scenarios when a white dwarf (WD) grows to the Chandrasekhar limit, it can undergo an accretion-induced collapse (AIC) to form a rapidly rotating neutron star. Nevertheless, the detection of such events has so far evaded discovery, likely because the optical, supernova-like emission is expected to be dim and short-lived. Here we propose a novel signature of AIC: a transient radio source lasting for a few months. Rapid rotation along with flux freezing and dynamo action can grow the WD's magnetic field to magnetar strengths during collapse. The spin-down of this newly born magnetar generates a pulsar wind nebula (PWN) within the {approx}10{sup -3}-10{sup -1} M{sub Sun} of ejecta surrounding it. Our calculations show that synchrotron emission from the PWN may be detectable in the radio, even if the magnetar has a rather modest magnetic field of {approx}2 Multiplication-Sign 10{sup 14} G and an initial spin period of {approx}10 ms. An all-sky survey with a detection limit of 1 mJy at 1.4 GHz would see {approx}4(f/10{sup -2}) above threshold at any given time, where f is the ratio of the AIC rate to Type Ia supernova rate. A similar scenario may result from binary neutron stars if some mergers produce massive neutron stars rather than black holes. We conclude with a discussion of the detectability of these types of transient radio sources in an era of facilities with high mapping speeds.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2003

Measurement of Aberration of Gravity Using Jupiter's Motion

On September 8, 2002 Jupiter passed within 3.7 min of the bright radio source, J0842+1835. The deflection predicted by General Relativity (GR) of this source at closest approach contains two major terms: an outward radial deflection from Jupiter of 1190 arcsec, and a deflection of 51 arcsec associated with the aberration of gravity. We used the Very Long Baseline Array with the Effelsberg telescope to measure the position of J0842+1835 with respect to two quasars a few degrees away during the period of September 4-12. With the use of two quasars on opposites sides of the radio source, we removed most of the tropospheric delay variations, and obtained the deflection of J0842+1835 on September 8 to an accuracy less than 10 arcsec. The aberrational part of the deflection that we measured is 0.98 +/- 0.19 times that predicted by GR. The speed of gravity associated with the measured aberration of the gravitational field of Jupiter is (1.06 +\- 0.21) times the speed of light.

Fomalont, Edward B.↗
NASA NTRS1976

A very-long-baseline interferometer system for geodetic applications

A very-long-baseline interferometer system was designed and built for geodetic applications. Each interferometer terminal records a 360-kHz spectral band of noise from a compact extragalactic radio source. The center frequency of the spectral band can be selected to sample sequentially bands covering a much wider frequency range to obtain subnanosecond accuracy in group-delay measurements. A tunnel-diode pulse generator is used to calibrate the delays in the receiver. The necessary sets of algorithms and computer programs have been developed to analyze the data and have allowed the system to be employed to make accurate determinations of vector baselines, radio-source positions, polar motion, and universal time.

Whitney, A. R.↗
DOE OSTI2024

A comparison of compact, presumably young with extended, evolved radio active galactic nuclei

Context.The triggering and evolution of active galactic nuclei (AGNs) and the interaction of the AGN with its host galaxy is an important topic in extragalactic astrophysics. Radio sources with peaked spectra (peaked spectrum sources, PSS) and compact symmetric objects (CSO) are powerful, compact, and presumably young AGNs and therefore particularly suitable to study aspects of the AGN-host connection. Aims.We use a statistical approach to investigate properties of a PSS-CSO sample that are related to host galaxies and could potentially shed light on the link between host galaxies and AGNs. The main goal is to compare the PSS-CSO sample with a matching comparison sample of extended sources (ECS) to see if the two have significant differences. Methods.We analysed composite spectra, diagnostic line diagrams, multi-band spectral energy distributions (MBSEDs), star formation (SF) indicators, morphologies, and cluster environments for a sample of 121 PSSs and CSOs for which spectra are available from the Sloan Digital Sky Survey (SDSS). The statistical results were compared with those of the ECS sample, where we generally considered the two subsamples of quasi-stellar objects (QSOs) and radio galaxies separately. The analysis is based on a large set of archival data in the spectral range from the ultraviolet to mid-infrared. Results.We find significant differences between the PSS-CSO and the ECS sample. In particular, we find that the ECS sample has a higher proportion of passive galaxies with a lower star formation activity. This applies to both sub-samples (QSOs or radio galaxies) as well as to the entire sample. The star formation rates of the PSS-CSO host galaxies with available data are typically in the range ∼0 to 5 ℳ ⊙ yr −1 , and the stellar masses are in the range 3 × 10 11 to 10 12 ℳ ⊙ . Secondly, in agreement with previous results, we find a remarkably high proportion of PSS-CSO host galaxies with merger signatures. The merger fraction of the PSS-CSO sample is 0.61 ± 0.07, which is significantly higher than that of the comparison sample (0.15 ± 0.06). We suggest that this difference can be explained by assuming that the majority of the PSSs and CSOs cannot evolve to extended radio sources and are therefore not represented in our comparison sample.

Astronomy & Astrophysics↗
NASA NTRS1987

Very long baseline interferometry using a radio telescope in Earth orbit

Successful Very Long Baseline Interferometry (VLBI) observations at 2.3 GHz were made using an antenna aboard an Earth-orbiting spacecraft as one of the receiving telescopes. These observations employed the first deployed satellite (TDRSE-E for East) of the NASA Tracking and Data Relay Satellite System (TDRSS). Fringes were found for 3 radio sources on baselines between TDRSE and telescopes in Australia and Japan. The purpose of this experiment and the characteristics of the spacecraft that are related to the VLBI observations are described. The technical obstacles to maintaining phase coherence between the orbiting antenna and the ground stations, as well as the calibration schemes for the communication link between TDRSE and its ground station at White Sands, New Mexico are explored. System coherence results and scientific results for the radio source observations are presented. Using all available calibrations, a coherence of 84% over 700 seconds was achieved for baselines to the orbiting telescope.

Ulvestad, J. S.↗
NASA NTRS1972

Precision geodesy via radio interferometry.

Very-long-baseline interferometry experiments, involving observations of extragalactic radio sources, were performed in 1969 to determine the vector separations between antenna sites in Massachusetts and West Virginia. The 845.130-kilometer baseline was estimated from two separate experiments. The results agreed with each other to within 2 meters in all three components and with a special geodetic survey to within 2 meters in length; the differences in baseline direction as determined by the survey and by interferometry corresponded to discrepancies of about 5 meters. The experiments also yielded positions for nine extragalactic radio sources and allowed the hydrogen maser clocks at the two sites to be synchronized.

Hinteregger, H. F.↗
DOE OSTI2013

VERY LONG BASELINE ARRAY MULTI-FREQUENCY POLARIMETRIC IMAGING OF RADIO-LOUD BROAD ABSORPTION LINE QUASARS

We conducted the first multi-frequency polarimetric imaging of four broad absorption line (BAL) quasars using the Very Long Baseline Array at milliarcsecond resolutions to investigate the inclination of the nonthermal jet and test the hypothesis that radio sources in BAL quasars are still young. Among these four sources, J0928+446, J1018+0530, and J1405+4056 show one-sided structures in parsec scales and polarized emission detected in the core. These characteristics are consistent with those of blazars. We set constraints on viewing angles to <66 Degree-Sign for these jets in the framework of a Doppler beaming effect. J1159+0112 exhibits an unpolarized gigahertz-peaked spectrum component and several discrete blobs with steep spectra on both sides of the central component across {approx}1 kpc. These properties are consistent with those of young radio sources. We discuss the structures of jets and active galactic nucleus wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Effect of VSR invariant Chern-Simons Lagrangian on photon polarization

We propose a generalization of the Chern-Simons (CS) Lagrangian which is invariant under the SIM(2) transformations but not under the full Lorentz group. The generalized lagrangian is also invariant under a SIM(2) gauge transformation. We study the effect of such a term on radiation propagating over cosmological distances. We find that the dominant effect of this term is to produce circular polarization as radiation propagates through space. We use the circular polarization data from distant radio sources in order to impose a limit on this term.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1976

Observations of intensity and linear polarization of CTB 80 at 6 and 2.8 cm

The paper presents observations of the distribution of intensity and linear polarization over the radio source CTB 80 at 6 and 2.8 cm. The physical nature of the source is examined, particularly the relation between the strong central component and a ridgelike structure that extends across the northern half in the E-W direction. The structure, nonthermal spectrum, and polarization of the central component are found to be very similar to those of a supernova remnant, but the ridge is shown to be unlike any feature of any other SNR. It is suggested that the ridge may have resulted from expansion of the remnant shell into a dense interstellar cloud or, alternatively, that CTB 80 might be a nearby extragalactic radio source similar to Centaurus A.

Velusamy, T.↗
NASA NTRS2004

On-the-Fly Mapping for Calibrating Directional Antennas

An improved method of calibrating a large directional radio antenna of the type used in deep-space communication and radio astronomy has been developed. This method involves a raster-scanning-and-measurement technique denoted on-the-fly (OTF) mapping, applied in consideration of the results of a systematic analysis of the entire measurement procedure. Phenomena to which particular attention was paid in the analysis include (1) the noise characteristics of a total-power radiometer (TPR) that is used in the measurements and (2) tropospherically induced radiometer fluctuations. The method also involves the use of recently developed techniques for acquisition and reduction of data. In comparison with prior methods used to calibrate such antennas, this method yields an order-of-magnitude improvement in the precision of determinations of antenna aperture efficiency, and improvement by a factor of five or more in the precision of determination of pointing error and beam width. Prerequisite to a meaningful description of the present method is some background information concerning three aspects of the problem of calibrating an antenna of the type in question: In OTF mapping measurements in which a TPR is used, the desired data are the peak temperature corresponding to a radio source, the pointing offset when the antenna is commanded to point toward the source, and the shape of the main lobe of the antenna beam, all as functions of the antenna beam elevation and azimuth angles. These data enable one to calculate the (1) antenna aperture efficiency by comparing the measured peak temperature with that expected for a 100-percent-efficient antenna, (2) the mechanical pointing error resulting from small misalignments of various parts of the antenna structure, and (3) misalignments of the antenna subreflector and other mirrors. For practical reasons having to do with obtaining adequate angular resolution and all-sky coverage, it is necessary to perform azimuth and elevation scans fairly rapidly. Many natural radio sources used in calibrating antennas are only approximately pointlike: some sources subtend angles greater than the beam width of a given antenna. In such a case, the antenna partially resolves the source structure and does not collect all of the radiation emitted by the source. This makes it necessary to estimate how much of the total known radiation from the source would actually be collected by the antenna if it were 100-percent efficient. The resulting estimate, leading to a source-size correction factor, introduces another degree of uncertainty to the measurements. OTF mapping can remove this uncertainty

Rochblatt, David↗
NASA NTRS1991

Statistical analysis of power-size-redshift distributions of extragalactic jets

This paper investigates whether a hot, sparse, yet cosmologically significant intergalactic medium is consistent with data collected from extragalactic radio sources. This is done by use of Monte Carlo simulations which employ previously run pseudohydrodynamical simulations to cover an observational parameter space. These observational parameters include the scale height, central density, and temperature of a (isothermal) galactic halo, and the power of the central engine which drives the jet. The Monte Carlo simulations generate distribution of sizes in bins of (received) power and redshift, which have been compared with observational data using Kolmogorov-Smirnov tests. Results of this analysis are consistent with the existence of an IGM with temperature and density mentioned above. In addition, this analysis suggests that the active lifetime of powerful extragalactic radio sources decreases with increasing power.

Rosen, Alexander↗
DOE OSTI2014

Diagnostics of the solar corona from comparison between Faraday rotation measurements and magnetohydrodynamic simulations

Polarized natural radio sources passing behind the Sun experience Faraday rotation as a consequence of the electron density and magnetic field strength in coronal plasma. Since Faraday rotation is proportional to the product of the density and the component of the magnetic field along the line of sight of the observer, a model is required to interpret the observations and infer coronal structures. Faraday rotation observations have been compared with relatively ad hoc models of the corona. Here for the first time we compare these observations with magnetohydrodynamic (MHD) models of the solar corona driven by measurements of the photospheric magnetic field. We use observations made with the NRAO Very Large Array of 34 polarized radio sources occulted by the solar corona between 5 and 14 solar radii. The measurements were made during 1997 May, and 2005 March and April. We compare the observed Faraday rotation values with values extracted from MHD steady-state simulations of the solar corona. We find that (1) using a synoptic map of the solar magnetic field just one Carrington rotation off produces poorer agreements, meaning that the outer corona changes in the course of one month, even in solar minimum; (2) global MHD models of the solar corona driven by photospheric magnetic field measurements are generally able to reproduce Faraday rotation observations; and (3) some sources show significant disagreement between the model and the observations, which appears to be a function of the proximity of the line of sight to the large-scale heliospheric current sheet.

79 ASTRONOMY AND ASTROPHYSICS↗