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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The coevolution of decimetric millisecond spikes and hard X-ray emission during solar flares

Results are presented of an analysis of a comprehensive data set of 27 solar flares with decimetric millisecond spikes between 1980 and 1989, simultaneously observed with the Zuerich radio spectrometers and the Hard X-ray Burst Spectrometer on the SMM spacecraft. Two contradictory relationships of the coevolution of hard X-ray and spiky radio emissions during flares are found: the temporal evolution of both emissions reveals a close functional dependence, but there is a substantial time delay between the two emissions. Five possible scenarios for the hard-X-ray-associated radio spike emission which may account for both their detailed coevolution and their substantial intervening time delay are discussed. All five scenarios are able to explain both the close coevolution of hard X-ray and radio emission as well as their mutual delay to some degree, but none of them can explain all observational aspects in a simple way.

Aschwanden, Markus J.↗
NASA NTRS1991

Milliarcsecond radio structure of weak-lined T Tauri stars

VLBI and VLA observations of six radio-bright weak-lined T Taur (WTT) stars are reported, as well as direct measurements of the sizes of the emitting regions. VLBI measurements established that essentially all the radio emission from these premain-sequence stars originates in regions 15 stellar radii or less in size. Corresponding brightness temperatures ranged from 10 exp 7.5 to not less than 10 exp 9 K, ruling out a thermal process such as free-free bremsstrahlung radiation from a circumstellar wind. The radio luminosity and structure of several stars changed significantly between measurements separated by 1 day. HD 283447 showed intraday radio variability on time scales as short as 1 hr. Corresponding VLBI measurements show a new unresolved component appearing after an increase in flux density, possibly indicating that the driving agent for larger radio flares originates close to the star. The high conformation rate of nonthermal radio emission from this initial sample of radio-bright WTT stars show that these solar-type premain-sequence stars alter their immediate environments via magnetic processes to an extent comparable to that shown by RS CVn or Algol close binaries.

Phillips, R. B.↗
NASA NTRS1995

Ulysses radio observations of the heliospheric activity in early June 1991

The radio receiver of the URAP (Unified Radio and Plasma Wave) experiment on Ulysses has recorded a heliospheric activity particularly intense between late May and early June 1991. Many solar radio emissions of types III and II were observed together with interplanetary (IP) shocks. In the same time, the radio spectrograph ARTEMIS at Nancay (France) observed several intense type II bursts. We investigate the association and/or interaction of these radio emissions, which are remotely observed, with some IP shocks detected in situ, in the context of a Coronal Mass Ejection (CME) induced scenario.

Hoang, S.↗
NASA NTRS1987

Radio continuum, far infrared and star formation

A very tight correlation was found between the radio emission and the far infrared emission from galaxies. This has been found for various samples of galaxies and is explained in terms of recent star formation. The tight correlation would imply that the total radio emission is a good tracer of star formation. The correlation between the radio power at 5 GHz and the far infrared luminosity is shown. The galaxies are of various morphological types and were selected from the various IRAS circulars, hence the sample is an infrared selected sample. The far infrared luminosities were corrected for the dust temperature. This is significant because it decreases the dispersion in the correlation.

Wielebinski, R.↗
NASA NTRS1989

Does the far-infrared/radio correlation in spiral galaxies extend to the spatial domain

A comparison is made between the spatial distribution of the thermal far-infrared and non-thermal radio emission of nearby spiral galaxies. This is done in an attempt to improve our understanding of the well known correlation between the integrated Infrared Astronomy Satellite (IRAS) far-infrared and radio emission of spiral galaxies, e.g., de Jong et al., 1985, Helou et al., 1986. A physical explanation for this correlation is not straight forward due to the ambiguous nature of the origin of the far-infrared and radio, and the dependence of the non-thermal radio on each galaxies' magnetic field. It is now widely believed that the infrared emission detected in the longer wavelength IRAS wavebands (less than 50 microns) arises from at least two distinct sources, e.g., Cox et al., 1986, Persson and Helou, 1987: (1) a warm (T approx. 40 K) component associated with dense dust clouds heated by embedded O and B type stars; and (2) a cooler (T approx. 20 K) component associated with diffuse dust distributed throughout the interstellar matter (ISM) heated by the interstellar radiation field. A link between the warm component and the radio via electrons originating in Type II supernovae (the ultimate fate of many of the O and B type stars responsible for the warm component) has been suggested by numerous authors. The supporting evidence is scarce and inconclusive. Researchers have attempted to provide some insight into the problem by looking at the spatial distribution of the different components in some nearby spiral galaxies, starting with the face-on spiral M51. The source of the far-infrared data is the IRAS chopped photometric channel (CPC) instrument. Warm and cold far-infrared fluxes integrated over all wavelengths and the radio intensity at two frequencies are plotted against radius. All plots are to a common resolution of 100 arcsec, the radio data originating from the Cambridge Low Frequency Synthesis Telescope (151 MHz) and the VLA (1490 MHz, from Condon, 1987). The warm and cold regions are assumed to be representedby a single galactic wide temperatures of 50 K and 20 K respectively. A dust emissivity of 1 has been assumed. The form of the plots is little effected by varying these assumptions. The radio and cold component curves appear to follow each other most closely, in contradiction to the simple OB star/type II supernovae hypothesis.

Howarth, Neil A.↗
NASA NTRS1986

Evidence for intrinsically asymmetrical radio structure in the nucleus of NGC 6251

High dynamic range VLBI hybrid maps of the nucleus of NGC 6251 at 18 cm have been made in order to study its structure. The maps are consistent with a simple two-sided relativistic beaming model of the nucleus only if the radio axis is aligned within 10 deg of the line of sight and the flow velocity in the two jets is greater than 0.7c. This implies an unreasonably large physical size for the extended radio emission associated with this galaxy. The most plausible alternative explanation is that both jets are transporting energy to the radio lobes continuously, but that the counterjet is much less efficient at converting a fraction of this energy into radio emission than the brighter jet is.

Jones, Dayton L.↗
NASA NTRS1993

Evolution of luminous IRAS galaxies: Radio imaging

In a recent study of IRAS galaxies' optical morphologies, we found that luminous IR sources lie in the IR color-luminosity plane in groups which separate out by optical spectroscopic type and also by degree of tidal disturbance. We found that the most luminous steep-IR-spectrum sources are generally galaxies in the initial stages of a major tidal interaction. Galaxies with active nuclei were generally found to have flatter IR spectra, to cover a range of IR luminosity, and to be in the later stages of a tidal interaction. We proposed a sequence of events by which luminous IR sources evolve: they start as interacting or merging galaxies, some develop active nuclei, and most undergo extensive star-formation in their central regions. Another way to study these objects and their individual evolution is to study their radio morphologies. Radio emission may arise at a detectable level from supernovae in star-forming regions and/or the appearance of an active nucleus can be accompanied by a nuclear radio source (which may develop extended structure). Therefore, the compact radio structure may trace the evolution of the inner regions of IRAS-luminous sources. If the radio sources are triggered by the interactions, we would expect to find the radio morphology related to the optical 'interactivity' of the systems. Here, we explore using the radio emission of IRAS galaxies as a possible tracer of galaxy evolution. We present and discuss observations of the compact radio morphology of 111 luminous IRAS-selected active galaxies covering a wide range of IR and optical properties.

Neff, S. G.↗
NASA NTRS1975

The earth as a radio source

The primary characteristics of radio emission from the earth's magnetosphere are summarized, the origins of these missions are considered and similarities to other astronomical radio sources discussed. The auroral kilometric radiation has features very similar to Io-related decametric radiation from Jupiter and from Saturn. The radiation at fp and 2 fp upstream of the bow shock appears to be generated by the same mechanism as type III solar radio bursts. The beaming of the auroral kilometric radiation into a cone shaped region over the polar cap has some similarity to the angular distribution of radiation from Io and to the beaming of radio emission from pulsars.

Gurnett, D. A.↗
NASA NTRS1987

Radio and infrared observations of (almost) one hundred non-Seyfert Markarian galaxies

The 13 cm flux densities of 96 non-Seyfert Markarian galaxies were measured at Arecibo Observatory. Far infrared flux densities have been published for 78 of these galaxies in the IRAS catalog. The radio, infrared, and optical fluxes of these galaxies and of a magnitude limited sample of normal galaxies were compared to clarify the nature of the radio emission in Markarian galaxies. It was found that Markarian galaxies of a given apparent magnitude and Hubble type generally have radio fluxes several times higher that the fluxes typical of normal galaxies of the same magnitude and type. Remarkably, the ratio of radio flux to far infrared flux is nearly the same for most of these starburst galaxies and for normal spiral disks. However, the compact and peculiar Markarian galaxies consistently have about 60% more radio flux per unit infrared flux than the other Markarian galaxies and the normal spirals. It is not clear whether this difference reflects a difference in the evolution of the starbursts in these galaxies or whether there is excess radio emission of nonstellar origin.

Dressel, Linda L.↗
NASA NTRS1993

Radio light curves of V471 Tauri

We have acquired light curves at a wavelength of 6 cm of the eclipsing binary V471 Tauri around the orbit, in order to determine the geometrical location of the radio emission in the binary. Each light curve shows a broad minimum near the time of optical eclipse, suggesting that the radio luminosity originates between the two stars. Other observations at X-ray, UV, and visual wavelengths are also supportive of the idea of a gas cloud more or less permanently located between the stars. This could be explained if the radio emission arises from the interaction of the magnetic fields of the secondary and the white dwarf near the line of centers.

Patterson, Joseph↗
NASA NTRS2009

The Atmospheric Dynamics of Jupiter, Saturn, and Titan

Comparative studies of Jupiter and Saturn often emphasize their similarities, but recent observations have highlighted important differences. The stratospheres of both planets exhibit an equatorial oscillation reminiscent of that in Earth's middle atmosphere. Jupiter's oscillation has a 4-5 year period, not linked to its season, and it has been modeled as an analog to the terrestrial quasi-biennial oscillation, driven by the stresses associated with vertically propagating waves. Saturn's equatorial oscillation is nearly semiannual, but wave activity may still be a driver. Jupiter's internal rotation rate is inferred from its steady modulated radio emission. Saturn's internal rotation is more enigmatic. It has been inferred from the modulation of the body's kilometric radio emission, but this period has varied by 1% over the last 25 years. Saturn's equatorial winds are also puzzling, as those inferred from cloud tracking by Cassini and more recent HST observations are weaker than those from Voyager. Whether this is attributable to a difference in altitudes of the tracked clouds in winds with vertical shear or a real temporal change in the winds is not known. Both winter and summer poles of Saturn exhibit very compact circumpolar vortices with warm cores, indicating subsidence. Titan's middle atmosphere is characterized by global cyclostrophic winds, particularly the strong circumpolar vortex in the winter hemisphere. In many ways, the spatial distribution of temperature, gaseous constituents, and condensates is reminiscent of conditions in terrestrial winter vortices, albeit with different chemistry. The meridional contrast in Titan's tropospheric temperatures is small, only a few kelvins.

Flasar, F. M.↗
DOE OSTI2021

The radio loudness of SDSS quasars from the LOFAR Two-metre Sky Survey: ubiquitous jet activity and constraints on star formation

ABSTRACT We examine the distribution of radio emission from ∼42 000 quasars from the Sloan Digital Sky Survey, as measured in the LOFAR Two-metre Sky Survey (LoTSS). We present a model of the radio luminosity distribution of the quasars that assumes that every quasar displays a superposition of two sources of radio emission: active galactic nuclei (jets) and star formation. Our two-component model provides an excellent match to the observed radio flux density distributions across a wide range of redshifts and quasar optical luminosities; this suggests that the jet-launching mechanism operates in all quasars but with different powering efficiency. The wide distribution of jet powers allows for a smooth transition between the ‘radio-quiet’ and ‘radio-loud’ quasar regimes, without need for any explicit bimodality. The best-fitting model parameters indicate that the star formation rate of quasar host galaxies correlates strongly with quasar luminosity and also increases with redshift at least out to z ∼ 2. For a model where star formation rate scales as $L_{\rm bol}^{\alpha } (1+z)^{\beta }$, we find α = 0.47 ± 0.01 and β = 1.61 ± 0.05, in agreement with far-infrared studies. Quasars contribute ≈0.15 per cent of the cosmic star formation rate density at z = 0.5, rising to 0.4 per cent by z ∼ 2. The typical radio jet power is seen to increase with both increasing optical luminosity and black hole mass independently, but does not vary with redshift, suggesting intrinsic properties govern the production of the radio jets. We discuss the implications of these results for the triggering of quasar activity and the launching of jets.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

Gamma ray emission from radio pulsars

While the proposed research received partial funding under this grant, during the term of support substantial progress was made on the development of a new model for the emission of gamma-rays from isolated rotation-powered pulsars. In phase one of the work, we showed how a modified version of the 'outer gap' model of pulsar emission could reproduce the double peaked profiles seen in CGRO pulsar observations. This work also demonstrated the spectrum of gap radiation varies significantly with position in the magnetosphere, and produced approximate computations of the emission from outer magnetosphere gap zones, including primary curvature radiation, gamma - gamma pair production and synchrotron radiation and inverse Compton scattering by the resulting secondary particles. This work was followed in phase two by a more complete treatment of the geometry of the radiation zone, and improved connections with observations at other wavelengths.

Romani, Roger W.↗
NASA NTRS1994

Do we have a radio powerful star without a chromosphere?

Two radio stars are selected, both equally powerful in microwave radio emission, but one of which, HD 101379, with a superpower chromosphere, and the other one, HD 132742, without a noticable chromosphere. On International Ultraviolet Explorer (IUE) recordings of HD 132742, a binary system like HD 101319, the doublet 2800 Mg II appears in absorption. However, in the last case the faint emission traces of k and h Mg II have been discovered in the lowest parts of absorption profiles. The reconstructed profiles of these emission details seem to be shifted to the relation of absorption profiles, which may be interpreted as a confirmation of the non-chromospheric nature of these details. In the case of radio star HD 101379, also a binary system, on the contrary, the doublet 2800 Mg II appears as a strong emission, without shift. These limiting cases may be combined assuming that the sources of both types of emission, radio and magnesium doublet, are located in the space between the components of binary systems. The parameters of these sources are obtained.

Gurzadyan, G. A.↗
DOE OSTI2024

``Beads on a String" Star Formation Tied to one of the most Powerful AGN Outbursts Observed in a Cool Core Galaxy Cluster

With two central galaxies engaged in a major merger and a remarkable chain of 19 young stellar superclusters wound around them in projection, the galaxy cluster SDSS J1531+3414 ($z=0.335$) offers an excellent laboratory to study the interplay between mergers, AGN feedback, and star formation. New Chandra X-ray imaging reveals rapidly cooling hot ($T\sim 10^6$ K) intracluster gas, with two "wings" forming a concave density discontinuity near the edge of the cool core. LOFAR $144$ MHz observations uncover diffuse radio emission strikingly aligned with the "wings," suggesting that the "wings" are actually the opening to a giant X-ray supercavity. The steep radio emission is likely an ancient relic of one of the most energetic AGN outbursts observed, with $4pV > 10^{61}$ erg. To the north of the supercavity, GMOS detects warm ($T\sim 10^4$ K) ionized gas that enshrouds the stellar superclusters but is redshifted up to $+ 800$ km s$^{-1}$ with respect to the southern central galaxy. ALMA detects a similarly redshifted $\sim 10^{10}$ M$_\odot$ reservoir of cold ($T\sim 10^2$ K) molecular gas, but it is offset from the young stars by $\sim 1{-}3$ kpc. We propose that the multiphase gas originated from low-entropy gas entrained by the X-ray supercavity, attribute the offset between the young stars and the molecular gas to turbulent intracluster gas motions, and suggest that tidal interactions stimulated the "beads on a string" star formation morphology.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1988

Compact radio cores in three Seyfert galaxies - High resolution studies of Mkn 3, Mkn 231, and Mkn 463

The small-scale morphologies of Mkn 231 and Mkn 463 have been investigated using the European VLBI Network (EVN) at 1.6 GHz and the VLA at 15 and 22 GHz. Mkn 3 has been studied using the EVN at 1.4 and 1.6 GHz. All of the galaxies have a substantial fraction of their core radio flux in linear or elongated structures, typically tens of parsecs in extent. Agreement between the parsec- and kiloparsec-scale position angles of continuum radio emission suggests collimated outflow from the galaxy centers that continues over several orders of magnitude in scale. Supernova rates of order one hundred per year on scales of tens of parsecs are required to explain the nuclear radio emission as the result of bursts of star formation. The luminosities, small extents, and linear morphologies of the radio sources all support the idea that the nuclear radio sources are energized by active galactic nuclei and therefore support the more general picture of Seyfert galaxy nuclei being low-luminosity quasars. All of the galaxies appear to have undergone violent interactions in the recent past.

Neff, S. G.↗
NASA NTRS2001

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source?

Most of the EGRET high-energy gamma-ray sources remain unidentified. It is highly likely that many of these are fainter blazars or pulsars, but there may also be new types of sources to be discovered. We have focussed our search for novel gamma-ray sources on 3EG 1835+5918, which is the brightest and most accurately positioned of the unidentified EGRET sources at high Galactic latitude (l, b = 89 deg, 25 deg). In this talk, we will summarize the results of X-ray, radio, and optical surveys of this location. In particular, we have made complete optical identifications of all of the ROSAT and ASCA sources in this region to a flux limit of approximately 1 x 10(exp -13) ergs/sq cm s. All of the X-ray sources within the EGRET error circle are radio-quiet quasars or coronally emitting stars. Previous radio pulsar searches have been unsuccessful. We set an upper limit of 3.8 mJy (at 1.4 GHz) on any possible radio counterpart to 3EG 1835+5918. We also find several quasars and white dwarfs using optical color selection, and we have monitored the entire field for variable optical objects on short and long time scales. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, we assert that 3EG 1835+5918 must be lacking in one or more of the physically essential attributes of those classes of gamma-ray emitters. In particular, its radio flux is at least two orders of magnitude fainter than any of the securely identified EGRET blazars, and its soft X-ray flux is at least 30 times fainter than that of Geminga and other EGRET pulsars. If it is an AGN it lacks the beamed radio emission of blazars. If it is an isolated neutron star, it lacks both the thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. As such, it is more problematic physically than Geminga, which is an ordinary pulsar that only lacks radio emission. As a pulsar, 3EG 1835+5918 would have to be either older or more distant than Geminga, and probably an even more efficient gamma-ray engine.

Mirabal, N.↗
NASA NTRS1978

Nonthermal galactic emission below 10 megahertz

The Radio Astronomy Explorer 2 (RAE 2) lunar-orbiting satellite has provided measurements of the nonthermal galactic radio emission at frequencies below 10 MHz. Measurements of the emission spectra are presented for the center, anticenter, north polar, and south polar directions at 22 frequencies between 0.25 and 9.18 MHz. Survey maps of the spatial distribution of the observed low-frequency Galactic emission at 1.31, 2.20, 3.93, 4.70, 6.55, and 9.18 MHz are presented. The observations were obtained with the 229-m traveling-wave V antenna on this lunar-orbiting spacecraft. The improved frequency coverage offers additional insights into the structure of the local Galactic neighborhood.

Milky Way↗