Discovery of a high-energy gamma-ray-emitting persistent microquasar

Microquasars are stellar x-ray binaries that behave as a scaled-down version of extragalactic quasars. The star LS 5039 is a new microquasar system with apparent persistent ejection of relativistic plasma at a 3-kiloparsec distance from the sun. It may also be associated with a gamma-ray source discovered by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the COMPTON-Gamma Ray Observatory satellite. Before the discovery of LS 5039, merely a handful of microquasars had been identified in the Galaxy, and none of them was detected in high-energy gamma-rays.     

Fast variability of tera-electron volt gamma rays from the radio galaxy M87

The detection of fast variations of the tera-electron volt (TeV) (10(12) eV) gamma-ray flux, on time scales of days, from the nearby radio galaxy M87 is reported. These variations are about 10 times as fast as those observed in any other wave band and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed toward the observer.     

Jets in extragalactic radio sources

Observations now require that there be a continuous supply of energy to the giant extragalactic radio sources. These observations also suggest that this energy input may be in the form of streams or jets of gas emanating from the centers of galaxies and quasi-stellar objects. Current data indicate that the large-scale jet structures are not moving with relativistic speeds, as previously proposed. Slow-moving jets, which possess turbulent interiors and are dominated by relatively cool gas, can account for the observed jet properties at optical and radio wavelengths. Extremely small-scale jets observed adjacent to the central energy source may or may not be in relativistic motion.     

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, most to within 1 arc second, and allowed the hydrogen maser clocks at the two sites to be synchronized a posteriori with an uncertainty of only a few nanoseconds.     

Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis

The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.     

Short-Lived s-Process Gamma-Ray Lines in Type II Supernovae

The nuclear abundances in the helium-burning shells of presupernova stellar models are calculated. For stars of over 20 solar masses the (22)Ne(alpha,n)(23) Mg reaction produces enough neutrons on a sufficiently short time scale for the s-process to produce (59)Fe (half-life = 45 days) and (60)Co (half-life = 5.3 years). These isotopes are expected to survive the passage of the shock, and gamma rays from their decays should be detectable from most galactic type II supernovae with the NASA Gamma Ray Observatory. Because of its great distance and low metallicity, these lines are not expected to be observable from supernova 1987A.     

Overview and initial results of the very long baseline interferometry space observatory programme

High angular resolution images of extragalactic radio sources are being made with the Highly Advanced Laboratory for Communications and Astronomy (HALCA) satellite and ground-based radio telescopes as part of the Very Long Baseline Interferometry (VLBI) Space Observatory Programme (VSOP). VSOP observations at 1.6 and 5 gigahertz of the milli-arc-second-scale structure of radio quasars enable the quasar core size and the corresponding brightness temperature to be determined, and they enable the motions of jet components that are close to the core to be studied. Here, VSOP images of the gamma-ray source 1156+295, the quasar 1548+056, the ultraluminous quasar 0014+813, and the superluminal quasar 0212+735 are presented and discussed.     

Pulsar test of a variation of the speed of light with frequency

The sharply defined optical and radio pulses from pulsars make possible a test of the variation of the speed of light with frequency, and of the possible existence of a photon mass. The data indicate that the mass of a real photon is less than 10(-44) gram. Detection of extragalactic pulsars could allow a substantial improvement of this limit.     

Discovery of intense gamma-ray flashes of atmospheric origin

Detectors aboard the Compton Gamma Ray Observatory have observed an unexplained terrestrial phenomenon: brief, intense flashes of gamma rays. These flashes must originate in the atmosphere at altitudes above at least 30 kilometers in order to escape atmospheric absorption and reach the orbiting detectors. At least a dozen such events have been detected over the past 2 years. The photon spectra from the events are very hard (peaking in the high-energy portion of the spectrum) and are consistent with bremsstrahlung emission from energetic (million-electron volt) electrons. The most likely origin of these high-energy electrons, although speculative at this time, is a rare type of high-altitude electrical discharge above thunderstorm regions.     

Relativistic Jets in SS 433

A variety of recent optical, radio, and x-ray observation have confirmed the hypothesis that the peculiar star SS 433 is ejecting two narrow, opposed, highly collimated jets of matter at one-quarter the speed of light. This unique behavior is probably driven by mass exchange between a relatively normal star and a compact companion, either a neutron star or a black hole. However, numerous details regarding the energetics, radiation, acceleration, and collimation of the jets remain to be understood. This phenomenon may well be a miniature example of similar collimated ejection of gas by active extragalactic objects such as quasars and radio galaxies.     

Numerical models of extragalactic radio sources

Numerical simulations with supercomputers allow analysis of the wide range of nonlinear physics inherent in the hydrodynamic and magnetohydrodynamic equations. When applied to extragalactic radio sources, these numerical models have begun to reproduce many of the complex structures observed on telescopic images. This combination of telescopic and numerical observations provides powerful probes of the physics of radio galaxies. In this review, some of the recent results from both two-dimensional and three-dimensional numerical simulations of the formation and evolution of extended radio morphologies are described. These numerical models have allowed the exploration of important physical phenomena including the role of magnetic fields in the dynamics and emissivity of extended radio galaxies, intermittent outflow from the cores of active galaxies, instabilities and disruption of fluid jets, and the bending of collimated outflows by motion through the intergalactic medium.     

The double quasar 0957+561: a radio study at 6-centimeters wavelength

The optical double quasar 0957+561 has been interpreted as the gravitational double image of a single object. A radio map made with the Very Large Array of the National Radio Astronomy Observatory shows unresolved sources coincident With the optical images as well as a complex of related extended emission. Although the results cannot rule out the gravitational lens hypothesis, the complex radio structure is more easily interpreted as two separate quasars. The optical and radio properties of the two quasars are so similar that the two must have been formed at the same time with similar initial conditions.     

Particle accelerators in the hot spots of radio galaxy 3C 445, imaged with the VLT

Hot spots (HSs) are regions of enhanced radio emission produced by supersonic jets at the tip of the radio lobes of powerful radio sources. Obtained with the Very Large Telescope (VLT), images of the HSs in the radio galaxy 3C 445 show bright knots embedded in diffuse optical emission distributed along the post-shock region created by the impact of the jet into the intergalactic medium. The observations reported here confirm that relativistic electrons are accelerated by Fermi-I acceleration processes in HSs. Furthermore, both the diffuse emission tracing the rims of the front shock and the multiple knots demonstrate the presence of additional continuous re-acceleration processes of electrons (Fermi-II).     

Radio emission from an ultraluminous x-ray source

The physical nature of ultraluminous x-ray sources is uncertain. Stellar-mass black holes with beamed radiation and intermediate black holes with isotropic radiation are two plausible explanations. We discovered radio emission from an ultraluminous x-ray source in the dwarf irregular galaxy NGC 5408. The x-ray, radio, and optical fluxes as well as the x-ray spectral shape are consistent with beamed relativistic jet emission from an accreting stellar black hole. If confirmed, this would suggest that the ultraluminous x-ray sources may be stellar-mass rather than intermediate-mass black holes. However, interpretation of the source as a jet-producing intermediate-mass black hole cannot be ruled out at this time.     

Peculiar galaxies and radio sources

Pairs of radio sources which are separated by from 2 degrees to 6 degrees on the sky have been investigated. In a number of cases peculiar galaxies have been found approximately midway along a line joining the two radio sources. The central peculiar galaxies belong mainly to a certain class in the recently compiled Atlas of Peculiar Galaxies. Among the radio sources so far associated with the peculiar galaxies are at least five known quasars. These quasars are indicated to be not at cosmological distances (that is, red shifts not caused by expansion of the universe) because the central peculiar galaxies are only at distances of 10 to 100 megaparsecs. The absolute magnitudes of these quasars are indicated to be in the range of brightness of normal galaxies and downward. Some of the radio sources which have been found to be associated with peculiar galaxies are galaxies themselves. It is therefore implied that ejection of material took place within or near the parent peculiar galaxies with speeds between 10(2) and 10(4) kilometers per second. After traveling for times of the order of 10(7) to 10(9) years, the luminous matter (galaxies) and radio sources (plasma) have reached their observed separations from the central peculiar galaxy. The large red shifts measured for the quasars would seem to be either (i) gravitational, (ii) collapse velocities of clouds of material falling toward the center of these compact galaxies, or (iii) some as yet unknown cause.     

A low-magnetic-field soft gamma repeater

Soft gamma repeaters (SGRs) and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, that is, neutron stars powered by extreme magnetic fields, B ~ 10(14) to 10(15) gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 × 10(12) gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far.     

Voyager 2 radio science observations of the uranian system: atmosphere, rings, and satellites

Voyager 2 radio occultation measurements of the Uranian atmosphere were obtained between 2 and 7 degrees south latitude. Initial atmospheric temperature profiles extend from pressures of 10 to 900 millibars over a height range of about 100 kilometers. Comparison of radio and infrared results yields mole fractions near the tropopause of 0.85 and 0.15 +/- 0.05 for molecular hydrogen and helium, respectively, if no other components are present; for this composition the tropopause is at about 52 kelvins and 110 millibars. Distinctive features in the signal intensity measurements for pressures above 900 millibars strongly favor model atmospheres that include a cloud deck of methane ice. Modeling of the intensity measurements for the cloud region and below indicates that the cloud base is near 1,300 millibars and 81 kelvins and yields an initial methane mole fraction of about 0.02 for the deep atmosphere. Scintillations in signal intensity indicate small-scale stucture throughout the stratosphere and upper troposphere. As judged from data obtained during occultation ingress, the ionosphere consists of a multilayer structure that includes two distinct layers at 2,000 and 3,500 kilometers above the 100-millibar level and an extended topside that may reach altitudes of 10,000 kilometers or more. Occultation measurements of the nine previously known rings at wavelengths of 3.6 and 13 centimeters show characteristic values of optical depth between about 0.8 and 8; the maxim value occurs in the outer region of the in ring, near its periapsis. Forward-scattered signals from this ring have properties that differ from those of any of Saturn's rings, and they are inconsistent with a discrete scattering object or local (three-dimensional) assemblies of orbiting objects. These signals suggest a new kdnd of planetary ring feature characterized by highly ordered cylindrical substructures of radial scale on the order of meters and azimuthal scale of kilometers or more. From radio data alone the mass of the Uranian system is GM(sys) = 5,794,547- 60 cubic kilometers per square second; from a combination of radio and optical navigation data the mass of Uranus alone is GM(u) = 5,793,939+/- 60 cubic kilometers per square second. From all available Voyager data, induding imaging radii, the mean uncompressed density of the five major satellites is 1.40+/- 0.07 grams per cubic centimeter; this value is consistent with a solar mix of material and apparently rules out a cometary origin of the satellites.     

A cosmic double helix in the archetypical quasar 3C273

Finding direct evidence for plasma instability in extragalactic jets is crucial for understanding the nature of relativistic outflows from active galactic nuclei. Our radio interferometric observations of the quasar 3C273 made with the orbiting radio telescope, HALCA, and an array of ground telescopes have yielded an image in which the emission across the jet is resolved, revealing two threadlike patterns that form a double helix inside the jet. This double helical structure is consistent with a Kelvin-Helmholtz instability, and at least five different instability modes can be identified and modeled by a light jet with a Lorentz factor of 2 and Mach number of 3.5. The model reproduces in detail the internal structure of the jet on scales of up to 30 milli-arc seconds ( approximately 300 parsecs) and is consistent with the general morphology of the jet on scales of up to 1 kiloparsec.     

Quasi-stellar objects: possible local origin

Many difficulties face the conventional interpretation of the red shift of quasars as a Hubble shift, with associated immense distances. These objects are not of galactic size or nature, and are not associated with galaxies or clusters of galaxies. The continuing energy source for such enormous powers for a period of 10(6) to 10(7) years has not been clearly revealed. The absence of the expected absorption for the Lyman-alpha spectral line of hydrogen is a new difficulty. Because of the relativistic limit on the diameter which can produce rapid fluctuations of light output, there may not be enough surface to radiate the required light.A similar and perhaps more serious difficulty exists for the fluctuating radio output. Calculations given here for synchrotron radiation self-absorption lead to a reasonably accurate formula for the angular diameter of a radio source. For the quasar 3C 273B these relations indicate a conflict with the usually assumed distance. However, the discrepancy may be explained in terms of strong variation of radio diameter with frequency. For CTA 102 the conflict is more serious, and could be explained -for cosmological distance-only by rejecting the data of Sholomitskii. These difficulties are removed by the hypothesis that the observed quasars were ejected from a gravitational collapse at the center of our own galaxy, which may have occurred roughly 5 million years ago. The resultant distances, of the order of a million lightyears, reduce the energy problem by a factor of 10(6) or 10(7). On this basis the optical diameter would be less than a light-hour, about the size of the earth's orbit. A rotating mass of a few thousand solar masses with this diameter would account for the unusual line width, could easily produce the required radiated energy, and could readily account for observed short fluctuation periods and variations in spectrum. It is suggested that the radio output may be produced by high-speed passage of the quasar through intergalactic gas. This would probably correspond to a radio size of a few light-years or less, in agreement with the fluctuations. Since the radio power would be considerably less than that of radio galaxies, it is suggested that radio galaxies may have ejected groups of quasars. This would explain the peculiarly distant locations of the radio sources for many such galaxies. The objections to this model that have been raised are apparently not fatal. In particular, the receding hydrogen cloud discovered by Koehler to be in the line of sight to 3C 273 is more plausibly interpreted as having been ejected from our own galaxy, in the manner observed for other galaxies, than as being associated with the Virgo cluster of galaxies. The latter interpretation, which would place 3C 273 further away, is in conflict with Lyman-alpha absorption data for 3C 9 and other quasars. Thus the local model seems to give a reasonable explanation not only of quasars but also of radio galaxies, bothv of which seem largely to defy explanation on other grounds. Whether or not this model is valid, it is clear that an understanding of quasars will radically change our understanding of the universe.     

Astrophysical jets

Astrophysical jets are linear structures associated with stars and galaxies which span about seven orders of magnitude in size; the largest jets emanating from galaxies are about 100 times the size of our galaxy and are the largest single objects in the universe. Jets associated with stars are composed of ionized gas moving away from the star with velocities of a few hundred kilometers per second. Extragalactic jets are composed of relativistic particles, magnetic field, and probably additional amounts of cooler ionized plasma either originally ejected in the jet or entired by it out of the surrounding gaseous medium. The initial outflow velocity for extragalactic jets may be relativistic, and average outflow speeds of several thousand kilometers per second are likely. The energy flux carried by extragalactic jets may be in excess of 10(46) ergs per second, depending upon the nature of the jet. A definition of jet properties, deduced from their interaction with the ambient medium, can place essential constraints on models for the central power source in the parent galaxy or quasi-stellar object where they originate.