Large-scale,decelerating, relativistic x-ray jets from the microquasar XTE J1550-564

We have detected, at x-ray and radio wavelengths, large-scale moving jets from the microquasar XTE J1550-564. Plasma ejected from near the black hole traveled at relativistic velocities for at least 4 years. We present direct evidence for gradual deceleration in a relativistic jet. The broadband spectrum of the jets is consistent with synchrotron emission from high-energy (up to 10 tera-electron volts) particles that were accelerated in the shock waves formed within the relativistic ejecta or by the interaction of the jets with the interstellar medium. XTE J1550-564 offers a rare opportunity to study the dynamical evolution of relativistic jets on time scales inaccessible for active galactic nuclei jets, with implications for our understanding of relativistic jets from Galactic x-ray binaries and active galactic nuclei.     

Very high energy gamma-ray binary stars

One of the major astronomical discoveries of the last two decades was the detection of luminous x-ray binary star systems in which gravitational energy from accretion is released by the emission of x-ray photons, which have energies in the range of 0.1 to 10 kiloelectron volts. Recent observations have shown that some of these binary sources also emit photons in the energy range of 10(12) electron volts and above. Such sources contain a rotating neutron star that is accreting matter from a companion. Techniques to detect such radiation are ground-based, simple, and inexpensive. Four binary sources (Hercules X-1, 4U0115+63, Vela X-1, and Cygnus X-3) have been observed by at least two independent groups. Although the discovery of such very high energy "gamma-ray binaries" was not theoretically anticipated, models have now been proposed that attempt to explain the behavior of one or more of the sources. The implications of these observations is that a significant portion of the more energetic cosmic rays observed on Earth may arise from the action of similar sources within the galaxy during the past few million years.     

Binary stars

Most stars in the solar neighborhood are either double or multiple systems. They provide a unique opportunity to measure stellar masses and radii and to study many interesting and important phenomena. The best candidates for black holes are compact massive components of two x-ray binaries: Cygnus X-1 and LMC X-3. The binary radio pulsar PSR 1913 + 16 provides the best available evidence for gravitational radiation. Accretion disks and jets observed in close binaries offer a very good testing ground for models of active galactic nuclei and quasars.     

Periodic emission from the gamma-ray binary 1FGL J1018.6-5856

Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.     

Iron emission lines from extended x-ray jets in SS 433: reheating of atomic nuclei

Powerful relativistic jets are among the most ubiquitous and energetic observational consequences of accretion around supermassive black holes in active galactic nuclei and neutron stars and stellar-mass black holes in x-ray binary (XRB) systems. But despite more than three decades of study, the structure and composition of these jets remain unknown. Here we present spatially resolved x-ray spectroscopy of arc second-scale x-ray jets from XRB SS 433 analyzed with the Chandra advanced charge-coupled device imaging spectrometer. These observations reveal evidence for a hot continuum and Doppler-shifted iron emission lines from spatially resolved regions. Apparently, in situ reheating of the baryonic component of the jets takes place in a flow that moves with relativistic bulk velocity even more than 100 days after launch from the binary core.     

Orbital motion in the radio galaxy 3C 66B: evidence for a supermassive black hole binary

Supermassive black hole binaries may exist in the centers of active galactic nuclei such as quasars and radio galaxies, and mergers between galaxies may result in the formation of supermassive binaries during the course of galactic evolution. Using the very-long-baseline interferometer, we imaged the radio galaxy 3C 66B at radio frequencies and found that the unresolved radio core of 3C 66B shows well-defined elliptical motions with a period of 1.05 +/- 0.03 years, which provides a direct detection of a supermassive black hole binary.     

Phase-resolved spectroscopy of Geminga shows rotating hot spot(s)

Isolated neutron stars are seen in x-rays through their nonthermal and/or surface thermal emissions. X-ray Multimirror Mission-Newton observations of the Geminga pulsar show a 43-electron volt spectrum from the whole neutron star surface, as well as a power-law component above 2 kiloelectron volts. In addition, we have detected a hot (170 electron volts) thermal emission from an approximately 60-meter-radius spot on the pulsar's surface. Such a thermal emission, only visible at selected phase intervals, may be coming from polar hot spot(s), long thought to exist as a result of heating from magnetospheric accelerated particles. It may provide the missing link between the x-ray and gamma-ray emission of the pulsar.     

A fossil record of galaxy encounters

The cosmic infrared background (CIRB) is a record of a large fraction of the emission of light by stars and galaxies over time. The bulk of this emission has been resolved by the Infrared Space Observatory camera. The dominant contributors are bright starburst galaxies with redshift z approximately 0.8; that is, in the same redshift range as the active galactic nuclei responsible for the bulk of the x-ray background. At the longest wavelengths, sources of redshift z >/= 2 tend to dominate the CIRB. It appears that the majority of present-day stars have been formed in dusty starbursts triggered by galaxy-galaxy interactions and the buildup of large-scale structures.     

An extremely luminous panchromatic outburst from the nucleus of a distant galaxy

Variable x-ray and γ-ray emission is characteristic of the most extreme physical processes in the universe. We present multiwavelength observations of a unique γ-ray-selected transient detected by the Swift satellite, accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z = 0.3534. Its high-energy emission has lasted much longer than any γ-ray burst, whereas its peak luminosity was ～100 times higher than bright active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a rare mechanism involving the massive black hole in the nucleus of that galaxy.     

Polarized gamma-ray emission from the crab

Pulsar systems accelerate particles to immense energies. The detailed functioning of these engines is still poorly understood, but polarization measurements of high-energy radiation may allow us to locate where the particles are accelerated. We have detected polarized gamma rays from the vicinity of the Crab pulsar using data from the spectrometer on the International Gamma-Ray Astrophysics Laboratory satellite. Our results show polarization with an electric vector aligned with the spin axis of the neutron star, demonstrating that a substantial fraction of the high-energy electrons responsible for the polarized photons are produced in a highly ordered structure close to the pulsar.     

A transient radio jet in an erupting dwarf nova

Astrophysical jets seem to occur in nearly all types of accreting objects, from supermassive black holes to young stellar objects. On the basis of x-ray binaries, a unified scenario describing the disc/jet coupling has evolved and been extended to many accreting objects. The only major exceptions are thought to be cataclysmic variables: Dwarf novae, weakly accreting white dwarfs, show similar outburst behavior to x-ray binaries, but no jet has yet been detected. Here we present radio observations of a dwarf nova in outburst showing variable flat-spectrum radio emission that is best explained as synchrotron emission originating in a transient jet. Both the inferred jet power and the relation to the outburst cycle are analogous to those seen in x-ray binaries, suggesting that the disc/jet coupling mechanism is ubiquitous.     

Polarized gamma-ray emission from the galactic black hole Cygnus X-1

Because of their inherently high flux allowing the detection of clear signals, black hole x-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the International Gamma-Ray Astrophysics Laboratory Imager on Board the Integral Satellite (INTEGRAL/IBIS) telescope. Spectral modeling of the data reveals two emission mechanisms: The 250- to 400-keV (kilo-electron volt) data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400-keV to 2-MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band.     

The very-high-energy gamma-ray sky

Over the past few years, very-high-energy gamma-ray astronomy has emerged as a truly observational discipline, with many detected sources representing different galactic and extragalactic source populations-supernova remnants, pulsar wind nebulae, giant molecular clouds, star formation regions, compact binary systems, and active galactic nuclei. It is expected that observations with the next generation of stereoscopic arrays of imaging atmospheric Cherenkov telescopes over a very broad energy range from 10(10) to 10(15) electron volts will dramatically increase the number of very-high-energy gamma-ray sources, thus having a huge impact on the development of astrophysics, cosmology, and particle astrophysics.     

Building black holes: supercomputer cinema

A new computer code can solve Einstein's equations of general relativity for the dynamical evolution of a relativistic star cluster. The cluster may contain a large number of stars that move in a strong gravitational field at speeds approaching the speed of light. Unstable star clusters undergo catastrophic collapse to black holes. The collapse of an unstable cluster to a supermassive black hole at the center of a galaxy may explain the origin of quasars and active galactic nuclei. By means of a supercomputer simulation and color graphics, the whole process can be viewed in real time on a movie screen.     

A 200-second quasi-periodicity after the tidal disruption of a star by a dormant black hole

Supermassive black holes (SMBHs; mass is greater than or approximately 10(5) times that of the Sun) are known to exist at the center of most galaxies with sufficient stellar mass. In the local universe, it is possible to infer their properties from the surrounding stars or gas. However, at high redshifts we require active, continuous accretion to infer the presence of the SMBHs, which often comes in the form of long-term accretion in active galactic nuclei. SMBHs can also capture and tidally disrupt stars orbiting nearby, resulting in bright flares from otherwise quiescent black holes. Here, we report on a ~200-second x-ray quasi-periodicity around a previously dormant SMBH located in the center of a galaxy at redshift z = 0.3534. This result may open the possibility of probing general relativity beyond our local universe.     

The galactic center: is it a massive black hole?

Studies of active galactic nuclei constitute one of the major efforts in astronomy. Massive black holes are the most likely source for the enormous energy radiated from such nuclei. Observations reviewed here suggest unusual activity and the possible existence of a massive black hole in the nucleus of our galaxy. Because of its proximity to Earth, our galactic nucleus can be observed in unsurpassed detail and may serve as the Rosetta stone both for deciphering active galactic nuclei and for confirming the existence of a massive black hole.     

Transient particle acceleration associated with solar flares

Understanding how individual charged particles can be accelerated to extreme energies (10(20) electron volts), remains a foremost problem in astrophysics. Within our solar system, the active sun is capable of producing, on a short time scale, ions with energies higher than 25 gigaelectron volts. Satellite and ground-based observation over the past 30 years have greatly increased our knowledge of the properties of transient bursts of energetic particles emitted from the sun in association with solar flares, but a real understanding of the solar flare particle acceleration process requires greatly refined experimental data. On the practical side, it is also imperative that this problem be solved if man is to venture, for long periods of time, beyond the protective umbrella of Earth's magnetic field, which excludes much of the biologically damaging solar energetic particles. It is only through an understanding of the basic acceleration problem that we can expect to be able to predict the occurrence of a solar flare with lethal solar radiations. For our knowledge of these effects to advance, a new space mission dedicated to studying the high-energy aspects of solar flares at high spatial and energy resolution will be required.     

Astrophysics on the GRAPE family of special-purpose computers

The GRAPE-4, the world's fastest computer in 1995-1997, has produced some major scientific results through a wide diversity of large-scale simulations in astrophysics. Applications have included planetary formation, the evolution of star clusters and galactic nuclei, and the formation of galaxies and clusters of galaxies.     

ROSAT Detection of an X-ray Shadow in the 1/4-keV Diffuse Background in the Draco Nebula

The detection by the Roentgen satellite (ROSAT) x-ray telescope of a shadow in the 1/4-kiloelectron volt (C band, 0.1 to 0.284 kiloelectron volt) cosmic diffuse background is reported. The location and morphology of the local minimum in x-rays are in clear agreement with a discrete H I cloud. The shadow is very deep with a minimum level at 50 percent of the surrounding emission; therefore, a minimum of 50 percent of the observed off-cloud flux must originate on the far side of the cloud. The analysis of H I velocity components links the cloud with the Draco nebula (distance approximately 600 parsecs); it then follows that there is significant 1/4-kiloelectron volt x-ray emission at a large distance (>400 parsecs) from the galactic plane along this line of sight. The extent of the distant emission region is uncertain, and, if it indicates the existence of a hot galactic corona, it must be patchy in nature.     

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.