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To investigate the formation and early evolution of stars, astronomers study the x-ray emission of T Tauri stars, which are young, solar-mass stars called pre-main sequence stars. Two Earth-orbiting x-ray satellites, the Rontgen X-ray Satellite (ROSAT) and the Advanced Satellite for Cosmology and Astrophysics (ASCA), have discovered x-ray emission from young protostars, called Class I objects. Many T Tauri stars were detected as x-ray sources by ROSAT. X-ray luminosity functions and correlations with other stellar parameters can be studied and used to investigate the x-ray emission mechanism. From the ROSAT data hundreds of T Tauri stars have been discovered, some of which are located outside regions of ongoing star formation. Stellar x-rays also irradiate circumstellar disks, regions where planets may form, so x-ray emission data from T Tauri stars may also be used to investigate the formation of planets., Stars are born during the collapse of gas and dust clouds, so the youngest protostars are best observed at infrared (IR) and millimeter wavelengths (1). Optical emission from the protostar [...]

Neutron stars and black holes can be studied by observation of the radiation produced as matter falls into their gravitational fields. X-ray binaries, which are systems consisting of a neutron star or black hole and a companion gaseous star, produce radiation in this manner. Recently, oscillations at frequencies near 1000 cycles per second have been detected from x-ray binaries. These oscillations are likely produced in regions of very strong gravitational fields within a few tens of kilometers of the compact star. The oscillations have been interpreted as evidence for the existence of an innermost stable orbit near a compact star, a key prediction of general relativity theory. The study of x-ray binaries has also advanced the search for definitive evidence of black holes. Recent developments in our understanding of accretion flows in x-ray binaries have provided evidence for the existence of event horizons in x-ray binaries thought to contain black holes., Neutron stars and stellar-mass black holes are born in the deaths of gaseous stars, when exhaustion of the nuclear energy of a star causes it to collapse under the pull [...]

Observations made with the x-ray satellite ROSAT (Roentgen Satellite) have produced the first spatially resolved x-ray image of a corona around a star like our sun. The star is the secondary in the eclipsing binary system α Coronae Borealis (CrB), which consists of one star of spectral type AOV and one of type G5V. The x-ray light curve of α CrB shows a total x-ray eclipse during secondary optical minimum, with the G star behind the A star. The totality of the eclipse demonstrates that the A-type component in α CrB is x-ray dark and that the x-ray flux arises exclusively from the later-type companion. The x-ray eclipse ingress and egress are highly asymmetric compared with the optical eclipse, indicating a highly asymmetric x-ray intensity distribution on the surface of the G star. From a detailed modeling of the ingress and egress of the x-ray light curve, an eclipse map of the G star was constructed by a method based on an optimization by simulated annealing., Soft x-ray observations with the Einstein and ROSAT satellites in the wavelength range 6 to 100 Å have shown the ubiquitous presence of hot, tenuous coronae around almost all types [...]

THE 1/4-keV COSMIC SOFT X-RAY diffuse background (SXRB) was first detected over two decades ago [1, 2], yet the spatial distribution of the source or sources responsible for this emission [...]

Photometry and spectroscopy of the object Cha H[Alpha] 1, located in the Chamaeleon I star-forming cloud, show that it is a ~[10.sup.6]-year-old brown dwarf with spectral type M7.5 to M8 and 0.04 ± 0.01 solar masses. Quiescent x-ray emission was detected in a 36-kilosecond observation with 31.4 ± 7.7 x-ray photons, obtained with the Rontgen Satellite (ROSAT), with 9 Σ detection significance. This corresponds to an x-ray luminosity of 2.57 x [10.sup.28] ergs per second and an x-ray to bolometric luminosity ratio of [10.sup.-3.44]. These are typical values for late M-type stars. Because the interior of brown dwarfs may be similar to that of convective late-type stars, which are well-known x-ray sources, x-ray emission from brown dwarfs may indicate magnetic activity., A brown dwarf (BD) is an object with a mass [is less than or equal to] 0.08 solar masses (M??), which is insufficient to sustain nuclear fusion of hydrogen (1). [...]

The instability resulting from the relative motion of newly picked-up cometary photoions and the solar wind generates lower hybrid waves that are capable of accelerating electrons to the keV range of energies. These electrons may be responsible for the x-rays from comet C/Hyakutake 1996 B2 seen by the Rontgen X-ray Satellite. The inferred x-ray photon power depends on the electron energy, with keV electrons providing values of photon power two orders of magnitude greater than 100 eV electrons. These observations and in particular, spectral resolution of the x-rays, should provide more insight into the composition of the comet., Observations by the Rontgen X-ray Satellite (ROSAT) on 26 to 28 March 1996 of x-ray emission from comet C/Hyakutake 1996 b2 (1) appear to have been a surprise to most [...]

Among the great astronomical discoveries of the 1960s was the totally unexpected finding by Giacconi et al. (1) of a bright x-ray star, now known as Scorpio X-1. Because cosmic [...]

During the last 30 years x-ray astronomy has become one of the cornerstones of observational astrophysics. At x-ray wavelengths, we see the 'hot universe'--objects at temperatures of millions to billions [...]

If you spin in your office chair while holding a full cup of coffee, centrifugal force is likely to leave you wet -- even if the entire office building and [...]

The soft x-ray spectra of the Virgo and Coma clusters cannot be modeled by emission from the well-known hot intracluster medium attenuated by photoelectric absorption along the line of sight in our galaxy. If the excess soft emission is due to thermal emission in the clusters from gas at about 1 million kelvin, then the gas should be rapidly cooling. The high rate of cooling poses problems for the origin of the gas and its sink. A model in which the component is due to turbulent mixing layers around clouds scattered throughout the intracluster medium may explain the excess emission. The gas in the mixing layer is reheated after cooling, so that the total mass remains relatively small., Lieu et al. (1) report excess soft x-ray emission in the Virgo and Coma clusters, greater than that expected from the known hot intracluster medium in those clusters. The excess [...]

The Hague--You wouldn't want to get too close to a quasar, but astronomers would love to get a better view of these enigmatic beasts. Seen only in the farthest reaches [...]

X-ray astronomers are burning the candle at both ends these days, thanks to the appearance of a bizarre and brilliant x-ray emitter that is doing its own kind of double [...]

The night sky,, seemingly serene, is abuzz with signals from our cosmic neighborhood: light, radio waves, and other radiation from stars and gas within our galaxy. This cosmic buzz tends [...]

It may not look like the sun, but the image at right shows what the Japanese satellite Yohkoh saw when it trained its instruments on the sun's surface just 2 [...]