Number 467, January 18, 2000 by Phillip F. Schewe and Ben Stein THE X-RAY BACKGROUND, the glow of x rays seen in all directions in space, has now largely been resolved into emissions from discrete sources by the Chandra X-Ray Telescope, ending the notion that the x rays come from distant hot gas. Previously only about 20-30% of the x-ray background had been ascribed to point sources (by such telescopes as ASCA). Chandra was launched in July 1999 and put in an elliptical orbit. With its high angular resolution and acute sensitivity it could tell apart x-ray objects (many of them thought to be accretion disks around black holes) that before had been blurred into a continuous x-ray curtain. (Of course, now that the background has been resolved into points it ceases to be a background.) Richard Mushotzky of Goddard Space Flight Center reported these Chandra results at last week's meeting in Atlanta of the American Astronomical Society (AAS). Resolving the x-ray background was not all. Mushotzky added that the Chandra survey had revealed the existence of two categories of energetic galaxies that had been imaged only poorly or not at all by optical telescopes. He referred to one category as "veiled galactic nuclei," objects (with a redshift of about 1) bright in x rays but obscured by dust at optical wavelengths. The other category was "ultra-faint galaxies." One interpretation of these galaxies is that optical emission is suppressed owing to absorption over what could be a very long pathway to Earth. Mushotzky speculated that such high redshift (z greater than 5) galaxies could be the most distant, and hence earliest, objects ever identified. The XMM x-ray telescope, just launched, should provide complementary information in the form of high-precision spectra (from which redshifts are derived) of the distant objects. OTHER CHANDRA RESULTS at the meeting included the mapping of a thousand x-ray stars in the Orion Nebula portion of our galaxy 1500 light years away, making this the highest density of x-ray sources yet recorded. Gordon Garmire of Penn State spoke about this finding as well as about the effort to find x-ray counterparts for objects cataloged in the Hubble Deep Field image made with visible light; some tentative matches were made. Meanwhile, Frederick Baganoff of MIT reported that Chandra's inspection of the center of the Milky Way revealed what might be the first recorded x-ray signal from the vicinity of the massive (2 million solar mass) black hole residing at or near the radio-bright object called Sagittarius A*. In x rays this object proved to be fainter than expected by a factor of 5. The supermassive black hole at the heart of our sister spiral galaxy, Andromeda, also is much cooler than expected. According to Stephen Murray from Harvard-Smithsonian, the measured temperature was only a few million K, compared to temperatures of tens of millions for much more modest x-ray stars in the same galaxy. None of this fits with theories of supermassive black holes. Finally, Claude Canizares of MIT summarized Chandra observations of supernova remnant E0102-72, located in the Small Magellanic Cloud. E0102-72 is the leftover from an explosion 1000 years ago of a huge star of 15-20 solar masses. A diffraction grating on the telescope was used to spread out incoming x rays into a spectrum which could be scanned for the presence of specific elements in the stellar debris. Canizares estimated that as much as 10 solar masses' worth of oxygen was present in the wreckage of the older star, enough to furnish thousands of solar systems like ours with the breathable element needed for much of life on Earth. SOLITARY, WANDERING BLACK HOLES, unheralded by any bright accretion disk or rapidly orbiting stars or gas, have been detected through the process of gravitational microlensing. The Massive Compact Halo Object (MACHO) collaboration regularly views millions of stars in the direction of the dense bulge of our galaxy hoping to observe, every now and then, stars brightening courtesy of the lensing caused by the passage of some nonluminous object (hovering in the galaxy's halo) between us and the star. The brightening can last as short as two days or as long as 1000. Longer durations suggest either large or very slow lensing objects. David Bennett of Notre Dame reported at the AAS meeting on two such long-duration events in which the mass of the lens was calculated to be roughly 6 solar masses, too heavy to be a neutron star and more likely to be a black hole. Bennett speculates that the lone-wolf black holes form from supernova collapse and might be as common as neutron stars in the galaxy.