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PHYSICS NEWS UPDATE, Number 368, April 23, 1998
The American Institute of Physics Bulletin of Physics News by Phillip F. Schewe and Ben Stein
THE STRONGEST GRAVITATIONAL FIELDS EVER MEASURED, corresponding to a spacetime warping of 30%, have been recorded by scientists using the Rossi X-Ray Timing Explorer (RXTE) satellite. By comparison, the proportional curvature of space is 100% at a black hole, but only about one part in a million near the sun's surface and one part per billion near the Earth's surface. RXTE was designed to monitor (over microsecond time intervals) the x rays coming from binary star systems in which matter from a conventional star is siphoned off into an accretion disk surrounding a nearby neutron star or black hole. In about 16 binary-star systems that contain neutron stars, blobs of gas in the disk are thought to spiral in toward the neutron star, picking up speed before they make a final plunge onto the surface. The x rays produced in this process are regularly dimmed when the hot gas is on the far side of the star. This leads to quasi-periodic oscillations (QPOs) in the x-ray brightness of the star. Also notable is the fact that the brightness variations only occur at certain well-defined rates, "pure tones" corresponding to special orbital periods for the gas going around the star. The spacetime encountered by the gas is so highly warped because the gas is able to skim within a few km of the neutron star, which itself is only about 10 km in diameter. At this week's meeting of the American Physical Society in Columbus, Ohio, Frederick Lamb of the University of Illinois (217-333-6363, f-lamb@uiuc.edu) described how the observed variations in the x-ray brightness can be used to deduce properties of the neutron star, such as its mass and size. At a press conference, Lamb and William Zhang of NASA Goddard concentrated on the binary-star system 4U1820-30, about 20,000 light years from Earth. The neutron star has a mass of 2.3 solar masses and orbits its companion star in only 11 minutes. Close observations of this system confirm a prediction made by Lamb and his colleagues Coleman Miller and Dimitrios Psaltis that the gas blobs would continue to spiral inward until they reached an "innermost stable orbit," where they would orbit before making the dive for the surface. This is a purely general relativistic (GR) effect; in Newton's mechanics, by contrast, the blob could have gotten arbitrarily close to the surface, providing it were going fast enough. The observations by Zhang and his collaborators now confirm Lamb's prediction, thus opening up a new "strong-gravitational field" era in GR studies. The measurements of the gas motion even provide hints as to the nature of the strong nuclear force sustaining the neutron star against further gravitational collapse. The new evidence indicates that the nuclear force is stiffer and more repulsive than has generally been thought.
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PHYSICS NEWS UPDATE, Number 364 March 27, 1998
ENERGETIC COSMIC RAYS MAY BE IRON NUCLEI. Cosmic rays are particles that come from far away (many likely to be of extra-galactic origin) and strike our atmosphere, where they engender a shower of new particles that show up in detectors on the ground. One of the biggest puzzles in cosmic ray physics is why there should be so many events with total energies above 100 EeV (10^20 eV). The main reason for this was the notion that if the primary particle were journeying from a distant galaxy, the particle's energy would be sapped by interactions with cosmic microwave background photons or with infrared background radiation (IBR). There is the additional problem of how cosmic rays with energies as high as 300 EeV could have been accelerated to such a degree if the first place (see Update 243). Floyd Stecker of the Goddard Space Flight Center has determined that a 200 EeV nucleus (starting out as an iron nucleus) could negotiate a 300-light- year journey through the IBR. The nucleus would partially disintegrate en route, but would still arrive at Earth with a potent energy. (Physical Review Letters, 2 March.)
The Flattest Surface in the Solar System is on Mars' northern lowlands. Over a 2000-km-around belt between 50 and 80 degrees north latitude, the surface tilts up only about 0.0 degrees to the south. This data and a number of other results from the Mars Global Surveyor---such as information on the Martian atmosphere, magnetism, plasma fields, topography, and mineralogy---are reported in the 13 March issue of Science.
PHYSICS NEWS UPDATE, Number 361 March 4, 1998 (The American Institute of Physics Bulletin of Physics News) by Phillip F. Schewe and Ben Stein
A REPULSIVE FORCE IN THE UNIVERSE seems to be at work on a cosmic scale, at least partly neutralizing the attractive force of gravity. Astronomers have arrived at this conclusion after a series of observations of distant supernovas showed that the expansion of the universe has not only not slowed (through the mutual attraction of galaxies) but seems to be accelerating. The most recent data to support this view were reported by an LBL group in January at the AAS meeting in Washington (Update 355) and by the multi-national High-z Supernova Search Team two weeks ago in California at a symposium on dark matter (Science, 27 February 1998). Several theories attempt to explain the positive push; one popular model sees the effect as coming from a huge reservoir of energy hidden in the universal vacuum. An early precedent for this notion was Einstein's use of an antigravity fudge factor called the cosmological constant in his gravitational equations. (See also Science News, 28 Feb, and Updates 345 and 360.)
Paper: astro-ph/9802174 From: eml@astro.caltech.edu (Erik Leitch) Date: Fri, 13 Feb 1998 05:19:44 GMT (48kb)
Title: Mass Extinctions and The Sun's Encounters with Spiral Arms Authors: Erik M. Leitch & Gautam Vasisht Comments: Latex, aaspp4.sty, 8 pages, 2 Ps figures, to appear in New Astronomy 3 (1998) 51-56
The terrestrial fossil record shows that the exponential rise in biodiversity since the Precambrian period has been punctuated by large extinctions, at intervals of 40 to 140 Myr. These mass extinctions represent extremes over a background of smaller events and the natural process of species extinction. We point out that the non-terrestrial phenomena proposed to explain these events, such as boloidal impacts (a candidate for the end-Cretaceous extinction), and nearby supernovae, are collectively far more effective during the solar system's traversal of spiral arms. Using the best available data on the location and kinematics of the Galactic spiral structure (including distance scale and kinematic uncertainties), we present evidence that arm crossings provide a viable explanation for the timing of the large extinctions.
( http://xxx.lanl.gov/abs/astro-ph/9802174 , 48kb)
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 355 January 20, 1998 by Phillip F. Schewe and Ben Stein
THE UNIVERSE WILL EXPAND FOREVER
This prediction is based on new studies of distant supernovas. Because Type Ia supernovas (supernovas in which material falling onto a white dwarf from a companion object ignites violently) brighten and fade in such a predictable way, their intrinsic brightness (and their distances from Earth) can be determined by carefully watching light emission over time. Combining these distances with the velocities of the host galaxies (determined from redshifts) allows one to calculate the expansion of the universe with some confidence. And the result appears to suggest that the universe does not have enough matter (visible or dark) to halt the current expansion. This view emerged two weeks ago at the meeting of the American Astronomical Society in Washington, where optical data for many new supernovas (including the most distant supernova ever observed, one with a redshift of 0.97) were reported by a group from LBL (led by Saul Perlmutter) and one from Harvard-Smithsonian (Peter Garnavich). The new findings are consistent with an age estimate for the universe of 15 billion years.
The hunt for a giant meteor is on in Greenland
In the dead of the Arctic night on December 9, flashes of light as bright as nuclear blasts lighted up the Southern tip of Greenland, and Danish scientists have begun a search for what they believe was a gigantic meteor impact.
Paper: astro-ph/9801052 From: "Andrew P. Gould" <gould@astronomy.ohio-state.edu> Date: Wed, 7 Jan 1998 16:45:24 GMT (21kb)
Title: No Death Star -- For Now Author: Jay A. Frogel and Andrew Gould Comments: 11 pages, 1 figure. Submitted to Astrophysical Journal Letters Report-no: OSU-TA-1/98
If a star passed within ~10,000 AU of the Sun, it would trigger a comet shower that would reach the inner solar system about 0.18 Myr later. We calculate the a priori probability to be about 0.4% that a star has passed this close to the Sun but that the comet shower has not yet reached the Earth. We search the HIPPARCOS catalog for such recent close-encounter candidates and find none. Because of the relatively bright completeness limit of the catalog, V~8, this search is sensitive to only about half the stars that could have had such a near encounter. However, we show that our search is sensitive to nearly all of the past encounters that would lead to a major shower in the future. We conclude that it is highly unlikely that a major shower will occur during the next 0.5 Myr.
( http://xxx.lanl.gov/abs/astro-ph/9801052 , 21kb)