by Paul Gilster | Oct 5, 2005 | Deep Sky Astronomy & Telescopes
Imagine two neutron stars colliding, or even worse, a neutron star and a black hole. The release of energy would be catastrophic, and has apparently now led to the first detection in visible light from a short gamma-ray burst. Thus we’re beginning to get a handle on the most powerful explosions in the known universe, whose identity has bedeviled astronomers for thirty years.
There are actually two different kinds of gamma-ray bursts. The longer ones have been linked to the explosion of a massive star as it collapses into a black hole. It’s the short-duration bursts that have proven the greater challenge. The new work, performed at La Silla (Chile) and at the European Southern Observatory’s Very Large Telescope, used data from NASA’s HETE-2 satellite to guide the observations, and the fading source was found.
“That was the clue we were waiting for,” said Garrett Jernigan, a research physicist at the Berkeley Space Sciences Laboratory. “Bursts seem to come mainly in two varieties – the long ones, which last about 20 seconds, and the short ones, which last a few tenths of a second. It’s the short ones that have still been puzzling us.”
Image: An artist’s conception of the merger of two neutron stars. Credit: European Southern Observatory.
This particular burst occurred 2400 million light years away (but see note below) in a still-forming dwarf galaxy. Astronomers are ruling out exploding stars as the cause, supporting the idea that these short-range events are the result of neutron star collisions. A second burst, detected on May 9 by the Swift satellite, pointed to a location in an elliptical galaxy 2700 million light years away. From an ESO press release:
“It is striking that the two short bursts that have finally been localised appear in quite different environments”, says Jesper Sollerman, a member of the team from Stockholm Observatory (Sweden) and Dark Cosmology Centre (Denmark). “The most important aspect of these discoveries is probably that we have finally shown that the short bursts are indeed cosmic explosions from far away in the Universe”, he adds.
All of which may support the collision hypothesis, in that elliptical galaxies are often rich in the kind of tight binary systems that might produce such collisions, according to the ESO materials. Centauri Dreams admits to finding this latter thought unconvincing as being based on insufficient evidence, and suspects the authors of the visible light study, which appears in the October 6 issue of Nature, would agree.
While progress in detecting sources for short-duration gamma rays is heartening, the amount we don’t know about these mysterious emmanations still dwarfs our understanding of the processes behind them. Centauri Dreams prefers this statement from one of the study’s authors: “Our observations do not prove the coalescence model, but we surely have found a lady with a smoking gun next to a dead body,” said Shri Kulkarni of the California Institute of Technology.
Four papers report on gamma-ray observations in the September 5 issue of Nature, all of which are abstracted here.
Sources: European Southern Observatory; California Institute of Technology; University of California at Berkeley. That press releases do not always agree would surprise no one, but the extreme difference in distance cited among these sources — 1 billion light years vs. 2.4 billion for the first gamma-ray source — does point out the dangers of fast reporting on still evolving stories.
by Paul Gilster | Oct 4, 2005 | Astrobiology and SETI |
The search for extraterrestrial intelligence (SETI) has seen a great deal of publicity, from television programs interviewing involved scientists to blockbuster movies like Contact. But the idea that there might be signs of extraterrestrial life closer to home has received relatively short shrift. Nonetheless, SETA (the search for extraterrestrial artifacts) has spawned interesting work, from Gregory Matloff’s examinations of anomalous Kuiper Belt objects to Robert Freitas’ surveys of ‘halo orbits’ around the Lagrangian points.
So far both kinds of search — SETI and SETA — have come up short, but a few curious things have been observed on each side. One interesting SETA investigation involved an object called 1991 VG, which made a close approach to Earth in 1991 (thanks to Adam Crowl for bringing this one to my attention). Discovered by Jim Scotti using the University of Arizona’s Spacewatch telescope (normally used to detect small asteroids near the Earth), 1991 VG seemed to be about 10 meters in size, but did not appear to be an asteroid. In a 1995 paper on 1991 VG, astronomer Duncan Steel (Anglo-Australian Observatory and University of Adelaide) noted “…strong, rapid brightness variations which can be interpreted as transient specular reflections from the surfaces of a rotating spacecraft.”
Fair enough — various man-made spacecraft have left rocket parts behind that could account for the object (and the thought that this could be leftover Apollo hardware immediately sprang to mind). But 1991 VG gets more interesting still given Steel’s finding that none of these known rocket bodies seem to have orbits returning to Earth at the time of the observation. That and the extremely close pass by the Earth suggested to Steel the possibility that 1991 VG could be an alien probe on a controlled reconnaissance mission.
Note this: Steel is hardly advocating the alien probe conclusion. What he is doing is to point out the interesting aspects of 1991 VG and examining it as an object of SETA interest. Studies like this are unlikely to turn up alien artifacts, but if we never investigate anomalies, we’ll sharply limit our understanding of what happens in space near the Earth. So how should such a study be followed up?
“It will be of interest,” says Steel, “to see whether sky-surveillance programmes reveal asteroids with similar orbital and light-curve properties as 1991 VG.” Steel reports that his personal bias is that this is a manmade artificial object; he calls for detailed investigation of rocket bodies left in heliocentric orbits that could account for the sighting. “If 1991 VG is a returned man-made rocket body,” he concludes, “it was very much a fluke that it was observed, and the normal process of science then requires that we consider the possibility of some other origin for it.”
Steel’s paper is “SETA and 1991 VG,” in The Observatory Vol. 115 (April, 1995), pp. 78-83. Also, see Jim Scotti’s recollections of the discovery, written in 1996. From Scotti’s posting:
“…the debate is still alive, and may continue to be so until someone can recover it next time it comes back around and then someday we can go out and take a close look. Does it have a regolith or a rocky surface or does it have “USA” or “CCCP” painted on the side? My guess is that it is indeed a natural object, but if it is manmade, perhaps it is a Saturn IVB stage from one of the early Apollo missions. As I recall, at least one left the 3rd stage in a high Earth orbit that would have eventually been perturbed into solar orbit.”
On SETA itself, one early Freitas paper is “A Search for Natural or Artificial Objects Located at the Earth-Moon Libration Points,” Icarus 42 (1980), pp. 442-47. Freitas argues in the paper that the Earth/Moon Lagrangian points are not in fact stable, being disturbed by solar gravity, but that large, stable orbits around these points do exist. Matloff’s recent work, discussed here, includes “Suggested Targets for an Infrared Search for Artificial Kuiper Belt Objects,” JBIS 58 (January/February 2005), pp. 51-61, written in collaboration with Anthony R. Martin (of Project Daedalus fame).
by Paul Gilster | Oct 3, 2005 | Deep Sky Astronomy & Telescopes
The sharp-eyed Jon Lomberg writes with a correction to today’s story on Xena and its moon Gabrielle. Specifically, my statement that adaptive optics ‘bounces’ the light of a laser off the atmosphere to create an artificial star used in refining the telescope’s images. Lomberg rightly points out that what the laser actually does is to excite sodium atoms at a specific height. The glow from this excitation is then tracked and used to adjust for atmospheric distortion. The results, as we have seen, are nothing short of spectacular.
What’s ahead for adaptive optics? “A future improvement of the technique,” writes Lomberg, “would use different lasers to excite other elements at other altitudes, thus giving a more detailed profile of distortion in the atmosphere resulting in more precise adjustments.”
by Paul Gilster | Oct 3, 2005 | Outer Solar System
Everyone is calling 2003 UB313, the Solar System’s 10th planet, Xena. The name comes from a TV warrior princess of whom the curmudgeonly Centauri Dreams, never one for television, was utterly unaware. Now Xena has been found to have a moon, inevitably named Gabrielle after an equally incrutable character on the series (apparently the sidekick of Xena herself). One-tenth the size of Xena, Gabrielle is slated for further observations with the Hubble Space Telescope that will allow more accurate determination of its mass.
The advent of Gabrielle is good news for those wishing to learn more about Xena. From a California Institute of Technology press release quoting Michael Brown, the 10th planet’s discoverer:
“A combination of the distance of the moon from the planet and the speed it goes around the planet tells you very precisely what the mass of the planet is,” explains Brown. “If the planet is very massive, the moon will go around very fast; if it is less massive, the moon will travel more slowly. It is the only way we could ever measure the mass of Xena-because it has a moon.”
Given the distances involved (Xena is now some 97 AU from the Sun, or 9 billion miles), it’s intriguing to learn how the moon was spotted. The work was performed using adaptive optics at the Keck Observatory in Mauna Kea. The technology allowed Brown and colleagues to remove the blur of atmospheric turbulence to create images as sharp as those from space-based instruments. Keck uses a laser guide system that actually creates an artificial ‘star’ with a laser beam, bouncing its light off the atmosphere. This allows for the needed correction for atmospheric effects and an unprecedentedly clear view from a ground-based system.
Image: The discovery of the moon of the 10th planet from the W.M. Keck Observatory. The planet appears in the center, while the moon is the small dot at the 3 o’clock position. Credit: W.M. Keck Observatory.
Centauri Dreams‘ take: we keep finding moons around these odd outer system objects. In fact, 2003 EL61 was found to have a moon (also discovered by Brown using adaptive optics) in January, and we know that both Pluto and Xena have moons. All of which may force a reexamination of conventional views of gravitational capture, which is how such moons have been explained until now. It is possible that actual collisions between Kuiper Belt objects may emerge as the prime factor in their formation, although we clearly have just begun our study of this strange realm.
For more, see this page from Michael Brown’s Web site.
