Keck II infrared image of HR 4796 showing "debris disk".(NASA/JPL)
Several press releases concerning the existence of black holes in our nearby Universe were presented at the American Astronomical Society (AAS) meeting in Atlanta the week of Jan 11-15, 2000. In particular, results from NRAO's VLA, have contributed to several of these investigations, particularly the imaging of galactic streamers and discovery of a nearby black hole. In addition, the debut of results from Chandra, NASA's new X-ray observatory, featured prominently in these results.
Also conspicuous at this meeting was the rejeuvanated Hubble Space Telescope. See the following stories, though the "Bubble" results were from observations taken in 1997 and April 1999, the "Lone Black Holes" in June 1999, with no date given for the Beta Pictoris Ring images, not recently as might be implied by the timing of the releases.
As with any scientific press releases, the probable importance (or even veracity) of the conclusions drawn from these often complicated observations should be viewed skeptically! Unfortunately, this is particularly true for the flurry of press releases that accompany the AAS meetings. However, it is clear that we are gaining ground in the fight to pin down elusive Black Holes and to discover their role in our Universe.
And speaking of dubious claims, on the lighter side...
What can you say? At least they make a concrete verifiable prediction! Too bad they wont comment when nothing happens...
For the first time, optical flashes from elusive objects known as "Gamma Ray Bursters" or GRBs have been captured only seconds after the gamma rays themselves were intercepted by the Compton Gamma Ray Observatory (CGRO). Up until 1997, the only things known about GRBs were that they emitted bursts of gamma rays, we got about one per day in the entire sky, and they were not located in any preferred direction. With the discovery of X-ray emission, as well as faint optical and radio afterglows, we finally knew that GRBs were hyperluminous and located at cosmological distances. But catching the optical flash, which lasts only a few seconds, has proved much more difficult.
However, a particularly bright GRB triggered the sensors onboard CGRO at 4:47 a.m. EST, on Jan. 23 1999 (giving it the designation GRB990123). Simultaneously, another satellite, Beppo-SAX, detected X-rays and reported the postion to a wide-field ground-based telescope in Los Alamos, NM, which snapped pictures only seconds later. The brightness of the flash reached nearly magnitude +9, which could have actually be seen with binoculars at a dark site, if you knew where and when to look! Within hours, however, it had faded to mag +18, but was bright enough for Caltech astronomers to determine the redshift as z=1.6 for the host galaxy. The total estimated luminosity for GRB 990123 was around 10^16 solar luminosities!
See the following sites for more information:
In the continuous quest to unravel the nature of the dark matter, several teams of astronomers have been working on microlensing surveys, where many images are taken of a very large number of stars to detect a variation of their apparent brightness. If a compact object, such as a star or a planet, passes near the line-of-sight to one of the stars being observed, the gravitational force of the intervening object deflects the light in the phenomenon known as gravitational lensing, and causes a variation in the observed flux. Given the velocities of stars in the halo of our galaxy, such microlensing events have a characteristic duration of several months if the mass of the lensing object is similar to the mass of the Sun, or several hours in the case of an Earth-like planet.
About 200 microlensing events have been detected so far in the bulge of the Milky Way, and over a dozen in the Large Magellanic Cloud (LMC). Most of these events have been found by The MACHO Collaboration . If the dark matter in the halo of the Milky Way was made of compact objects, they would lie on the line-of-sight to the LMC and produce observable microlensing events. But if the dark matter is made of some particles that do not form compact objects and are distributed smoothly in space, then only the microlensing events caused by the normal, luminous stars that are known to exist would be observed, which would be many less. These stars that can act as lenses would be the stars in the LMC itself, stars in the disk of the Milky Way that are close to us, and stars in the halo that account for less than 1% of the total mass of the halo.
So far, the number of observed LMC events seem to indicate that there are more events than would be expected only from known stars, although maybe not as many as would be expected if all the dark matter was made of compact objects. But curiously, the durations of the events suggest that the masses of these lenses are similar to the mass of the Sun. Objects made of normal baryonic matter of this mass would have to emit light and be visible. Thus, if they are not normal stars, what are they? Black holes? A new type of object making up most of the Universe that nobody is expecting? Or may be they are just LMC stars and we have somehow underestimated the number of lensing events they can produce? Or are they some foreground group of stars , maybe from a disrupted galaxy tidal tail, which is maliciously lying in front of the LMC and causing the events that puzzle the astronomers of the Earth? Observations are continuing and a debate about the nature of the lensing objects is going on.
Recently, there was also an event observed on a star in the Small Magellanic Cloud. The detection of this event is in agreement with the lensing optical depth inferred from the LMC.
The Hipparcos Satellite, which has measured accurate parallaxes for a large number of stars, has recently produced some results which have refined the distance scale, and more fundamentally, claimed to have brought the ages of globular clusters in line with the expansion age of the Universe as determined by the standard cosmological parameters (Hubble constant and density).
Some preprints related to this topic:
An X-ray satellite, combined with ground-based optical and radio measurements, may have discovered the "smoking gun" that will identify the source of the mysterious Gamma-Ray Bursts (GRBs).
On December 14, 1997, a short but intense burst of Gamma Rays was detected in Earth orbit by the BeppoSAX satellite. This week, in a press conference, astronomers announced that this Gamma Ray Burst was the most luminous known. The key clue has been the measurement of the distance to the host galaxy of the GRB, using the W.M. Keck telescope. This discovery supports the notion that GRBs are the result of the merging of two ultradense compact objects, perhaps a neutron star and a black hole.
Previously in a May 14, 1997 press release, a team of Caltech astronomers has reportedly clinched the identification of the source of one of the mysterious gamma-ray bursts as cosmological - originating in distant galaxies as opposed to in our own galaxy or in the outer reaches of our solar system as some competing models would have it. The identification of spectral features and determination of its redshift, and hence distance, is the smoking gun for a cosmological origin model.
The leading model for the origin of these bursts is a merger between two neutron stars (or perhaps black holes) where because of the liberation of vast amounts of gravitational potential energy the object can outshine an entire galaxy for a short time. Note that a supernova, due to the core collapse inside a massive star, is powered by graviational binding energy also (though the luminosity comes from a different mechanism).
Keck II infrared image of HR 4796 showing "debris
disk".(NASA/JPL)
In a press conference today (21Apr98), David Koerner of University of Pennsylvania and Mike Werner of JPL announced the discovery of a "debris-disk" around the young star HR 4796. The observations were made with the Keck II Telescope in Hawaii. The elongated structure surrounding the star seen in the infrared is interpreted as a disk of dust particles similar to one seen around the nearer star Beta Pictoris. Furthermore, there appears to be a hole in the center of the disk, large enough to contain a solar system.
The NCSA has put together a fascinating exposition Cosmos in a Computer featuring some of the latest state-of-the-art simulations of our Universe. Be sure to try the exhibit map to navigate the site.
smyers@nrao.edu Steven T. Myers