Universe Today

The center of our galaxy is hidden behind a “brick wall” of obscuring dust so thick that not even the Hubble Space Telescope can penetrate it. Astronomers Silas Laycock and Josh Grindlay (Harvard-Smithsonian Center for Astrophysics) and colleagues have lifted that veil to reveal a beautiful vista swarming with stars. Moreover, their hunt for specific stars associated with X-ray-emitting sources has ruled out one of two options for the nature of these X-ray sources: most apparently are not associated with massive stars, which would have shown up as bright counterparts in their deep infrared images. This points to the X-ray sources being white dwarfs, not black holes or neutron stars, accreting matter from low-mass binary companion stars.

Their study is being presented today at a press conference at the 205th meeting of the American Astronomical Society in San Diego, Calif.

To peer into the galactic center, Laycock and Grindlay used the unique capabilities of the 6.5-meter-diameter Magellan Telescope in Chile. By gathering infrared light that more easily penetrates dust, the astronomers were able to detect thousands of stars that otherwise would have remained hidden. Their goal was to identify stars that orbit, and feed, X-ray-emitting white dwarfs, neutron stars or black holes – any of which could yield the faint X-ray sources discovered originally with NASA’s Chandra X-ray Observatory.

Chandra previously detected more than 2000 X-ray sources in the central 75 light-years of our galaxy. About four-fifths of the sources emitted mostly hard (high-energy) X-rays. The precise nature of those hard X-ray sources remained a mystery. Two possibilities were suggested by astronomers: 1) high-mass X-ray binary systems, containing a neutron star or black hole with a massive stellar companion; or, 2) cataclysmic variables, containing a highly magnetized white dwarf with a low-mass stellar companion. Determining the nature of the sources can teach us about the star formation history and dynamical evolution of the region near the galactic center.

“If we found that most of the hard X-ray sources were high-mass X-ray binaries, it would tell us that there had been a lot of recent star formation because massive stars don’t live long,” says Laycock. “Instead, we found that most of the X-ray sources are likely to be older systems associated with low-mass stars.”

That conclusion comes from a null result: that is, most of the counterparts to the X-ray sources must be fainter than the brightness expected if the X-ray sources had massive companions. Since massive stars are both rare and bright, an association with the X-ray sources would have been easy to spot. Smaller stars are more common and fainter, making it difficult to match them to a specific X-ray source. Analysis of the infrared images found only a chance number of matches between stars and the locations of X-ray sources. Many of those matches likely were due to the crowded field of view.

“The fact that we found no significant excess of bright infrared counterparts means that the galactic center Chandra sources are probably low-mass binaries. Since by far the most common low-mass binaries with X-ray luminosities, spectra, and variability similar to the galactic center Chandra sources are accreting magnetic white dwarfs, we conclude these are the most likely identification,” says Grindlay.

If the X-ray sources near the galactic center are accreting white dwarfs, the large numbers of compact low-mass binaries required could suggest that they formed in the very dense star cluster around the galactic center or that they have been “deposited” there by the destruction of globular clusters. Deeper infrared observations and spectra of the sources are needed to make actual identifications and constrain the masses of the accreting compact objects.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

Image credit: NASA
1.Begin the New Year by observing Saturn. Saturn will be at opposition (i.e. opposite the sun in the sky) on January 13th at 08:00 UTC. Saturn is in the constellation Gemini the Twins, about 6 degrees south east of the star Pollux.

2.Begin learning the names of 20 constellations. The inclination is to start with the brightest, most obvious ones. But sometimes it can be a fun challenge to find a less conspicuous one. Obviously this is a project that could last throughout the entire year. It is also a good chance to learn which constellations are visible during each season.

3.Learn the names of 20 bright stars. While many of us know the names of the constellations, the names of the stars are somewhat less well known. Knowing the star names is a good way to find and learn the constellations as well. Sometimes a constellation will have only one or two bright stars. These can act as guide posts to finding the entire constellation. As with learning the constellations, this is a project that can last all year.

4.Count the stars in the Pleiades, also known as the Seven Sisters. This little star cluster is west of the constellation Taurus the Bull. From a dark location you may see 5 or 6 stars. Then look at them with a pair of binoculars, what do you see?

5.Observe the moon for a month. Notice how much of its surface is illuminated from night to night. Kids can use a calendar and draw what shape the moon is each night (or even day!) Use binoculars or a telescope to observe the lunar features especially near the line that divides day from night (called the terminator).

6.In June look for Mercury and Venus in the evening sky between 0:300 and 04:00 UTC. The pair will be only 0.1 degrees apart. At that distance you will probably need binoculars to tell them apart. Venus will be the brightest object in the sky while Mercury will be much fainter. Saturn will be the bright object to the North West.

7.Observe a meteor shower. The famed Perseids is visible from July 17th to August 24th with the peak occurring before dawn on August 12th . Meteor showers get their names from the constellation they appear to radiate from, in this case Perseus the Hero. The best way to observe meteors is lying back on the ground or in a lawn chair. Kids can have fun counting how many they see in an hour.

8.On September 1st, about 03:00 UTC. Jupiter and Venus will be about 1 degree apart in the west just after sunset. No need to use binoculars to find this pair, but it might be fun to see them both through a telescope. Don’t forget to look for Jupiter’s 4 large moons: Ganymede, Callisto, Europa and Io.

9. Hunt for double stars. Many of the stars that we see in the sky have one or more companion stars. In many cases they are not visible to the naked eye. Here is a good opportunity to hone your binocular or telescope skills. Begin with the star Mizar in the constellation Ursa Major (the Big Dipper); it is the middle star in the Dipper’s handle.

10.Share the sky with your children or other young people. Kids love stories, and most never tire of hearing the same ones over and over again. Learn the myths and legends of your favorite constellations and tell them to your kids. Who knows, maybe your love for the sky will spark an interest in them that will last a lifetime.

This list should give even the most timid of beginners a starting point for their celestial quest. And don’t forget, there are lots of other enthusiasts out there, don’t be shy in contacting them and asking questions and sharing your experiences. Enjoy 2005, and clear skies to you all.

Written by Rod Kennedy.

Hubble Space Telescope data, analyzed by a Yale astronomer using gravitational lensing techniques, has generated a spatial map demonstrating the clumped substructure of dark matter inside clusters of galaxies.

Clusters of galaxies (about a million, million times the mass of our sun), are typically made up of hundreds of galaxies bound together by gravity. About 90 percent of their mass is darkmatter. The rest is ordinary atoms in the form of hot gas and stars.

Although little is known about it, cold dark matter is thought to have structure at all magnitudes. Theoretical models of the clumping properties were derived from detailed, high resolution simulations of the growth of structure in the Universe. Although previous evidence supported the ?concordance model? of a Universe mostly composed of cold, dark matter, the predicted substructure had never been detected.

In this study, Yale assistant professor of astronomy and physics Priyamvada Natarajan and her colleagues demonstrate that, at least in the mass range of typical galaxies in clusters, there is an excellent agreement between the observations and theoretical predictions of the concordance model.

Using gravitational lensing made it possible for the observers to visualize light from distant galaxies as it bent around mass in its way. This allowed the researchers to measure light deflections that indicated structural clumps in the dark matter.

?We used an innovative technique to pick up the effect of precisely the clumps which might otherwise be obscured by the presence of more massive structures,? said Natarajan. ?When we compared our results with theoretical expectations of the concordance model, we found extremely good agreement, suggesting that the model passes the substructure test for the mass range we are sensitive to with this technique.?

?We think the properties of these clumps hold a key to the nature of dark matter ? which is presently unknown,? said Natarajan. ?The question remains whether these predictions and observations agree for smaller mass clumps that are as yet undetected.?

Co-author on the study, funded by Yale University, is Volker Springel, MPA, Garching, Germany. Other collaborators include. Jean-Paul Kneib, LAM ? OAMP, Marseille, France, Ian Smail, University of Durham, U.K., and Richard Ellis of Caltech.

Original Source: Yale News Release

Image credit: NASA
The NASA-led Swift mission has opened its doors to a flurry of gamma-ray burst action.

Scientists were still calibrating the main instrument, the Burst Alert Telescope (BAT), when the first burst appeared on December 17. Three bursts on December 19, and one on December 20, followed.

Swift’s primary goal is to unravel the mystery of gamma ray bursts. The bursts are random and fleeting explosions, second only to the Big Bang in total energy output. Gamma rays are a type of light millions of times more energetic than light human eyes can detect. Gamma ray bursts last only from a few milliseconds to about one minute. Each burst likely signals the birth of a black hole.

“The optimists among us were hoping to detect two bursts a week, not three in one day just after turning the telescope on,” said Dr. Scott Barthelmy, the BAT lead scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md. “Maybe we got lucky, or maybe we’ve underestimated the true rate of these bursts. Only time will tell,” he added.

Once the BAT, that covers about one-seventh of the sky at any time, detects a gamma ray burst, it quickly relays a location to the ground. Within about one minute, the satellite automatically turns toward the burst. The move brings the burst within view of Swift’s two other telescopes: the X-ray Telescope (XRT) and the Ultraviolet/Optical Telescope (UVOT).

Once all three instruments are turned on and calibrated, Swift will get down to the business of analyzing gamma ray bursts. “The universe kept up its side of the bargain, and we kept up ours,” said Dr. Neil Gehrels, Swift’s Principal Investigator at Goddard. “This is going to be an exciting mission,” he said.

The Swift team tested the BAT by observing Cygnus X-1, a well-known bright source that produces gamma rays in our galaxy. It is thought to be a black hole in orbit around a star. The team called this BAT’s “first light.”

The BAT is the most sensitive gamma ray detector ever flown. The BAT employs a novel technology to image and locate gamma ray bursts. Unlike visible light, gamma rays pass right through telescope mirrors and cannot be reflected onto a detector. The BAT uses a technique called “coded aperture mask” to create a gamma ray shadow on its detectors. The mask contains 52,000 randomly placed lead tiles that block some gamma rays from reaching the detectors. With each burst, some detectors light up while others remain dark, shaded by the lead tiles. The angle of the shadow points back to the gamma ray burst.

“The BAT coded aperture mask is about the size of a pool table, the largest and most intricate ever fabricated,” said Ed Fenimore of Los Alamos National Laboratory, N.M. Los Alamos created the BAT software. “BAT can accurately pinpoint a burst within seconds and detect bursts five times fainter than previous instruments,” he added.

Swift, a medium-class explorer mission managed by Goddard, was launched from Cape Canaveral on November 20, 2004. The mission is in participation with the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom.

Swift was built at Goddard in collaboration with General Dynamics, Ariz.; Penn State University, College Station, Pa.; Sonoma State University, Rohnert Park, Calif.; Los Alamos; Mullard Space Science Laboratory, Surrey, England; the University of Leicester, England; the Brera Observatory, Milan, Italy; and ASI Science Data Center, Rome.

Original Source: NASA News Release