Astrophysicists Explore Blazars

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ScienceDaily (Mar. 18, 2009) — An international team of astrophysicists using telescopes on the ground and in space have uncovered surprising changes in radiation emitted by an active galaxy. The picture that emerges from these first-ever simultaneous observations with optical, X-ray and new-generation gamma-ray telescopes is much more complex than scientists expected and challenges current theories of how the radiation is generated.

The galaxy in question is PKS 2155-304, a type of object known as a "blazar." Like many active galaxies, a blazar emits oppositely directed jets of particles traveling near the speed of light as matter falls into a central supermassive black hole; this process is not well understood. In the case of blazars, the galaxy is oriented such that we're looking right down the jet.
PKS 2155-304 is located 1.5 billion light-years away in the southern constellation of Piscis Austrinus and is usually a detectable but faint gamma-ray source. But when its jet undergoes a major outburst, as it did in 2006, the galaxy can become the brightest source in the sky at the highest gamma-ray energies scientists can detect -- up to 50 trillion times the energy of visible light. Even from strong sources, only about one gamma ray this energetic strikes a square yard at the top of Earth's atmosphere each month.
The four identical telescopes of the High Energy Stereoscopic System in Namibia detect faint atmospheric flashes caused by the absorption of ultrahigh-energy gamma rays. Credit: H.E.S.S Atmospheric absorption of one of these gamma rays creates a short-lived shower of subatomic particles. As these fast-moving particles rush through the atmosphere, they produce a faint flash of blue light. The High Energy Stereoscopic System (H.E.S.S), an array of telescopes located in Namibia, captured these flashes from PKS 2155-304.
Gamma rays at lower energies were detected directly by the Large Area Telescope (LAT) aboard NASA's orbiting Fermi Gamma-ray Space Telescope. "The launch of Fermi gives us the opportunity to measure this powerful galaxy across as many wavelengths as possible for the first time," says Werner Hofmann, spokesperson for the H.E.S.S. team at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany.
With the gamma-ray regime fully covered, the team turned to NASA's Swift and Rossi X-ray Timing Explorer (RXTE) satellites to provide data on the galaxy's X-ray emissions. Rounding out the wavelength coverage was the H.E.S.S. Automatic Telescope for Optical Monitoring, which recorded the galaxy's activity in visible light.
Between August 25 and September 6, 2008, the telescopes monitored PKS 2155-304 in its quiet, non-flaring state. The results of the 12-day campaign are surprising. During flaring episodes of this and other blazars, the X- and gamma-ray emission rise and fall together. But it doesn't happen this way when PKS 2155-304 is in its quiet state -- and no one knows why.
What's even stranger is that the galaxy's visible light rises and falls with its gamma-ray emission. "It's like watching a blowtorch where the highest temperatures and the lowest temperatures change in step, but the middle temperatures do not," says Berrie Giebels, an astrophysicist at France's École Polytechnique who works with both the H.E.S.S. and Fermi LAT teams.
"Astronomers are learning that the various constituents of the jets in blazars interact in fairly complicated ways to produce the radiation that we observe," says Fermi team member Jim Chiang at Stanford University, Calif. "These observations may contain the first clues to help us untangle what's really going on deep in the heart of a blazar."
The findings have been submitted to The Astrophysical Journal.
The H.E.S.S. team includes scientists from Germany, France, the United Kingdom, Poland, the Czech Republic, Ireland, Armenia, South Africa and Namibia. The Fermi mission is an astrophysics and particle physics partnership, developed by NASA in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.
Adapted from materials provided by NASA/Goddard Space Flight Center.

Hearts Of Galaxies Close In For Cosmic Train Wreck

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ScienceDaily (Mar. 17, 2009) — A new image from NASA's Spitzer Space Telescope offers a rare view of an imminent collision between the cores of two merging galaxies, each powered by a black hole with millions of times the mass of the sun.

The galactic cores are in a single, tangled galaxy called NGC 6240, located 400-million light years away in the constellation Ophiuchus. Millions of years ago, each core was the dense center of its own galaxy before the two galaxies collided and ripped each other apart. Now, these cores are approaching each other at tremendous speeds and preparing for the final cataclysmic collision. They will crash into each other in a few million years, a relatively short period on a galactic timescale.
The spectacular image combines visible light from NASA's Hubble Space Telescope and infrared light from Spitzer. It catches the two galaxies during a rare, short-lived phase of their evolution, when both cores of the interacting galaxies are still visible but closing in on each other fast.
"One of the most exciting things about the image is that this object is unique," said Stephanie Bush of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a new paper describing the observation in an upcoming issue of the Astrophysical Journal. "Merging is a quick process, especially when you get to the train wreck that is happening. There just aren't many galactic mergers at this stage in the nearby universe."
NGC 6240 is already putting out huge amounts of infrared light, an indication that a burst of star formation is underway. The extra infrared radiation is common in interacting galaxies; as the two galaxies interact, dust and gas swept up by the collision form a burst of new stars that give off infrared light. Such galaxies are called luminous infrared galaxies. Spitzer's infrared array camera can image the extra heat from newly formed stars, even though their visible light is obscured by thick dust clouds around them.
The blob-like shape of the galaxy is due to the sustained violence of the collision. Streams of millions of stars are being ripped off the galaxy, forming wispy "tidal tails" that lead off NGC 6240 in several directions. But things are about to get even more violent as the main event approaches and the two galactic cores meld into one.
In the center of NGC 6240, the two black holes in the cores will whip up a frenzy of radiation as they careen towards one another head-on, likely transforming the galaxy into a monster known as an ultra-luminous infrared galaxy, thousands of times as bright in infrared as our Milky Way.
Another fascinating aspect of this rare object is that no two galactic mergers are the same. "Not only are there few objects at this stage, but each object is unique because it came from different progenitor galaxies," said Bush. "These observations give us another layer of information about this galaxy, and galactic mergers in general."
Infrared light taken by Spitzer's infrared array camera at 3.6 and 8.8 microns (red) shows cold dust and radiation from star formation; visible light from Hubble (green and blue) shows hot gas and stars.
Other authors of this paper include Zhong Wang, Margarita Karovska and Giovanni Fazio, all of the Harvard-Smithsonian Center for Astrophysics.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.
More information about Spitzer is at http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer.
Adapted from materials provided by NASA/Jet Propulsion Laboratory.

Four Of Saturn's Moons Parade By Their Parent

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ScienceDaily (Mar. 17, 2009) — A new Hubble image shows four of Saturn's moons circling the ringed planet.

On 24 February 2009, the NASA/ESA Hubble Space Telescope captured a photo sequence of four moons of Saturn passing in front of their parent planet. The moons, from far left to right, are the white icy moons Enceladus and Dione, the large orange moon Titan, and icy Mimas. Due to the angle of the Sun, they are each preceded by their own shadow.
These rare moon transits only happen when the tilt of Saturn's ring plane is nearly "edge on" as seen from the Earth. Saturn's rings will be perfectly edge on to our line of sight on 10 August and 4 September 2009. Unfortunately, Saturn will be too close to the Sun to be seen by viewers on Earth at that time. This "ring plane crossing" occurs every 14-15 years. In 1995-96 Hubble witnessed the previous ring plane crossing, as well as many moon transits, and helped to discover several new moons of Saturn.
Early 2009 was a favourable time for viewers with small telescopes to watch moon and shadow transits crossing the face of Saturn. Titan, Saturn's largest moon, crossed Saturn on four separate occasions: 24 January, 9 February, 24 February and 12 March, although not all events were visible from all locations on Earth.
Italian Galileo Galilei — often referred to as the father of astronomy — was the first to observe Saturn through a telescope in 1610. Dutch mathematician and astronomer Christian Huygens discovered Titan in 1655 and, 350 years later, the ESA probe named for him touched down on Titan (on 14 January 2005), giving the world its first views of the surface of the mysterious, icy world. Giovanni Domenico Cassini, a French/Italian astronomer, discovered Dione (in addition to others) and the German-born Englishman, William Herschel, discovered Mimas and Enceladus.
These pictures were taken with Hubble's Wide Field Planetary Camera 2 on 24 February 2009, when Saturn was at a distance of roughly 1.25 billion kilometres from Earth. Hubble can see details as small as 300 kilometres across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet.
Adapted from materials provided by ESA/Hubble Information Centre.

Curious Pair Of Galaxies

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ScienceDaily (Mar. 16, 2009) — The ESO Very Large Telescope has taken the best image ever of a strange and chaotic duo of interwoven galaxies. The images also contain some surprises — interlopers both far and near.

Sometimes objects in the sky that appear strange, or different from normal, have a story to tell and prove scientifically very rewarding. This was the idea behind Halton Arp’s catalogue of Peculiar Galaxies that appeared in the 1960s. One of the oddballs listed there is Arp 261, which has now been imaged in more detail than ever before using the FORS2 instrument on ESO’s Very Large Telescope. The image proves to contain several surprises.
Arp 261 lies about 70 million light-years distant in the constellation of Libra, the Scales. Its chaotic and very unusual structure is created by the interaction of two galaxies that are engaged in a slow motion, but highly disruptive close encounter. Although individual stars are very unlikely to collide in such an event, the huge clouds of gas and dust certainly do crash into each other at high speed, leading to the formation of bright new clusters of very hot stars that are clearly seen in the picture. The paths of the existing stars in the galaxies are also dramatically disrupted, creating the faint swirls extending to the upper left and lower right of the image. Both interacting galaxies were probably dwarfs not unlike the Magellanic Clouds orbiting our own galaxy.
The images used to create this picture were not actually taken to study the interacting galaxies at all, but to investigate the properties of the inconspicuous object just to the right of the brightest part of Arp 261 and close to the centre of the image. This is an unusual exploding star, called SN 1995N, that is thought to be the result of the final collapse of a massive star at the end of its life, a so-called core collapse supernova. SN 1995N is unusual because it has faded very slowly — and still shows clearly on this image more than seven years after the explosion took place! It is also one of the few supernovae to have been observed to emit X-rays. It is thought that these unusual characteristics are a result of the exploding star being in a dense region of space so that the material blasted out from the supernova ploughs into it and creates X-rays.
Apart from the interacting galaxy and its supernova the image also contains several other objects at wildly different distances from us. Starting very close to home, two small asteroids, in our Solar System between the orbits of Mars and Jupiter, happened to cross the images as they were being taken and show up as the red-green-blue trails at the left and top of the picture. The trails arise as the objects are moving during the exposures and also between the exposures through different coloured filters. The asteroid at the top is number 14670 and the one to the left number 9735. They are probably less than 5 km across. The reflected sunlight from these small bodies takes about fifteen minutes to get to the Earth.
The next closest object is probably the apparently bright star at the bottom. It may look bright, but it is still about one hundred times too faint to be seen with the unaided eye. It is most likely a star rather like the Sun and about 500 light-years from us — 20 million times further away than the asteroids. Arp 261 itself, and the supernova, are about 140 000 times further away again than this star, but still in what astronomers would regard as our cosmic neighbourhood. Much more distant still, perhaps some fifty to one hundred times further away than Arp 261, lies the cluster of galaxies visible on the right of the picture. There is no doubt, however, that a much more remote object lies, unrecognised, amongst the faint background objects seen in this marvellous image.
Adapted from materials provided by ESO.

The Day The Sun Brought Darkness

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ScienceDaily (Mar. 16, 2009) — On March 13, 1989 the entire province of Quebec, Canada suffered an electrical power blackout. Hundreds of blackouts occur in some part of North America every year. The Quebec Blackout was different, because this one was caused by a solar storm.

Solar flares and coronal mass ejections (CMEs), associated giant clouds of plasma in space, are the largest explosions in the solar system. They are caused by the buildup and sudden release of magnetic stress in the solar atmosphere above the giant magnetic poles we see as sunspots. CMEs can cause magnetic storms affecting communication systems, power grids and astronauts in space.
On Friday March 10, 1989 astronomers witnessed a powerful explosion on the sun. Within minutes, tangled magnetic forces on the sun had released a billion-ton cloud of gas. It was like the energy of thousands of nuclear bombs exploding at the same time. The storm cloud rushed out from the sun, straight towards Earth, at a million miles an hour. The solar flare that accompanied the outburst immediately caused short-wave radio interference, including the jamming of radio signals from Radio Free Europe into Russia. It was thought that the signals had been jammed by the Kremlin, but it was only the sun acting up.
On the evening of Monday, March 12 the vast cloud of solar plasma (a gas of electrically charged particles) finally struck Earth's magnetic field. The violence of this 'geomagnetic storm' caused spectacular 'northern lights' that could be seen as far south as Florida and Cuba. The magnetic disturbance was incredibly intense. It actually created electrical currents in the ground beneath much of North America. Just after 2:44 a.m. on March 13, the currents found a weakness in the electrical power grid of Quebec. In less than 2 minutes, the entire Quebec power grid lost power. During the 12-hour blackout that followed, millions of people suddenly found themselves in dark office buildings and underground pedestrian tunnels, and in stalled elevators. Most people woke up to cold homes for breakfast. The blackout also closed schools and businesses, kept the Montreal Metro shut during the morning rush hour, and closed Dorval Airport.
The Quebec Blackout was by no means a local event. Some of the U.S. electrical utilities had their own cliffhanger problems to deal with. New York Power lost 150 megawatts the moment the Quebec power grid went down. The New England Power Pool lost 1,410 megawatts at about the same time. Service to 96 electrical utilities in New England was interrupted while other reserves of electrical power were brought online. Luckily, the U.S. had the power to spare at the time…but just barely. Across the United States from coast to coast, over 200 power grid problems erupted within minutes of the start of the March 13 storm. Fortunately none of these caused a blackout.
In space, some satellites actually tumbled out of control for several hours. NASA's TDRS-1 communication satellite recorded over 250 anomalies as high-energy particles invaded the satellite's sensitive electronics. Even the Space Shuttle Discovery was having its own mysterious problems. A sensor on one of the tanks supplying hydrogen to a fuel cell was showing unusually high pressure readings on March 13. The problem went away just as mysteriously after the solar storm subsided.
Twenty years later, the March 1989 'Quebec Blackout' has reached legendary stature, at least among electrical engineers and space scientists. It is a dramatic example of how solar storms can affect us even here on the ground. Fortunately, storms as powerful as this are rather rare. It takes quite a solar wallop to cause anything like the conditions leading up to a Quebec-style blackout. Typical solar activity 'sunspot' cycles can produce least two or three large storms, so it really is just a matter of chance whether one will cause a blackout or not. As it is for hurricanes and tornadoes, the more we can learn about the sun's 'space weather,' the better we can prepare for the next storm when it arrives.
Adapted from materials provided by NASA.

Galactic Dust Bunnies Found To Contain Carbon After All

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ScienceDaily (Mar. 17, 2009) — Using NASA's Spitzer Space Telescope, researchers have found evidence suggesting that stars rich in carbon complex molecules may form at the center of our Milky Way galaxy.

This discovery is significant because it adds to our knowledge of how stars form heavy elements -- like oxygen, carbon and iron -- and then blow them out across the universe, making it possible for life to develop.
Astronomers have long been baffled by a strange phenomenon: Why have their telescopes never detected carbon-rich stars at the center of our galaxy even though they have found these stars in other places? Now, by using Spitzer's powerful infrared detectors, a research team has found the elusive carbon stars in the galactic center.
"The dust surrounding the stars emits very strongly at infrared wavelengths," says Pedro García-Lario, a research team member who is on the faculty of the European Space Astronomy Center, the European Space Agency's center for space science. He co-authored a paper on this subject in the February 2009 issue of the journal Astronomy & Astrophysics.
"With the help of Spitzer spectra, we can easily determine whether the material returned by the stars to the interstellar medium is oxygen-rich or carbon-rich."
The team of scientists analyzed the light emitted from 40 planetary nebulae – blobs of dust and gas surrounding stars -- using Spitzer's infrared spectrograph. They analyzed 26 nebulae toward the center of the Milky Way -- a region called the "Galactic Bulge" -- and 14 nebulae in other parts of the galaxy. The scientists found a large amount of crystalline silicates and polycyclic aromatic hydrocarbons, two substances that indicate the presence of oxygen and carbon.
This combination is unusual. In the Milky Way, dust that combines both oxygen and carbon is rare and is usually only found surrounding a binary system of stars. The research team, however, found that the presence of the carbon-oxygen dust in the Galactic Bulge seems to be suggestive of a recent change of chemistry experienced by the star.
The scientists hypothesize that as the central star of a planetary nebula ages and dies, its heavier elements do not make their way to the star's outer layers, as they do in other stars. Only in the last moments of the central star's life, when it expands and then violently expels almost all of its remaining outer gasses, does the carbon become detectable. That's when astronomers see it in the nebula surrounding the star.
"The carbon produced through these recurrent 'thermal pulses' is very inefficiently dredged up to the surface of the star, contrary to what is observed in low-metallicity, galactic disk stars," said García-Lario. "It only becomes visible when the star is about to die." This study supports a hypothesis about why the carbon in some stars does not make its way to the stars' surfaces. Scientists believe that small stars -- those with masses up to one-and-a-half times that of our sun -- that contain lots of metal do not bring carbon to their surfaces as they age. Stars in the Galactic Bulge tend to have more metals than other stars, so the Spitzer data support this commonly held hypothesis. Before the Spitzer study, this hypothesis had never been supported by observation.
This aging and expelling process is typical of all stars. As stars age and die, they burn progressively heavier and heavier elements, beginning with hydrogen and ending with iron. Towards the end of their lives, some stars become what are called "red giants." These dying stars swell so large that if one of them were placed in our solar system, where the sun is now, its outermost border would touch Earth's orbit. As these stars pulsate – losing mass in the process – and then contract, they spew out almost all of their heavier elements. These elements are the building blocks of all planets, including our own Earth (as well as of human beings and any other life forms that may exist in the universe).
The paper is co-authored by José Vicente Perea-Calderón of the European Space Astronomy Center in Villanueva de la Cañada, Spain; Domingo Anibal García-Hernández of the Instituto de Astrofísica de Canarias, on Spain's Tenerife island; Ryszard Szczerba of the Nicolaus Copernicus Astronomical Center in Torun, Poland; and Matt Bobrowsky of the University of Maryland, College Park.
Adapted from materials provided by NASA.

Best-ever View Of The Cosmos In Gamma Rays

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ScienceDaily (Mar. 12, 2009) — A new map combining nearly three months of data from NASA's Fermi Gamma-ray Space Telescope is giving astronomers an unprecedented look at the high-energy cosmos. To Fermi's eyes, the universe is ablaze with gamma rays from sources ranging from within the solar system to galaxies billions of light-years away.

"Fermi has given us a deeper and better-resolved view of the gamma-ray sky than any previous space mission," said Peter Michelson, the lead scientist for the spacecraft's Large Area Telescope (LAT) at Stanford University, Calif. "We're watching flares from supermassive black holes in distant galaxies and seeing pulsars, high-mass binary systems, and even a globular cluster in our own."
A paper describing the 205 brightest sources the LAT sees has been submitted to The Astrophysical Journal Supplement. "This is the mission's first major science product, and it's a big step toward producing our first source catalog later this year," said David Thompson, a Fermi deputy project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
The LAT scans the entire sky every three hours when operating in survey mode, which is occupying most of the telescope’s observing time during Fermi's first year of operations. These snapshots let scientists monitor rapidly changing sources.
The all-sky image just released shows us how the cosmos would look if our eyes could detect radiation 150 million times more energetic than visible light. The view merges LAT observations spanning 87 days, from August 4 to October 30, 2008.
The map includes one object familiar to everyone: the sun. "Because the sun appears to move against the background sky, it produces a faint arc across the upper right of the map," Michelson explained. During the next few years, as solar activity increases, scientists expect the sun to produce growing numbers of high-energy flares. "No other instrument will be able to observe solar flares in the LAT's energy range," he said.
To better show individual sources, the new map was processed to suppress emissions from gas in the plane of our galaxy, the Milky Way. As a way of underscoring the variety of the objects the LAT is seeing, the Fermi team created a "top ten" list comprising five sources within the Milky Way and five beyond our galaxy.
The top sources within our galaxy include the sun; a star system known as LSI +61 303, which pairs a massive normal star with a superdense neutron star; PSR J1836+5925, which is one of many new pulsars, a type of spinning neutron star that emits gamma-ray beams; and the globular cluster 47 Tucanae, a sphere of ancient stars 15,000 light-years away.
Top extragalactic sources include NGC 1275, a galaxy that lies 225 million light-years away and is known for intense radio emissions; the dramatically flaring active galaxies 3C 454.3 and PKS 1502+106, both more than 6 billion light-years away; and PKS 0727-115, which is thought to be a type of active galaxy called a quasar.
The Fermi top ten also includes two sources -- one within the Milky Way plane and one beyond it -- that researchers have yet to identify. More than 30 of the brightest gamma-ray sources have no obvious counterparts at other wavelengths. "That's good news. It means we're seeing new objects," Michelson said. "It also means that we have lots of work to do."
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership mission, developed in collaboration with the U.S. Department of Energy and important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the U.S.
Adapted from materials provided by NASA/Goddard Space Flight Center.