Fermi Maps An Active Galactic Smokestack Plumes

The gamma-ray output from Cen A's lobes exceeds their radio output by more than ten times. High-energy gamma rays detected by Fermi's Large Area Telescope are depicted as purple in this gamma ray/optical composite of the galaxy. Credit: NASA/DOE/Fermi LAT Collaboration, Capella Observatory

If our eyes could see radio waves, the nearby galaxy Centaurus A (Cen A) would be one of the biggest and brightest objects in the sky, nearly 20 times the apparent size of a full moon. What we can't see when looking at the galaxy in visible light is that it lies nestled between a pair of giant radio-emitting gas plumes ejected by its supersized black hole. Each plume is nearly a million light-years long.

NASA's Fermi Gamma-Ray Space Telescope maps gamma rays, radiation that typically packs 100 billion times the energy of radio waves. Nevertheless, and to the surprise of many astrophysicists, Cen A's plumes show up clearly in the satellite's first 10 months of data. The study appears in Thursday's edition of Science Express.

"This is something we've never seen before in gamma rays," said Teddy Cheung, a Fermi team member at the Naval Research Laboratory in Washington. "Not only do we see the extended radio lobes, but their gamma-ray output is more than ten times greater than their radio output." If gamma-ray telescopes had matured before their radio counterparts, astronomers would have instead classified Cen A as a "gamma-ray galaxy."

Also known as NGC 5128, Cen A is located about 12 million light-years away in the constellation Centaurus and is one of the first celestial radio sources identified with a galaxy.

"A hallmark of radio galaxies is the presence of huge, double-lobed radio-emitting structures around otherwise normal-looking elliptical galaxies," said Jurgen Knodlseder, a Fermi collaborator at the Center for the Study of Space Radiation in Toulouse, France. "Cen A is a textbook example."

Astronomers classify Cen A as an "active galaxy," a term applied to any galaxy whose central region exhibits strong emissions at many different wavelengths. "What powers these emissions is a well-fed black hole millions of times more massive than our sun," said Yasushi Fukazawa, a co-author of the study at Hiroshima University in Japan. "The black hole somehow diverts some of the matter falling toward it into two oppositely directed jets that stream away from the center."

Fueled by a black hole estimated at hundreds of millions of times the sun's mass, Cen A ejects magnetized particle jets moving near the speed of light. Over the course of tens of millions of years, these jets puffed out two giant bubbles filled with magnetic fields and energetic particles - the radio lobes we now see. The radio waves arise as high-speed electrons spiral through the lobes' tangled magnetic fields.

But where do gamma rays - the highest-energy form of light - come from?

The entire universe is filled with low-energy radiation - radio photons from the all-pervasive cosmic microwave background, as well as infrared and visible light from stars and galaxies. The presence of this radiation is the key to understanding Cen A's gamma rays.

"When one of these photons collides with a super-fast particle in the radio lobes, the photon receives such an energy boost, it becomes a gamma ray," explained co-author Lukasz Stawarz at the Japan Aerospace Exploration Agency in Sagamihara, Japan.

Although it sounds more like billiards than astrophysics, this process, called inverse Compton scattering, is a common way of making cosmic gamma rays. For Cen A, an especially important aspect is the case where photons from the cosmic microwave background ricochet off of the highest-energy particles in the radio lobes.

In dozens of active galaxies, this process has been shown to produce X-rays. But the Cen A study marks the first case where astronomers have solid evidence that microwave photons can be kicked up to gamma-ray energies.

Fermi cataloged hundreds of blazars and other types of active galaxies in its first year. Before its mission ends, that number may reach several thousand. But because Cen A is so close, so large and so vigorous, it may be the only active galaxy Fermi will view this way.

With Centaurus A, Fermi hit the jackpot.

Centaurus A Emits in Both Radio and Gamma-ray Wavelengths

In orbit only since June 2008, the Fermi Gamma-ray Space Telescope has already produced some remarkable science. One of the most remarkable findings it made so far was the fact that the closest active galaxy to the Milky Way, called Centaurus A, is capable of emitting both gamma-rays and radio radiation. In previous studies, it was proposed that a galaxy could either emit one or the other, but the new data seems to indicate that both types of emissions are possible at the same time.

An active galaxy is a space structure whose central region emits radiation across a very wide portion of the electromagnetic spectrum. Generally, the core is occupied by a supermassive black hole, which gobbles up matter, and then releases vast amounts of radiation in exchange. In the case of Centaurus A, which has for a long time been determined to be one of the most potent and bright sources of radio wavelengths in the sky, the core produced vast amounts of extremely high-energy gamma-rays as well, Space reports.

“This is something we've never seen before in gamma-rays,” explains of the new Fermi findings expert Teddy Cheung. He is a member of the team managing the observatory, and is based in Washington DC, at the Naval Research Laboratory (NRL). As far as physicists go, gamma-rays represent the most energetic form of light possible in the Universe. But, inside Centaurus A, these photons are ramped up to even higher energies than usual. The huge lobes of the active galaxy contain super-strong magnetic fields, in which a wide variety of particles get accelerated. The gamma-rays entering these highly-active regions also get a massive energy boost.

This results in what can perhaps be best described as the most energetic photons in the known Universe. Astrophysicists suspect that the situation is not unique and say that many of the active, massive galaxies out there may be producing the same amplification effect on their own gamma-ray sources. “Not only do we see the extended radio lobes, but their gamma-ray output is more than 10 times greater than their radio output,” Cheung adds. More details of the study appear in the April 2 issue of the esteemed publication Science.

Vela Pulsar And Dozens Of Others Get Probed

Two studies published in Science Express show the analysis of gamma-rays from two dozen pulsars, including 16 discovered by NASA's Fermi Gamma-ray Space Telescope. Fermi is the first spacecraft able to identify pulsars by their gamma-ray emissions alone.

A pulsar is the rapidly spinning and highly magnetized core left behind when a massive star explodes. Most of the currently cataloged pulsars, some 1800 of them, were found through their periodic radio emissions; pulses caused by narrow, lighthouse-like radio beams emanating from the pulsar's magnetic poles, according to current theory.

The Vela pulsar, which spins 11 times a second, is the brightest persistent source of gamma rays in the sky. Yet gamma rays -- the most energetic form of light -- are few and far between. Even Fermi's Large Area Telescope sees only about one gamma-ray photon from Vela every two minutes.

"That's about one photon for every thousand Vela rotations," said Marcus Ziegler, a member of the team reporting on the new pulsars at the University of California, Santa Cruz. "From the faintest pulsar we studied, we see only two gamma-ray photons a day."

Radio telescopes on Earth can detect a pulsar easily only if one of the narrow radio beams happens to swing our way. If not, the pulsar can remain hidden.

A pulsar's radio beams represent only a few parts per million of its total power, whereas its gamma rays account for 10 percent or more. Somehow, pulsars are able to accelerate particles to speeds near that of light. These particles emit a broad beam of gamma rays as they arc along curved magnetic field lines.

The new pulsars were discovered as part of a comprehensive search for periodic gamma-ray fluctuations using five months of Fermi Large Area Telescope data and new computational techniques.

"Before launch, some predicted Fermi might uncover a handful of new pulsars during its mission," Ziegler added. "To discover 16 in its first five months of operation is really beyond our wildest dreams."

Like spinning tops, pulsars slow down as they lose energy. Eventually, they spin too slowly to power their characteristic emissions and become undetectable.

This all-sky map shows the positions and names of 16 new pulsars (yellow) and eight millisecond pulsars (magenta) studied using Fermi's LAT. The famous Vela, Crab, and Geminga pulsars (right) are the brightest ones Fermi sees. The pulsars Taz, Eel, and Rabbit have taken the nicknames of nebulae they are now known to power. The Gamma Cygni pulsar resides within a supernova remnant of the same name. Credit: NASA/DOE/Fermi LAT Collaboration

But pair a slowed dormant pulsar with a normal star, and a stream of stellar matter from the companion can spill onto the pulsar and increase its spin. At rotation periods between 100 and 1,000 times a second, ancient pulsars can resume the activity of their youth. In the second study, Fermi scientists examined gamma rays from eight of these "born-again" pulsars, all of which were previously discovered at radio wavelengths.

"Before Fermi launched, it wasn't clear that pulsars with millisecond periods could emit gamma rays at all," said Lucas Guillemot at the Center for Nuclear Studies in Gradignan, near Bordeaux, France. "Now we know they do. It's also clear that, despite their differences, both normal and millisecond pulsars share similar mechanisms for emitting gamma rays."

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed 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 U.S.

NASA's Fermi Telescope reveals a population of radio-quiet gamma-ray pulsars

A new class of pulsars detected by NASA's Fermi Gamma-ray Space Telescope is solving the mystery of previously unidentified gamma-ray sources and helping scientists understand the mechanisms behind pulsar emissions. A study to be published by an international team of scientists in the July 2 edition of Science Express describes 16 pulsars discovered by Fermi based on their pulsed emissions of high-energy gamma rays. A pulsar is a rapidly spinning neutron star, the dense core left behind after a supernova explosion. Most of the 1,800 known pulsars were found through their periodic radio emissions.

"These are the first pulsars ever detected by gamma rays alone, and already we've found 16," said coauthor Robert Johnson, professor of physics at the University of California, Santa Cruz. "The existence of a large population of radio-quiet pulsars was suspected prior to this, but until Fermi was launched, only one radio-quiet pulsar was known, and it was first detected in x-rays."

Johnson and other physicists at UCSC's Santa Cruz Institute for Particle Physics (SCIPP) identified the gamma-ray pulsars using computational techniques they developed to comb through data from Fermi's Large Area Telescope (LAT). Marcus Ziegler, a postdoctoral researcher at SCIPP and corresponding author of the paper, said detection of gamma-ray pulsations from a typical source requires weeks or months of data from the LAT.

"From the faintest pulsar we studied, the LAT sees only two gamma-ray photons a day," Ziegler said.

Of the 16 gamma-ray pulsars found by Fermi, 13 are associated with unidentified gamma-ray sources detected previously by the EGRET instrument on the Compton Gamma-ray Observatory. EGRET detected nearly 300 gamma-ray point sources, but was unable to detect pulsations from those sources, most of which have remained unidentified, said Pablo Saz Parkinson, also a SCIPP postdoctoral researcher and corresponding author of the paper.

"It's been a longstanding question what could be powering those unidentified sources, and the new Fermi results tell us that a lot of them are pulsars," Saz Parkinson said. "These findings are also giving us important clues about the mechanism of pulsar emissions."

A pulsar emits narrow beams of radio waves from the magnetic poles of the neutron star, and the beams sweep around like a lighthouse beacon because the magnetic poles are not aligned with the star's spin axis. If the radio beam misses the Earth, the pulsar cannot be detected by radio telescopes. Fermi's ability to detect so many radio-quiet gamma-ray pulsars indicates that the gamma-rays are emitted in a beam that is wider and more fan-like than the radio beam.

"This favors models in which the gamma rays are emitted from the outer magnetosphere of the pulsar, as opposed to the polar cap much closer to the surface of the star," Saz Parkinson said.

The very intense magnetic and electric fields of a pulsar accelerate charged particles to nearly the speed of light, and these particles are ultimately responsible for the gamma-ray emissions.

Because the rotation of the star powers the emissions, isolated pulsars slow down as they age and lose energy. But a binary companion star can feed material to a pulsar and spin it up to a rotation rate of 100 to 1,000 times a second. These are called millisecond pulsars, and Fermi scientists detected gamma-ray pulsations from eight millisecond pulsars that were previously discovered at radio wavelengths. Those results are reported in a second study also published in the July 2 edition of Science Express.

"Fermi has truly unprecedented power for discovering and studying gamma-ray pulsars," said Paul Ray of the Naval Research Laboratory in Washington. "Since the demise of the Compton Gamma Ray Observatory a decade ago, we've wondered about the nature of unidentified gamma-ray sources it detected in our galaxy. These studies from Fermi lift the veil on many of them."

Source: University of California - Santa Cruz