NASA's Fermi Satellite Detects First Gamma-ray Pulsar in Another Galaxy

Explore Fermi's discovery of the first gamma-ray pulsar detected in a galaxy other than our own.

Credits: NASA's Goddard Space Flight Center.  

Download the video in HD at NASA's Scientific Visualization Studio

The pulsar lies in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a small galaxy that orbits our Milky Way and is located 163,000 light-years away. The Tarantula Nebula is the largest, most active and most complex star-formation region in our galactic neighborhood. It was identified as a bright source of gamma rays, the highest-energy form of light, early in the Fermi mission. Astronomers initially attributed this glow to collisions of subatomic particles accelerated in the shock waves produced by supernova explosions.

   

"It's now clear that a single pulsar, PSR J0540-6919, is responsible for roughly half of the gamma-ray brightness we originally thought came from the nebula," said lead scientist Pierrick Martin, an astrophysicist at the National Center for Scientific Research (CNRS) and the Research Institute in Astrophysics and Planetology in Toulouse, France. "That is a genuine surprise."

NASA's Fermi Gamma-ray Space Telescope has detected the first extragalactic gamma-ray pulsar, PSR J0540-6919, near the Tarantula Nebula (top center) star-forming region in the Large Magellanic Cloud, a satellite galaxy that orbits our own Milky Way. Fermi detects a second pulsar (right) as well but not its pulses. PSR J0540-6919 now holds the record as the highest-luminosity gamma-ray pulsar. The angular distance between the pulsars corresponds to about half the apparent size of a full moon. Background: An image of the Tarantula Nebula and its surroundings in visible light.Credits: NASA's Goddard Space Flight Center; background: ESO/R. Fosbury (ST-ECF). Hi-res image

A gamma-ray view of the same region shown above in visible wavelengths. Lighter colors indicate greater numbers of gamma rays with energies between 2 and 200 billion electron volts. For comparison, visible light ranges between 2 and 3 electron volts. The two pulsars, PSR J0540−6919 (left) and PSR J0537−6910, clearly stand out. Credits: NASA/DOE/Fermi LAT Collaboration. Hi-res image

When a massive star explodes as a supernova, the star's core may survive as a neutron star, where the mass of half a million Earths is crushed into a magnetized ball no larger than Washington, D.C. A young isolated neutron star spins tens of times each second, and its rapidly spinning magnetic field powers beams of radio waves, visible light, X-rays and gamma rays. If the beams sweep past Earth, astronomers observe a regular pulse of emission and the object is classified as a pulsar.

The Tarantula Nebula was known to host two pulsars, PSR J0540-6919 (J0540 for short) and PSR J0537−6910 (J0537), which were discovered with the help of NASA's Einstein and Rossi X-ray Timing Explorer (RXTE) satellites, respectively. J0540 spins just under 20 times a second, while J0537 whirls at nearly 62 times a second -- the fastest-known rotation period for a young pulsar.

Nevertheless, it took more than six years of observations by Fermi's Large Area Telescope (LAT), as well as a complete reanalysis of all LAT data in a process called Pass 8, to detect gamma-ray pulsations from J0540. The Fermi data establish upper limits for gamma-ray pulses from J0537 but do not yet detect them.

Martin and his colleagues present these findings in a paper to be published in the Nov. 13 edition of the journal Science.

"The gamma-ray pulses from J0540 have 20 times the intensity of the previous record-holder, the pulsar in the famous Crab Nebula, yet they have roughly similar levels of radio, optical and X-ray emission," said coauthor Lucas Guillemot, at the Laboratory for Physics and Chemistry of Environment and Space, operated by CNRS and the University of Orléans in France. "Accounting for these differences will guide us to a better understanding of the extreme physics at work in young pulsars."

J0540 is a rare find, with an age of roughly 1,700 years, about twice that of the Crab Nebula pulsar. By contrast, most of the more than 2,500 known pulsars are from 10,000 to hundreds of millions of years old.

Despite J0540's luminosity, too few gamma rays reach the LAT to detect pulsations without knowing the period in advance. This information comes from a long-term X-ray monitoring campaign using RXTE, which recorded both pulsars from the start of the Fermi mission to the end of 2011, when RXTE operations ceased.

"This campaign began as a search for a pulsar created by SN 1987A, the closest supernova seen since the invention of the telescope," said co-author Francis Marshall, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "That search failed, but it discovered J0537."

Prior to the launch of Fermi in 2008, only seven gamma-ray pulsars were known. To date, the mission has found more than 160.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

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Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Source: NASA/Fermi Space Telescope

NASA's Fermi Finds A 'Transformer' Pulsar

These artist's renderings show one model of pulsar J1023 before (top) and after (bottom) its radio beacon (green) vanished. Normally, the pulsar's wind staves off the companion's gas stream. When the stream surges, an accretion disk forms and gamma-ray particle jets (magenta) obscure the radio beam. Image Credit: NASA's Goddard Space Flight Center

Zoom into an artist's concept of AY Sextantis, a binary star system whose pulsar switched from radio emissions to high-energy gamma rays in 2013. This transition likely means the pulsar's spin-up process is nearing its end. 


In late June 2013, an exceptional binary containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed. The pulsar's radio beacon vanished, while at the same time the system brightened fivefold in gamma rays, the most powerful form of light, according to measurements by NASA's Fermi Gamma-ray Space Telescope.

"It's almost as if someone flipped a switch, morphing the system from a lower-energy state to a higher-energy one," said Benjamin Stappers, an astrophysicist at the University of Manchester, England, who led an international effort to understand this striking transformation. "The change appears to reflect an erratic interaction between the pulsar and its companion, one that allows us an opportunity to explore a rare transitional phase in the life of this binary."

A binary consists of two stars orbiting around their common center of mass. This system, known as AY Sextantis, is located about 4,400 light-years away in the constellation Sextans. It pairs a 1.7-millisecond pulsar named PSR J1023+0038 -- J1023 for short -- with a star containing about one-fifth the mass of the sun. The stars complete an orbit in only 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion.

When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar, an object squeezing more mass than the sun's into a sphere no larger than Washington, D.C. Young isolated neutron stars rotate tens of times each second and generate beams of radio, visible light, X-rays and gamma rays that astronomers observe as pulses whenever the beams sweep past Earth. Pulsars also generate powerful outflows, or "winds," of high-energy particles moving near the speed of light. The power for all this comes from the pulsar's rapidly spinning magnetic field, and over time, as the pulsars wind down, these emissions fade.

More than 30 years ago, astronomers discovered another type of pulsar revolving in 10 milliseconds or less, reaching rotational speeds up to 43,000 rpm. While young pulsars usually appear in isolation, more than half of millisecond pulsars occur in binary systems, which suggested an explanation for their rapid spin.

"Astronomers have long suspected millisecond pulsars were spun up through the transfer and accumulation of matter from their companion stars, so we often refer to them as recycled pulsars," explained Anne Archibald, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy (ASTRON) in Dwingeloo who discovered J1023 in 2007.

During the initial mass-transfer stage, the system would qualify as a low-mass X-ray binary, with a slower-spinning neutron star emitting X-ray pulses as hot gas raced toward its surface. A billion years later, when the flow of matter comes to a halt, the system would be classified as a spun-up millisecond pulsar with radio emissions powered by a rapidly rotating magnetic field.

To better understand J1023's spin and orbital evolution, the system was regularly monitored in radio using the Lovell Telescope in the United Kingdom and the Westerbork Synthesis Radio Telescope in the Netherlands. These observations revealed that the pulsar's radio signal had turned off and prompted the search for an associated change in its gamma-ray properties.

A few months before this, astronomers found a much more distant system that flipped between radio and X-ray states in a matter of weeks. Located in M28, a globular star cluster about 19,000 light-years away, a pulsar known as PSR J1824-2452I underwent an X-ray outburst in March and April 2013. As the X-ray emission dimmed in early May, the pulsar's radio beam emerged.

While J1023 reached much higher energies and is considerably closer, both binaries are otherwise quite similar. What's happening, astronomers say, are the last sputtering throes of the spin-up process for these pulsars.

In J1023, the stars are close enough that a stream of gas flows from the sun-like star toward the pulsar. The pulsar's rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind. When the radio beam is detectable, the pulsar wind holds back the companion's gas stream, preventing it from approaching too closely. But now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disk.

Gas in the disk becomes compressed and heated, reaching temperatures hot enough to emit X-rays. Next, material along the inner edge of the disk quickly loses orbital energy and descends toward the pulsar. When it falls to an altitude of about 50 miles (80 km), processes involved in creating the radio beam are either shut down or, more likely, obscured.

The inner edge of the disk probably fluctuates considerably at this altitude. Some of it may become accelerated outward at nearly the speed of light, forming dual particle jets firing in opposite directions -- a phenomenon more typically associated with accreting black holes. Shock waves within and along the periphery of these jets are a likely source of the bright gamma-ray emission detected by Fermi.

The findings were published in the July 20 edition of The Astrophysical Journal. The team reports that J1023 is the first example of a transient, compact, low-mass gamma-ray binary ever seen. The researchers anticipate that the system will serve as a unique laboratory for understanding how millisecond pulsars form and for studying the details of how accretion takes place on neutron stars.

"So far, Fermi has increased the number of known gamma-ray pulsars by about 20 times and doubled the number of millisecond pulsars within in our galaxy," said Julie McEnery, the project scientist for the mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Fermi continues to be an amazing engine for pulsar discoveries."

Related Links:

• Download HD video and additional images from NASA Goddard's Scientific Visualization Studio

• Paper: "A State Change In The Missing Link Binary Pulsar System PSR J1023+0038"

• Paper: "A Radio Pulsar/X-ray Binary Link"

• "Astronomers Uncover a 'Transformer' Pulsar" (09.25.2013)

• Interactive: Fermi Pulsar Explorer

• List of rotation- and accretion-powered millisecond pulsars

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Home Computers Discover Gamma-ray Pulsars

Einstein@Home volunteers find four cosmic lighthouses in data from NASA's Fermi Gamma-ray Space Telescope

The combination of globally distributed computing power and innovative analysis methods proves to be a recipe for success in the search for new pulsars. Scientists from the Max Planck Institutes for Gravitational Physics and Radio Astronomy together with international colleagues have now discovered four gamma-ray pulsars in data from the Fermi space telescope. The breakthrough came using the distributed computing project Einstein@Home, which connects more than 200,000 computers from 40,000 participants around the world to a global supercomputer. The discoveries include volunteers from Australia, Canada, France, Germany, Japan, and the USA.

All four gamma-ray pulsars discovered by Einstein@Home lie in the plane of our Milky Way, as shown in this sky map using data from Fermi's Large Area Telescope (LAT). The plane is apparent as an area of particularly intense gamma radiation; brighter colors indicate more intense radiation. The insets show the four pulsars as point sources. The flags indicate the nationalities of the Einstein@Home volunteers whose computers made the discoveries.© Knispel/Pletsch/AEI/NASA/DOE/Fermi LAT Collaboration

 
Since its launch in 2008, the Fermi satellite has been observing the entire sky in gamma-rays. It has discovered thousands of previously unknown gamma-ray sources, among which are possibly hundreds of yet undiscovered pulsars – compact and rapidly rotating remnants of exploded stars. Identifying these new gamma-ray pulsars, however, is computationally very expensive – wide parameter ranges have to be “scanned” at very high resolution.

“Our innovative solution for the compute intensive search for gamma-ray pulsars is the combination of particularly efficient methods along with the distributed computing power of Einstein@Home,” says Holger Pletsch, Independent Research Group Leader at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI), and lead author of the study. “The volunteers from around the world enable us to deal with the huge computational challenge posed by the Fermi data analysis. In this way, they provide an invaluable service to astronomy,” says Pletsch.

A gamma-ray pulsar is a compact neutron star that accelerates charged particles to relativistic speeds in its extremely strong magnetic field. This process produces gamma radiation (violet) far above the surface of the compact remains of the star, for example, while radio waves (green) are emitted over the magnetic poles in the form of a cone. The rotation moves the emission regions across the terrestrial line of sight, making the pulsar light up periodically in the sky. © NASA/Fermi/Cruz de Wilde 

 Distributed Computing for Astronomy
Einstein@Home is a joint project of the Center for Gravitation and Cosmology at the University of Wisconsin–Milwaukee and the AEI in Hannover. It is funded by the National Science Foundation and the Max Planck Society. Since mid-2011, Einstein@Home has been searching for signals from gamma-ray pulsars in Fermi data. The project was founded in 2005 to search for gravitational-wave signals in data from the LIGO detectors – still the main task of Einstein@Home. Since early 2009, the project has also been conducting successful searches for new radio pulsars.

“The first-time discovery of gamma-ray pulsars by Einstein@Home is a milestone – not only for us but also for our project volunteers. It shows that everyone with a computer can contribute to cutting-edge science and make astronomical discoveries,“ says co-author Bruce Allen, director at the AEI and principal investigator of Einstein@Home. “I'm hoping that our enthusiasm will inspire more people to help us with making further discoveries.”

Using his home computer, Hans-Peter Tobler from Rellingen, Germany, discovered one of the four new pulsars. With hundreds of thousands of computers teaming up, he never expected that his PC would discover anything. © Hans-Peter Tobler

Pulsars for everyone
The volunteers who contributed to the discoveries are thrilled. “At first I was a bit dumbfounded and thought someone was playing a hoax on me. But after I did some research everything checked out. That someone as insignificant as myself could make a difference was amazing,” says Thomas M. Jackson from Kentucky in the USA, who runs Einstein@Home on his quad-core processor.

Hans-Peter Tobler from Rellingen, Germany, has been participating in Einstein@Home since 2005 and has now helped in the discovery of a gamma-ray pulsar: “I'm fascinated by astronomy. Einstein@Home allows me to contribute to this field of science, even though I'm not a professional astronomer myself.” With hundreds of thousands of computers teaming up, he never expected that his PC would discover anything.

All Einstein@Home volunteers are acknowledged for their contributions in the scientific publication. The astronomers particularly mention the eight volunteers whose computers made the discoveries. The volunteers are from Australia, Canada, France, Germany, Japan, and the USA. As a token of appreciation, they receive special certificates of discovery.

New Window for the Discovery of Neutron Stars
Not only are the four gamma-ray pulsars the first to be found with a distributed volunteer computing project, but also the pulsars are special, too. “It is exciting that all four pulsars are in the plane of our Milky Way,” says co-author Michael Kramer, director at the Max Planck Institute for Radio Astronomy (MPIfR). Earlier surveys with radio telescopes have been thoroughly searching this part of the sky, but the four new pulsars had remained hidden and only one comparable neutron star had been found.

Apparently, the pulsars are only visible in gamma-rays. The radio and gamma-ray emission are produced in different regions around the pulsar. Depending on the orientation of the pulsar, the narrow radio beam might miss Earth, while the wider beam of gamma-ray photons could be detectable. Dedicated follow-up observations of all four new discoveries with the MPIfR's 100-meter Effelsberg radio telescope and the Australian Parkes Observatory confirm the absence of any detectable radio emission.

“With the successful blind searches for gamma-ray pulsars, we use a new window for the discovery of neutron stars,” says Kramer. The new searches employ methods inspired by gravitational-wave data analysis. Using these, astronomers around Pletsch discovered all of the eleven gamma-ray pulsars found in the last three years of blind searches in Fermi data.

A neutron star is the densest object astronomers can observe directly, crushing half a million times Earth's mass into a sphere about 20 kilometers across. This illustration compares the size of a neutron star to the area around Hannover, Germany, hometown of the Albert Einstein Institute Hannover. © NASA's Goddard Space Flight Center

Young Neutron Stars with a Hiccup
Two of the newly discovered pulsars exhibited a sudden change in their otherwise perfectly regular rotation – they suffered a so-called pulsar glitch. During a glitch, the neutron star's rotation suddenly speeds up, then gradually becomes slower and returns to the initial rotation period after a few weeks. “We don't know the exact cause of these glitches yet, but measuring them can provide new insights into the incompletely understood neutron star interior,” says co-author Lucas Guillemot, who worked as a researcher at MPIfR when the discoveries were made and has recently taken up a position at the LPC2E in Orléans.

Glitches mostly happen in newly born pulsars. According to the measurements of the astronomers, the four pulsars discovered now are between 30,000 and 60,000 years old – youngsters among neutron stars.

Discovery Potential

In the future, the efficient search methods will become increasingly important, since Fermi is scheduled to take data for at least another five years. The longer the measurement time, the weaker the pulsars the scientists can discover. With increasing measurement time, however, the computational costs grow quickly. Conventional methods are already too costly at present, but there is still headroom for the new methods.

“Only our methods will enable efficient blind pulsar searches in Fermi data in the future. Using the distributed computing power provided by the Einstein@Home volunteers, we hope to discover gamma-ray pulsars that are particularly far away or faint,” says Pletsch.

Pulsars

Neutron stars are exotic objects. They are made up of matter much more densely packed than normal, giving the entire star a density comparable to an atomic nucleus. The diameter of our sun would shrink to less than 30 km if it was that dense.

Neutron stars also have extremely strong magnetic fields. Charged particles accelerated along the field lines emit electromagnetic radiation in different wavelengths. This radiation is bundled into a cone along the magnetic field axis. As the neutron star turns about its rotational axis, the cones of emitted radiation sweep through the sky like a lighthouse beam because the rotational axis is usually inclined relative to the magnetic field axis. The neutron star is then visible as a pulsar. Pulsars rotate with cycles of a few seconds up to only milliseconds with a precision that makes them the most accurate clocks in the world.

These cosmic lighthouses were first discovered in 1967 by Jocelyn Bell Burnell and identified as radio pulsars. X-ray and gamma-ray pulsars are also known to exist today. Even though not all pulsars are observable in all wavelengths, scientists assume that they still emit radiation in the entire electromagnetic spectrum. However the mechanisms which govern radiation emission in different frequency ranges are not yet completely understood.

Gamma-ray Pulsars and Radio Pulsars

A plausible explanation could be that lower-energy radio waves are bundled in a tighter cone at the magnetic poles than high-energy gamma-radiation. Since radiation is mainly emitted along the surface of the cone and different wavelengths are emitted in cones with a different spread, radio waves and gamma waves would leave the neutron star in different directions. A pulsar might thus become visible as a gamma-ray or radio pulsar to a distant observer (depending on which cone sweeps across the observers position). Another model has gamma radiation originating not in the polar regions of the magnetic field but rather the equatorial plane where the field lines are disrupted. It is therefore very important to observe as many pulsars as possible in all wavelengths to better understand these mechanisms.

Data Analysis

When analyzing data from gravitational wave detectors, scientists have to rely on very effective algorithms and high computing power. This is necessary, because a possible gravitational wave signal would be scarcely stronger than the background noise at the current measurement accuracy.

The data is analyzed in several steps. First, the astrophysicists scan large areas of the sky for signals. If there is a conspicuous signal in one direction, they investigate the vicinity with an algorithm which has a narrower search grid and thus requires more computing time. If the signal is confirmed, the scientists analyse its temporal characteristic and examine whether it can be assigned to a specific pulsar period, for example. The Hanover scientists have modified the algorithm to search for continuous sources of gravitational waves and used it successfully to search for gamma-ray pulsars in Fermi data.

Einstein@Home

This project for distributed volunteer computing connects PC users from all over the world, who voluntarily donate spare computing time on their home and office computers. So far more than 350,000 volunteers have participated and it is therefore one of the largest projects of this kind. Scientific supporters are the Center for Gravitation and Cosmology at the University of Wisconsin-Milwaukee and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, Hanover) with financial support from the National Science Foundation and the Max Planck Society. Since 2005, Einstein@Home has examined data from the gravitational wave detectors within the LIGO-Virgo-Science Collaboration (LVC) for gravitational waves from unknown, rapidly rotating neutron stars.

As of March 2009, Einstein@Home has also been involved in the search for signals from radio pulsars in observational data from the Arecibo Observatory in Puerto Rico and the Parkes Observatory in Australia.

Since the first discovery of a radio pulsar by Einstein@Home in August 2010, the global computer network has discovered more than 50 new radio pulsars.

A new search for gamma-ray pulsars in data of the Fermi satellite was added in August 2011. It made the four discoveries reported now. The project is looking for, among other things, the first millisecond pulsar, visible only in the gamma-ray range.


Science Contacts

Dr. Holger J. Pletsch

Independent Research Group Leader

Phone:+49 511 762-17171Fax:+49 511 762-2784

Email: holger.pletsch@aei.mpg.de

Homepage Holger J. Pletsch

Prof. Dr. Bruce Allen

Director

Phone:+49 511 762-17148Fax:+49 511 762-17182

Email: bruce.allen@aei.mpg.de

Homepage of Bruce Allen


Media Contacts

Dr. Benjamin Knispel

Press Officer AEI Hannover

Phone:+49 511 762-19104Fax:+49 511 762-17182

Email: benjamin.knispel@aei.mpg.de
   

Dr. Norbert Junkes

Press Officer MPIfR

Phone:+49 228 525-399Fax:+49 228 525-438

Email: njunkes@mpifr-bonn.mpg.de

 Source: Max Planck Institute for Gravitacional Physics

NASA's Fermi Finds Youngest Millisecond Pulsar, 100 Pulsars To-Date

In three years, NASA's Fermi has detected more than 100 gamma-ray pulsars, but something new has appeared. Among a type of pulsar with ages typically numbering a billion years or more, Fermi has found one that appears to have been born only millions of years ago. (Credit: NASA's Goddard Space Flight Center). Download this video and related content from NASA Goddard's Scientific Visualization Studio

A pulsar is a type of neutron star that emits electromagnetic energy at periodic intervals. A neutron star is the closest thing to a black hole that astronomers can observe directly, crushing half a million times more mass than Earth into a sphere no larger than a city. This matter is so compressed that even a teaspoonful weighs as much as Mount Everest.

This plot shows the positions of nine new pulsars (magenta) discovered by Fermi and of an unusual millisecond pulsar (green) that Fermi data reveal to be the youngest such object known. With this new batch of discoveries, Fermi has detected more than 100 pulsars in gamma rays. Credit: NASA/DOE/Fermi LAT Collaboration. Larger image

This image shows the on and off state of gamma rays from pulsar J1823-3021A as seen by Fermi's Large Area Telescope (LAT). The object pulses 183.8 times a second and has a spin period of 5.44 milliseconds, which translates to 11,000 rpm. Credit: NASA/DOE/Fermi LAT Collaboration. Larger image

"With this new batch of pulsars, Fermi now has detected more than 100, which is an exciting milestone when you consider that, before Fermi's launch in 2008, only seven of them were known to emit gamma rays," said Pablo Saz Parkinson, an astrophysicist at the Santa Cruz Institute for Particle Physics at the University of California Santa Cruz, and a co-author on two papers detailing the findings.

One group of pulsars combines incredible density with extreme rotation. The fastest of these so-called millisecond pulsars whirls at 43,000 revolutions per minute.

Millisecond pulsars are thought to achieve such speeds because they are gravitationally bound in binary systems with normal stars. During part of their stellar lives, gas flows from the normal star to the pulsar. Over time, the impact of this falling gas gradually spins up the pulsar's rotation.

The strong magnetic fields and rapid rotation of pulsars cause them to emit powerful beams of energy, from radio waves to gamma rays. Because the star is transferring rotational energy to the pulsar, the pulsar's spin eventually slows as the star loses matter.

Typically, millisecond pulsars are around a billion years old. However, in the Nov. 3 issue of Science, the Fermi team reveals a bright, energetic millisecond pulsar only 25 million years old.

The object, named PSR J1823−3021A, lies within NGC 6624, a spherical collection of ancient stars called a globular cluster, one of about 160 similar objects that orbit our galaxy. The cluster is about 10 billion years old and lies about 27,000 light-years away toward the constellation Sagittarius.

Fermi's Large Area Telescope (LAT) showed that eleven globular clusters emit gamma rays, the cumulative emission of dozens of millisecond pulsars too faint for even Fermi to detect individually. But that's not the case for NGC 6624.

"It's amazing that all of the gamma rays we see from this cluster are coming from a single object. It must have formed recently based on how rapidly it's emitting energy. It's a bit like finding a screaming baby in a quiet retirement home," said Paulo Freire, the study's lead author, at the Max Planck Institute for Radio Astronomy in Bonn, Germany.

J1823−3021A was previously identified as a pulsar by its radio emission, yet of the nine new pulsars, none are millisecond pulsars, and only one was later found to emit radio waves.

Fermi's LAT records the precise time and position of the gamma rays it detects, but to identify a pulsar requires additional information -- its position in the sky, its pulse period, and the way the pulse changes over time. Additionally, even Fermi's sensitive LAT detects few gamma rays from these objects -- as few as one photon per 100,000 rotations. New analysis methods allow computers to check many different combinations of position and period against 8,000 photons Fermi's LAT has collected during its three years in orbit. When photons from the pulses align in time, a new gamma-ray pulsar has been discovered. (Credit: AEI/NASA's Goddard Space Flight Center). Download this video and related content from NASA Goddard's Scientific Visualization Studio

Despite its sensitivity, Fermi's LAT may detect only one gamma ray for every 100,000 rotations of some of these faint pulsars. Yet new analysis techniques applied to the precise position and arrival time of photons collected by the LAT since 2008 were able to identify them.

"We adapted methods originally devised for studying gravitational waves to the problem of finding gamma-ray pulsars, and we were quickly rewarded," said Bruce Allen, director of the Max Planck Institute for Gravitational Physics in Hannover, Germany. Allen co-authored a paper on the discoveries that was published online today in The Astrophysical Journal.

Allen also directs the Einstein@Home project, a distributed computing effort that uses downtime on computers of volunteers to process astronomical data. In July, the project extended the search for gamma-ray pulsars to the general public by including Femi LAT data in the work processed by Einstein@Home users.

To celebrate these achievements, the Fermi team has created an interactive feature on pulsars. Called the Fermi Pulsar Explorer, the interactive includes an all-sky map that links to information about each of the 101 Fermi pulsars known so far, as well as video and background information on Fermi and gamma-ray astronomy. View interactive

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership. It is managed by NASA's Goddard Space Flight Center in Greenbelt, Md. It was developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

Related Link

Albert Einstein Institute release

Crab Pulsar Dazzles Astronomers with its Gamma-Ray Beams

An artist's conception of the pulsar at the center of the Crab Nebula, with a Hubble Space Telescope photo of the nebula in the background. Researchers using the Veritas telescope array have discovered pulses of high-energy gamma rays coming from this object. Credit: David A. Aguilar / NASA / ESA. High Resolution Image (jpg) - Low Resolution Image (jpg)

This artist's conception shows the Crab Nebula pulsar, which astronomers discovered to be sending out pulses of gamma rays with energies exceeding 100 billion electron-volts (100 GeV). A pulsar is a spinning neutron star - the collapsed core of a massive star that exploded as a supernova. Credit: David A. Aguilar (CfA). High Resolution Image (jpg) - Low Resolution Image (jpg)

An artist’s rendering of the VERITAS array detecting gamma-ray pulses from the Crab Nebula. Credit: José Francisco Salgado based on images by M. SubbaRao, S. Criswell, B. Humensky, and J.F. Salgado. Low Resolution Image (jpg)

Cambridge, MA - A thousand years ago, a brilliant beacon of light blazed in the sky, shining brightly enough to be seen even in daytime for almost a month. Native American and Chinese observers recorded the eye-catching event. We now know that they witnessed an exploding star, which left behind a gaseous remnant known as the Crab Nebula.

The same object that dazzled skygazers in 1054 C.E. continues to dazzle astronomers today by pumping out radiation at higher energies than anyone expected. Researchers have detected pulses of gamma rays with energies exceeding 100 billion electron-volts (100 GeV) -- a million times more energetic than medical X-rays and 100 billion times more than visible light.

"If you asked theorists a year ago whether we would see gamma-ray pulses this energetic, almost all of them would have said, 'No.' There's just no theory that can account for what we've found," said corresponding author Martin Schroedter of the Harvard-Smithsonian Center for Astrophysics (CfA).

The gamma rays come from an extreme object at the Crab Nebula's center known as a pulsar. A pulsar is a spinning neutron star -- the collapsed core of a massive star. Although only a few miles across, a neutron star is so dense that it weighs more than the Sun.

Rotating about 30 times a second, the Crab pulsar generates beams of radiation from its spinning magnetic field. The beams sweep around like a lighthouse beacon because they're not aligned with the star's rotation axis. So although the beams are steady, they're detected on Earth as rapid pulses of radiation.

The discovery was reported by an international team of scientists in a paper in the October 7 issue of Science. Corresponding author Nepomuk Otte, a postdoctoral researcher at the University of California, Santa Cruz, said that some researchers had told him he was crazy to even look for pulsar emission in this energy realm.

"It turns out that being persistent and stubborn helps," Otte said. "These results put new constraints on the mechanism for how the gamma-ray emission is generated."

Some possible scenarios to explain the data have been put forward, but it will take more data, or even a next-generation observatory, to really understand the mechanisms behind these gamma-ray pulses.

The gamma-ray pulses were detected by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) -- the most powerful very-high-energy gamma-ray observatory in the Northern Hemisphere. VERITAS is located at the Smithsonian's Whipple Observatory, just south of Tucson, Ariz.

Astronomers observe very-high-energy gamma rays with ground-based Cherenkov telescopes. These gamma rays, coming from cosmic "particle accelerators," are absorbed in Earth's atmosphere, where they create a short-lived shower of subatomic particles. The Cherenkov telescopes detect the faint, extremely short flashes of blue light that these particles emit (named Cherenkov light) using extremely sensitive cameras. The images can be used to infer the arrival direction and initial energy of the gamma rays.

This technique is used by gamma-ray observatories throughout the world, and was pioneered under the direction of CfA's Trevor Weekes using the 10-meter Cherenkov telescope at Whipple Observatory. The Whipple 10-meter telescope was used to detect the first Galactic and extragalactic sources of very-high-energy gamma rays.

VERITAS continues the tradition of Whipple's 10-meter telescope. It is comprised of an array of four 12-meter-diameter Cherenkov telescopes. VERITAS began full-scale observations in September 2007. The telescopes are used to study the remnants of exploded stars, distant galaxies, powerful gamma-ray bursts, and to search for evidence of mysterious dark matter particles.

VERITAS is funded by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Smithsonian Institution, the Natural Sciences and Engineering Research Council of Canada, the Science Foundation Ireland and the STFC of the U.K.

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.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Nature's Most Precise Clocks May Make "Galactic GPS" Possible

Radio searches netted 17 new millisecond pulsars by examining the Fermi Gamma-ray Space Telescope's list of unidentified sources. Colored circles indicate the positions of the new pulsars on the Fermi one-year all-sky map. Credit: NASA/DOE/Fermi LAT Collaboration. Larger image

Pulsars slow down their rotation as they age and eventually cease their characteristic emissions. That can change if an aging pulsar is a member of a binary system containing a normal star. Gas flowing from the star can spin the pulsar up to hundreds of revolutions a second and allow it to resume its lighthouse-like beams of radiation. Credit: NASA. Watch animation

Radio astronomers have uncovered 17 millisecond pulsars in our galaxy by studying unknown high-energy sources detected by NASA's Fermi Gamma-ray Space Telescope. The astronomers made the discovery in less than three months. Such a jump in the pace of locating these hard-to-find objects holds the promise of using them as a kind of "galactic GPS" to detect gravitational waves passing near Earth.

A pulsar is the rapidly spinning and highly magnetized core left behind when a massive star explodes. Because only rotation powers their intense gamma-ray, radio and particle emissions, pulsars gradually slow as they age. But the oldest pulsars spin hundreds of times per second -- faster than a kitchen blender. These millisecond pulsars have been spun up and rejuvenated by accreting matter from a companion star.

"Radio astronomers discovered the first millisecond pulsar 28 years ago," said Paul Ray at the Naval Research Laboratory in Washington. "Locating them with all-sky radio surveys requires immense time and effort, and we've only found a total of about 60 in the disk of our galaxy since then. Fermi points us to specific targets. It's like having a treasure map."

Millisecond pulsars are nature's most precise clocks, with long-term, sub-microsecond stability that rivals human-made atomic clocks. Precise monitoring of timing changes in an all-sky array of millisecond pulsars may allow the first direct detection of gravitational waves -- a long-sought consequence of Einstein's relativity theory.

"The Global Positioning System uses time-delay measurements among satellite clocks to determine where you are on Earth," explained Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Va. "Similarly, by monitoring timing changes in a constellation of suitable millisecond pulsars spread all over the sky, we may be able to detect the cumulative background of passing gravitational waves."

The sources Fermi detected are not associated with any known gamma-ray emitting objects and did not show evidence of pulsing behavior. However, scientists considered it likely that many of the unidentified sources would turn out to be pulsars.

For a more detailed look at radio wavelengths, Ray organized the Fermi Pulsar Search Consortium and recruited a handful of radio astronomers with expertise in using five of the world's largest radio telescopes -- the National Radio Astronomy Observatory, Robert C. Byrd Green Bank Telescope in W.Va., the Parkes Observatory in Australia, the Nancay Radio Telescope in France, the Effelsberg Radio Telescope in Germany and the Arecibo Telescope in Puerto Rico.

After studying approximately 100 targets, and with a computationally intensive data analysis still under way, the discoveries have started to pour in.

"Other surveys took a decade to find as many of these pulsars as we have," said Ransom, who led one of the discovery groups. "Having Fermi tell us where to look is a huge advantage."

Four of the new objects are "black widow" pulsars, so called because radiation from the recycled pulsar is destroying the companion star that helped spin it up.

"Some of these stars are whittled down to masses equivalent to tens of Jupiters," said Ray. "We've doubled the known number of these systems in the galaxy's disk, and that will help us better understand how they evolve."

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the U.S. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Francis Reddy
Goddard Space Flight Center

Gamma-ray sources guide astronomers to pulsars

The rapidly rotating ultradense remnant of a massive star, a pulsar emits jets of radiation that sweep across space like a lighthouse beam, as shown in this illustration. For the first time, gamma-ray observations have provided clues to radio astronomers about where to find millisecond pulsars. Credit: NASA

Fermi Telescope observations provide candidates to check for radio emissions
By Ron Cowen

WASHINGTON — Energetic gamma rays are providing astronomers with a new way to hunt those hard-to-find whirling dervishes known as pulsars.

“We usually have to look over the whole sky” to find pulsars, said Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Va. “Now we can use the gamma-ray point sources as guides, telling us exactly where to look.”

Ultradense, collapsed remnants of massive stars, pulsars rotate up to hundreds of times a second and emit beacons of light that sweep across the sky like lighthouse beams. Pulsars are not only predicted to be key sources of gravitational waves — the subtle ripples in spacetime predicted by Einstein’s theory of general relativity — but their clockwork pulses can be used to detect those waves in surrounding space.

Although researchers have calculated that the Milky Way galaxy contains tens of thousands of the fastest spinning pulsars, known as millisecond pulsars, radio telescopes have found only about 100. Because these celestial bodies can reside anywhere in the galaxy, searching for millisecond pulsars was like looking for the proverbial needle in a haystack, requiring radio surveys of the entire sky, noted Ransom, who presented his new work November 2 during the 2009 Fermi Symposium.

Last year, pulsar hunters got some encouraging news. Researchers using the Fermi Gamma-ray Space Telescope demonstrated that many millisecond pulsars emit gamma rays. However, many pulsars emit so few high-energy gamma rays — only 300 to 1,000 over an entire year — that it has been difficult to use the gamma rays to identify the period of many of these pulsars (or even to clearly identify if any of these rapid rotators are the source of the gamma rays).

But Ransom and colleagues calculated that a significant number of the unidentified gamma-ray sources found by Fermi might turn out to be pulsars. So the researchers recently aimed a giant radio dish, the Green Bank Telescope in West Virginia, at four of these sources and found that three were indeed radio-emitting millisecond pulsars, Ransom reported.

“For the first time, gamma-ray observations are guiding radio observations,” commented Fermi researcher Peter Michelson of Stanford University in Palo Alto, Calif.

The gamma-ray technique may soon reveal dozens of previously unknown pulsars, Ransom estimated, including radio-bright pulsars whose radio emissions could be clocked to reveal the presence of gravitational waves (SN: 6/6/09, p. 14).

NASA'S Fermi Telescope Probes Dozens of Pulsars

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

Larger image

Larger image (unlabeled)

This movie shows one cycle of pulsed gamma rays from the Vela pulsar as constructed from photons detected by Fermi's Large Area Telescope. The movie includes data from August 4 to Sept. 15, 2008. The bluer color in the latter part of the pulse indicates the presence of gamma rays with energies exceeding a billion electron volts. For comparison, visible light has energies between two and three electron volts. Credit: NASA/DOE/Fermi LAT Collaboration

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With NASA's Fermi Gamma-ray Space Telescope, astronomers now are getting their best look at those whirling stellar cinders known as pulsars. In two studies published in the July 2 edition of Science Express, international teams have analyzed gamma-rays from two dozen pulsars, including 16 discovered by Fermi. Fermi is the first spacecraft able to identify pulsars by their gamma-ray emission alone.

A pulsar is the rapidly spinning and highly magnetized core left behind when a massive star explodes. Most of the 1,800 cataloged pulsars were found through their periodic radio emissions. Astronomers believe these pulses are caused by narrow, lighthouse-like radio beams emanating from the pulsar's magnetic poles.

"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."

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.

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.

Francis Reddy

NASA's Goddard Space Flight Center