Fermi Catches a ‘Transformer’ Pulsar

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.” Continue reading Fermi Catches a ‘Transformer’ Pulsar

Fermi’s Close Call with a Soviet Satellite

NASA scientists don’t often learn that their spacecraft is at risk of crashing into another satellite. But when Julie McEnery, the project scientist for NASA’s Fermi Gamma-ray Space Telescope, checked her email on March 29, 2012, she found herself facing this precise situation.
While Fermi is in fine shape today, continuing its mission to map the highest-energy light in the universe, the story of how it sidestepped a potential disaster offers a glimpse at an underappreciated aspect of managing a space mission: orbital traffic control.
As McEnery worked through her inbox, an automatically generated report arrived from NASA’s Robotic Conjunction Assessment Risk Analysis (CARA) team based at NASA’s Goddard Space Flight Center in Greenbelt, Md. On scanning the document, she discovered that Fermi was just one week away from an unusually close encounter with Cosmos 1805, a dead Cold-War era spy satellite.
fermiThe two objects, speeding around Earth at thousands of miles an hour in nearly perpendicular orbits, were expected to miss each other by a mere 700 feet.
Although the forecast indicated a close call, satellite operators have learned the hard way that they can’t be too careful. The uncertainties in predicting spacecraft positions a week into the future can be much larger than the distances forecast for their closest approach.
With a speed relative to Fermi of 27,000 mph, a direct hit by the 3,100-pound Cosmos 1805 would release as much energy as two and a half tons of high explosives, destroying both spacecraft.
The update on Friday, March 30, indicated that the satellites would occupy the same point in space within 30 milliseconds of each other. Fermi would have to move out of the way if the threat failed to recede. Because Fermi’s thrusters were designed to de-orbit the satellite at the end of its mission, they had never before been used or tested, adding a new source of anxiety for the team.
By Tuesday, April 3, the close approach was certain, and all plans were in place for firing Fermi’s thrusters. Shortly after noon EDT, the spacecraft stopped scanning the sky and oriented itself along its direction of travel. It then parked its solar panels and tucked away its high-gain antenna to protect them from the thruster exhaust.
The maneuver was performed by the spacecraft based on previously developed procedures. Fermi fired all thrusters for one second and was back doing science within the hour.
In 2012, the Goddard CARA team participated in collision-avoidance maneuvers for seven other missions. A month before the Fermi conjunction came to light, Landsat 7 dodged pieces of Fengyun-1C, a Chinese weather satellite deliberately destroyed in 2007 as part of a military test. And in May and October, respectively, NASA’s Aura and CALIPSO Earth-observing satellites took steps to avoid fragments from Cosmos 2251, which in 2009 was involved in the first known satellite-to-satellite collision with Iridium 33. http://youtu.be/npVgLM7Zd3M

Fermi Provides New Insights on Dark Matter


There’s more to the cosmos than meets the eye. About 80 percent of the matter in the universe is invisible to telescopes, yet its gravitational influence is manifest in the orbital speeds of stars around galaxies and in the motions of clusters of galaxies. Yet, despite decades of effort, no one knows what this “dark matter” really is. Many scientists think it’s likely that the mystery will be solved with the discovery of new kinds of subatomic particles, types necessarily different from those composing atoms of the ordinary matter all around us. The search to detect and identify these particles is underway in experiments both around the globe and above it.

Scientists working with data from NASA’s Fermi Gamma-ray Space Telescope have looked for signals from some of these hypothetical particles by zeroing in on 10 small, faint galaxies that orbit our own. Although no signals have been detected, a novel analysis technique applied to two years of data from the observatory’s Large Area Telescope (LAT) has essentially eliminated these particle candidates for the first time.

WIMPs, or Weakly Interacting Massive Particles, represent a favored class of dark matter candidates. Some WIMPs may mutually annihilate when pairs of them interact, a process expected to produce gamma rays — the most energetic form of light — that the LAT is designed to detect.

The team examined two years of LAT-detected gamma rays with energies in the range from 200 million to 100 billion electron volts (GeV) from 10 of the roughly two dozen dwarf galaxies known to orbit the Milky Way. Instead of analyzing the results for each galaxy separately, the scientists developed a statistical technique — they call it a “joint likelihood analysis” — that evaluates all of the galaxies at once without merging the data together. No gamma-ray signal consistent with the annihilations expected from four different types of commonly considered WIMP particles was found.

For the first time, the results show that WIMP candidates within a specific range of masses and interaction rates cannot be dark matter. A paper detailing these results appeared in the Dec. 9, 2011, issue of Physical Review Letters.
Learn more at: www.nasa.gov

Fermi Finds Youthful Pulsar Among Ancient Stars


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

An international team of scientists using NASA’s Fermi Gamma-ray Space Telescope has discovered a surprisingly powerful millisecond pulsar that challenges existing theories about how these objects form.
At the same time, another team has located nine new gamma-ray pulsars in Fermi data, using improved analytical techniques


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

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.

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. 


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

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)

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.


Fermi gamma-ray space telescope confirms puzzling preponderance of positrons

This illustration shows how the electron-positron sky appears to the Large Area Telescope. The purple region contains positrons while electrons are blocked by the Earth's bulk, the orange region contains electrons but is inaccessible to positrons, and the green region is completely out of the Earth's shadow for both positrons and electrons. Image courtesy Justin Vandenbroucke, Fermi-LAT collaboration

By finding a clever way to use the Earth itself as a scientific instrument, members of a SLAC-led research team turned the Fermi Gamma-ray Space Telescope into a positron detector – and confirmed a startling discovery from 2009 that found an excess of these antimatter particles in cosmic rays, a possible sign of dark matter…..
Read more: http://www.physorg.com

Fermi’s latest gamma-ray census highlights cosmic mysteries

This all-sky image, constructed from two years of observations by NASA's Fermi Gamma-ray Space Telescope, shows how the sky appears at energies greater than 1 billion electron volts (1 GeV). Brighter colors indicate brighter gamma-ray sources. For comparison, the energy of visible light is between 2 and 3 electron volts. A diffuse glow fills the sky and is brightest along the plane of our galaxy (middle). Discrete gamma-ray sources include pulsars and supernova remnants within our galaxy as well as distant galaxies powered by supermassive black holes.

Every three hours, NASA’s Fermi Gamma-ray Space Telescope scans the entire sky and deepens its portrait of the high-energy universe. Every year, the satellite’s scientists reanalyze all of the data it has collected, exploiting updated analysis methods to tease out new sources. These relatively steady sources are in addition to the numerous transient events Fermi detects, such as gamma-ray bursts in the distant universe and flares from the sun.

Earlier this year, the Fermi team released its second catalog of sources detected by the satellite’s Large Area Telescope (LAT), producing an inventory of 1,873 objects shining with the highest-energy form of light. “More than half of these sources are active galaxies, whose massive  are responsible for the gamma-ray emissions that the LAT detects,” said Gino Tosti, an astrophysicist at the University of Perugia in Italy and currently a visiting scientist at SLAC National Accelerator Laboratory in Menlo Park, Calif.

One of the scientists who led the new compilation, Tosti today presented a paper on the catalog at a meeting of the American Astronomical Society’s High Energy Astrophysics Division in Newport, R.I. “What is perhaps the most intriguing aspect of our new catalog is the large number of sources not associated with objects detected at any other wavelength,” he noted.

Indeed, if the Fermi catalog were a recipe, the two major ingredients would be active galaxies and pure mystery. To them, add in a pinch of pulsars, a dollop of supernova remnants, and a dash of other celestial objects, such as globular star clusters and galaxies like our own Milky Way.

Astronomers delight in the possibility of finding new types of gamma-ray-emitting objects within the “unassociated sources” that constitute roughly a third of the catalog. But Fermi’s LAT is revealing gamma-rays from an increasing — and sometimes, surprising — variety of astronomical objects. To highlight the range of LAT discoveries, the Fermi team created the following “top ten” list of five sources within the Milky Way and five beyond our galaxy.

Active galaxies called blazars constitute the single largest source class in the second Fermi LAT catalog, but nearly a third of the sources are unassociated with objects at any other wavelength. Their natures are unknown.

The Crab Nebula. The famous Crab Nebula, located in the constellation Taurus, is the wreckage of an exploded star whose light reached Earth in 1054. Located 6,500 light-years away, the Crab is one of the most studied objects in the sky. At the heart of an expanding gas cloud lies what’s left of the original star’s core, a superdense neutron star (also called a pulsar) that spins 30 times a second. Until recently, all of the Crab’s high-energy emissions were thought to be the result of physical processes near the pulsar that tapped into this rapid spin…… Continue reading Fermi’s latest gamma-ray census highlights cosmic mysteries

Fermi catalogue update shows ‘violent Universe’ changes

The catalogue that lists the most violent neighbourhoods in the Universe has been updated.

Fermi spotted that the Crab Nebula, once thought to be constant, flares violently with gamma rays

The Fermi space telescope captures gamma rays – the highest-energy light in nature, which hints at the cosmos’ most extreme conditions and processes.

The second Fermi catalogue represents a full two years of data, improving on the first edition’s 11 months.

It lists 1,873 gamma-ray sources; some 589 remain unidentified and could represent entirely new cosmic objects.

Dave Thompson, a Nasa astrophysicist who co-led the catalogue’s production, told BBC News that the effort was more than just an expanded list.

“The new catalogue is a new data set,” he said. “We’ve reanalysed all the data, reduced our background, developed new methods of analysis. We’re convinced that not only is this quantitatively a better catalogue – it’s qualitatively a better catalogue.”

It is also a snapshot from a slowly unfolding film of the Universe’s most extreme environments.

“It’s very important to understand that the gamma-ray sky is not static, it’s changing all the time,” explained Steven Ritz, deputy principal investigator for the Fermi mission’s Large-Area Telescope….. Continue reading Fermi catalogue update shows ‘violent Universe’ changes