Apr 13

Einstein Was Right — So Far— Record-breaking pulsar takes tests of general relativity into new territory

Source: ESO

Artist’s impression of the pulsar PSR J0348+0432 and its white dwarf companion.
Image credits: ESO/L. Calçada.

Astronomers have used ESO’s Very Large Telescope, along with radio telescopes around the world, to find and study a bizarre stellar pair consisting of the most massive neutron star confirmed so far, orbited by a white dwarf star. This strange new binary allows tests of Einstein’s theory of gravity — general relativity — in ways that were not possible up to now. So far the new observations exactly agree with the predictions from general relativity and are inconsistent with some alternative theories. The results appeared in the journal Science on 26 April 2013.

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Jul 12

A pulsar with a tremendous hiccup

Source: Max Planck Institute for Gravitational Physics

Artist's impression of a gamma-ray pulsar.
Image credits: NASA/Fermi/Cruz de Wilde

Max Planck scientists discover a young and energetic neutron star with unusually
irregular rotation.

Pulsars are superlative cosmic beacons. These compact neutron stars rotate about their axes many times per second, emitting radio waves and gamma radiation into space. Using ingenious data analysis methods, researchers from the Max Planck Institutes for Gravitational Physics and for Radio Astronomy, in an international collaboration, dug a very special gamma-ray pulsar out of data from the Fermi Gamma-ray Space Telescope. The pulsar J1838-0537 is radio-quiet, very young, and, during the observation period, experienced the strongest rotation glitch ever observed for a gamma-ray-only pulsar.

Pure gamma-ray pulsars are difficult to identify because their characteristics, such as its sky position, the period of rotation and its change in time, are unknown. And astronomers can only determine their approximate position in the sky from the original Fermi observations. They must therefore check many combinations of these characteristics in a blind search, which requires a great deal of computing time. This is the only way of finding a hidden periodicity in the arrival times of the gamma-ray photons.

Even high-performance computers quickly reach their limit in this process. Therefore, the researchers used algorithms originally developed for the analysis of gravitational-wave data to conduct a particularly efficient hunt through the Fermi data. “By employing new optimal algorithms on our ATLAS computer cluster, we were able to identify many previously-missed signals,” says Bruce Allen, Director of the AEI.

Back in November 2011, Allen’s team announced the discovery of nine new Fermi gamma-ray pulsars, which had escaped all previous searches. Now the scientists have made a new extraordinary find with the same methods.

The name of the newly discovered pulsar – J1838-0537 – comes from its celestial coordinates. “The pulsar is, at 5,000 years of age, very young. It rotates about its own axis roughly seven times per second and its position in the sky is towards the Scutum constellation,” says Holger Pletsch, a scientist in Allen’s group and lead author of the study which has now been published. “After the discovery we were very surprised that the pulsar was initially only visible until September 2009. Then it seemed to suddenly disappear.”

Only a complex follow-up analysis enabled an international team led by Pletsch to solve the mystery of pulsar J1838-0537: it did not disappear, but experienced a sudden glitch after which it rotated 38 millionths of a Hertz faster than before. “This difference may appear negligibly small, but it’s the largest glitch ever measured for a pure gamma-ray pulsar,” explains Allen. And this behaviour has consequences.

“If the sudden frequency change is neglected, then after only eight hours, a complete rotation of the pulsar is lost in our counting, and we can no longer determine at which rotational phase the gamma-ray photons reach the detector aboard Fermi,” adds Pletsch. The “flashing” of the neutron star then disappears. If the researchers take the glitch into account and correct the change in rotation, the pulsar shows up again in the observational data.

The precise cause of the glitches observed in many young pulsars is unknown. Max-Planck-Institut für Radioastronomie Astronomers consider  “star quakes” of the neutron star crust or interactions between the superfluid stellar interior and the crust to be possible explanations. “Detecting a large number of strong pulsar glitches makes it possible to learn more about the inner structure of these compact celestial bodies,” says Lucas Guillemot from the Max  Planck Institute for Radio Astronomy in Bonn, the second author of the study. “This is a good example of the collaboration of two Max Planck institutes with complementary research foci,” says Michael Kramer, Director and Head of the Fundamental Physics in Radio Astronomy research group.

After the discovery in data from the Fermi satellite, the researchers pointed the radio telescope in Green Bank, West Virginia/USA at the celestial position of the gamma-ray pulsar. In an observation of almost two hours and by analysing a further, older, one-hour observation of the source they found no indications of pulsations in the radio range, indicating that J1838-0537 is a rare gamma-ray-only pulsar.

There were, however, noticeable overlays with observations of the High Energy Stereoscopic System (H.E.S.S.) in Namibia, which searches for very-high-energy gamma radiation from the depths of space. In a survey with H.E.S.S., astronomers found an extended source of this radiation near the now discovered pulsar, but have not yet been able to clarify its nature.

The discovery of the pulsar suggests that the H.E.S.S. source is a pulsar wind nebula. These are produced by particles moving at almost the speed of light, which the pulsar accelerates in its extremely strong magnetic field. Since the exact position of the pulsar is now known, H.E.S.S. can take this into account in the future and to make more precise measurements than before in this celestial region.

The ATLAS computer cluster of the Albert Einstein Institute has thus already assisted in the discovery of the tenth previously unknown gamma-ray pulsar; however, Allen’s team has meanwhile mobilised further computing capacity. “Since August 2011, our search has also been running on the distributed computing project Einstein@Home, which has computing power a factor of ten greater than the ATLAS cluster. We are very optimistic about finding more unusual gamma-ray pulsars in the Fermi data,” says Bruce Allen.

One goal of the expanded search is to discover the first gamma-ray-only pulsar with a rotation period in the millisecond range.

Original publication
Holger J. Pletsch, L. Guillemot, B. Allen, M. Kramer et al.
PSR J1838-0537: Discovery of a young, energetic gamma-ray pulsar
The Astrophysical Journal Letters, gone to press

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Nov 11

Fermi Finds a Youthful Pulsar Among Ancient Stars

Source: NASA Science Cast


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.

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Nov 11

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

Source: NASA Fermi

Image credits: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.(read more)

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Jul 11

A Pulsar and its mysterious tail

Source: Chandra

Image credit: X-ray: NASA/CXC/IUSS/A.De Luca et al; Optical: DSS

A spinning neutron star is tied to a mysterious tail - or so it seems. Astronomers using NASA's Chandra X-ray Observatory have found that this pulsar, known as PSR J0357+3205 (or PSR J0357 for short), apparently has a long, X-ray bright tail streaming away from it.

This composite image shows Chandra data in blue and Digitized Sky Survey data in yellow. The position of the pulsar at the upper right end of the tail is seen by mousing over the image. The two bright sources lying near the lower left end of the tail are both thought to be unrelated background objects located outside our galaxy.

PSR J0357 was originally discovered by the Fermi Gamma Ray Space Telescope in 2009. Astronomers calculate that the pulsar lies about 1,600 light years from Earth and is about half a million years old, which makes it roughly middle-aged for this type of object.

If the tail is at the same distance as the pulsar then it stretches for 4.2 light years in length. This would make it one of the the longest X-ray tails ever associated with a so-called "rotation-powered" pulsar, a class of pulsar that get its power from the energy lost as the rotation of the pulsar slows down. (Other types of pulsars include those driven by strong magnetic fields and still others that are powered by material falling onto the neutron star.)

The Chandra data indicate that the X-ray tail may be produced by emission from energetic particles in a pulsar wind, with the particles produced by the pulsar spiraling around magnetic field lines.

Other X-ray tails around pulsars have been interpreted as bow-shocks generated by the supersonic motion of pulsars through space, with the wind trailing behind as its particles are swept back by the pulsar's interaction with the interstellar gas it encounters.

However, this bow-shock interpretation may or may not be correct for PSR J0357, with several issues that need to be explained. For example, the Fermi data show that PSR J0357 is losing a very small amount of energy as its spin slows down with time. This energy loss is important, because it is converted into radiation and powering a particle wind from the pulsar.

This places limits on the amount of energy that particles in the wind can attain, and so might not account for the quantity of X-rays seen by Chandra in the tail.

Another challenge to this explanation is that other pulsars with bow-shocks show bright X-ray emission surrounding the pulsar, and this is not seen for PSR J0357. Also, the brightest portion of the tail is well away from the pulsar and this differs from what has been seen for other pulsars with bow-shocks.

Further observations with Chandra could help test this bow-shock interpretation. If the pulsar is seen moving in the opposite direction from that of the tail, this would support the bow-shock idea.

These results were published in the June 1st, 2011 issue of The Astrophysical Journal. The first author is Andrea De Luca of Institute of Advanced Study in Pavia, Italy (IUSS), the National Institute of Nuclear Physics (INFN) in Rome, and the National Institute for Astrophysics (INAF) in Milano.

The co-authors are M. Marelli of INAF, Milano and the University of Insubria in Italy; R. Mignani of University College London, UK and University of Zielona Gora, Poland; P. Caraveo of INAF, Milano; W. Hummel of ESO, Germany; S. Collins and A. Shearer of National University of Ireland; P. Saz Parkinson of University of California at Santa Cruz; A. Belfiore of University of California at Santa Cruz and University of Pavia; and, G. Bignami of IUSS, Pavia and INAF, Milano.

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