Youngest Millisecond Pulsar Discovered

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

By Max Planck Institute for Radio Astronomy, Bonn, Germany, Max Planck Society, Munich, Germany

Published : November 3, 2011

Paulo Freire from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn and his collaborators have discovered the first gamma-ray pulsar in a globular cluster using the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The pulsar, labeled J1823-3021A, is located in the globular cluster NGC 6624 in Sagittarius, not far from the direction to the galactic center. At a distance of approximately 27,000 light-years, it is also the most distant pulsar ever detected in gamma rays. Its extreme gamma-ray luminosity implies that it is the youngest millisecond pulsar discovered to date and that its magnetic field is much larger than previously predicted by pulsar recycling theories. It suggests the existence of a whole new population of such extreme objects forming at the same rate as the more normal millisecond pulsars.

When the cores of massive stars run out of nuclear fuel, they collapse catastrophically, a phenomenon known as a supernova. This spectacular event marks the birth of a neutron star: a ball of neutrons, a single giant atomic nucleus with a radius of about 6-10 miles (10-16 kilometers) and about half a million times the Earth's mass. A pulsar is a rapidly rotating neutron star for which we can detect pulsations (normally at radio, but now also at gamma-ray wavelengths), modulated by the rotation of the object — like a lighthouse. Ordinary pulsars have rotation periods between 16 milliseconds and 8 seconds. Even faster rotating are the so-called millisecond pulsars (MSPs), which can have rotation periods as fast as 1.4 milliseconds — corresponding to 43,000 rotations per minute. They are thought to have been spun up by accretion of matter from a companion star, a theory that is supported by the observation that roughly 80 percent of MSPs are found in binary systems.

MSPs possess extraordinary long-term rotational stability, which is in some cases similar to those of the best atomic clocks on Earth. They are basically giant flywheels in space where nothing disturbs their rotation. They are being used to test Einstein's general theory of relativity, search for gravitational waves, and study the properties of the super-dense matter at their center.

"We have discovered more than 100 of these objects in globular clusters with radio telescopes," said Freire. "Thanks to the sensitivity of the Large Area Telescope on the Fermi satellite, we have been able, for the first time, to see one of them in gamma rays."

Globular clusters are ancient swarms of hundreds of thousands of stars bound together by their mutual gravity. They produce many binary systems of the kind that lead to the formation of millisecond pulsars. One of these clusters is NGC 6624 in Sagittarius. At a distance of about 27,000 light-years, it is in the proximity of the galactic center. A total of six pulsars have been discovered in this globular cluster to date, three of these to be announced soon. The first pulsar found in NGC 6624 was J1823-3021A. With a rotation period of 5.44 milliseconds (11,000 rotations/minute), it is the most luminous radio pulsar found in a globular cluster to date. It has been timed since its discovery in 1990 with several large radio telescopes, in particular with the Lovell Telescope of the University of Manchester/England and with the radio telescope at Nançay/France.

"To our surprise, we found the pulsar to be extremely bright in gamma rays, as well," said Damien Parent from the Center for Earth Observing and Space Research. "Millisecond pulsars were not supposed to be that bright. This implies an unexpectedly high magnetic field for such a fast pulsar."

"This challenges our current theories for the formation of such objects," saud Michael Kramer from MPIfR. "We are currently investigating a number of possibilities. Nature might even be forming millisecond pulsars in a way we have not anticipated."

"Whichever way these anomalous pulsars are formed, one thing appears to be clear," said Freire. "At least in globular clusters, they are so young that they are probably forming at rates comparable to the large known population of normal millisecond pulsars."

Geriatric Pulsar Still Kicking

Artist concept of ancient pulsar J0108
Image credit: X-ray: NASA/CXC/Penn State/G.Pavlov et al.
Optical: ESO/VLT/UCL/R.Mignani et al. Illustration: CXC/M. Weiss

February 27, 2009

The oldest isolated pulsar ever detected in X-rays has been found with NASA's Chandra X-ray Observatory. This very old and exotic object turns out to be surprisingly active.

The pulsar, PSR J0108-1431 (J0108 for short) is about 200 million years old. Among isolated pulsars -- ones that have not been spun-up in a binary system -- it is over 10 times older than the previous record holder with an X-ray detection. At a distance of 770 light years, it is one of the nearest pulsars known.

Pulsars are born when stars that are much more massive than the Sun collapse in supernova explosions, leaving behind a small, incredibly weighty core, known as a neutron star. At birth, these neutron stars, which contain the densest material known in the Universe, are spinning rapidly, up to a hundred revolutions per second. As the rotating beams of their radiation are seen as pulses by distant observers, similar to a lighthouse beam, astronomers call them "pulsars".

Astronomers observe a gradual slowing of the rotation of the pulsars as they radiate energy away. Radio observations of J0108 show it to be one of the oldest and faintest pulsars known, spinning only slightly faster than one revolution per second.

The surprise came when a team of astronomers led by George Pavlov of Penn State University observed J0108 in X-rays with Chandra. They found that it glows much brighter in X-rays than was expected for a pulsar of such advanced years.

Some of the energy that J0108 is losing as it spins more slowly is converted into X-ray radiation. The efficiency of this process for J0108 is found to be higher than for any other known pulsar.

"This pulsar is pumping out high-energy radiation much more efficiently than its younger cousins," said Pavlov. "So, although it's clearly fading as it ages, it is still more than holding its own with the younger generations."

It's likely that two forms of X-ray emission are produced in J0108: emission from particles spiraling around magnetic fields, and emission from heated areas around the neutron star's magnetic poles. Measuring the temperature and size of these heated regions can provide valuable insight into the extraordinary properties of the neutron star surface and the process by which charged particles are accelerated by the pulsar.

The younger, bright pulsars commonly detected by radio and X-ray telescopes are not representative of the full population of objects, so observing objects like J0108 helps astronomers see a more complete range of behavior. At its advanced age, J0108 is close to the so- called “pulsar death line,” where its pulsed radiation is expected to switch off and it will become much harder, if not impossible, to observe.

"We can now explore the properties of this pulsar in a regime where no other pulsar has been detected outside the radio range," said co- author Oleg Kargaltsev of the University of Florida. "To understand the properties of ‘dying pulsars,’ it is important to study their radiation in X-rays. Our finding that a very old pulsar can be such an efficient X-ray emitter gives us hope to discover new nearby pulsars of this class via their X-ray emission."

The Chandra observations were reported by Pavlov and colleagues in the January 20, 2009, issue of The Astrophysical Journal. However, the extreme nature of J0108 was not fully apparent until a new distance to it was reported on February 6 in the PhD thesis of Adam Deller from Swinburne University in Australia. The new distance is both larger and more accurate than the distance used in the Chandra paper, showing that J0108 was brighter in X-rays than previously thought.

"Suddenly this pulsar became the record holder for its ability to make X-rays," said Pavlov, "and our result became even more interesting without us doing much extra work." The position of the pulsar seen by Chandra in X-rays in early 2007 is slightly different from the radio position observed in early 2001. This implies that the pulsar is moving at a velocity of about 440,000 miles per hour, close to a typical value for pulsars.

Currently the pulsar is moving south from the plane of the Milky Way galaxy, but because it is moving more slowly than the escape velocity of the Galaxy, it will eventually curve back towards the plane of the Galaxy in the opposite direction.

The detection of this motion has allowed Roberto Mignani of University College London, in collaboration with Pavlov and Kargaltsev, to possibly detect J0108 in optical light, using estimates of where it should be found in an image taken in 2000. Such a multi-wavelength study of old pulsars is critical for understanding the long-term evolution of neutron stars, such as how they cool with time, and how their powerful magnetic fields evolve.

The team of astronomers that worked with Pavlov also included Gordon Garmire and Jared Wong at Penn State. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

Pulsars could help unravel space-time

photo:A pulsar and a neutron star whirl around each other in this illustration.

The discovery of a pulsar in a faint double-star system is giving scientists new hope of glimpsing one of nature's most elusive phenomena: gravity waves. While their existence has been inferred indirectly, scientists have yet to come face-to-face with these warpings and billowings of space-time that Einstein predicted in 1918 as a fundamental consequence of his theory of general relativity. Massive objects moving at extreme speeds, the theory explains, create ripples in the very fabric of the universe — ripples that, if detected, will help confirm Einstein's elegant vision of the workings of the cosmos.

Compact objects like neutron-star binaries are just the kind of cosmic dancers capable of shaking up space-time. As they tauntingly spiral toward one another, neutron stars give off energy in the form of gravitational radiation. Recently discovered pulsar J0737-3039 and its neutron-star companion now are engaged in the closest, fastest tango of any neutron-star system ever observed, orbiting one another once every 2.4 hours. The pair was discovered by a team of scientists from Italy, Australia, the United Kingdom, and the United States using the 64-meter CSIRO Parkes radio telescope in eastern Australia.The team found that this system is moving much faster than, for example, the Hulse-Taylor pulsar — the first neutron-star binary ever detected, which has an orbital rate of 7.75 hours. It was discovered in 1974, winning Russell Hulse and Joseph Taylor of Princeton University a Nobel Prize. Since this discovery, only one other system capable of producing significant gravitational radiation has been found. That made the outlook for catching gravity waves rather bleak. So when LIGO (the Laser Interferometer Gravitational Wave Observatory) set out to do just that, many were skeptical of its chances.

Now, the prospect of detecting gravitational waves appears much more favorable. Because the newly discovered neutron stars are orbiting one another so quickly, the system's life span will be significantly shorter than those of its two predecessors. These stars are expected to complete their dance eighty-five million years from now, when they will collide. Of course, that in itself is not entirely helpful, given that it is only in the last two or three minutes before they merge that they will produce gravity waves strong enough for LIGO to register. But the finding suggests that this type of tight, fast, and relatively short-lived system actually is common in our Milky Way — and much more common than binaries like the Hulse-Taylor."This is a very faint system and the pulsar's spin period is very short, only 22 milliseconds, which makes it very hard for observers to find compared to the Hulse-Taylor," says Northwestern University's Vicky Kalogera, a member of the team who made the discovery. "So if we have something that is easy to discover and we find only one and we have something that is difficult to discover and we also find one, then in the second case, there are probably many more of these in the galaxy."

photo:The 64-meter Parkes Radio Telescope has been hard at work since 1961. A movable 18-meter radio dish seen here in the background assists with interferometry studies. John Sarkissian / CSIRO

In fact, the new data boosts the neutron-star merger rate by a factor of six or seven, which means LIGO should be able to detect such an event once every year and a half, a vast leap from previous estimates of only one every ten or twenty years. And if that's the case, we won't have long to wait to witness firsthand the extraordinary rolling landscape of Einstein's universe.

Cosmic firehose caught in action

photo:The Chandra X-ray Observatory captured these images of an erratic jet of high-energy particles associated with the Vela pulsar. These images are part of a series of 13 taken over a period covering two and a half years. NASA / CXC / PSU / G. Pavlov et al.

July 10, 2003

Using NASA's orbiting Chandra X-ray Observatory, astronomers have captured stunning views of a twisting column of x rays whipping around a pulsar. Propelled by voltages 100 million times more powerful than a lightning bolt and traveling near half the speed of light, such highly erratic jets have never been seen before.

Snapping a series of x-ray images of the 1,000-light-year-distant Vela pulsar, a team of Pennsylvania State University researchers has made a short movie spanning a two-and-a-half-year period that brings this spinning dead star to life. The time-lapse animation clearly shows that, in a span of a few weeks, a gigantic serpentine jet of high-energy particles bends and coils in all directions.

"This jet is half a light-year in length and is shooting out ahead of the moving pulsar," said George Pavlov, lead author of a report in the July 10 issue of The Astrophysical Journal. "The most striking thing about this jet is how rapidly it changes both its shape and brightness. Such strong, fast variability has never been observed in astrophysical jets."
Composed either of supercharged matter or antimatter electrons, this wild, outflowing jet appears to be fairly constant in width (about 200 billion miles) — held in check by the pulsar's monstrous magnetic field, researchers say. Moving through the surrounding gas at 200,000 miles per hour, the pulsar creates strong head winds that may trigger the jet's erratic behavior. Laboratory versions of magnetically confined jets have shown similar erratic behavior, caused by an effect called the "firehose instability."

"Imagine a firehose lying on the ground," explains co-investigator Marcus Teter. "After you turn on the water, you will see different parts of the hose kinking up and moving rapidly in different directions, pushed by the increased pressure at the bends in the hose. The Vela jet resembles a hose made of magnetic fields, which confines the electrically charged particles."

Pavlov and his teammates believe these intimate observations also suggest that the bright arcs encircling the Vela pulsar, once thought to be equatorial rings, are shockwaves resulting from the jet smashing into clouds of dust and gas surrounding the stellar corpse.

With Vela showing incredibly fast changes in its jet morphology, this novel cinematic study will begin to unveil the inner clockwork of pulsars. Astronomers hope the observations will go one step further and even shed light on how bizarre jet formations work around more energetic objects like supermassive black holes. But, as team member Oleg Kargaltsev adds, "Those jets may also vary, but on time scales of millions of years, instead of weeks as in the Vela pulsar jet."