Chapter 30:

Pulsar Distance Measured with VLBA Parallaxes

"Getting an accurate distance to this pulsar gave us a real bonanza," said Walter Brisken, of the National Radio Astronomy Observatory (NRAO) in Socorro, NM.

The pulsar, called PSR B0656+14, is in the constellation Gemini, and appears to be near the center of a circular supernova remnant that straddles Gemini and its neighboring constellation, Monoceros, and is thus called the Monogem Ring. Since pulsars are superdense, spinning neutron stars left over when a massive star explodes as a supernova, it was logical to assume that the Monogem Ring, the shell of debris from a supernova explosion, was the remnant of the blast that created the pulsar.

However, astronomers using indirect methods of determining the distance to the pulsar had concluded that it was nearly 2500 light-years from Earth. On the other hand, the supernova remnant was determined to be only about 1000 light-years from Earth. It seemed unlikely that the two were related, but instead appeared nearby in the sky purely by a chance juxtaposition.

Brisken and his colleagues used the VLBA to make precise measurements of the sky position of PSR B0656+14 from 2000 to 2002. They were able to detect the slight offset in the object's apparent position when viewed from opposite sides of Earth's orbit around the Sun. This effect, called parallax, provides a direct measurement of distance.

"Our measurements showed that the pulsar is about 950 light-years from Earth, essentially the same distance as the supernova remnant," said Steve Thorsett, of the University of California, Santa Cruz. "That means that the two almost certainly were created by the same supernova blast," he added.

With that problem solved. the astronomers then turned to studying the pulsar's neutron star itself. Using a variety of data from different telescopes and armed with the new distance measurement, they determined that the neutron star is between 16 and 25 miles in diameter. In such a small size, it packs a mass roughly equal to that of the Sun.

The next result of learning the pulsar's actual distance was to provide a possible answer to a longstanding question about cosmic rays. Cosmic rays are subatomic particles or atomic nuclei accelerated to nearly the speed of light. Shock waves in supernova remnants are thought to be responsible for accelerating many of these particles.

Scientists can measure the energy of cosmic rays, and had noted an excess of such rays in a specific energy range. Some researchers had suggested that the excess could come from a single supernova remnant about 1000 light-years away whose supernova explosion was about 100,000 years ago. The principal difficulty with this suggestion was that there was no accepted candidate for such a source.

"Our measurement now puts PSR B0656+14 and the Monogem Ring at exactly the right place and at exactly the right age to be the source of this excess of cosmic rays," Brisken said.

With the ability of the VLBA, one of the telescopes of the NRAO, to make extremely precise position measurements, the astronomers expect to improve the accuracy of their distance determination even more.

"This pulsar is becoming a fascinating laboratory for studying astrophysics and nuclear physics," Thorsett said.

In addition to Brisken and Thorsett, the team of astronomers includes Aaron Golden of the National University of Ireland, Robert Benjamin of the University of Wisconsin, and Miller Goss of NRAO. The scientists are reporting their results in papers appearing in the Astrophysical Journal Letters in August.

The VLBA is a continent-wide system of ten radio- telescope antennas, ranging from Hawaii in the west to the U.S. Virgin Islands in the east, providing the greatest resolving power, or ability to see fine detail, in astronomy. Dedicated in 1993, the VLBA is operated from the NRAO's Array Operations Center in Socorro, New Mexico.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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Chandra Makes a Movie of the Vela Pulsar Jet

An X-ray movie of the Vela pulsar, made from a series of observations by NASA's Chandra X-ray Observatory, reveals a spectacularly erratic jet that varies in a way never seen before. The jet of high-energy particles whips about like an untended firehose at about half the speed of light. This behavior gives scientists new insight into the nature of jets from pulsars and black holes.

http://chandra.harvard.edu/photo/2003/vela_pulsar/

First Direct Measurements of a Neutron Star's Magnetic Field

Using the superior sensitivity of ESA's X-ray observatory, XMM-Newton, a team of European astronomers has made the first direct measurement of a neutron star's magnetic field. The results provide deep insights into the extreme physics of neutron stars and reveal a new mystery yet to be solved about the end of this star's life.

A neutron star is very dense celestial object that usually has something like the mass of our Sun packed into a tiny sphere only 20-30 km across. It is the product of a stellar explosion, known as a supernova, in which most of the star is blasted into space, but its collapsed heart remains in the form of a super-dense, hot ball of neutrons that spins at a incredible rate.

Despite being a familiar class of object, individual neutron stars themselves remain mysterious. Neutron stars are extremely hot when they are born, but cool down very rapidly. Therefore, only few of them emit highly energetic radiation, such as X-rays. This is why they are traditionally studied via their radio emissions, which are less energetic than X-rays and which usually appear to pulse on and off. Therefore, the few neutron stars which are hot enough to emit X-rays can be seen by X-ray telescopes, such as ESA's XMM-Newton.

One such neutron star is 1E1207.4-5209. Using the longest ever XMM-Newton observation of a galactic source (72 hours), Professor Giovanni Bignami of the Centre d'Etude Spatiale des Rayonnements (CESR) and his team have directly measured the strength of its magnetic field. This makes it the first ever isolated neutron star where this could be achieved. All previous values of neutron star magnetic fields could only be estimated indirectly. This is done by theoretical assumptions based on models that describe the gravitational collapse of massive stars, like those which lead to the formation of neutron stars. A second indirect method is to estimate the magnetic field by studying how the neutron star's rotation slows down, using radio astronomy data.

In the case of 1E1207.4-5209, this direct measurement using XMM-Newton reveals that the neutron star's magnetic field is 30 times weaker than predictions based on the indirect methods.

How can this be explained? Astronomers can measure the rate at which individual neutron stars decelerate. They have always assumed that 'friction' between its magnetic field and its surroundings was the cause. In this case, the only conclusion is that something else is pulling on the neutron star, but what? We can speculate that it may be a small disc of supernova debris surrounding the neutron star, creating an additional drag factor.

The result raises the question of whether 1E1207.4-5209 is unique among neutron stars, or it is the first of its kind. The astronomers hope to target other neutron stars with XMM-Newton to find out.

X-rays emitted by a neutron star like 1E1207.4-5209, have to pass through the neutron star's magnetic field before escaping into space. En route, particles in the star's magnetic field can steal some of the outgoing X-rays, imparting on their spectrum tell-tale marks, known as 'cyclotron resonance absorption lines'. It is this fingerprint that allowed Prof. Bignami and his team to measure the strength of the neutron star's magnetic field.

These results are being published in this week's issue of Nature.

Prof. Giovanni Bignami
Director of Centre d'Etude Spatiale des Rayonnements (CESR)

LIGO Evaluated

May 19, 2002

See Scientific American for April 2002: "Ripples in Spacetime" by W. Wayt Gibbs. He discusses the LIGO program extensively.
He quotes
Jerry Ostriker, Cambridge University, then at Princeton: "I have always believed that detecting gravitational waves will provide us insights obtainable in no other way. That said, I think that the LIGO program has been an egregious waste of funds--funds that could have been used for more productive science."
Kip Thorne, Caltech:
"Theorists have a very poor track record for predicting whqat we will see when a new window is opened on the universe. Early radio telescopes discovered that the signals were much stronger than theorists expected. That happened again when the x-ray window opened in the 1960s.... In some sense, opening the gravitational window will give us a more radically different view on the universe than those previous advances did."

Quark Stars Instead of Only Neutron Stars?

The "standard" stellar-evolution track generally accepted as of 2002 has neutron stars held up by neutron degeneracy intermediate between white dwarfs held up by electron degeneracy and black holes, which are not held up against gravity at all. But two independent measurements, both made with the Chandra X-ray Observatory and reported in April 2002, both indicate objects that are too small and too cold to match the expectation of the neutron-star model. Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics and colleagues reported that RXJ1856 is "only" 0.7 million kelvins, lower than expected for a neutron star, and a diameter of "only" 20 km, smaller than expected. David Helfand of Columbia and colleagues observed 3C58, the radio designationn for the remnant of a supernova, and found a temperature of 1.0 million kelvins, also lower than expected.

The solution may be that these stars are not made only of the up and down quarks that make up neutrons but also have "strange" quarks (described in the cosmology chapter of the textbook). Such "strange matter" may leave the resulting stars in a state different from that of ordinary neutron stars. (In the early 1900s, some particles were found that took longer to decay than expected, and were called "strange particles." They were later explained as incorporating a third type of quark beyond the "up" and "down" quarks that make up ordinary matter, and it was called a "strange quark." The "charmed quark," found later, is the other member of its pair.)
jmp

Age Discrepancy Throws Pulsar Theories into Turmoil

NRAO Press Release, March 11, 2002

Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope have found a pulsar -- a spinning, superdense neutron star -- that apparently is considerably younger than previously thought. This finding, combined with the discovery in 2000 of a pulsar that was older than previously thought, means that many assumptions astronomers have made about how pulsars are born and age must be reexamined, according to the researchers.

"We are learning that each individual pulsar is a very complicated object, and we should assume nothing about it," said Bryan Gaensler, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "Our work makes it more difficult to put pulsars into neat categories, but ultimately will yield new insights into how pulsars are born," he added. The research is reported in the March 10 edition of the Astrophysical Journal Letters.

The astronomers studied a pulsar called B1951+32 and a supernova remnant called CTB 80, both nearly 8,000 light-years from Earth. The supernova remnant is the shell of debris from the explosion of a giant star. The explosion resulted from the giant star's catastrophic collapse into the superdense neutron star. By observing the pulsar and the supernova remnant from 1989 to 2000 with the VLA, the scientists were able to measure the movement of the pulsar, which, they found, is moving directly outward from the center of the shell of explosion debris.

"We've always felt that, if you see a pulsar and a supernova remnant close together, the pulsar had been born in an explosion at the center of the supernova remnant, but this is the first time that actual observational measurement shows a pulsar moving away from the center of the supernova remnant. It's nice to finally have such an example," said Joshua Migliazzo of the Center for Space Research at the Massachusetts Institute of Technology, another one of the researchers.

By tracking the pulsar's motion for more than a decade, the astronomers were able to calculate that it is traveling through space at more than 500,000 miles per hour. At that speed, the pulsar required about 64,000 years to travel from its birthplace -- the site of the supernova explosion -- to its present location. That means, the astronomers say, that the pulsar is about 64,000 years old.

This age, however, differs significantly from the age estimated by another method which has been used by astronomers for decades. This method uses measurements of the rotation rate of the neutron star and the tiny amount by which that rotation slows over time to arrive at an estimate called the pulsar's "characteristic age." For B1951+32, that method produced an estimated age of 107,000 years.

"Now we have a pulsar that is much younger than we thought. In 2000, a different pulsar was shown to be significantly older than we thought. That means that some of the assumptions that have gone into estimating the ages of these objects are unjustified," Migliazzo said.

The pulsar's rotation is thought to slow because the neutron star's powerful magnetic field acts as a giant dynamo, emitting light, radio waves and other electromagnetic radiation as the star rotates. The energy lost by emitting the radiation results in the star's rotation slowing down.

Previous estimates of pulsar ages have assumed that all pulsars are born spinning much faster than we see them now, that the physical characteristics of the pulsar such as its mass and magnetic-field strength do not change with time, and that the slowdown rate can be estimated by applying the physics of a magnet spinning in a vacuum.

"With one pulsar older than the estimates and one younger, we now realize that we have to question all three of these assumptions," said Gaensler.

Further research, the scientists say, should help them understand more about the conditions under which pulsars form and just how they get their spin in the first place. Neutron stars are formed in a fraction of a second as a massive star collapses onto itself, compressing its matter to the density of an atomic nucleus. During the collapse, the neutron star is thought to receive numerous "kicks" that spin it up.

The measurements of B1951+32's position were made in 1989, 1991, 1993 and 2000, with the VLA. The 2000 observations also used the Pie Town station of NSF's Very Long Baseline Array (VLBA), which improved the precision of the measurements. The other pulsar, which was found to be older than its estimated age, is called B1757-24 or "the duck." The report on its motion and age was published in Nature in July of 2000.

In addition to Gaensler and Migliazzo, the researchers are: Donald Backer of the University of California-Berkeley; Benjamin Stappers of ASTRON in the Netherlands; Eric Van Der Swaluw of the Dublin Institute for Advanced Studies in Ireland; and Richard Strom of ASTRON and the University of Amsterdam in the Netherlands.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

http://www.nrao.edu/pr/pulsarage.html

Probable pulsar in a supernova

NEW BRUNSWICK/PISCATAWAY, N.J. A team of astronomers led by Rutgers Professor John P. Hughes has made an important new discovery using NASA's orbital Chandra X-ray Observatory. The astronomers have found what appears to be a pulsar at the center of the exploded remains of a 1,600-year-old supernova. Pulsars, first discovered in 1967, are known to be rapidly rotating neutron stars, formed in supernova explosions. They emit regular bursts or pulses of radio waves, X-rays and optical light.

"For the first time, we have an oxygen-rich supernova remnant close enough for detailed study, with almost incontrovertible evidence for the existence of an associated pulsar," said Hughes. "Based on the pattern of elements now revealed by Chandra throughout this remnant, we will be able to ascertain the mass and composition of the star that gave rise to what we now see. This will allow us to make a much closer connection between pulsars and the massive stars from which they formed."

Supernovae are of great interest to astronomers because they are one of the primary sources of the heavy elements necessary to form planets and people. Supernovae are rare, occurring only once every 50 years or so in a galaxy like our own.

Located in the Southern Hemisphere in the constellation Centaurus, the supernova remnant (labeled G292.0+1.8) studied by Hughes and his group shows a rapidly expanding shell of gas 36 light-years across surrounding the apparent pulsar. It is one of three known oxygen-rich supernovae in our galaxy and is among the 10 brightest supernova remnants known.

A full account of the discovery can be found in "A Pulsar Wind Nebula in the Oxygen-Rich Supernova Remnant G292.0+1.8," published in the Oct. 1 issue of Astrophysical Journal Letters.

The research team also included Patrick Slane (Smithsonian Astrophysical Observatory), David Burrows, Gordon Garmire and John Nousek (Pennsylvania State University), and Charles Olbert and Jonathan Keohane (North Carolina School of Science and Mathematics).

An image of G292.0+1.8 may be downloaded from http://ur.rutgers.edu/medrel/photos/NovaRem-sm.jpg or http://ur.rutgers.edu/medrel/photos/NovaRem.jpg. The Astrophysical Journal article on the discovery is available at http://www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v559n2/15520/15 520.web.pdf.