Pulsars

pulsars
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Pulsars are thought to be rotating neutron stars with strong magnetic fields of approximately about 10¹¹ to 10¹² Gauss. While similar to magnetars, they have a much weaker magnetic field strength. Pulsars are believed to form when a large star goes supernova and compresses its core as it blows off its outer layers, or when a white dwarf accretes enough material to force gravitational collapse. The pulsation attribute comes from the rapid rotation of the neutron star whose magnetic field axis is not aligned with its spin axis. As material is funneled onto the magnetic poles of the star, energy from this accreted matter is released in the form of X-rays. The concentration of emitted X-rays from the magnetic poles appears to a distant observer as a periodic change in the intensity of the source, like a lighthouse.

Left: A diagrammatic representation of why we believe the pulsation mechanism in a Pulsar works. (Click the thumbnail for a larger image.)

Pulsars get their name because their emissions appear to turn on and off, or pulse, very rapidly. Astronomers believe the stars channel some of their energy into a beam of radiation, and as the star spins the beam sweeps through space like a lighthouse beacon. -- By counting how rapidly the beam flashes at Earth, scientists can calculate a pulsar's rate of spin.

When a star explodes as a supernova it leaves behind a lingering core about 15 miles across but packed with as much matter as in the Earth's Sun. The star is so dense that neutrons are the only form of matter that exist within its boundaries, thus earning the name "neutron star." Those whose rapid and consistent spin can be detected are called "pulsars." Neutron star matter is the densest form of matter known to exist. Theoretically, a piece of neutron star surface weighing as much as a fleet of battleships would be small enough to be held in the palm of your hand.

Pulsars are among the most intriguing objects in the sky. They were found in 1965 when radio astronomers discovered several objects that emitted radio waves with clock-like precision. The sources soon were identified as rapidly rotating neutron stars with intense magnetic fields. Where radio pulsars have the regularity of a atomic watch, accretion pulsars (i.e. - GRO J2058+42) are like cheap alarm clocks that easily gain and lose time - and go off when you least expect it. Radio pulsars typically have short periods, less than 10 seconds, and accretion-powered pulsars have longer periods, up to 10,000 seconds. Non- pulsing neutron stars may be old, dead pulsars, with ages of more than a million years, or they may never have been pulsars.

A leading theory holds that all pulsars start out fast, but accreting pulsars go through a blackout period when they are surrounded by material that blocks them from view. In the "propeller effect," the pulsar's magnetosphere bats the material away while the material drags on the magnetic field, slowing the star's rotation. Eventually, the star slows to the point that the magnetosphere can no longer bat the material away, and the star accretes enough to clear the field of view.

In addition to pulsations, they can emit occasional flashes of X-rays or bursts in two different classes. Type I bursts occur when accreted material accumulates on the star and becomes hot enough to trigger a thermonuclear instability. "Basically it's a big fusion bomb on the surface," Woods said. (These are not to be confused with more energetic and enigmatic gamma ray bursts from deeper in the universe.)

Type II bursts occur when the mass accretion rate is too high to allow this instability to arise. It is believed that GRO J1744-28 emits Type II bursts which are due to an uneven flow of material onto the star. "Instead of having a constant stream of matter, you have a big blob that falls in and causes the burst," he explained. The model was first proposed by Dr. John Cannizzo of NASA's Goddard Space Flight Center.

How did the bursting pulsar form? Neutron stars can be formed in at least two ways: from the collapse of an initially massive normal star (greater than 10 solar masses), called Type II supernova, or by the accretion-induced collapse of a white dwarf. Dr. Jan van Paradijs of the University of Alabama in Huntsville and the University of Amsterdam believes the latter applies to GRO J1744-28. Scientists know from the companion star which is donating material that this binary is old and a lot of material has been transferred between the two stars already.

As neutron stars accrete material, their magnetic field is slowly buried and gradually weakens. In the case of GRO J1744-28, the magnetic field is too strong to be this old. That leaves the option that the neutron star was only formed recently when a white dwarf accreted enough material to collapse under its own gravitational pull.

On Dec. 1, 1998, the original astrophysical "one-man band" sounded off again, this time for a encore that wasn't quite as long or loud as its debut. The band is a pulsar rotating around a low mass star, in a binary system called GRO J1774-28. (Such binaries are also called low-mass X-ray binaries.)

"It was originally discovered by the Burst and Transient Source Experiment (BATSE) in December 1995," said Peter Woods, a graduate student working at NASA's Marshall Space Flight Center. Woods is using data from BATSE which rides aboard the Compton Gamma Ray Observatory (CGRO). "It burst and pulsed for five months during which the source released about 40 bursts per day. Then it became inactive again for seven months. In December 1996, it started up for the second time, lasting 4 months but only becoming half as bright as before. During these two stints of activity, the source released about 10,000 bursts!" It's been quiet since then.

"What's unique about this object is that it does so many different things all at once," said Dr. Fred Lamb, an astrophysicist at the University of Illinois at Urbana-Champaign. He gave J1744-28 its nickname in 1996. "We've seen some sources that play the drums, some that crash cymbals, and a few that play the trumpet, but this source is a one-man band."

Right: Seven frames from a NASA animation depict GRO J1744-28 tuning up and sounding off as it is orbited by its low-mass companion (in the distance). Click to get a 240x1260-pixel, 40KB copy.

Left: A sky map showing the approximate location of GRO J1744-28. The pulsar is located between the familiar constellations Sagittarius and Scorpius. Sagittarius is just visible above the south-western horizon as the sun sets in early December. The yellow dot shows the location of Venus on December 1st, 1998. GRO J1744-28 is invisible to the naked eye.

Right: Neutron stars are usually formed in violent supernova explosions, but GRO J1744-28 may have been born in the collapse of a white dwarf. For more information about neutron stars and pulsars see our pulsar tutorial (java required)
Right: A typical tune - a burst profile - from GRO J1744-28 as seen by the BATSE. It rises from count of near zero to a peak of almost 1,900 counts per second, then fades almost to zero again. Through it all, the period is clearly visible as a series of spikes. Click here for a 785x605-pixel, 48KB JPG.

Pulsar GRO J2058+42, was discovered in 1995 by Dr. Colleen Wilson-Hodge of NASA's Marshall Space Flight Center. She found the pulsar's signature was buried in data from the Burst and Transient Source Experiment (BATSE), which she was mining with the hope of finding discrete, regular sources. During a giant outburst period lasting the 46 days after the pulsar's discovery, the pulsar's rotation accelerated from 198 to 196 seconds per rotation. GRO J2058+42 was 2 seconds faster - a phenomenal increase.

GRO J2058+42 was dubbed the 2-in-1 pulsar in 1998 because it appeared to burst twice on each orbit, instead of once like others of its kind, apparently as it orbited through an extended accretion disk of gas spun out from its companion star. This pulsar is an accretion-powered pulsar, one in which the neutron star's emissions come from matter, probably emitted by a visible-light companion, making a irrecoverable descent to the pulsar's surface.

While freshly hammered iron will give a dull infrared glow, an accretion pulsar will shine in X-rays and gamma rays - but only when its surface experiences a rain of matter from space. "For the first nine outbursts we were seeing this odd-even pattern," Wilson-Hodge explained.

The first outburst observed was about 10 times brighter than all subsequent bursts. Subsequent to that event, the pattern was that odd-numbered outbursts were about 1.5 to 2 times brighter than the even-numbered outbursts. That supported the theory that the pulsar was orbiting through an excretion disk, most likely spewed from the equator of a rapidly rotating type Be star into space. A type-Be star is a hot blue-white star with emission lines from glowing gas it has blown off.

J2058+42's location has some uncertainty, and the companion has not been identified. The strong odd-numbered bursts could be in close to the companion, where the disk would be thicker, and the even-numbered bursts would be farther out, where the disk is thinner.

Wilson made her discovery with the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory, and built on it with additional observations by the Rossi X-ray Timing Explorer. This is one of 12 known transient accreting X-ray pulsars with no visible companion. Since the April 1991 launch of BATSE, a NASA/Marshall instrument, astrophysicists have detected 22 of 70 known pulsars (as of February 1999) and discovered another 6 new X-ray pulsars. These accreting pulsars are NOT the same as the mysterious gamma-ray bursts that appear to be coming from the edge of the universe. They are energetic enough to be recorded by BATSE, but don't always get noticed right away.

Wilson reviewed BATSE data in September 1995, and found a burst that registered 140 milliCrabs - 140/1,000ths the brightness of the Crab Nebula. Using a computer to fold the data on itself, Wilson found that the source repeated every 198 seconds, an indication of a massive, compact object spinning at high speed. Further data established a regular strong-faint outburst pattern.

Then the outbursts of GRO J2058+42 became more irregular and weaker," she explained, "and the pattern seems to go away or reversed itself. Now we're back to not being able to explain this pattern." Part of the uncertainty stems from the fact that GRO J2058+42 is so faint that it's near the limit of what BATSE can detect. On a few cycles, BATSE saw only the odd-numbered (bright) outbursts and none of the even-numbered (faint) ones.

It has been theorized that the rotational increase may be due to GRO J2058+42 swallowing more of the matter streaming off of its companion star, or it could be an observational effect from the pulsar's motion. A larger question is why it has such a long rotational period when the great majority of pulsars have periods ranging from less than 10 seconds to as fast as a few milliseconds.

"The change in the period during the first outburst was most likely due to matter being dumped from the companion star," Wilson-Hodge explained. "In the weaker outbursts, where the period change is smaller, the observed period decrease may be, produced by the neutron star moving toward us in its orbit or it may be due to matter being dumped from the companion."

We now see an older, slower pulsar. What the outbursts actually show is a hot spot, probably a magnetic pole, offset from the pulsar's north or south geographic pole. Stellar material is funneled along the pulsar's strong magnetic field lines, somewhat like Earth's own magnetic field directing materials to the polar cusps and forming the aurora borealis.The change in the outbursts, including the flip in the odd-even pattern, is leading Wilson-Hodge to reconsider her initial belief that the pulsar has a 110-day orbital period and instead has a 55-day period.

Then why would the Be companion or the excretion disk, neither with a link to the pulsar, act in apparent concert with the pulsar?"That's a very good question," Wilson-Hodge replied. "It may be in a disk with an instability, or in the material coming off the Be companion. Why it should seem to be two times the orbital period is really hard to explain. Maybe it will have the courtesy to brighten up again," she said. "That would help a lot."

Scientists have announced the discovery of a superdense star spinning at more than 60 times per second, and calculate it could have been spinning as fast as 150 times per second or more when it formed some 4,000 years ago. Most astronomers had not previously believed this class of star, called a pulsar, could form with such a rapid spin. "This shatters the glass ceiling," said astrophysicist John Middleditch of the U.S. Department of Energy's Los Alamos National Laboratory in New Mexico. "This is the fastest high-energy pulsar of its type we know about."

"The pulsar is spinning twice as fast as any young pulsar that we have seen before," adds Dr. Frank Marshall of NASA's Goddard Space Flight Center, Greenbelt, MD, who led the team making the discovery. "To put it in perspective, this pulsar is spinning more than 6 million times as rapidly as the Earth."^76c * The newly discovered pulsar establishes a link between fast-spinning pulsars with relatively weak magnetic fields and slow-spinning ones with strong fields. There may be a natural continuum between the two known types.

The pulsar was found by Dr. Marshall and his colleagues Drs. William Zhang and Eric Gotthelf of Goddard, and Middleditch, by examining X-ray emissions recorded by NASA's Rossi X-ray Timing Explorer spacecraft in 1996, and confirmed with observations using the joint Japanese/U.S. Advanced Satellite for Cosmology and Astrophysics (ASCA) spacecraft.

The team identified the pulsar as most likely being associated with the remnant of a supernova, catalogued as N157B by astronomers, that exploded in the Large Magellanic Cloud about 4,000 years ago. The age estimate comes from other X-ray and visible observations of the spreading, tattered gas cloud from the supernova blast and is in agreement with that predicted by theoretical models. Data from both the Rossi and ASCA satellites were used to calculate the rate at which the pulsar's spin is slowing, which in turn provides an estimate of its age: 5,000 years old, a close match to the age estimate for the supernova remnant.

The other well-known high energy pulsar, in the Crab Nebula, spins just under 30 times per second, and is generally thought to have been spinning at only 60 times a second at its birth in 1054 AD. Since the Crab pulsar's discovery in 1968, astronomers have spotted pulsars spinning as fast as hundreds of times per second. These so-called "millisecond pulsars" have magnetic fields a thousand times weaker than the Crab pulsar.

Most astronomers believe that the weak-field, millisecond pulsars were born with a slow spin and were "spun up" after sucking in gaseous material from an orbiting stellar companion, but astronomers have not located enough suitable binary star systems to account for the large numbers of millisecond pulsars being discovered.

The pulsar found in N157B, whose magnetic field is only a few times weaker than the Crab pulsar's, suggests an evolutionary link between the strong-field, slower-spinning energetic pulsars and the weak-field millisecond pulsars. Its discovery confirms a prediction published by Gotthelf and Dr. Q. Daniel Wang of Northwestern University. "This is a fantastic confirmation of our hypotheses; that the central source of X-ray light from N157B is a fast pulsar associated with a supernova remnant, like that seen in the Crab nebula," commented Gotthelf. "Now, clearly, it seems that the weaker the magnetic field, the faster the pulsar will spin at birth -- possibly all the way down to one- or two-millisecond periods (corresponding to spin rates of 1,000 to 500 times per second) for fields of the strength measured for the weak-field pulsars," Middleditch said. Marshall and his team encouraged other researchers to study N157B at other regions of the spectrum to see if its pulsations are observable there, too.

Astronomers continue to search for a pulsar at the heart of SN1987A, a supernova that appeared in the southern skies Feb. 23, 1987. Most astronomers who study this supernova expect that a rapidly spinning, weak-field pulsar should eventually reveal itself for observation, which would provide another link in theories of how fast pulsars are born.

Astronomers using NASA's Hubble Space Telescope have taken their first direct look, in visible light, at a lone neutron star. This offers a unique opportunity to pinpoint its size and to narrow theories about the composition and structure of this bizarre class of gravitationally collapsed, burned-out stars. By successfully characterizing the properties of an isolated neutron star, astrophysicists have an opportunity to better understand the transitions matter undergoes when subjected to the extraordinary pressures and temperature found in the intense gravitational field of a neutron star.

The Hubble results show the star is very hot, and can be no larger than 16.8 miles (28 kilometers) across. These results prove that the object must be a neutron star, for no other known type of object can be this hot and small.^76b "This puts the neutron star uncomfortably close to the theoretical limit of how small a neutron star should be," says Fred Walter of the State University of New York (SUNY) at Stony Brook. "With this observation we can begin to rule out some of the many models of the internal structure of neutron stars." The observation results, made by Walter and Lynn Matthews (also of SUNY), are reported in the Sept. 25 issue of Nature magazine.

The Hubble observations, combined with earlier data, promise to help astronomers refine the mathematical description -- called the equation of state -- of the complex transformations matter undergoes at extraordinary densities not found on Earth. Equations of state are well understood for "everyday" matter such as water, which can transition between gaseous, liquid and solid states. The behavior of matter under extreme temperature and pressure found on a neutron star is not well understood.

Several hundred million neutron stars should exist in our galaxy. All neutron stars now known have either been found orbiting other stars in X-ray binary systems or emitting machine-gun blasts of radio energy as pulsars (a class of neutron star). The neutron star seen by Hubble is not a member of a binary system, and is not known to pulse at X-ray or radio wavelengths (it has not been detected as a radio source).

Only a few lone neutron star candidates have been pinpointed through X- ray observations, and this is the first optical counterpart to be identified. The first clue that there was a neutron star at this location came in 1992, when the ROSAT (the Roentgen Satellite) found a bright X-ray source without any optical counterpart in optical sky surveys. It drew the attention of astronomers because objects this hot and bright, without counterparts at other wavelengths, are extremely rare.

Hubble's Wide Field Planetary Camera 2 was used in October 1996 to undertake a sensitive search for the optical object, and found a stellar pinpoint of light within only 2 arc seconds (1/900th the diameter of the Moon) of the X-ray position. Astronomers haven't directly measured the neutron star's distance but fortunately the neutron star lies in front of a molecular cloud known to be about 400 light-years away in the southern constellation Coronae Australis. Using the distance to the cloud as an upper limit, the astronomers calculated a diameter by next comparing the neutron star's brightness and color as measured by Hubble, along with X- ray brightness from the ROSAT and EUVE (Extreme Ultraviolet Explorer) satellites.

This object is brightest at X-ray wavelengths. In the two Hubble images, the object is brighter at ultraviolet wavelengths than at visible wavelengths. They concluded they are directly seeing an ultracompact surface sizzling at about 1.2 million degrees Fahrenheit. To be so hot, yet so dim (below 25th magnitude in visual light) and relatively close to Earth, the object must be extremely small - below the size of a white dwarf, a more common stellar cinder.

A hot white dwarf at this magnitude would lie 150,000 light-years away (outside our galaxy), and have 1/70,000 as much X-ray emission. The 16.8-mile diameter estimate comes from assuming the neutron star is at the farthest it can be, just in front of the obscuring "wall" of the molecular cloud. If instead the neutron star is significantly closer to us, say midway to the molecular cloud, it would be smaller still, and present an even bigger challenge to the theories of the equation of state of nuclear matter.

Although neutron stars in binary systems allow astronomers to measure their mass, which turn out to be consistent with theory, it's much harder for astronomers to estimate the diameter of the neutron stars. Since the neutron stars "feed" on their companion stars in these systems, the light does not come exclusively from the surface but from jets, disks and other phenomenon that occur around the star. This can lead to inaccurate size estimates.

Astronomers predict that the gaseous shells left over from supernova explosions should hold rapidly-spinning radio pulsars, but few such stars have actually been observed within these nebulae. Astronomers from Columbia University and the California Institute of Technology propose an explanation: The expected pulsars do exist, but they are slowly spinning neutron stars invisible to radio probes and have magnetic fields a quadrillion times denser that that of our Sun -- so-called "magnetars."

Radio pulsars, the first observed in the Crab nebula, are believed to be neutron stars that spin at velocities of up to 600 revolutions a second, sending a beacon of radio waves whirling across the heavens. The new observation of slower X-ray pulsars confirms the existence of slower-spinning neutron stars that are believed to have huge magnetic fields that would sweep matter along in their wake. This may account for their slower rotational velocities.^76a

The new work overturns a 30-year-old hypothesis that implies that most supernova remnants should hold a rapidly-rotating radio pulsar. In addition, this new revelation offers new insights into the formation of neutron stars, and also confirms that a slowly-rotating pulsar in the Kes 73 supernova remnant - first announced by the National Aeronautics and Space Administration in October 1997 - which is presumed to be a magnetar, which would make it the first one ever observed.

Astronomers have long held that young neutron stars, born in the cataclysmic explosion of supernovae, should be formed rapidly spinning the most famous being the radio pulsar in the Crab nebula. Young pulsars are expected to be visible across the spectrum of electromagnetic radiation, from radio waves to the highest-energy gamma rays. Searches for radio emissions from pulsars near young remnants of supernovae have failed to confirm this prediction.

New observations by Eric Gotthelf of Columbia and Gautam Vasisht of Caltech provide evidence that most neutron stars are born and evolve in a manner very different than that of the Crab pulsar. The "poster" they will exhibit is available at http://www.astro.columbia.edu/~evg/newpulsars.html. An earlier report of this research was presented to the Italian Astronomical Society meeting last year in Elba, Italy, for which a reprint is available at http://xxx.lanl.gov/abs/astro-ph/9809139.

Dr. Gotthelf and his colleagues have discovered several new neutron star candidates in supernova remnants that show these pulsars are far more numerous than previously believed. He presents a compilation of the latest results from these and other radio-quiet, slow X-ray pulsars, and suggests that these objects are likely the "missing" pulsars in supernova remnants. "The surprise is that the rapidly rotating pulsars are in fact the rare examples of young neutron stars, not the typical example, as was thought by most researchers until now," Dr. Gotthelf said. As of this time (18 November 1999 - AFTER the 5th Gamma Ray Burst Symposium in Huntsville, AL) confirming evidence hasn't yet been assembled for all the candidates.

Dr. Gotthelf, Dr. Vasisht and others believe these objects can be distinguished from their radio-bright cousins by their enormous magnetic fields, about 1000 times stronger than expected. With such intense magnetic fields, these pulsars must have evolved very differently from young radio pulsars. "If these pulsars are truly isolated pulsars in the hearts of supernova remnants, as the evidence suggests, then their inferred magnetic fields are enormous, unlike anything known on Earth or elsewhere in the heavens," Dr. Gotthelf said. "The strength would be on the order of 10¹⁵ times stronger than our Sun's magnetic field."

The Crab nebula is the remains of a supernova explosion recorded by Chinese astronomers in 1054. In 1968, David H. Staelin and E.C. Reifenstein of Massachusetts Institute of Technology, using radio telescopes, discovered that the Crab nebula contained a rapidly-rotating central star. The Columbia and Caltech scientists have found that most young pulsars in supernova remnants are, in fact, not powerful radio beacons, as is the Crab pulsar. In addition, they are seen only by the light of the X-rays they give off, and in no other light. These so-called anomalous pulsars (AXPs) are found to be rotating 1000 times slower then the Crab pulsar, and slowing down 100 times faster. "If these pulsars started out rotating as fast as a normal pulsar, it would require more time than the universe has existed for it to slow down to its present rate," Dr. Vasisht said.

The existence of highly magnetized neutron stars was postulated in 1992 by Robert Duncan of the University of Texas at Austin and Christopher Thompson of the University of North Carolina at Chapel Hill. It is believed that the repetitious burst of gamma-rays from so-called soft gamma-ray repeaters, first observed in 1986, could be explained if such stars had huge magnetic fields, and so they were dubbed "magnetars."

The first compelling evidence for a magnetar was provided by the discovery in 1997 of a slow pulsar in the supernova remnant Kes 73 by Drs. Vasisht and Gotthelf. A NASA press release (97-131) is available at the URL ftp://pao.gsfc.nasa.gov/pub/pao/releases/1997/97-131.htm.

This pulsar has the slowest known period of any isolated pulsar. Since then, several new slow X-ray pulsars have been discovered in supernova remnants. The new observations of the soft gamma-ray repeaters, along with the discovery of a new one, show that they, too, are slow pulsars associated with supernova remnants. As of this time, researchers suggest that soft gamma-ray repeaters and other slow X-ray pulsars are related by their slow periods and comparable spectra. "We have now discovered several new slow young pulsars at the center of supernova remnants," Dr. Gotthelf said. "Along with other recent discoveries, we are able to show that these slow pulsars now outnumber the observed census of Crab-like pulsars." Of the 300 or so visible supernova remnants, the latest tally finds seven with slow pulsars, while there are only four confirmed Crab-like pulsars.

Victoria M. Kaspi, assistant professor of physics at MIT, has presented evidence that most supernova remnants do not contain rapidly spinning radio pulsars, as had been assumed, but that the few that are seen can be explained by chance alignment. (See related press release available from Deborah Halber, dhalber@mit.edu ).

The Columbia-Caltech results were obtained using pictures from the Advanced Satellite for Cosmology and Astrophysics, or ASCA. The satellite allows imaging of astronomical sources of X-rays that are more energetic than those previous X-ray satellites could see. Drs. Vasisht and Gotthelf were able to find several new slow pulsars invisible to radio searches. At least two more sensitive X-ray satellites are to be launched in the next few years, and Dr. Gotthelf believes they will find many more radio-quiet pulsars.

In mid-1999, NASA is due to launch the Advanced X-ray Astrophysics Facility, AXAF-I, named Chandra. In early 2000, the European Space Agency will send aloft the High-Throughput X-Ray Spectroscopy Mission (XMM). Both will allow imaging and spectroscopy of a wide range of X-rays from a variety of cosmic sources.

Recent comprehensive radio surveys suggest that most radio pulsars near supernova remnants (SNRs) can be attributed to chance overlap (e.g. Lorimer et al. 1998; Gaensler & Johnston 1995; see Kaspi et al. 1996 for a review). Traditional arguments for the lack of observed radio pulsars associated with SNRs, such as those invoking beaming and large ``kick'' velocities, are less compelling.^71b Evidence is accumulating that many young neutron stars (NSs) are slowly rotating (P ~ 10 s) X-ray pulsars, lacking in detectable radio emission.

There are currently about a dozen slow X-ray pulsars apparently associated with young SNRs. These include the four known soft Gamma-ray repeaters (SGR), which have recently been confirmed as slow rotators. This means that there are now more known slow, radio-quiet X-ray pulsars in the center of identified SNRs than confirmed Crab-like radio pulsars.

The observational properties of these radio-quiet NS candidates associated with supernova remnants suggests that alternative evolutionary-paths exist for young pulsars. Neutron stars are thought to be born as rapidly rotating (~ 10 ms) radio pulsars created during a Type II/Ib supernova. Their existence was postulated in 1934 by Baade & Zwicky (1934) based on theoretical arguments, but awaited the 1970s for observational support, provided by the remarkable discoveries of the Crab and Vela pulsars in their respective supernova remnants (SNRs).

Most supernovae (non-Type Ia) are expected to produce a NS, whose unpulsed emission should be easily discernible in the radio-band during the lifetime of a typical SNR (> 10,000 yrs) as a radio-loud ``plerion'' (Weiler & Sramek 1988). Despite detailed radio searches, few of the hundreds of known SNRs have yielded a NS candidate. Most radio pulsars near SNRs are now considered to be consistent with chance superpositions.

The properties of the Crab and Vela pulsars were found to be uniquely explained in the context of rapidly rotating, magnetized neutron stars emitting beamed non-thermal radiation. Their fast rotation rates and large magnetic fields (~ 10¹² Gauss) are consistent with those of a main-sequence star collapsed to NS dimension and density. A fast period essentially precluded all but a NS hypothesis and thus provided direct evidence for the reality of NSs (see Shapiro & Teukolsky 1983 for a brief history and intro to NS physics). Their inferred age and association with SNRs provided strong evidence that NSs are indeed born in supernova explosions.

The "Missing" Young Neutron Star Problem
(excerpted from Dr. Eric Gotthelfs' "Radio-quiet X-ray pulsars in Supernova Remnants and the "Missing" Pulsar," )

* Most of the new slow pulsar candidates where first detected with ASCA, The Advance Satellite for Cosmology and Astrophysics, which has a spectro-imaging capability in the 0.5-10 keV energy range allows us to locate X-ray pulsars previously "hidden" to earlier X-ray missions.

Recent X-ray observations of known SNRs have revealed several X-ray bright, but radio-quiet compact objects at their centers. Some of these sources have been found to be slowly rotating pulsars with unique properties. Their temporal signal is characterized by spin periods in the range of 5 - 12 s, steady spin-down rates, and highly modulated sinusoidal pulse profiles (~ 30%).

The slow X-ray pulsars likely represent a population of ultra-magnetized neutron stars, or ``magnetars'' (Thompson & Duncan 1993). It is theorized that the luminosity of this object is powered by magnetic field decay instead of spin down energy. In addition, the magnetic field may be the critical parameter which governs the evolution of young NSs. These pulsars were likely born as fast rotators (P ~ 10 ms), but spun down rapidly due to their enormous magnetic fields.

The three classes of pulsars are distinguished by their periods and derived age and magnetic fields. The inferred magnetic field for the AXPs and SGRs are well above the quantum critical field B_qed = 4.4 X 10¹³ Gauss. AXP 1841-045 in Kes 73 (see poster) and the recently discovered SGR 1900+14 (and perhaps SGR 1806-20) are likely magnetar candidates, whose luminosity is dominated by magnetic field decay.

The driving force behind that magnetic field is at ground zero in the Crab. It has been theorized that there is a neutron star about 20 km wide but as massive as our Sun, and spinning at 33 rpm located somewhere within the Crab. Its magnetic field was another remnant that was frozen in when the star that became the Crab imploded. The ultimate power source is anticipated to be a spinning magnetized neutron star too small to be seen even by Chandra. Both rotate at about the same speed, but the Crab clearly is the heavyweight of the two - it is 10,000 times bigger than any manmade dynamo, has a magnetic field 1 billion times stronger, produces voltages a million billion times higher, and a power output 100 million billion times greater.

It is not visible to the naked eye because 99 percent of the energy is shed in ways that are not fully understood. These mechanisms may be a) through the star's own magnetic field, b) through electrons and positrons (anti-electrons) somehow accelerated to speeds that are spectacularly close to the speed of light, and c) through ions lifted from the surface of the neutron star and also accelerated to near the speed of light. What proportion goes where remains an open question - as does the process behind the nearly 100 percent efficient electron and positron acceleration.

The strength and regularity of the source are such that the Crab is used as a "standard candle" for calibrating X-ray telescopes including Chandra. The Crab turned out to be pulsing in visible light and X-rays as well.

The Crab Nebula - spectacular remains of a stellar explosion - reveals something never seen before: a brilliant ring around the nebula's heart. Combined with observations from the Hubble Space Telescope, the images provides important clues to the puzzle of how a pulsing neutron star energizes the nebula, which still glows brightly almost 1,000 years after the explosion.

"The inner ring is unique," said Professor Jeff Hester of Arizona State University, Tempe, AZ. "It has never been seen before, and it should tell us a lot about how the energy from the pulsar gets into the nebula. It's like finding the transmission lines between the power plant and the light bulb."^11a

Professor Mal Ruderman of Columbia University, New York, N.Y., agreed. "The X-rays Chandra sees are the best tracer of where the energy is - we can directly diagnose what is going on." What is going on, according to Dr. Martin Weisskopf, Chandra Project Scientist from NASA's Marshall Space Flight Center, Huntsville, AL., is that the Crab pulsar is accelerating particles up to the speed of light and flinging them out into interstellar space at an incredible rate. Hubble Space Telescope images have shown moving knots and wisps around the neutron star. Previous X-ray images have shown the outer parts of the jet and hinted at the ring structure. Now, the jet can be traced all the way in to the neutron star, and the ring pattern clearly appears.

The image, made with Chandra's Advanced Charge-Coupled Device Imaging Spectrometer and High Energy Transmission Grating, shows tilted rings or waves of high-energy particles that appear to have been flung outward over the distance of a light year from the central star, and high-energy jets of particles blasting away from the neutron star in a direction perpendicular to the spiral.

The Crab Nebula is easily the most intensively studied object beyond our solar system. It is the remnant of a star that was observed to explode in 1054 A.D. - a "guest star" that appeared suddenly and remained visible for weeks, even during daytime. From gamma-ray telescopes to radio telescopes, the Crab has been observed using virtually every astronomical instrument that could see that part of the sky. The Crab convincingly tied the origin of enigmatic "pulsars" to the stellar cataclysms known as supernovae.

Observations of the expanding cloud of filaments in the Crab were instrumental in confirming the cosmic origin of the chemical elements from which planets (and people) are made. The nebula is located 6,000 light years from Earth in the constellation Taurus.

The Crab pulsar, which was discovered by radio astronomers in 1968, is a neutron star rotating 30 times per second. Neutron stars are formed in the seconds before a supernova explosion when gravity crushes the central core of the star to densities 50 trillion times that of lead and a diameter of only 12 miles. Another consequence of the dramatic collapse is that neutron stars are rapidly rotating and highly magnetized. The rotating magnetar generates 10 quadrillion volts of electricity, 30 million times that of a typical lightning bolt. "It is an incredibly efficient generator," Ruderman explained. "More than 95 percent efficient. There's nothing like it on Earth."

12,000 light years away in the contellation Sagittarius, an extremely rapidly spinning neutron star is providing a long-sought "missing link" in our understanding of the evolution of stars. This star, spotted by NASA's Rossi X-ray Timing Explorer (RXTE) satellite during a month-long outburst of X-rays that began in April, could be a millisecond radar pulsar in the making. It provides proof for the theory that these unusual neutron stars are propelled to mind-boggling speeds by the force of material spiralling onto their surfaces from a companion star.^51a

Pulsars are rotating, magnetized neutron stars that often emit radio waves, whose surface is moving at almost one-fifth the speed of light. The first millisecond radio pulsar was discovered more than 15 years ago. Scientists have sought to prove that these stars are "spun up" by matter spiraling down onto them from a binary companion, a normal adjacent star, according to postdoctoral fellow Deepto Chakrabarty and research scientist Edward H. Morgan, both at MIT's Center for Space Research.

Every 2.5 seconds, the new star found by the RXTE emitted clocklike X-ray pulses, thought to be caused by a "hot spot" on the spinning neutron star's surface from the impact of the companion star's matter. Chakrabarty and Morgan noticed that these pulsations underwent periodic Doppler shifts that mirrored the neutron star's revolving dance with its companion. They deduced that this new millisecond X-ray pulsar is part of a highly compact binary system with an orbital period of just two hours and a companion star that is much less massive than our sun.

The two stars are so close together, that mass is lost from the low-mass companion onto the pulsar. This is generating X-rays from the monstrous heat of the impacts, and "spinning up" the pulsar through the "accretion" of angular momentum, like a top given an extra twirl.

* Chakrabarty and Morgan believe that this low-mass X-ray binary, as it is known, may be related to "black widow" radio pulsars. These objects accumulate matter from their companions while gradually evaporating away these companions through intense radiation. This newly discovered star, designated SAX J1808.4-3658, is the first clearly emitting the regular X-ray pulsations characteristic of a rapidly spinning star accumulating material from its companion. It is considered "a dramatic vindication of the basic model theorists had put forward for millisecond pulsars." said Chakrabarty "This fills an important niche in the history of stars." said MIT Physics Professor Hale Bradt, who helped design instruments for the satellite named for Bruno B. Rossi, an MIT pioneer in the field of X-ray astronomy.

If radio pulsations commence from SAX J1808.4-3658 now that the transient X-ray outburst has ended, this may be the first millisecond radar pulsar to provide conclusive proof for the theory of how these unusual neutron stars are propelled to mind-boggling speeds.

Victoria Kaspi is leading an international team that is searching for radio pulsations from SAX J1808.4-3658 using the Parkes radio telescope in Australia. The discovery of an accretion-powered millisecond X-ray pulsar is powerful evidence in favor of the theory.

?? -- why is it pulsing while none of the more than 30 other known low-mass X-ray binary systems have detectable pulsations?

The main focus of this research is to understand the nature of radio-quiet X-ray pulsars, to explore possible alternative evolutionary paths for NSs, which might help resolve the mystery of the missing NSs and further our understanding of NS physics.

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The "Missing" Young Neutron Star Problem (excerpted from Dr. Eric Gotthelfs' "Radio-quiet X-ray pulsars in Supernova Remnants and the "Missing" Pulsar," )

The "Missing" Young Neutron Star Problem
(excerpted from Dr. Eric Gotthelfs' "Radio-quiet X-ray pulsars in Supernova Remnants and the "Missing" Pulsar," )