Earthdate 2004-04-23's issue of the journal Science (typically requires subscription or pay-per-view) has a special section on pulsars, which is chock full of interesting information.  There are eight parts to the series, but we'll only consider three of those articles here — scroll down or click on the following index:

Magnetars and Pulsars
→  The Pulsar Menagerie
→  The Physics of Neutron Stars
→  Crushed by Magnetism
→  References

Labels: astronomy, pulsars

Scores of extrasolar planetary bodies have been discovered over the past dozen years or so, but it somehow had escaped my notice anyway that the very first such planetary system beyond our own to be discovered orbits round a pulsar.  As Robert Irion points out in his piece “The Pulsar Menagerie” in Science's pulsar special: ¹  “Indeed, more than 100 other planets are now known, although PSR B1257+12 is still the only burned-out corpse of a dead star known to have a planetary system.  […]  The masses and relative positions of the three planets are ‘shockingly similar to our inner solar system.’”  Shades of Arthur C. Clarke's most poignant short story “The Star”!

Impearls has featured several articles recently discussing the testing of Einstein's general relativity (here, here, and here), and it's worth mentioning in this context the pair of neutron stars (one pulsing, one not) orbiting round each other known as the “Hulse-Taylor binary” (PSR B1913+16), discovered in 1974.  “[T]he team showed that the two bodies inexorably spiral together, at exactly the rate predicted by Einstein 60 years earlier.  Gravitational waves carry away the lost orbital energy.  ‘It's indirect, like showing that radio waves exist because you know the radio transmitter uses power,’ Hulse says.  ‘But it was the first evidence for the existence of gravitational waves.’”

Just earlier this year came the sequel:

The latest stunner was anticipated for years: two pulsars deadlocked in a tight orbit.  The new system, detected by the Parkes Radio Telescope in Australia and announced in January, will likely provide even more stringent tests of general relativity than the Hulse-Taylor binary (Science, 9 January, p. 153).

Already, astrophysicists are mystified by the energetic interplay between the neutron stars.  Intense winds from the faster rotating pulsar create a tear-shaped shock wave around the slower pulsar.  Teams are probing this process as one pulsar dips behind the other, every 2.4 hours.

In one interpretation of the data, the fast pulsar is churning out 100,000 to 1 million times more charged gas than expected from the seething region above its surface, says theorist Jonathan Arons of UC Berkeley.  “The physics is not quite incredible, but it's close,” he says.

Labels: astronomy, pulsars

Continuing review of the Science series on pulsars, in the article “The Physics of Neutron Stars,” J. M. Lattimer and M. Prakash reveal some of the mind-boggling physics employed by neutron stars (the physical core of pulsars). ²

Neutron stars may exhibit conditions and phenomena not observed elsewhere, such as hyperon-dominated matter, deconfined quark matter, superfluidity and superconductivity with critical temperatures near 10¹⁰ kelvin, opaqueness to neutrinos, and magnetic fields in excess of 10¹³ Gauss.  […]

The term “neutron star” as generally used today refers to a star with a mass M on the order of 1.5 solar masses (M_[Sun]), a radius R of ∼12 km, and a central density n_c as high as 5 to 10 times the nuclear equilibrium density n₀ ≅ 0.16 fm⁻³ of neutrons and protons found in laboratory nuclei.  A neutron star is thus one of the densest forms of matter in the observable universe.  Although neutrons dominate the nucleonic component of neutron stars, some protons (and enough electrons and muons to neutralize the matter) exist.  At supranuclear densities, exotica such as strangeness-bearing baryons, condensed mesons (pion or kaon), or even deconfined quarks may appear.  Fermions, whether in the form of hadrons or deconfined quarks, are expected to also exhibit superfluidity and/or superconductivity.

Neutron stars encompass “normal” stars, with hadronic matter exteriors in which the surface pressure and baryon density vanish (the interior may contain any or a combination of exotic particles permitted by the physics of strong interactions), and “strange quark matter” (SQM) stars.  An SQM star could have either a bare quark-matter surface with vanishing pressure but a large, supranuclear density, or a thin layer of normal matter supported by Coulomb forces above the quark surface.  The name “SQM star” originates from the conjecture that quark matter with up, down, and strange quarks (the charm, bottom, and top quarks are too massive to appear inside neutron stars) might have a greater binding energy per baryon at zero pressure than iron nuclei have.  If true, such matter is the ultimate ground state of matter.  Normal matter is then metastable, and compressed to sufficiently high density, it would spontaneously convert to deconfined quark matter.  Unlike normal stars, SQM stars are self-bound, not requiring gravity to hold them together.  It is generally assumed that pulsars and other observed neutron stars are normal neutron stars.  If SQM stars have a bare quark surface, calculations suggest that photon emission from SQM stars occurs primarily in the energy range 30 keV < E < 500 keV.

After a discussion of how neutron stars form (fascinating, with an illuminating diagram), Lattimer and Prakash go on to discuss a proto-neutron star's possible collapse into a black hole.

The proto-neutron star, in some cases, might not survive its early evolution, collapsing instead into a black hole.  This could occur in two different ways.  First, proto-neutron stars accrete mass that has fallen through the shock.  This accretion terminates when the shock lifts off, but not before the star's mass has exceeded its maximum mass.  It would then collapse and its neutrino signal would abruptly cease.  If this does not occur, a second mode of black hole creation is possible.  A proto-neutron star's maximum mass is enhanced relative to a cold star by its extra leptons and thermal energy.  Therefore, following accretion, the proto-neutron star could have a mass below its maximum mass, but still greater than that of a cold star.  If so, collapse to a black hole would occur on a diffusion time of 10 to 20 s, longer than in the first case.  Perhaps such a scenario could explain the enigma of SN 1987A.  The 10-s duration of the neutrino signal confirmed the birth and early survival of a proto-neutron star, yet there is no evidence that a neutron star exists in this supernova's remnant.  The remnant's observed luminosity is fully accounted for by radioactivity in the ejected matter, meaning that any contribution from magnetic dipole radiation, expected from a rotating magnetized neutron star, is very small.  Either there is presently no neutron star, or its spin rate or magnetic field is substantially smaller than those of typical pulsars.  A delayed collapse scenario could account for these observations.

Lattimer and Prakash proceed to discuss neutron stars' internal structure and composition (also including an illuminating diagram).

A neutron star has five major regions: the inner and outer cores, the crust, the envelope, and the atmosphere.  The atmosphere and envelope contain a negligible amount of mass, but the atmosphere plays an important role in shaping the emergent photon spectrum, and the envelope crucially influences the transport and release of thermal energy from the star's surface.  The crust, extending about 1 to 2 km below the surface, primarily contains nuclei.  The dominant nuclei in the crust vary with density, and range from ⁵⁶Fe for matter with densities less than about 10⁶ g cm⁻³ to nuclei with A ∼ 200 but x ∼ (0.1 to 0.2) near the core-crust interface at n ≈ n₀⁄3.  Such extremely neutron-rich nuclei are not observed in the laboratory, but rare-isotope accelerators hope to create some of them.

Within the crust, at densities above the neutron drip density 4 × 10¹¹ g cm⁻³ where the neutron chemical potential (the energy required to remove a neutron from the filled sea of degenerate fermions) is zero, neutrons leak out of nuclei.  At the highest densities in the crust, more of the matter resides in the neutron fluid than in nuclei.  At the core-crust interface, nuclei are so closely packed that they are almost touching.  At somewhat lower densities, the nuclear lattice can turn insideout and form a lattice of voids, which is eventually squeezed out at densities near n₀.  If so, beginning at about 0.1 n₀, there could be a continuous change of the dimensionality of matter from three-dimensional (3D) nuclei (meatballs), to 2D cylindrical nuclei (spaghetti), to 1D slabs of nuclei interlaid with planar voids (lasagna), to 2D cylindrical voids (ziti), to 3D voids (ravioli, or Swiss cheese) before an eventual transition to uniform nucleonic matter (sauce).  This series of transitions is known as the nuclear pasta.

For temperatures less than 0.1 MeV, the neutron fluid in the crust probably forms a ¹S₀ superfluid.  Such a superfluid would alter the specific heat and the neutrino emissivities of the crust, thereby affecting how neutron stars cool.  The superfluid would also form a reservoir of angular momentum that, being loosely coupled to the crust, could cause pulsar glitch phenomena.

The core constitutes up to 99% of the mass of the star.  The outer core consists of a soup of nucleons, electrons, and muons.  The neutrons could form a ³P₂ superfluid and the protons a ¹S₀ superconductor within the outer core.  In the inner core, exotic particles such as strangeness-bearing hyperons and/or Bose condensates (pions or kaons) may become abundant.  It is possible that a transition to a mixed phase of hadronic and deconfined quark matter develops, even if strange quark matter is not the ultimate ground state of matter.  Delineating the phase structure of dense cold quark matter has yielded novel states of matter, including color-superconducting phases with and without condensed mesons.

Lattimer and Prakash end their review with a discussion of future prospects in neutron star and pulsar physics.

A new generation of neutrino observatories also hold great potential for studies of proto-neutron star evolution and neutron star structure.  Neutrino observations of supernovae, validated by the serendipitous observations of SN 1987A, which yielded about 20 neutrinos, should detect thousands of neutrinos from a galactic supernova.  This could yield neutron star binding energies to a few percent accuracy and provide estimates of their masses, radii, and interior compositions, as well as details of neutrino opacities in dense matter.  Neutrino fluxes from proto-neutron stars with and without exotica (hyperons, Bose condensates, and quarks) have been investigated […].

Gravitational radiation is expected from asymmetric spinning compact objects, from mergers involving neutron stars and black holes, and from gravitational-collapse supernovae.  Depending on the internal viscous forces in rotating neutron stars, gravitational radiation could drive an instability in r-modes of nonradial pulsations to grow on a time scale of tens of seconds.  Mergers can be observed to great distances.  Detectors due to begin operation over the next decade, including LIGO (Laser Interferometer Gravitational-Wave Observatory), VIRGO (Italian-French Laser Interferometer Collaboration), GEO600 (British-German Cooperation for Gravity Wave Experiment), and TAMA (Japanese Interferometric Gravitational-Wave Project), could see up to hundreds of mergers per year.  Binary mergers can yield important information, including the masses and mass-to-radius ratios of the binary's components and possibly details of their inspiraling orbits.

Labels: astronomy, pulsars

Perhaps the oddest pulsar phenomena which Science's pulsar series delves into is that of those entities known as “magnetars.”  As Robert Irion writes in his piece “Crushed by Magnetism”: ³

Some of the strangest mutations in space create superenergetic but short-lived cousins of pulsars, called magnetars.  Like a pulsar, a magnetar is a neutron star forged at the center of a supernova when a massive star explodes.  But something odd happens during a magnetar's birth.  An unknown process — perhaps ultrafast rotation within the dying star's collapsing core — endows each magnetar with a crushing magnetic field.  This magnetism, up to 1000 times more intense than that of a typical pulsar, is the strongest known in space.

As the magnetic forces subside, they rupture the brittle crust of the neutron star and drive fierce bursts of gamma rays and x-rays.  But the pyrotechnics takes a toll.  The magnetism acts as a brake, grinding each magnetar to a near-halt within thousands of years and short-circuiting its spin power.  In contrast, an ordinary pulsar can sweep the galaxy with rotation-powered beams of radio waves for millions of years.

Astrophysicists have found just 11 magnetars, but their brief lives and sporadic tantrums point to a far larger population that we can't see.  “There probably are hundreds of thousands of these dead relics, undetected and undetectable, now spinning in our galaxy,” says x-ray astronomer Chryssa Kouveliotou of NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama.  Indeed, some proponents think the objects might not be mutants at all but common offspring of supernovas.  “It's quite possible that a majority of neutron stars are magnetars rather than radio pulsars,” says astrophysicist Robert Duncan of the University of Texas, Austin.

It was theoreticians who first came up with the concept of neutron stars possessed of magnetic fields of stupendous magnitude.

Duncan and fellow theorist Christopher Thompson of the Canadian Institute for Theoretical Astrophysics in Toronto, Ontario, have swayed skeptics before.  They first calculated that powerful magnetic fields could lace through newborn neutron stars in 1987, when Duncan was a postdoctoral researcher at Princeton University and Thompson was a graduate student.  But their solution for the strengths of such fields — 10¹⁵ gauss — was so startling that they weren't sure what to make of it for several years.

For perspective, Earth's global magnetic field is about 0.6 gauss.  Magnetic resonance imagers for medical scans attain 10,000 gauss.  Radio pulsars cluster around 10¹² gauss, a deduction based on magnetism's gradual braking effect on their spins.  Such fields are impressive, but a radio pulsar's main power comes from its rotation, not its magnetism.  The magnetic fields act as conveyor belts to carry radiation spawned as the neutron star slows down and sheds rotational energy.  No one expected the fields to soar much higher.

But Thompson and Duncan realized that ultrastrong fields could explain some mysteries.  Notably, astrophysicists were puzzled by soft gamma repeaters (SGRs).  These unidentified objects emitted erratic flares of soft gamma rays — a notch above the most piercing x-rays — then fell quiet.  In 1979, an SGR in a neighboring galaxy unleashed a giant flare that packed as much energy into its first 0.2 seconds as the sun produces in 10,000 years.  The source was close to the remains of a recent supernova.  However, the flare ebbed and flowed just once every 8 seconds as it gradually subsided, seemingly far too slow to come from a pulsar.

The theorists postulated that the bursts arose from a slow-spinning neutron star that had spun breathtakingly fast at birth.  Astrophysicists Adam Burrows of the University of Arizona in Tucson and James Lattimer of the State University of New York, Stony Brook, had shown that during a neutron star's first 10 seconds of existence, its hot nuclear fluid would convect about 100 times every second.  If the neutron star whirled between 100 and 1000 times each second during those birth pangs, Thompson and Duncan calculated, it would spark a furious dynamo — a self-sustaining generator of an intense magnetic field, 10¹⁵ gauss and beyond.

Once magnetism suffuses the dense superfluid of a neutron star, it's tough to disperse.  Still, the magnetic fields and the electric currents that support them try to shift into patterns that are less taut with pent-up energy.  “The magnetic field is strongly wound up in a tight spiral inside the star,” Thompson explains.  “It is the progressive unwinding of the field that drives the [SGR] flares.”  Each shift strains the solid crust of the neutron star.  At a critical point the crust snaps, creating faults that may span a kilometer.  Once the surface cracks, the magnetic fields above it whip into new positions as well.  The violent motions blast particles along the magnetic fields, triggering gamma rays and x-rays.

Duncan and Thompson published this scenario in 1992, discarding their initial “burstar” term for the more descriptive “magnetar.”  Three years later, they noted that the magnetic fields should confine a burst's energy in a fireball lasting a few minutes, exactly the pattern observed.

After initial skepticism from the physics community, Irion notes, “observations won the day.”

First, a team led by Kouveliotou used NASA's Rossi X-ray Timing Explorer (RXTE) satellite to measure pulsations once every 7.47 seconds in an SGR with frequent outbursts.  The periodic fluctuations were visible only during bright bursts; at other times the SGR did not emit ordinary pulsarlike beams.  The object's rotational “clock” was slowing down by an astonishing 0.26 seconds per century — an effect that could result only from the strong drag of a magnetic field around 10¹⁵ gauss.

Then on 27 August 1998 [Earthdate 1998-08-27], a wave of gamma rays and x-rays more intense than the 1979 flare swept through the solar system.  The source was an SGR across the Milky Way.  Despite the distance, the radiation was powerful enough to affect radio transmissions on Earth by strongly ionizing the upper atmosphere.  Slow, 5.16-second pulsations modulated the flare.  Kouveliotou's team also studied it with RXTE to show that the SGR's spin decelerated at a magnetar-like clip.

With those findings, magnetars passed into mainstream science.  Peers honored the work last year when Duncan, Thompson, and Kouveliotou jointly received the 2003 Bruno Rossi Prize, the top research award from the AAS High-Energy Astrophysics Division.

Since establishment of magnetars on a firm basis, observational studies have extended the classes of phenomena that magnetars have been invoked to explain.

In recent years, astronomers have broadened the magnetar family.  Most now agree that objects called anomalous x-ray pulsars (AXPs), which pulsate slowly in x-rays but not in radio waves, are another flavor of magnetar.  Astronomer Victoria Kaspi of McGill University in Montreal, Canada, and her colleagues have shown that AXPs can spew impulsive bursts, although not quite as vehemently as SGRs.

Curiously, the 11 known SGRs and AXPs all spin at nearly the same rate: between 5 and 12 seconds for each rotation.  Magnetic fields stifle a young magnetar's spin so severely that its rotation stutters from a few milliseconds down to a few seconds within centuries — such a brief interval that astronomers would have to get lucky to see a furiously spinning magnetar.  “And if they were active for more than a few thousand years, we'd expect to see some with periods of tens of seconds, but we don't,” says astronomer Peter Woods of MSFC.  “So it appears to be a very short life cycle when they are x-ray bright.”

Two new studies to appear in the Astrophysical Journal suggest that magnetars are more common than their measly statistics indicate.  In one report, astronomers led by Woods describe an AXP that flickered intensely for 4 hours in June 2002, then just as quickly faded.  Similar outbursts elsewhere in the galaxy might go undetected by current instruments, says Woods, because telescopes that monitor the whole sky aren't yet sensitive enough.  In another study, astronomers led by Alaa Ibrahim of NASA's Goddard Space Flight Center in Greenbelt, Maryland, exposed a “transient” magnetar.  The object was too faint to attract attention throughout the 1990s, but it suddenly grew 100 times brighter in early 2003.

In their quiet states, these misbehaving magnetars bear some resemblance to faint sources of x-rays in supernova remnants, called central compact objects.  They also look similar to another mysterious class of bodies called dim isolated neutron stars.  Kaspi, a collaborator on both studies, agrees that the magnetar family tree may include some of these branches.  “Dim isolated neutron stars could be dead magnetars with some residual heat,” she says.  “I think the numbers are consistent with half the neutron star population being born as magnetars.”  But better counts — and a firmer handle on the strengths of magnetic fields — are needed before anyone accepts that logic.

Fascinating theoretical work has continued as well.

On the theoretical side, several groups are probing possible links between magnetars and gamma ray bursts (GRBs), the most energetic explosions in the cosmos.  Many astrophysicists now think the most viable triggers of long-duration GRBs, lasting seconds to minutes, are powerful supernovas that create newborn black holes.  However, a magnetically dominated wind from a new magnetar makes more sense as a coherent driving force, says astrophysicist Maxim Lyutikov of McGill University.  “The dissipation of magnetic energy can be very efficient,” he notes.  In contrast, blasts of matter from close to a black hole might lose too much energy within violent shocks.

In related work, modeling by Hubble postdoctoral fellow Todd Thompson of the University of California, Berkeley, shows that a brand-new magnetar will sling matter into space along stiff magnetic “spokes” at nearly the speed of light.  This outpouring of mass expels so much momentum that if the magnetar spins 1000 times per second at birth, it takes merely 10 seconds to slam the brakes down to about 300 spins per second.  That deceleration releases a whopping 90% of the object's energy.  Thompson thinks all that energy can propel a hyperenergetic supernova or, under the right conditions, a GRB.

The heaviest elements in nature could arise in this turbulent setting as well, Thompson adds.  Astrophysicists haven't yet identified a convincing site for the “r-process,” the creation of heavy atomic nuclei by rapid bombardment with a fierce wind of neutrons.  Ultrastrong magnetic fields might keep a hot bath of neutrons and protons close enough to a new magnetar to push element synthesis up the periodic table to uranium and beyond.

Duncan, advocate of all things magnetar, loves the idea.  “It's possible that all elements heavier than bismuth are synthesized in magnetar winds,” he says.  “If that's true, nuclear bombs and reactors are running on magnetar energy.”  Since supernovas supply the iron in our blood, it's only fair that magnetars get in on the action as well.

Labels: astronomy, magnetism, pulsars

¹ Robert Irion, “The Pulsar Menagerie,” Science, Vol. 304, Issue No. 5670 (Issue dated 2004-04-23), pp. 532-533 [DOI: 10.1126/science.304.5670.532].

² J. M. Lattimer and M. Prakash, “The Physics of Neutron Stars,” Science, Vol. 304, Issue No. 5670 (Issue dated 2004-04-23), pp. 536-542 [DOI: 10.1126/science.1090720].

³ Robert Irion, “Crushed by Magnetism,” Science, Vol. 304, Issue No. 5670 (Issue dated 2004-04-23), pp. 534-535 [DOI: 10.1126/science.304.5670.534].

Labels: astronomy, pulsars