An eccentric binary millisecond pulsar in the galactic plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric (e = 0.44) 95-day orbit around a solar mass (M(middle dot in circle)) companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster, then ejecting it into the Galactic disk, or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74 +/- 0.04 M solar symbol, an unusually high value.     

Pulsars in binary systems: probing binary stellar evolution and general relativity

Radio pulsars in binary orbits often have short millisecond spin periods as a result of mass transfer from their companion stars. They therefore act as very precise, stable, moving clocks that allow us to investigate a large set of otherwise inaccessible astrophysical problems. The orbital parameters derived from high-precision binary pulsar timing provide constraints on binary evolution, characteristics of the binary pulsar population, and the masses of neutron stars with different mass-transfer histories. These binary systems also test gravitational theories, setting strong limits on deviations from general relativity. Surveys for new pulsars yield new binary systems that increase our understanding of all these fields and may open up whole new areas of physics, as most spectacularly evidenced by the recent discovery of an extremely relativistic double-pulsar system.     

Durability of the accretion disk of millisecond pulsars

Pulsars with pulsation periods in the millisecond range are thought to be neutron stars that have acquired an extraordinarily short spin period through the accretion of stellar material spiraling down onto the neutron star from a nearby companion. Nearly all the angular momentum and most of the mass of the companion star is transferred to the neutron star. During this process, wherein the neutron star consumes its companion, it is required that a disk of stellar material be formed around the neutron star. In conventional models it is supposed that the disk is somehow lost when the accretion phase is finished, so that only the rapidly spinning neutron star remains. However, it is possible that, after the accretion phase, a residual disk remains in stable orbit around the neutron star. The end result of such an accretion process is an object that looks much like a miniature (about 100 kilometers), heavy version of Saturn: a central object (the neutron star) surrounded by a durable disk.     

An x-ray nebula associated with the millisecond pulsar B1957+20

We have detected an x-ray nebula around the binary millisecond pulsar B1957+20. A narrow tail, corresponding to the shocked pulsar wind, is seen interior to the known Halpha bow shock and proves the long-held assumption that the rotational energy of millisecond pulsars is dissipated through relativistic winds. Unresolved x-ray emission likely represents the shock where the winds of the pulsar and its companion collide. This emission indicates that the efficiency with which relativistic particles are accelerated in the postshock flow is similar to that for young pulsars, despite the shock proximity and much weaker surface magnetic field of this millisecond pulsar.     

Twenty-one millisecond pulsars in Terzan 5 using the Green Bank Telescope

We have identified 21 millisecond pulsars (MSPs) in globular cluster Terzan 5 by using the Green Bank Telescope, bringing the total of known MSPs in Terzan 5 to 24. These discoveries confirm fundamental predictions of globular cluster and binary system evolution. Thirteen of the new MSPs are in binaries, of which two show eclipses and two have highly eccentric orbits. The relativistic periastron advance for the two eccentric systems indicates that at least one of these pulsars has a mass 1.68 times greater than the mass of the Sun at 95% confidence. Such large neutron star masses constrain the equation of state of matter at or beyond the nuclear equilibrium density.     

Observational properties of pulsars

Pulsars are remarkable clocklike celestial sources that are believed to be rotating neutron stars formed in supernova explosions. They are valuable tools for investigations into topics such as neutron star interiors, globular cluster dynamics, the structure of the interstellar medium, and gravitational physics. Searches at radio and x-ray wavelengths over the past 5 years have resulted in a large increase in the number of known pulsars and the discovery of new populations of pulsars, posing challenges to theories of binary and stellar evolution. Recent images at radio, optical, and x-ray wavelengths have revealed structures resulting from the interaction of pulsar winds with the surrounding interstellar medium, giving new insights into the physics of pulsars.     

X-ray flares from postmerger millisecond pulsars

Recent observations support the suggestion that short-duration gamma-ray bursts are produced by compact star mergers. The x-ray flares discovered in two short gamma-ray bursts last much longer than the previously proposed postmerger energy-release time scales. Here, we show that they can be produced by differentially rotating, millisecond pulsars after the mergers of binary neutron stars. The differential rotation leads to windup of interior poloidal magnetic fields and the resulting toroidal fields are strong enough to float up and break through the stellar surface. Magnetic reconnection-driven explosive events then occur, leading to multiple x-ray flares minutes after the original gamma-ray burst.     

The physics of neutron stars

Neutron stars are some of the densest manifestations of massive objects in the universe. They are ideal astrophysical laboratories for testing theories of dense matter physics and provide connections among nuclear physics, particle physics, and astrophysics. 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 10(10) kelvin, opaqueness to neutrinos, and magnetic fields in excess of 10(13) Gauss. Here, we describe the formation, structure, internal composition, and evolution of neutron stars. Observations that include studies of pulsars in binary systems, thermal emission from isolated neutron stars, glitches from pulsars, and quasi-periodic oscillations from accreting neutron stars provide information about neutron star masses, radii, temperatures, ages, and internal compositions.     

Transformation of a star into a planet in a millisecond pulsar binary

Millisecond pulsars are thought to be neutron stars that have been spun-up by accretion of matter from a binary companion. Although most are in binary systems, some 30% are solitary, and their origin is therefore mysterious. PSR J1719-1438, a 5.7-millisecond pulsar, was detected in a recent survey with the Parkes 64-meter radio telescope. We show that this pulsar is in a binary system with an orbital period of 2.2 hours. The mass of its companion is near that of Jupiter, but its minimum density of 23 grams per cubic centimeter suggests that it may be an ultralow-mass carbon white dwarf. This system may thus have once been an ultracompact low-mass x-ray binary, where the companion narrowly avoided complete destruction.     

Producing ultrastrong magnetic fields in neutron star mergers

We report an extremely rapid mechanism for magnetic field amplification during the merger of a binary neutron star system. This has implications for the production of the short class of gamma-ray bursts, which recent observations suggest may originate in such mergers. In detailed magnetohydrodynamic simulations of the merger process, the fields are amplified by Kelvin-Helmholtz instabilities beyond magnetar field strength and may therefore represent the strongest magnetic fields in the universe. The amplification occurs in the shear layer that forms between the neutron stars and on a time scale of only 1 millisecond, that is, long before the remnant can collapse into a black hole.     

Radio studies of extragalactic supernovae

Some exploding stars (supernovae) are powerful emitters of centimeter radio radiation. Detailed observations have shown that these supernovae quickly become detectable in the radio range, first at shorter wavelengths (higher frequencies) and later at progressively longer and longer wavelengths (lower frequencies). This part of the phenomenon appears to be well explained by a monotonic decrease in the amount of ionized material surrounding the radio-emitting regions as the shock from the explosion travels outward. The radio emission itself is of a nonthermal, synchrotron origin, as is the case in most bright cosmic radio sources. Once the absorption effects become negligible, the radio intensity declines with time until reaching the detection limit of the telescope. Models suggest that the absorbing material originates in a dense wind of matter lost by the supernova progenitor star, or by its companion if it is in a binary system, in the last stages of evolution before the explosion. The synchrotron radio emission can be generated either externally by the shock wave from the explosion propagating through this same high density stellar wind or internally by a rapidly rotating neutron star, which is the collapsed core of the exploded star. Present results appear to favor the former model for at least the first several years after the supernova explosion, although the latter model remains viable.     

A double-pulsar system: a rare laboratory for relativistic gravity and plasma physics

The clocklike properties of pulsars moving in the gravitational fields of their unseen neutron-star companions have allowed unique tests of general relativity and provided evidence for gravitational radiation. We report here the detection of the 2.8-second pulsar J0737-3039B as the companion to the 23-millisecond pulsar J0737-3039A in a highly relativistic double neutron star system, allowing unprecedented tests of fundamental gravitational physics. We observed a short eclipse of J0737-3039A by J0737-3039B and orbital modulation of the flux density and the pulse shape of J0737-3039B, probably because of the influence of J0737-3039A's energy flux on its magnetosphere. These effects will allow us to probe magneto-ionic properties of a pulsar magnetosphere.     

Very high energy gamma-ray binary stars

One of the major astronomical discoveries of the last two decades was the detection of luminous x-ray binary star systems in which gravitational energy from accretion is released by the emission of x-ray photons, which have energies in the range of 0.1 to 10 kiloelectron volts. Recent observations have shown that some of these binary sources also emit photons in the energy range of 10(12) electron volts and above. Such sources contain a rotating neutron star that is accreting matter from a companion. Techniques to detect such radiation are ground-based, simple, and inexpensive. Four binary sources (Hercules X-1, 4U0115+63, Vela X-1, and Cygnus X-3) have been observed by at least two independent groups. Although the discovery of such very high energy "gamma-ray binaries" was not theoretically anticipated, models have now been proposed that attempt to explain the behavior of one or more of the sources. The implications of these observations is that a significant portion of the more energetic cosmic rays observed on Earth may arise from the action of similar sources within the galaxy during the past few million years.     

A radio pulsar spinning at 716 Hz

We have discovered a 716-hertz eclipsing binary radio pulsar in the globular cluster Terzan 5 using the Green Bank Telescope. It is the fastest spinning neutron star found to date, breaking the 24-year record held by the 642-hertz pulsar B1937+21. The difficulty in detecting this pulsar, because of its very low flux density and high eclipse fraction (approximately 40% of the orbit), suggests that even faster spinning neutron stars exist. If the pulsar has a mass less than twice the mass of the Sun, then its radius must be constrained by the spin rate to be <16 kilometers. The short period of this pulsar also constrains models that suggest that gravitational radiation, through an r-mode (Rossby wave) instability, limits the maximum spin frequency of neutron stars.     

Binary Pulsar PSR 1913 + 16: Model for Its Origin

The existing observational data for the binary pulsar PSR 1913 + 16 are sufficient to give a rather well-defined model for the system. On the basis of evolutionary considerations, the pulsar must be a neutron star near the upper mass limit of 1.2 solar masses (M.). The orbital inclination is probably high, i>/= 700, and the mass of the unseen companion probably lies close to the upper limit of the range 0.25 M. to 1.0 M.. The secondary cannot be a main sequence star and is probably a degenerate helium dwarf. At the 5.6-kiloparsec distance indicated by the dispersion measure, the magnetic dipole model gives an age of approximately 4 x 104 years, a rate of change of the pulsar period of P approximately 2 nanoseconds per day, and a surface magnetic field strength approximately (1/3) that of the Crab pulsar. The pulsar is fainter than an apparent magnitude V approximately + 26.5 and is at least approximately 80 times fainter than the Crab pulsar in the x-ray band. The companion star should be fainter than V approximately + 30, and a radio supernova remnant may be detectable near the position of the pulsar at a flux level of 相似文献   

Periodic emission from the gamma-ray binary 1FGL J1018.6-5856

Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.     

A long-period, violently variable X-ray source in a young supernova remnant

Observations with the Newton X-ray Multimirror Mission satellite show a strong periodic modulation at 6.67 +/- 0.03 hours of the x-ray source at the center of the 2000-year-old supernova remnant RCW 103. No fast pulsations are visible. If genetically tied to the supernova remnant, the source could either be an x-ray binary, composed of a compact object and a low-mass star in an eccentric orbit, or an isolated neutron star. In the latter case, the combination of its age and period would indicate that it is a peculiar magnetar, dramatically slowed down, possibly by a supernova debris disc. Both scenarios require nonstandard assumptions about the formation and evolution of compact objects in supernova explosions.     

Relativistic Jets in SS 433

A variety of recent optical, radio, and x-ray observation have confirmed the hypothesis that the peculiar star SS 433 is ejecting two narrow, opposed, highly collimated jets of matter at one-quarter the speed of light. This unique behavior is probably driven by mass exchange between a relatively normal star and a compact companion, either a neutron star or a black hole. However, numerous details regarding the energetics, radiation, acceleration, and collimation of the jets remain to be understood. This phenomenon may well be a miniature example of similar collimated ejection of gas by active extragalactic objects such as quasars and radio galaxies.     

Physics in strong magnetic fields near neutron stars

Electromagnetic phenomena occurring in the strong magnetic fields of neutron stars are currently of great interest in high-energy astrophysics. Observations of rotation rate changes and cyclotron lines in pulsars and gamma-ray bursts indicate that surface magnetic fields of neutron stars often exceed 10(12) gauss. In fields this strong, where electrons behave much as if they were in bound atomic states, familiar processes undergo profound changes, and exotic processes become important. Strong magnetic fields affect the physics in several fundamental ways: Energies perpendicular to the field are quantized, transverse momentum is not conserved, and electron-positron spin is important. Neutron stars therefore provide a unique laboratory for the study of physics in extremely high fields that cannot be generated on Earth.