High-resolution x-ray imaging of a globular cluster core: compact binaries in 47Tuc

We have obtained high-resolution (approximately 1") deep x-ray images of the globular cluster 47Tucanae (NGC 104) with the Chandra X-ray Observatory to study the population of compact binaries in the high stellar density core. A 70-kilosecond exposure of the cluster reveals a centrally concentrated population of faint (Lx approximately 10(30-33) ergs per second) x-ray sources, with at least 108 located within the central 2' x 2.5' and greater, similar half with Lx approximately 10(30.5) ergs per second. All 15 millisecond pulsars (MSPs) recently located precisely by radio observations are identified, though 2 are unresolved by Chandra. The x-ray spectral and temporal characteristics, as well as initial optical identifications with the Hubble Space Telescope, suggest that greater, similar50 percent are MSPs, about 30 percent are accreting white dwarfs, about 15 percent are main-sequence binaries in flare outbursts, and only two to three are quiescent low-mass x-ray binaries containing neutron stars, the conventional progenitors of MSPs. An upper limit of about 470 times the mass of the sun is derived for the mass of an accreting central black hole in the cluster. These observations provide the first x-ray "color-magnitude" diagram for a globular cluster and census of its compact object and binary population.     

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.     

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.     

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.     

Fermi detection of a luminous γ-ray pulsar in a globular cluster

We report on the Fermi Large Area Telescope's detection of γ-ray (>100 mega-electron volts) pulsations from pulsar J1823-3021A in the globular cluster NGC 6624 with high significance (~7 σ). Its γ-ray luminosity, L(γ) = (8.4 ± 1.6) × 10(34) ergs per second, is the highest observed for any millisecond pulsar (MSP) to date, and it accounts for most of the cluster emission. The nondetection of the cluster in the off-pulse phase implies that it contains <32 γ-ray MSPs, not ~100 as previously estimated. The γ-ray luminosity indicates that the unusually large rate of change of its period is caused by its intrinsic spin-down. This implies that J1823-3021A has the largest magnetic field and is the youngest MSP ever detected and that such anomalous objects might be forming at rates comparable to those of the more normal MSPs.     

Probing the faintest stars in a globular star cluster

NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed an ultradeep color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main-sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn toward bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H(2) and the resultant collision-induced absorption cause their atmospheres to become largely opaque to infrared radiation.     

Spin-down of radio millisecond pulsars at genesis

Millisecond pulsars are old neutron stars that have been spun up to high rotational frequencies via accretion of mass from a binary companion star. An important issue for understanding the physics of the early spin evolution of millisecond pulsars is the impact of the expanding magnetosphere during the terminal stages of the mass-transfer process. Here, I report binary stellar evolution calculations that show that the braking torque acting on a neutron star, when the companion star decouples from its Roche lobe, is able to dissipate >50% of the rotational energy of the pulsar. This effect may explain the apparent difference in observed spin distributions between x-ray and radio millisecond pulsars and help account for the noticeable age discrepancy with their young white dwarf companions.     

Pulsar test of a variation of the speed of light with frequency

The sharply defined optical and radio pulses from pulsars make possible a test of the variation of the speed of light with frequency, and of the possible existence of a photon mass. The data indicate that the mass of a real photon is less than 10(-44) gram. Detection of extragalactic pulsars could allow a substantial improvement of this limit.     

A young white dwarf companion to pulsar B1620-26: evidence for early planet formation

The pulsar B1620-26 has two companions, one of stellar mass and one of planetary mass. We detected the stellar companion with the use of Hubble Space Telescope observations. The color and magnitude of the stellar companion indicate that it is an undermassive white dwarf (0.34 +/- 0.04 solar mass) of age 480 x 10(6) +/- 140 x 10(6) years. This places a constraint on the recent history of this triple system and supports a scenario in which the current configuration arose through a dynamical exchange interaction in the cluster core. This implies that planets may be relatively common in low-metallicity globular clusters and that planet formation is more widespread and has happened earlier than previously believed.     

Pulse structure of four pulsars

The pulse structure of the four known pulsars is given. The pulse is about 38 milliseconds for the two pulsars of longest period, and within the pulsewidth three subpulses typically appear. The pulsar of next longest period typically radiates two pulses separated about 23 milliseconds in time. The one short-period pulsar emits single pulses of constant shape. The first subpulses of all pulsars have nearly the same shape. The shape of the first subpulse agrees well with the pulse shape expected from a radio-emitting sphere which is excited by a spherically expanding disturbance, and in which the radio emission, once excited, decays exponentially.     

Explaining the color distributions of globular cluster systems in elliptical galaxies

The colors of globular clusters in most large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the nonlinear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that horizontal-branch stars are the main drivers behind the empirical nonlinearity. We show that the scenario gives simple and cohesive explanations for all the key observations and could simplify theories of elliptical galaxy formation.     

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.     

密度依赖口袋常数下奇异夸克物质及奇异星

在有效质量口袋模型(EMBM)下采用密度依赖口袋常数描述奇异夸克物质,研究了口袋常数的密度依赖性对奇异夸克物质的状态方程及奇异星性质的影响.结果表明,密度依赖口袋常数在低密度处较大,随着密度增大而明显变小,很好地体现了低密度处色禁闭、高密度处夸克退紧闭的量子色动力学(QCD)基本精神.与常数口袋常数的情形相比,密度依赖口袋常数下奇异夸克物质的状态方程变硬,奇异星的引力质量和对应的半径均有明显变大,且均在脉冲星的观测值范围内,表明该模型适合于描述质量和半径较大的奇异星内的夸克物质.     

Diffuse Extreme-Ultraviolet Emission from the Coma Cluster: Evidence for Rapidly Cooling Gases at Submegakelvin Temperatures

The central region of the Coma cluster of galaxies was observed in the energy band from 0.065 to 0.245 kiloelectron volts by the Deep Survey telescope aboard the Extreme Ultraviolet Explorer. A diffuse emission halo of angular diameter approximately 30 arc minutes was detected. The extreme-ultraviolet (EUV) emission level exceeds that expected from the x-ray temperature gas in Coma. This halo suggests the presence of two more phases in the emitting gas, one at a temperature of approximately 2 x 10(6) kelvin and the other at approximately 8 x 10(5) kelvin. The latter phase cools rapidly and, in steady state, would have produced cold matter with a mass of approximately 10(14) solar masses within the EUV halo. Although a similar EUV enhancement was discovered in the Virgo cluster, this detection in Coma applies to a noncooling flow system.     

Interstellar scintillations of pulsar radiation

Time fluctuations in the intensity of pulsed radiation from CP 0834, CP 1133, AP 1237, and CP 1919 have been investigated. Power spectra, modulation indices, frequency distributions, and decorrelation frequencies are consistent with scintillation theory. If it is assumed that these scintillations are due to irregularities in the interstellar medium that travel at a velocity of 20 kilometers per second, the irregularities have a scale size on the order of 10(4) kilometers and a distance from the earth of approximately 70 parsecs. These interstellar scintillations would not have been observed if the apparent angular diameters of the pulsars were larger than 0.3 X 10(-5) second of arc, and they would cause even a point radio source to have an apparent angular diameter of approximately 10(-3) second of arc at 318 megahertz.     

摩擦对偏心轮推杆行星传动接触应力及疲劳强度的影响

为给偏心轮推杆行星传动的设计提供参考，将偏心轮与活齿内滚柱两接触体视为完全弹性且不蚺荷的动力效应，在考虑摩擦的前提下，对偏心轮与内滚柱接触的主应力、最大剪应力、最大八面体剪应力、最大正交剪应力进行了分析和计算．分析计算结果表明，摩擦对偏心轮推杆行星传动的接触应力与疲劳强度有重要影响，摩擦会加速各受载元件裂纹的形成和疲劳点蚀的出现．     

Absorption features in the x-ray spectrum of an ordinary radio pulsar

The vast majority of known nonaccreting neutron stars (NSs) are rotation-powered radio and/or γ-ray pulsars. So far, their multiwavelength spectra have all been described satisfactorily by thermal and nonthermal continuum models, with no spectral lines. Spectral features have, however, been found in a handful of exotic NSs and were thought to be a manifestation of their unique traits. Here, we report the detection of absorption features in the x-ray spectrum of an ordinary rotation-powered radio pulsar, J1740+1000. Our findings bridge the gap between the spectra of pulsars and other, more exotic, NSs, suggesting that the features are more common in the NS spectra than they have been thought so far.     

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.     

Age estimates of globular clusters in the Milky Way: constraints on cosmology

Recent observations of stellar globular clusters in the Milky Way Galaxy, combined with revised ranges of parameters in stellar evolution codes and new estimates of the earliest epoch of globular cluster formation, result in a 95% confidence level lower limit on the age of the Universe of 11.2 billion years. This age is inconsistent with the expansion age for a flat Universe for the currently allowed range of the Hubble constant, unless the cosmic equation of state is dominated by a component that violates the strong energy condition. This means that the three fundamental observables in cosmology-the age of the Universe, the distance-redshift relation, and the geometry of the Universe-now independently support the case for a dark energy-dominated Universe.