摘要:

I review the observational evidence for the existence of a population of high‐velocity pulsars in our Galaxy. It begins with a discussion of methods, showing how we measure pulsar velocities and what the accuracy and limitations are of the various methods. Recent developments, which have led to substantial changes in our understanding of the pulsar velocity distribution, will be discussed in detail. It will end by touching on a few relevant issues and questions pertaining to the gamma ray burster problem. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50261

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We discuss pulsar space velocities using available proper motion data and other properties of the radio pulsar population. Using a likelihood analysis, we show that all pulsars are consistent with an origination within the galactic disk (other than those in globular clusters or the Magellanic clouds). The same analysis yields estimates for radial velocities of the pulsars and allows indentification of objects whose ages and distances are significantly less than their nominal values. Evidence in favor of asymmetric kicks as the origin for high velocities is summarized. The fraction of objects that escape the Galaxy is estimated. Methods for correcting selection effects that bias the estimated velocity distribution function and the fraction of escaping objects are discussed. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50234

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We investigate the form of the neutron star velocity kick distribution functionf(v) by modeling the Galactic population of neutron stars. Using mock pular data sets drawn from models with known parameters, we discuss the recovery off(v). We illustrate the relationship betweenf(v) and several kinematic properties of thedetectedobjects: the distribution of kicks imparted at birth, the distribution of total space velocity today, and the distribution of the perpendicular component of the velocity today. The pulsar luminosity parameters strongly influence the determination off(v). ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50241

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

I will discuss the galactic distribution of low‐mass binaries in which the accreting compact object is a neutron star. The r.m.s. value of their distances,z, to the galactic plane is ∼1 kpc. This widezdistribution cannot be explained by systematic velocities that increased as a result of sudden mass loss at the formation of the neutron star alone. Kick velocities imparted on the neutron star, according to the radio pulsar velocity distribution recently derived by Lyne and Lorimer can account for the LMXBzdistribution. This distribution is consistent with formation of neutron stars in LMXBs from direct collapse of a helium star, and also from accretion induced collapse of a white dwarf. The triple‐star evolutionary scenario proposed by Eggleton and verbunt can only produce a minor fraction of the LMXB. A side result of the above work is the conclusion that if gamma‐ray bursts are caused by neutron star mergers one would expect the location distribution of these events to be enhanced near (the bigger) galaxies. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50249

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

It is now believed that pulsars comprise the fastest population of stars in the galaxy. With inferred mean, root‐mean‐square, and maximum 3‐D pulsar speeds of ∼300–500 km/s, ∼500 km/s, and ∼2000 km/s, respectively, the question of the origin of such singular proper motions becomes acute. What mechanism can account for speeds that range from zero to twice the galactic escape velocity? We speculate that a major vector component of a neutron star’s proper motion comes from the hydrodynamic recoil of the nascent neutron star during the supernova explosion in which it is born. Recently, theorists have shown that asymmetries and instabilities are a natural aspect of supernova dynamics. In this paper, we highlight two phenomena: 1) the ‘‘Brownian‐like’’ stochastic motion of the core in response to the convective ‘‘boiling’’ of the mantle of the protoneutron star during the post‐bounce, pre‐explosion accretion phase, and 2) the asymmetrical bounce and explosion of an aspherically collapsing Chandrasekhar core. In principle, either phenomenon can leave the young neutron star with a speed of hundreds of kilometers per second. However, neither has yet been adequately simulated or explored. The two‐dimensional radiation/hydrodynamic calculations we present here provide only crude estimates of the potential impulses due to mass motions and neutrino emissions. A comprehensive and credible investigation will require fully three‐dimensional numerical simulations not yet possible. Nevertheless, we have in the asymmetric hydrodynamics of supernovae a natural means of imparting respectable kicks to neutron stars at birth, though speeds approaching 1000 km/s are still problematic. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50256

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Formation of a neutron star is accompanied by neutrino emission carrying about 10% of the rest energy of the star. Toroidal field produced by twisting of a dipole field in differentially rotating star is antisymmetric. Its summation with antisymmetric toroidal field results in braking of mirror symmetry of the magnetic field. For large magnetic field the neutron decay rate depends on its strength. Neutrino is emitted more in one direction leading to flux asymmetry and recoil of the neutron star. Estimations show that the neutron star can reach velocities ∼1000 km/s for 3% asymmetry of the neutrino flux. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50262

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We consider a close binary system with separation &bartil;0.2R⊙which consists of a (C+O) star of mass &bartil;4M⊙and a neutron star as a progenitor of soft gamma‐ray repeater (SGR) and a high velocity pulsar. After the event of the supernova explosion, both new and old neutron stars have high relative velocity of &bartil;1000 km s−1to the center of mass of the ejecta. SGR activities can be induced by the episodic accretion of the supernova ejecta onto the old neutron star or the strange high rotating pulsar of the new neutron star. Future observations with fine positional resolution can clarify the position ambiguity between the gamma ray and X‐ray sources. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50263

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The oft expressed characteristics of soft gamma ray repeaters (SGRs) have been determined from observations of the three acknowledged sources: 0526−66, 1806−20, and 1900+14. These general properties are short durations (<1 second), simple profiles, rapid risetimes (<100 ms), soft spectra (kT∼35 keV), lack of spectral evolution, and repetition. But, as we shall see, there are individual bursts that violate each of these properties. Quite recently, counterparts at other wavelengths have been proposed and the resulting association with supernova remnants and, therefore, neutron stars, has strengthened the hypothesis of a galactic origin for SGRs. This review will concentrate on the three SGRs and what we have come to know about them, and in the process, have determined to be the observational qualities of SGRs. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50264

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Much progress has been made in our understanding of soft gamma ray repeaters primarily due to multiwavelength observations. Two of the three SGRs are definitely associated with quiescent X‐ray emission and all three can be associated with SNRs. SGR 1806−20 has been identified with a plerion, a non‐thermal nebula usually identified with pulsar powered SNRs. High resolution VLA observations reveal a compact source with outflow. This source coincides with a bright star, probably the rare blue luminous blue variables. This may well represent the first stellar identification of an SGR. A case for a binary model for SGRs can be made from the observations of SGR 1806−20 but this model is not supported from multiwavelength observations of SGR 0526−66. SGR 1900+14 has been plausibly identified with a quiescent X‐ray source which has no radio counterpart, akin to the situation of SGR 0526‐66. Thus there is no definite pattern that has emerged save the following. SGRs are associated with young neutron stars and in some cases with massive stars. The absence of radio emission in two out of three cases is suggestive that the bursts themselves may inject particles. Why SGR 1806−20 has a plerion remains a mystery. Radio observations that are currently in progress should be able to shed light on the important issue whether particles accompany photon outbursts. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50265

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The ASCA satellite has observed the locations of two of the three SGRs. Observations of 1806‐20 made simultaneously with the CGRO provided a quiescent x‐ray source coincident with the burst source, and therefore established the first x‐ray identification of a soft gamma ray repeater. This identification has lead to optical and infrared observations by others at this meeting. Observations of N49 do not show a hard x‐ray spectral component in addition to the nebular flux, but upper limits to a hard component are not in conflict with the 2‐3 level reported from ROSAT. Further details can be found in this author’s review of ASCA results on SGRs in the Proceedings of the 29th ESLAB Symposium. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50266

出版商:AIP    

年代:1996

数据来源: AIP