摘要:

Coherent millisecond x‐ray pulsations are expected from low‐mass x‐ray binaries (LMXB), but remain undetected. Using the single‐parameter Quadratic Coherence Recovery Technique (QCRT) to correct for unknown binary orbital motion we have performed Fourier transform searches for coherent oscillations in all long, continuous segments of data obtained at 1 millisecond time resolution during Ginga observations of LMXB. We have searched the six known Z‐sources (GX 5‐1, Cyg X‐2, Sco X‐1, GX 17+2, GX 340+0, and GX 349+2), seven of the ten known atoll sources (GX 3+1, GX 9+1, GX 9+9, 1728‐33, 1820‐30, 1636‐53, and 1608‐52), the ‘‘peculiar’’ source Cir X‐1, and the high mass binary Cyg X‐3. We find no evidence for coherent pulsations in any of these sources, with 99% confidence limits on the pulsed fraction between 0.3% and 5.0% at frequencies below the Nyquist frequency of 512 Hz. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45939

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We discuss the possibility of detecting the microsecond granularity expected in the flux from wind‐fed x‐ray binary pulsars. This microsecond structure is predicted by the noisy accretion scenario; we derive the physical characteristics of such burst structure in the framework of this model. We investigate the temporal dispersion induced by the scattering of burst photons in their passage through the neutron star magnetosphere and conclude that microsecond structure could indeed by observable for energies less than the cyclotron energy. We show that the coincidence timing mode for the Proportional Counter Array on‐board XTE can be used very effectively to statistically discriminate these microsecond bursts from accidental spurious detections. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45940

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The Unconventional Stellar Aspect (USA) experiment to be launched in September 1995 on the Advanced Research and Global Observations Satellite (ARGOS) is a low‐cost, quick—yet scientifically ambitious—x‐ray timing experiment. It is designed for the dual purpose of scientific research in x‐ray timing and time resolved spectroscopy and also for exploration of applications of x‐ray sensor technology. Bright galactic x‐ray binaries are used simultaneously for both scientific and applied objectives. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45941

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

In x‐ray pulsars, the emission from the bottom of the accretion column in the magnetic pole propagates to the observers through the surrounding environment: the magnetosphere, the accretion disk and the disk corona, the stellar wind, the companion’s atmosphere, and finally the interstellar medium. The photoionization zone of the stellar wind, which is formed surrounding the neutron star by x‐ray irradiation, plays an important role as a reprocessing site. The spectrum originating from the polar caps is modified by the photoelectric absorption and subsequent fluorescent emission, Thomson/Compton scattering and synchrotron resonant scattering that occur in these environments. Modifications in the spectrum due to the reprocessing are briefly reviewed based on the recent results obtained from observations by Tenma, EXOST, and Ginga. A preliminary result from Vela X‐1 with ASCA is presented, exhibiting a new aspect in studying x‐ray pulsars. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.46033

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The x‐ray source in high mass x‐ray binaries (HMXB’s) provides a unique ‘‘active’’ probe of the winds of massive stars. As the binary system orbits around the center of mass, our line of sight changes continuously, allowing us to map the distribution of x‐ray absorbing gas in the system. At the same time, the strong x‐ray flux alters the local dynamics of the wind, providing us with an opportunity to study the physics of radiatively‐driven winds. Some HMXB systems even provide a variable x‐ray source, from which we can measure the effects of a local x‐ray flux. New hydrodynamic models, coupled with recent observations, allow us to take advantage of this natural x‐ray machine, providing new insight into the dynamics of radiatively driven winds. The various physical processes that affect the wind dynamics will be reviewed, as well as the observational consequences of these processes as gleaned from multidimensional hydrodynamic models. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45942

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

By a Monte Carlo computation we have estimated the spectrum of x‐rays scattered into the eclipse of 4U1538‐52 by the atmosphere of its supergiant companion. The estimate fails to account for a soft component below 4.5 keV which amounts to approximately 1.4% of the average uneclipsed flux in the same energy range. The intensity of the soft component exhibits an initial downward trend following eclipse ingress as expected of a component scattered by interstellar dust grains. Comparing the intensity with the optical extinction of QV Nor, we derive an upper limit on a quantityRXV(E) which we call the scattering/extinction ratio of interstellar dust grains and define as (E/1 keV)2times the ratio of optical depth for scattering x‐rays of energyEto the total optical extinction. In the Rayleigh‐Gans approximation to the x‐ray scattering efficiency, this quantity is independent of energy. Our upper limit onRXVis 0.06 mag−1, which implies that the x‐ray scattering efficiency of interstellar dust is less than expected for solid grains with a size distribution of the formng(a)∼a−3.5in the range from 0.005 &mgr;m to 0.25 &mgr;m and composed of silicate (RXV=0.22 mag−1) or a silicate‐graphite mixture (RXV=0.11 mag−1) as derived from the calculations of Martin Rouleau (1991). This lends support to the idea (Mathis & Whiffen 1989) that interstellar grains are ‘‘fluffy’’ aggregates with an average bulk density less than that of their constituent particles. Such aggregates would have a smaller ratio of x‐ray scattering efficiency to optical extinction efficiency compared to solid grains of the same material. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45943

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Simple estimates indicate there should be ≳103low luminosity x‐ray pulsars (L≳1034erg s−1) in the Galaxy undergoing ‘‘low‐state’’ wind accretion in Be/x‐ray binary systems, and ∼108–109isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures (kTe≲300 eV), we predict the low luminosity accreting neutron stars with magnetic fieldsB∼(0.7−7)×1012G should emit a substantial fraction (0.5–5%) of their total luminosity in a narrow (E/&Dgr;E∼2–4) cyclotron emission line which peaks in the energy range ∼5–20 keV. In sharp contrast to the underlying thermal emission, thisnonthermalcyclotron component will not be strongly absorbed by the intervening H igas, and consequently it may be the only observable signature for the bulk of these low luminosity sources. We propose a search for this cyclotron emission feature in long pointed observations by ASCA of the Be/x‐ray transient pulsars V0331+53 and 4U0115+63 in their quiescent ‘‘low state’’. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45944

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A model for the expansion phase of a neutron star atmosphere during strong x‐ray bursts is presented. General relativistic hydrodynamical and radiative transfer equations are integrated from the star surface outwards, taking into account for helium burning in the dense, inner shells. The role of Compton heating in placing a lower limit on the mass loss rate for the existence of stationary, supersonic winds is analyzed. Comparison of our results with observational data of 4U/MXB 1820‐30 provides an estimate of both the spectral hardening and the accretion rate in this source. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45945

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We study numerically the quasi‐spherical accretion of matter on to a compact object (neutron star or black hole). Anisotropic x‐ray luminosity, powered by mass accretion, heats the accreting gas through Compton scattering. When the gas temperature increases above the local escape temperature, part of the accreting gas will flow outwards as a result of the action of buoyancy force. The direction of the outflow coincides with the maximum of the x‐ray luminosity. The depth of outflow is correlated with the energy of x‐ray quanta. In spite of its quantum nature, Compton heating markedly affects the gas, forcing the matter outflow at x‐ray luminosities as small as three or four orders of magnitude less than the Eddington limit. The phenomenon of hot gas outflow takes place in the case of accretion on to a wind‐fed x‐ray source in a wide binary with massive OB or Be‐star. We propose a new spin‐down mechanism for accreting neutron stars that explains the existence of a number of long‐period (p∼100–1000 s) x‐ray pulsars in these binaries. The spin‐down is a result of efficient angular‐momentum transfer from the rotating magnetosphere of the accreting star to an outflowing stream, when the outflow forms so deep as to capture the magnetic‐field lines from the rotating magnetosphere. The balance between angular‐momentum gain by accreting gas and loss by outflow matter takes place at a particular value of the equilibrium spin period (peq∼100–1000 s). ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45946

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Stationary, spherical accretion onto an unmagnetized neutron star is here reconsidered on the wake of the seminal paper by Zel’dovich & Shakura (1969). It is found that new ‘‘hot’’ solutions may exist for a wide range of luminosities. These solutions are characterized by a high temperature, 109÷1011K, and arise from a stationary equilibrium model where the dominant radiative mechanisms are multiple Compton scattering and bremsstrahlung emission. For low luminosities, ≲10−2LE, only the ‘‘cold’’ (a` la Zel’dovich and Shakura) solution is present. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.45947

出版商:AIP    

年代:1994

数据来源: AIP