Search for millisecond periodicities in type I X-ray bursts of the Rapid Burster

We have searched the rising portion of type I X-ray bursts observed from the Rapid Burster with the Rossi X-ray Timing Explorer for the presence of periodicities. The 95 per cent confidence upper limit on the average rms variation of near coherent pulsations with a width of ≲1 Hz (in 60–2048 Hz) during the first second of the bursts is <8.8 per cent. We find a possible detection (>98 per cent significance) at 306.5 Hz.     

A model for X-ray emission from loop prominences

A study is made of X-ray line emission observed during the developing stages of a set of post-flare loop prominences. The time behaviour of the line emission can be described by a model consisting of two flux tubes containing plasma heated impulsively at the flash phase; the plasma cools by radiation and by conduction to the chromosphere. These ideas are extended to the possible formation of H prominences from low-lying hot loops.     

A predictive model for space-based X-ray ccd degradation

The first generation of X-ray telescopes to use Charge-Coupled Devices (CCDs) is being launched this decade. With a read noise of a few electrons, CCDs provide Fano-limited spectral resolution across the soft X-ray band (0.1 – 10 keV). However, degradation of resolution due to charge transfer losses becomes noticeable as Charge Transfer Inefficiency (CTI) increases to 10^–5. In this paper, we present a model which calculates the effects of radiation damage in low Earth orbit in order to predict CCD lifetimes over which good charge transfer is maintained. The model presented here considers damage mechanisms within the CCD, environmental conditions in which the CCD operates, and experiment shielding. We find that the predicted CTI approaches 10^–5 after a one to two year mission for the flight instruments considered here.     

A new model for X-ray emission from NGC 4151

We present an analysis of the reported spectral features of NGC 4151 in X-rays. It is shown that the origin of X-rays from the source is inconsistent with a single production mechanism. We suggest a new two-component model in which soft X-rays arise from the black-body emission of a tiny hot nucleus withT2×10⁷ K and the hard X-ray photons are generated in an extended region by inverse Compton scattering of electrons with the infrared photons.     

A model of two-stream non-radial accretion for binary X-ray pulsars

The general case of non-radial accretion is assumed to occur in real binary systems containing X-ray pulsars. The structure and the stability of the magnetosphere, the interaction between the magnetosphere and accreted matter, as well as evolution of neutron star in close binary system are examined within the framework of the two-stream model of nonradial accretion onto a magnetized neutron star. Observable parameters of X-ray pulsars are explained in terms of the model considered.     

A cocoon model for thermal X-ray sources and ‘Oscillars’

A gas cocoon surrounding a neutron star can be heated to a high temperature by the low frequency radiation emitted by the neutron star whose rotation axis is inclined to its magnetic axis. This heated gas can emit X-rays and may be identified with thermal X-ray sources. If the neutron star emission shows periodicities larger than the cooling time of the gas, these will be reflected in the emission of X-ray; the recently observed X-ray sources which show oscillations and quasiperiodicities (Oscillars) may be such sources.     

X-ray spectra of various bursts from the rapid burster

About 20 type-I bursts and 3000 type-II bursts were observed with Tenma and Hakucho in 1983. Spectral evolutions were analyzed for both types of bursts and results are presented.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.Purple Mountain Observatory, Academia Sinica, Nanjing, China.     

The eccentric-orbit binary model for the transient X-ray sources

Eccentric-orbit binary models for transient X-ray sources are investigated. In these models, a compact star is in an eccentric orbit around a more massive star. As the compact star accretes mass from the stellar wind of the massive star, the accretion rate becomes time-dependent. The accretion rate is determined by Bondi's accretion radius, which depends on both the relative velocity of the stellar wind to the compact star and the sound velocity through the stellar wind. With reasonable sets of the eccentricity, the semi-major axis, the stellar wind velocity and the sound velocity, we obtain the variations of the light curves compatible with observations for the transient X-ray sources. It is likely that many transient X-ray sources are explainable by eccentric-orbit binary models.     

A disk-corona model for the low/hard state of black hole X-ray binaries

A disk-corona model for fitting the low/hard（LH）state of the associated steady jet in black hole X-ray binaries（BHXBs）is proposed based on the large-scale magnetic field configuration that arises from the coexistence of the Blandford-Znajek（BZ）and Blandford-Payne（BP）processes,where the magnetic field configuration for the BP process is determined by the requirement of energy conversion from Poynting energy flux into kinetic energy flux in the jet.It is found that corona current is crucial to guarantee the consistency of the jet launching from the accretion disk.The relative importance of the BZ and BP processes in powering jets from black hole accretion disks is discussed,and the LH state of several BHXBs is fitted based on our model.In addition,we suggest that magnetic field configuration can be regarded as the second parameter for governing the state transition of BHXBs.     

A two-component thermal model of X-ray burst sources

Solar Physics - A semi-empirical model of the soft X-ray source associated with solar flares is presented, based on the results of the two temperature analysis of OSO-5 data obtained by Herring and...     

A model for X-ray bright points due to unequal cancelling flux sources

A recent discovery from the Big Bear Solar Observatory has linked the cancellation of opposite polarity magnetic fragments in the photosphere (i.e., so-called cancelling magnetic features) to X-ray bright points and has stimulated the setting up of a converging flux model for the process. Cancelling magnetic features can occur between magnetic fragments of differing strengths in many different situations. Here, therefore, we model two opposite polarity fragments of different strengths in the photosphere by two unequal sources in an overlying uniform field. Initially in thepre-interaction phase these sources are assumed to be unconnected, but as they move closer together theinteraction phase starts with an X-type neutral point forming, initially on the photosphere, then rising up into the chromosphere and corona before lowering back down to the photosphere. Thecapture phase then follows with the sources fully connected as they move together. Finally, after they come in to contact, during thecancellation phase the weaker source is cancelled by part of the stronger source. The height of the X-type neutral point varies with the separation of the sources and the ratio of the source strengths, as do the positions of the neutral points before connection and after complete reconnection of the two sources. The neutral point is the location of magnetic reconnection and therefore energy release which is believed to power the X-ray bright point in the corona. By using a current sheet approximation, where it is assumed no reconnection takes place as the two sources move together, the total amount of energy released during reconnection may be estimated. The typical total free magnetic energy is found to be of the order of 10²⁰–10²¹ J, which is as required for an X-ray bright point. It is also found that, as the ratio of the source strengths increases, the height of the X-type neutral point decreases, as do the total energy released, and the lifetime of the bright point.     

A model of rapid burster MXB 1730-335

A model of rapid burster MXB 1730-335, the source of type II X-ray burst is proposed, based on the Rayleigh-Taylor instability due to interaction of relativistic electrons produced by the rapid rotation of a highly magnetized neutron star, and the infalling accreted matter through the magnetic funnel at the poles. Conclusions are made that type II X-ray burster may be a constant source of cosmic rays and such a mechanism may be the progenitor of some forms of nebulae.Permanent address: Imphal College, Imphal, Manipur.     

Solar cycle model for oscillating transient X-ray sources

The oscillations observed recently in the transient X-ray source A1118-61 are attributed to variations in the magnetic activity of a rotating white dwarf similar to the 11-year solar activity cycle. The transient nature, in analogy with the sun, is attributed to the variation in the strength of the cycle.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Magnetic reconnection model for X-ray flare loop interaction

The subtle interactions between the magnetohydrodynamics (MHD) and transverse plasmons are investigated. It is shown that there is a resistive instability by the plasmon's soliton in a current sheet, which eventually turns into an eruptive instability at the magnetic field reconnection. In the case of ion-acoustic turbulence, the high temperature current sheet model must adopt the aromalous conductivity instead of the Coulomb conductivity. The numerical results are consistent with the observations obtained by Hanaoka (1994). Thus the flare caused by X-ray loop coalescence can be basically interpreted by this model of magnetic field reconnection driven by ponderomotive force.     

Nonthermal electrons in the thick-target reverse-current model for hard X-ray bremsstrahlung

The behaviour of the accelerated electrons escaping from a high-temperature source of primary energy in a solar flare is investigated. The direct current of fast electrons is supposed to be balanced by the reverse current of thermal electrons in the ambient colder plasma inside flare loops. The self-consistent kinetic problem is formulated; and the reverse-current electric field and the fast electron distribution function are found from its solution. The X-ray bremsstrahlung polarization is then calculated from the distribution function. The difference of results from those in the case of thermal runaway electrons (Diakonov and Somov, 1988) is discussed. The solutions with and without account of the affect of a reverse-current electric field are also compared.     

A hydrodynamic model for asymmetric weak explosion of collapsing supernovae with rapid initial rotation

We further investigate the two-dimensional hydrodynamic explosion model for rapidly rotating and collapsing supernovae (Aksenov et al. 1997), in which the initial energy release inside a fragmenting low-mass neutron star of critical mass ≈0.1M _⊙ moving in a circular orbit at a velocity of ≈18000 km s^?1 is reduced considerably. This velocity closely corresponds to a pulsar escape velocity of ≈1000 km s^?1 (at a total mass of ≈1.9M _⊙ for the binary of neutron stars). Compared to our previous study (Zabrodina and Imshennik 1999), this energy release was reduced by more than a half. Otherwise, the model in question does not differ from the explosion model with a self-consistent chemical composition of nuclides investigated in the above paper. In particular, the initial energy release was carefully reconciled with a chemical composition. Our numerical solution shows that the reduction in energy release due to the time scales of β processes and neutrino energy losses being finite does not alter the qualitative results of our previous studies (Aksenov et al. 1997; Imshennik and Zabrodina 1999). An intense undamped diverging shock wave (with a total post-shock energy ? 10⁵¹ erg at a front radius of ≈10 000 km) is formed; a large asymmetry of explosion with a narrow cone (with a solid angle of ≈π/4) around the leading direction, which coincides with the velocity direction of the low-mass neutron star at the instant of its explosive fragmentation in the two-dimensional model, emerges. A jet of synthesized radioactive nickel, whose mass is estimated by using simple threshold criteria to be M _Ni≈(0.02?0.03)M _⊙ is concentrated inside this cone. This appears to be the integrated parameter that is most sensitive to the specified reduction in initial energy release; it is also reduced by almost a half compared to our previous estimate (Imshennik and Zabrodina 1999). The time of propagation of the shock wave inferred in our model to the presupernova surface was estimated for SN 1987A to be 0.5–1.0 h, in agreement with observations.