Long term observations of Cyg X-3 with Vela 5B

We present long-term (1969–1979) observations of Cygnus X-3, obtained by the Vela 5B satellite. The 3–12 keV light curve has 10 day time resolution. Cyg X-3 is a peculiar high-luminosity X-ray source, radiating from the radio region to hard gamma rays of more than 10¹⁶ eV. It has a 4.8 hour period, probably orbital, which is not resolved by our present analysis. Long term periodicities of 17, 20, and 33–34 days have been reported by several authors, and explained as the effects of apsidal motion, precession, or an eccentric orbit. We do not observe the 17 and 33–34 day variations, and set upper limits significantly lower than the reported amplitude of the 33–34 day variation. There is weak evidence for a 20 day flux variation. The light curve shows high and low states which alternate with a characteristic timescale of 1 year. There is no counterpart, at this time resolution, of the giant radio outburst of 1972 September.     

Periodic or random acceleration in solar flares?

The issue whether acceleration and injection of electron beams is coherently modulated by a single quasi-periodic source, or whether the injection is driven by a stochastic process in time or (eventually fragmented) in space, is investigated by menas of a periodicity analysis of metric type III bursts.We analyze 260 continuous type III groups observed byIkarus (ETH Zurich) in the frequency range of 100–500 MHz during 359 solar flares with simultaneous 25 keV hard X-ray emission, in the years 1980–1983. Pulse periods have been measured between 0.5 and 10 s, and can be described by an exponential distribution, i.e.N(P) e ^–P/1.0s. We measure the mean periodP and its standard deviation p in each type III group, and quantify the degree of periodicity by the dimensionless parameter p/P. The representative sample of 260 type III burst groups shows a mean periodicity of p/P=0.37±0.12, while Monte-Carlo simulations of an equivalent set of truly random time series show a distinctly different value of p/P=0.93±0.26. This result suggests that the injection of electron beams is periodically modulated by a particle acceleration source which is either compact or has a global organization on a time scale of seconds.     

Solar flares and particle acceleration

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 10¹⁰ cm^–3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 10³⁰ erg, and an average number of 10³⁸ electrons with E 15 keV is accelerated. In subsecond pulses these values are about 10³⁵ electrons and about 10²⁷ erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 10⁴ km above the solar photosphere, a region permeated by a large number ( 10) of fluxthreads, each carrying electric currents of 10¹⁰–10¹¹ A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of ³He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived ( 0.1 s) superhot ( 10⁸ K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs by fast reconnection in magnetic fields 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities ( 10¹⁰ cm^–3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and vestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).     

A 0535 + 26/HDE 245770: A typical X-ray/Be system

Among the X-ray/Be systems, A 0535 + 26/HDE 245770 has been noted, since its discovery, for its peculiar features in several respects, in a wide energy range. For this reason and for a series of concomitant favorable causes, this system has been one of the most studied among the massive X-ray binary systems. The most remarkable incident was that its optical identification with an early-type-emission-line star (O9.7IIIe) has led to a deep studies on Be stars and their interactions with neutron stars, which have allowed to discover, without unbiguity, the presence of optical indicators of consequent X-ray flares, as well as that Be stars in X-ray/Be systems behave just as normal Be stars. Overmore, thanks to the multifrequency coordinated observations of this system, the X-ray emissions from binary companion of the Be stars are best explained by assuming the presence of a thick equatorial disk with low expansion velocity and a thin polar region with high expansion velocity. This picture reconciled the strong discrepancy in mass loss rate evaluations coming from IR and from UV measurements, assuming that the observed regions are enterely distinct from each other, one being a high-density, low-velocity region, and the other being a low-density, very hot, rapidly-expanding disk-like zone.Since, this picture seems to be the best up-to-date frame to cuckold all the experimental panorama available on X-ray/Be systems, we would like to paint in this paper the multifrequency behaviour of A 0535 + 26/HDE 245770, which is the best studied among such systems, in order to stimulate future coordinated experimental-theoretical works on this very interesting class of objects.     

Stellar flares

Radio and X-ray observations of stellar flares provide the most direct probes of energy relaase particle acceleration, and energy transport on stars other than the Sun. In this review, the observational basis for our understanding of the flare phenomenon on other stars is briefly described and outstanding interpretive and theoretical issues are discussed. I shall confine my attention to objects which are solar-like, to the extent that they possess deep convective envelopes and display activity which is presumed to be magnetic in origin. These include pre-main sequence objects, classical flare stars, and close binaries. Future directions are briefly discussed.     

Wolf–Rayet star nucleosynthesis and the isotopic composition of the Galactic Cosmic Rays

There is now strong observational evidence that the composition of the Galactic Cosmic Rays (GCRs) exhibits some significant deviations with respect to the abundances measured in the local (solar neighbourhood) interstellar medium (ISM). Two main scenarios have been proposed in order to account for these differences (`anomalies). The first one, referred to as the `two-component scenario, invokes two distinct components to be accelerated to GCR energies by supernova blast waves. One of these components is just made of ISM material of `normal solar composition, while the other one emerges from the wind of massive mass-losing stars of the Wolf–Rayet (WR) type. The second model, referred to as the `metallicity-gradient scenario, envisions the acceleration of ISM material whose bulk composition is different from the local one as a result of the fact that it originates from inner regions of the Galaxy, where the metallicity has not the local value. In both scenarios, massive stars, particularly of the WR type, play an important role in shaping the GCR composition. After briefly reviewing some basic observations and predictions concerning WR stars (including s-process yields), this paper revisits the two proposed scenarios in the light of recent non-rotating or rotating WR models.     

Emission mechanism for low-frequency radiation in the outer heliosphere

The question of how low-frequency radio emissions in the outer heliosphere might be generated is considered. It is argued that the free energy contained in an electron beam distribution is first transformed into electrostatic Langmuir waves. The nonlinear interactions of these waves which can produce electromagnetic waves are then treated in the semi-classical formalism. Comparison of the results of the discussed model with electromagnetic radiation coming from upstream of the Earth's bow shock shows that the model adequately explains the generation of plasma waves at planetary shocks. By analogy, this model can provide a quantitative explanation of intensity of radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere provided that the electron beams generating Langmuir waves exist also in the postshock plasma due to secondary shocks in the compressed solar wind beyond the termination shock. The field strength of Langmuir waves required to generate the second harmonic emissions are approximately of 100–200 V m^–1 for the primary and 50–100 V m^–1 for the secondary foreshocks. However, only in the secondary foreshock the expected density is consistent with the observed frequency.     

X-ray Emission From Snrs Undergoing Efficient Shock Acceleration

In nonlinear, diffusive shock acceleration, compression ratios will be higher and the shocked temperature lower than test-particle, Rankine–Hugoniot relations predict. The heating of the gas to X-ray emitting temperatures is strongly coupled to the acceleration of cosmic-ray ions. We have developed a simple hydrodynamical supernova remnant model which includes the effects of nonlinear acceleration (Berezhko and Ellison, 1999). We show how efficient particle acceleration modifies the dynamics of supernova remnants, and use the X-ray spectral data on Keplers remnant to illustrate the effects on the thermal X-ray emission, including non-equilibrium ionization effects (Decourchelle et al., 2000a).     

The complex Hα profile of the hypergiant HR 8752 during 1970–1992

The profiles of H observed during the 1970–1992 period in the binary hypergiant HR 8752 (G0 Ia) are presented. We distinguish five typical H profiles designated as A, B, C, D and E types according to the number of emission and absorption features. The profiles of H are complex and contain several emission and absorption components, with: –130 km/s in emission or absorption, –84 km/s in absorption, –49 km/s in emission and about +6 km/s in emission. All of them are rather stable in radial velocities except of the main absorption component in the P Cygni profile with –84 km/s. The frequency of appearance and the periods of duration of the occurrence of the components is discussed. The duration times range between about 3 to 10 months for various components. The red emission component E₂ is particularly interesting. Possible explanations of its origin are discussed.A long-term acceleration of the absorption component in the P Cygni profile is found; it can be interpreted as monotonous acceleration of the stellar wind.     

An optical outburst from the periodic recurrent X-ray transient A0538-66

We present optical spectroscopy and photometry and IUE spectroscopy of the counterpart of the LMC recurrent X-ray transient A0538-66 during an outburst at the end of December 1980 which was consistent with the 16.6 day X-ray period (Skinner, 1980). The optical spectra show steadily increasing Balmer and HeI emission (indicative of a shell phase) superposed on a B2 IV spectrum with a substantial brightness increase of 2^m and the sharp turn-on of HeII 686 at the peak. Significant radial velocity changes have been detected but they show no correlation with the 16.6 day period. IUE spectra during a subsequent outburst show very strong and broad (5000 km s^–1) emission from C IV 1550 and HeII 1640. This behaviour is compared with other galactic transients and shell/Be stars.     

Properties of Ground Level Enhancement Events and the Associated Solar Eruptions During Solar Cycle 23

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenomena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average ～2000?km/s). An M7.1 flare and a 1200?km/s CME are the weakest events in the list of 16 GLE events. Most (80?%) of the CMEs are full halos with the three non-halos having widths in the range 167 to 212?degrees. The active regions in which the GLE events originate are generally large: 1290?msh (median 1010?msh) compared to 934?msh (median: 790?msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type?II onset and GLE particle release, we estimated the initial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3?km/s² vs. 1?km/s²). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type?II bursts, low frequency type?III bursts, soft X-ray flares and CMEs. The presence of metric type?II radio bursts some 17?min (median: 16?min; range: 3 to 48?min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of ～3.09?R _s (median: 3.18?R _s ; range: 1.71 to 4.01?R _s ) for well-connected events (source longitude in the range W20–W90). For poorly connected events, the average CME height at GLE particle release is ～66?% larger (mean: 5.18?R _s ; median: 4.61?R _s ; range: 2.75–8.49?R _s ). The longitudinal dependence is consistent with shock accelerations because the shocks from poorly connected events need to expand more to cross the field lines connecting to an Earth observer. On the other hand, the CME height at metric type?II burst onset has no longitudinal dependence because electromagnetic signals do not require magnetic connectivity to the observer. For several events, the GLE particle release is very close to the time of first appearance of the CME in the coronagraphic field of view, so we independently confirmed the CME height at particle release. The CME height at metric type?II burst onset is in the narrow range 1.29 to 1.8?R _s , with mean and median values of 1.53 and 1.47?R _s . The CME heights at metric type?II burst onset and GLE particle release correspond to the minimum and maximum in the Alfvén speed profile. The increase in CME speed between these two heights suggests an increase in Alfvénic Mach number from?2 to?3. The CME heights at GLE particle release are in good agreement with those obtained from the velocity dispersion analysis (Reames in Astrophys. J. 693:812, 2009a; Astrophys. J. 706:844, 2009b) including the source longitude dependence. We also discuss the implications of the delay of GLE particle release with respect to complex type?III bursts by ～18?min (median: 16?in; range: 2 to 44?min) for the flare acceleration mechanism. A?similar analysis is also performed on the delay of particle release relative to the hard X-ray emission.     

First detection of an X-ray burst and a one hour intensity dip in 4U1323-62

We report the results of a 1.4 10⁴s observation of the region of 4U 1323-62 with the EXOSAT ME. The source has a flux of 7–8 10^-11 erg/cm²s (2–10 keV) and a power-law spectrum with 1.1 < < 1.8. During our observation, the source showed a symmetric 60% dip in its X-ray flux of R~1 hr. The spectrum hardens during the dip. Inside the dip we observed an X-ray burst with a 2–10 keV peak flux of 7 10^-10 erg/cm²s. The burst spectrum is black-body, and shows evidence of cooling during the burst decay. The discovery of a burst from 4U 1323-62 settles the classification of the source; the observation of a dip suggests that we may be able to measure its orbital period in the near future.     

The Be stars

Classical Be stars are defined and their relationship to normal B-type stars stated. Spectral classification of the underlying stars suggests that, on the average, Be stars are located 0.5–1.0 magnitude above the main sequence. Struve's rotational model for Be stars, and several tests which support the model, are reviewed. The best evidence at this time suggests that Be stars may not rotate with the critical velocity at which centrifugal force just balances the equatorial gravitational force, but a number of mechanisms for getting material out into the shell have been proposed and are discussed.The physical characteristics of Be shells were first derived from optical observations of shell stars, supplemented more recently by ultraviolet, infrared, radio, and polarization measurements. These data suggest that Be shells are probably lenticular with radii 3 to 20 times the radius of the underlying star, excitation temperatures lower than those of the reversing layers, and electron densities in the range 10¹⁰-10¹³ cm^-3.Variability of Be stars, from spectroscopic, photometric, and polarimetric observations, seems well established over time scales of years and months, but the evidence for night-to-night and hourly changes is somewhat conflicting. Of special interest are recent X-ray observations of several Be stars.Models for the envelopes of Be stars are reviewed, including state-state stellar wind models, time-dependent stellar wind models, the elliptical ring model, disk models, and binary models. Finally, the evolutionary status of Be stars is discussed, and some recommendations for future work made.     

Theoretical studies of the flux and energy spectrum of gamma radiation from the Sun

The purpose of this work is to study the various -ray-production mechanisms in solar flares and to calculate the flux, the spectrum, and the decay curves of radiation. Using the continuity equation and taking into account the energy losses for solar-flare-accelerated particles, we obtain the time-dependent particle distribution and thus the time behavior of the resulting rays. The important processes for producing rays in solar flares are found to be nonthermal electron bremsstrahlung, decay of neutral mesons, positron annihilation, neutron capture, and decay of excited nuclei. The results are applied to several known solar flares. For a large flare such as the class 3⁺ on February 23, 1956, continuous rays with energies up to 100 MeV from electron bremsstrahlung and neutral meson decays are observable at the orbit of the Earth by existing -ray detectors. Line rays from positron annihilation (0.51 MeV), neutron capture (2.23 MeV), and deexcitation of excited nuclei O¹⁶ (6.14 and 7.12 MeV) and C¹² (4.43 MeV) are particularly strong and well above the continuous -ray background due to electron bremsstrahlung. These lines can be detected at the Earth.NASA-NRC Resident Research Associate.     

Microwave burst of November 17, 1991: Evidence of fragmented particle injection into a coronal loop

The unusual event of November 17, 1991 07:04 UT, observed at 2.5 and 2.85 GHz, is analysed. The event reveals the sophisticated superfine time structure including sudden reductions and quasi-periodic pulsations. We shown that the sudden reductions (30–100 ms) can be driven by upward injected 100 keV electron beams filling the losscone of the coronal magnetic trap. The nonlinear oscillations of Langmuir waves provide the best fit for the pulsation observed. A reduced steady-state level of the pulsation phase is connected with quasi-continuous injection of electrons. The physical parameters of the radio source are found for the event.     

EXOSAT observations of active stellar coronae in the Cygnus Loop,T Tau & NGC 2264 regions

A highly variable point X-ray source, first seen by the Einstein IPC, has been positioned with the EXOSAT CMA and identified with a bright (V = 8.5) K0 star. Although in the direction of the southern half of the Cygnus Loop, this star is almost certainly a foreground object and typical of other active cool stars that are related to RS CVn systems.An EXOSAT program to study T Tauri stars failed to detect T Tau itself. However, a strong X-ray source was observed 15 from T Tau, which in its turn had not been seen by Einstein. This new source has been identified with a hitherto unstudied 13 mag star which is likely to be a dMe flare star.The young star cluster NGC 2264 was observed with the EXOSAT CMA in an attempt to identify the sources found during an Einstein IPC study of S Mon. Apart from S Mon itself, only UV-bright objects were seen, but several of these are considered likely counterparts of the Einstein sources.     

A study of M100 in X-rays

During a search for X-ray emission from Supernova 1979c, the parent galaxy M100 (NGC 4321) was repeatedly observed with the IPC and HRI instruments aboard the Einstein X-ray Observatory. The X-ray data reveal two possible sources in the arms of the spiral galaxy, two components in the nuclear bulge and extended X-ray emission from the central part of the galaxy (160x160 square arc seconds centered on the nucleus). We find that the estended X-ray emission cannot be explained in terms of inverse Compton effect on radio, optical or 3 K blackbody photons but rather it is likely to originate from supernova remnants (M100 is indeed a prolific supernova producer) and/or early type stars. As for M100 as a whole, the ratio of X-ray to optical liminosity places it half way between normal galaxies e.g. M31 or M33 and peculiar or active galaxies.     

The transient highly excited solar flare plasma

Recent observations of the energetic particles produced in solar flares indicate that the production of electrons, with energies up to about 100 keV, is a fairly common feature of small flares. In those flares the acceleration of protons and other nuclei does not extend beyond about 1 MeV.The X-ray emission often exhibits two distinct components of which the first one is produced by non-thermal, the second by thermal electrons through bremsstrahlung collisions with the ambient ions. Along with these X rays, radio emission, in the microwave region, is observed. This radio emission is usually interpreted as due to gyrosynchrotron radiation from the same electrons.In this review a discussion is presented of the processes occurring in solar flares with special reference to the acceleration and radiation processes.     

A comparison between observed and predicted mass-loss rates and wind momentum of O stars

Empirical mass-loss rates were derived for 28 luminous O stars from radio fluxes and H equivalent widths. Comparison with theoretical values predicted by the theory of radiatively driven winds reveals a discrepancy of 0.30±0.05 dex, with the theoretical values being too low. We show that there is not only a mass-loss discrepancy but also a momentum flux discrepancy. The theoretically predicted momentum fluxes are too low by 0.17±0.04 dex. This discrepancy is independent of the adopted stellar mass. We demonstrate that the momentum discrepancy in the most luminous O stars is comparable to the one found in the least extreme Wolf-Rayet stars. We suggest that the physical reason for the break-down of the theory in Wolf-Rayet stars and the most luminous O stars may be related.     

Imaging diagnostics of solar non-thermal particles

Non-thermal hard X-ray, gamma-ray and radio emission are the most direct signatures of the presence of energetic particles in the solar atmosphere. This paper lays emphasis on hard X-ray and radio imaging data, obtained during and outside flares, which reveal the sites where particles interact with the ambient medium. These observations, which provide more or less direct information on the topology and dynamics of the magnetic structures in which particles are accelerated and propagate, are discussed in the framework of the statistical flare scenario.