Peculiarities of the plasma mechanism of radio emission from late-type stars

The plasma mechanism of radio emission in the coronas of late-type stars is shown to be considerably more efficient than that in the solar corona because of the high plasma temperature in their magnetic arches. This is attributable to an increase in the length of plasma-wave conversion into electromagnetic waves and a decrease in the optical depth of collisional wave absorption. Magnetic-arch filamentation results in a decrease in the intensity of the fundamental-tone radio emission and in the relative dominance of the second-harmonic radio emission. The efficiency of the fundamental-tone radio emission increases with plasma density in a coronal arch. The plasma mechanism accounts for the high brightness temperature of the flare radio emission from stars (≥10¹⁴ K).     

High excitation emission lines in binary systems with roundchroms

An unexpected empirical fact, a dependence of the observed luminosities inhigh excitation emission lines – 1240 NV, 1400 SiIV, 1550 CIV, 1640 HeII– on the intercomponent distance a of RS CVn type close binary systems,is revealed. It is assumed that those high excitation emission lines aregenerated most probably in a cone-like region between the Lagrangian pointL ₁ and the surface of the primary component of the system. Thebehavior of high excitation emission lines at various phases of theeclipse in the case of two binary systems, SX Cas and 22 Vul, indicatesthe possibility of existence of such a `Lagrangian cone' in the structureof common chromospheres – roundchroms – of close binary systems as amain source of generation of high excitation emission lines.     

High energy gamma-ray emission from binaries

The current Cherenkov telescopes together with GLAST are opening up a new window into the physics at work close to black holes and rapidly rotating neutron stars with great breakthrough potential. Very high energy gamma-ray emission up to 10 TeV is now established in several binaries. The radiative output of gamma-ray binaries is in fact dominated by emission above 1–10 MeV. Most are likely powered by the rotational spindown of a young neutron star that generates a highly relativistic wind. The interaction of this pulsar wind with the companion’s stellar wind is responsible for the high energy gamma-ray emission. There are hints that microquasars, accretion-powered binaries emitting relativistic jets, also emit gamma-ray flares that may be linked to the accretion–ejection process. Studying high energy gamma-ray emission from binaries offers good prospects for the study of pulsar winds physics and may bring new insights into the link between accretion and ejection close to black holes.     

The far-infrared–radio relationship at high and low redshift

We use the results of the SCUBA Local Universe Galaxy Survey, a submillimetre (submm) survey of galaxies in the nearby Universe, to investigate the relationship between the far-infrared (FIR)–submm and radio emission of galaxies at both low and high redshift. At low redshift we show that the correlation between radio and FIR emission is much stronger than the correlation between radio and submm emission, which is evidence that massive stars are the source of both the FIR and radio emission. At high redshift we show that the submm sources detected by SCUBA are brighter sources of radio emission than are predicted from the properties of galaxies in the local Universe. We discuss possible reasons for the cosmic evolution of the relationship between radio and FIR emission.     

Green corona and solar sector structure

Analysis of the green line corona for the interval 1947–1970 suggests the existence of largescale organization of the emission. The green line emission at high northern latitudes (≈ 40°–60°) is correlated with the emission at high southern latitudes 6, 15 and 24 days later, while the low latitude green corona seems to be correlated on both sides of the equator with no time lag. These coronal features are recurrent with a 27-day period at all latitudes between ± 60 °, and we associate these large-scale structures with the solar magnetic sector structure. The high correlation between northern and southern high-latitude emission at 15 days time lag is explained as a signature of a two-sector structure, while four sectors are associated with the 6 and 24 day peaks.     

日球边界射电辐射的研究进展

日球边界射电辐射是太阳系最强的射电辐射现象,辐射功率至少达1013 W,能够提供日球边界附近高能电子束和背景磁等离子体结构的重要物理信息.自1983年旅行者号卫星首次探测到日球边界射电辐射后,其便受到研究者们的广泛持续关注.日球边界射电辐射大致有两类:辐射频率相对较高的瞬时辐射或称漂移辐射以及辐射频率相对较低的持续辐射或称非漂移辐射.通常两类辐射都从大约2 kHz开始,漂移辐射具有向高频率漂移的特征,频漂率约为1–3 kHz/yr,频率范围1.8–3.6 kHz,持续时间较短大致100–300 d;非漂移辐射没有明显的频率漂移,频率范围1.8–2.6 kHz,持续时间较长大致3 yr.目前普遍认为日球边界射电辐射与激波有关.介绍了该射电辐射可能的辐射产生源区、辐射物理机制以及与辐射相关的激波来源,并且讨论了尚存在的科学问题以及展望了未来可以进一步开展的研究.     

脉冲星的射电发射

脉冲星的射电辐射与其他天体物理辐射源有很大的不同 ,因为它们有着极高的亮温度和高度的线或圆偏振。极高的亮温度意味着起作用的发射机制一定是相干的。至今尚无对这种辐射普遍接受的模型。本文讨论了关于脉冲星的射电辐射产生和传播研究中的新进展。     

Fe K emission in the ultraluminous infrared galaxy Arp 220

Prominent Fe Kα line emission is detected in the XMM–Newton spectrum of the ultraluminous infrared galaxy Arp 220. The centroid of the line is found at an energy of 6.7 keV and the equivalent width of the line is  EW ∼ 1.9 keV  (at 3.5σ significance). A few other spectral features are found at various degrees of significance in the lower energy range on a hard 2.5–10 keV continuum  (Γ∼ 1)  . The large EW of the Fe K line poses a problem with interpreting the hard X-ray emission as integrated X-ray binary emission. A thermal emission spectrum with a temperature of   kT ∼ 7 keV  modified by absorption of   N _H≃ 3 × 10²² cm⁻²  , can describe the 2.5–10 keV continuum shape and the Fe K emission. A hot bubble that is shocked internally in a starburst region would have a similar temperature and gives a good explanation for the observed X-ray properties with a high star formation rate. An ensemble of radio supernovae in a dense environment, as suggested from VLBI imaging, could be another possibility, if such powerful supernovae are produced continuously at a high rate. However, the apparent lack of emission from X-ray binaries is incompatible with the high supernova rate (∼2 SNe yr⁻¹) required by both interpretations. Highly photoionized, low-density gas illuminated by a hidden Compton-thick active galactic nucleus is a possible alternative for the hard X-ray emission, which can be tested by examining whether radiative recombination continua from highly ionized Ca and Fe are present in better quality data from a forthcoming observation.     

Photoionising shocks in SNRs and AGN

A high velocity radiative shock, or one moving into high-metallicity gas, provides an efficient means to generate a strong local UV photon field. The optical emission from the shock and precursor region is dominated by the photoionised gas, rather than by the cooling region, and the total optical + UV emission scales as the mechanical energy flux through the shock. In this paper, such models are applied to oxygen-rich supernova remnants and AGN. For AGN, the degree of magnetic support in the post-shock gas is an important parameter. LINER and cooling flow spectra can be understood as resulting from high velocity shocks without precursors, while Seyfert 1.5–2 galaxy emission line ratios result from high velocity shocks with their photoionised precursor HII regions. This model explains the problem of the high electron temperatures observed in both classes of object.     

Emission from the galaxy NGC 1275 at high and very high energies and its origin

The Seyfert galaxy NGC 1275 is the central, dominant galaxy in the Perseus cluster of galaxies. NGC1275 is known as a powerful source of radio and X-ray emission. The well-known extragalactic object NGC 1275 has been observed by the SHALON high-altitude mirror Cherenkov telescopes within the framework of long-term studies of metagalactic gamma-ray sources. In 1996, the SHALON observations revealed a new metagalactic source of very high energy gamma-ray emission coincident in its coordinates with the galaxy NGC 1275. Having analyzed the SHALON data, we have determined such characteristics of NGC 1275 as the spectral energy distributions and images at energies >800 GeV for the first time. The results obtained at high and very high energies are needed for understanding the emission generation processes in an entire wide energy range.     

Evidence for a jet contribution to the optical/infrared light of neutron star X-ray binaries

Optical/near-infrared (optical/NIR, OIR) light from low-mass neutron star X-ray binaries (NSXBs) in outburst is traditionally thought to be thermal emission from the accretion disc. Here we present a comprehensive collection of quasi-simultaneous OIR and X-ray data from 19 low magnetic field NSXBs, including new observations of three sources: 4U 0614+09, LMC X−2 and GX 349+2. The average radio–OIR spectrum for NSXBs is  α≈+ 0.2  (where   L _ν∝ν^α  ) at least at high luminosities when the radio jet is detected. This is comparable to, but slightly more inverted than the  α≈ 0.0  found for black hole X-ray binaries. The OIR spectra and relations between OIR and X-ray fluxes are compared to those expected if the OIR emission is dominated by thermal emission from an X-ray or viscously heated disc, or synchrotron emission from the inner regions of the jets. We find that thermal emission due to X-ray reprocessing can explain all the data except at high luminosities for some NSXBs, namely, the atolls and millisecond X-ray pulsars. Optically thin synchrotron emission from the jets (with an observed OIR spectral index of  α_thin < 0  ) dominate the NIR light above     and the optical above     in these systems. For NSXB Z-sources, the OIR observations can be explained by X-ray reprocessing alone, although synchrotron emission may make a low-level contribution to the NIR, and could dominate the OIR in one or two cases.     

Physical mechanisms of the spectral variability of α Orionis with IUE

We present a spectroscopic study of Mg II k&h emission lines in the 2000–3000 Å region of 91 high resolution IUE spectra of α Orionis obtained during the period 1978–1996. There are absorption and emission components on the blue sides of the k&h emission lines, these components may be coming from the dust envelope located inside the extended atmosphere of α Orionis. A set of 91 Mg II k&h emission lines have been identified and measured to determine their fluxes and widths. We found that there is a spectral variability for these physical parameters with phase, similar to that found for the light curve, which we attribute to the changes of density and temperature of the regions from which these emission lines are coming, as a result of the semi-regular pulsation and the variability of mass loss of the red supergiant.Also we present a study of C II and Fe II emission features in the 2000–3000 Å region of 55 high resolution IUE spectra during the period 1978–1996. A set of 55 C II and Fe II emission lines have been identified and measured to determine their fluxes and widths. We found the same result as with the Mg II emission lines.     

2800 Mg II emission in the most active radio stars

Spectral recordings in the region near 2800 Å are examined from the IUE archives for the twenty most active radio stars, which are also close binary systems. In all of these spectra, the doublet k and h Mg II is seen in strong emission. We find that the observed fluxes of the magnesium doublet emission are directly proportional to the mean fluxes of the observed radio emission at the frequency of 8.4 GHz. The same correlation is found between the absolute luminosities of the radio emission and magnesium doublet emissions. It is argued that the source of both emissions, radio and magnesium doublet, is related to a high temperature stratified cloud located between the components of the binary system. The fluxes of the magnesium doublet emission of these sources are much larger, by one or two orders of magnitude, compared with the usual chromospheric magnesium emission seen in single stars.     

Gamma ray signatures of ultra high energy cosmic ray accelerators: electromagnetic cascade versus synchrotron radiation of secondary electrons

We discuss the possibility of observing ultra high energy cosmic ray sources in high energy gamma rays. Protons propagating away from their accelerators produce secondary electrons during interactions with cosmic microwave background photons. These electrons start an electromagnetic cascade that results in a broad band gamma ray emission. We show that in a magnetized Universe (B≳10⁻¹² G) such emission is likely to be too extended to be detected above the diffuse background. A more promising possibility comes from the detection of synchrotron photons from the extremely energetic secondary electrons. Although this emission is produced in a rather extended region of size ∼10 Mpc, it is expected to be point-like and detectable at GeV energies if the intergalactic magnetic field is at the nanogauss level.      

Resolved OH megamaser emission in the nuclear region of the ultraluminous infrared galaxy Markarian 273

We present high angular resolution MERLIN observations of the 18-cm OH maser and continuum emission associated with the active core of the ultraluminous infrared galaxy Markarian 273. The continuum emission comes from three distinct regions in the central arcsecond of the galaxy. The brightest region of emission has a double-peaked structure which is spatially coincident with similar structures observed at 6 cm and 2.2 μm. The peak of the OH maser emission is spatially coincident with the peak in the continuum. For the first time the maser emission is spatially resolved, allowing us to measure the gas motion within the central 100 pc of the galaxy. Maser emission is found in both the 1665- and 1667-MHz lines, with no systematic offset found in the spatial locations of the two lines. The brighter component of the maser emission shows ordered motion and is aligned along the axis of the double-peaked structure in the brightest continuum region. The gas motion enables us to estimate the central mass density to be 850±50 M_⊙ pc⁻³, which corresponds to a total mass of ≈1.5×10⁸ M_⊙.     

An ionized disc reflection component for the X‐ray spectrum of NGC 4051 and IRAS13224–3809?

A spectral variability study of the two Narrow Line Seyfert 1 galaxies NGC 4051 and IRAS13224–3809 is presented. Both sources show a high degree of flux and spectral variability. The nuclear emission, lightly absorbed by warm material, has been decomposed into a direct power law emission and an ionized disc reflection plus constant emission from distant material. The ionized disc reflection component does not follow the variations of the primary component. Its flux is linearly correlated with the one of the power law component only at low fluxes, while it is almost constant at medium high‐flux. This behavior is expected when the light bending effect is important. If so, most of the primary emission comes from only a few gravitational radii from the black hole. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Extended inverse-Compton emission from distant, powerful radio galaxies

We present Chandra observations of two relatively high redshift FR II radio galaxies, 3C 432 and 3C 191 (   z = 1.785  and 1.956, respectively), both of which show extended X-ray emission along the axis of the radio jet or lobe. This X-ray emission is most likely to be due to inverse-Compton scattering of cosmic microwave background (CMB) photons. Under this assumption, we estimate the minimum energy contained in the particles responsible. This can be extrapolated to determine a rough estimate of the total energy. We also present new, deep radio observations of 3C 294, which confirm some association between radio and X-ray emission along the north-east–south-west radio axis and also that radio emission is not detected over the rest of the extent of the diffuse X-ray emission. This together with the offset between the peaks of the X-ray and radio emissions may indicate that the jet axis in this source is precessing.     

Cosmic Čerenkov emission

In real intensive cosmic sources it is necessary to take into account the ?erenkov reabsorption which restricts the brightness temperature of ?erenkov emission by energies of emitted relativistic electrons. As ?erenkov optical depth is given by τ_c~N/E, the brightness temperature of emission on frequencies near gyrofrequency is determined by electron energyE _r in which τ_c(E _r)=1. It is shown that most of the synchrotron sources (quasars, pulsars) must be powerful sources of ?erenkov emission. In these sources the high density of low frequency ?erenkov emission can give rise to many parametric effects.     

A survey of molecular hydrogen in the central galaxies of cooling flows

We present a large sample of H - and K -band spectra of 32 optically line-luminous central cluster galaxies. We find significant rovibrational H₂ emission in 23 of these galaxies as well as H recombination and/or [Fe  ii ] emission in another five. This represents a fourfold increase in the number of molecular line detections known. A number of the detections are of extended emission (5–20 kpc). In several objects we find significant [Si  vi ] emission that appears to correlate with the strength of high ionization lines in the optical (e.g. [O  iii ]). This comprehensive sample builds on previous work and confirms that warm (1000–2500 K) molecular hydrogen is present wherever there is ionized material in the cores of cooling flows and in most cases it also coincides with CO molecular line emission.     

Investigations of Stephenson's Hα stars

Hitherto unstudied objects from Stephenson's list of Hα emission line objects at high galactic latitude were observed spectroscopically to prove their nature. 9 out of 11 objects show Hα in emission. Spectroscopy combined with photometric information indicates most of them being classical Be stars, while one object is a Post‐AGB star and one a T‐Tauri star. The classification of two objects, which are showing Hα in emission, is unclear. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)