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Cosmic rays accelerated by a shock form a streaming distribution of outgoing particles in the foreshock region. If the ambient fields are negligible compared to the shock and cosmic ray energetics, a stronger magnetic field can be generated in the shock upstream via the streaming (Weibel-type) instability. Here we develop a self-similar model of the foreshock region and calculate its structure, e.g., the magnetic field strength, its coherence scale, etc., as a function of the distance from the shock. Our model indicates that the entire foreshock region of thickness $\sim R/(2\Gamma_{\rm sh}^2)$, being comparable to the shock radius in the late afterglow phase when $\Gamma_{\rm sh}\sim1$, can be populated with large-scale and rather strong magnetic fields (of sub-gauss strengths with the coherence length of order $10^{17} {\rm cm}$) compared to the typical interstellar medium magnetic fields. The presence of such fields in the foreshock region is important for high efficiency of Fermi acceleration at the shock. Radiation from accelerated electrons in the foreshock fields can constitute a separate emission region radiating in the UV/optical through radio band, depending on time and shock parameters. We also speculate that these fields being eventually transported into the shock downstream can greatly increase radiative efficiency of a gamma-ray burst afterglow shock., Comment: 10 pages, 1 figure. Submitted to ApJ

We address the question of whether numerical particle-in-cell (PIC) simulations and laboratory laser-plasma experiments can (or will be able to, in the near future) model realistic gamma-ray burst (GRB) shocks. For this, we compare the radiative cooling time, t_cool, of relativistic electrons in the shock magnetic fields to the microscopic dynamical time of collisionless relativistic shocks -- the inverse plasma frequency of protons, omega_pp^{-1}. We obtain that for t_cool*omega_pp^{-1}\lesssim ~few hundred, the electrons cool efficiently at or near the shock jump and are capable of emitiing away a large fraction of the shock energy. Such shocks are well-resolved in existing PIC simulations; therefore, the microscopic structure can be studied in detail. Since most of the emission in such shocks would be coming from the vicinity of the shock, the spectral power of the emitted radiation can be directly obtained from finite-length simulations and compared with observational data. Such radiative shocks correspond to the internal baryon-dominated GRB shocks for the conventional range of ejecta parameters. Fermi acceleration of electrons in such shocks is limited by electron cooling, hence the emitted spectrum should be lacking a non-thermal tail, whereas its peak likely falls in the multi-MeV range. Incidentally, the conditions in internal shocks are almost identical to those in laser-produced plasmas; thus, such GRB-like plasmas can be created and studied in laboratory experiments using the presently available Petawatt-scale laser facilities. An analysis of the external shocks shows that only the highly relativistic shocks, corresponding to the extremely early afterglow phase, can have efficient electron cooling in the shock transition. We emphasize the importance of radiative PIC simulations for further studies., Comment: 15 pages, submitted to ApJ

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and microquasars commonly exhibit power-law emission spectra. Recent PIC simulations of relativistic electron-ion (or electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In collisionless, relativistic shocks, particle (electron, positron, and ion) acceleration is due to plasma waves and their associated instabilities (e.g., the Weibel (filamentation) instability) created in the shock region. The simulations show that the Weibel instability is responsible for generating and amplifying highly non-uniform, small-scale magnetic fields. These fields contribute to the electron's transverse deflection behind the jet head. The resulting "jitter" radiation from deflected electrons has different properties compared to synchrotron radiation, which assumes a uniform magnetic field. Jitter radiation may be important for understanding the complex time evolution and/or spectra in gamma-ray bursts, relativistic jets in general, and supernova remnants., Comment: : 4 pages, 1 figure and 1 table, typos are corrected, submitted for the Proceedings of The 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, July 7-11, 2008, in Heidelberg, Germany

We consider radiation emitted by the jitter mechanism in a Blandford-McKee self-similar blastwave. We assume the magnetic field configuration throughout the whole blastwave meets the condition for the emission of jitter radiation and we compute the ensuing images, light curves and spectra. The calculations are performed for both a uniform and a wind environment. We compare our jitter results to synchrotron results. We show that jitter radiation produces slightly different spectra than synchrotron, in particular between the self-absorption and the peak frequency, where the jitter spectrum is flat, while the synchrotron spectrum grows as \nu^{1/3}. The spectral difference is reflected in the early decay slope of the light curves. We conclude that jitter and synchrotron afterglows can be distinguished from each other with good quality observations. However, it is unlikely that the difference can explain the peculiar behavior of several recent observations, such as flat X-ray slopes and uncorrelated optical and X-ray behavior., Comment: 11 pages, 7 postscript figures. Accepted for publication in MNRAS

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations of relativistic electron-ion (electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In the presence of relativistic jets, instabilities such as the Buneman instability, other two-streaming instability, and the Weibel (filamentation) instability create collisionless shocks, which are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The ``jitter'' radiation from deflected electrons in small-scale magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation, a case of diffusive synchrotron radiation, may be important to understand the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants., Comment: 8 pages,3 figures, accepted for the Proceedings of Science of the Workshop on Blazar Variability across the Electromagnetic Spectrum, April 22 to 25, 2008

It has been suggested that galactic shock asymmetry induced by our galaxy's infall toward the Virgo Cluster may be a source of periodicity in cosmic ray exposure as the solar system oscillates perpendicular to the galactic plane. Here we investigate a mechanism by which cosmic rays might affect terrestrial biodiversity, ionization and dissociation in the atmosphere, resulting in depletion of ozone and a resulting increase in the dangerous solar UVB flux on the ground, with an improved ionization background computation averaged over a massive ensemble (about 7 x 10^5) shower simulations. We study minimal and full exposure to the postulated extragalactic background. The atmospheric effects are greater than with our earlier, simplified ionization model. At the lower end of the range effects are too small to be of serious consequence. At the upper end of the range, ~6 % global average loss of ozone column density exceeds that currently experienced due to effects such as accumulated chlorofluorocarbons. The intensity is less than a nearby supernova or galactic gamma-ray burst, but the duration would be about 10^6 times longer. Present UVB enhancement from current ozone depletion ~3% is a documented stress on the biosphere, but a depletion of the magnitude found at the upper end of our range would double the global average UVB flux. For estimates at the upper end of the range of the cosmic ray variability over geologic time, the mechanism of atmospheric ozone depletion may provide a major biological stress, which could easily bring about major loss of biodiversity. Future high energy astrophysical observations will resolve the question of whether such depletion is likely., Comment: 22 pages, 5 figures, to be published in Journal of Geophysical Research--Planets. This is an update and replacement for our 2008 paper, with a much more extensive simulation of air shower ionization. Ionization effects and ozone depletion are somewhat larger

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks e.g. gamma-ray bursts (GRBs) active galactic nuclei (AGNs) and microquasars commonly exhibit power-law emission spectra. Recent PIC simulations of relativistic electron-ion (or electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In collisionless relativistic shocks particle (electron, positron and ion) acceleration is due to plasma waves and their associated instabilities (e.g. the Weibel (filamentation) instability) created in the shock region. The simulations show that the Weibel instability is responsible for generating and amplifying highly non-uniform small-scale magnetic fields. These fields contribute to the electron's transverse deflection behind the jet head. The resulting ``jitter'' radiation from deflected electrons has different properties compared to synchrotron radiation which assumes a uniform magnetic field. Jitter radiation may be important for understanding the complex time evolution and/or spectra in gamma-ray bursts, relativistic jets in general and supernova remnants., Comment: 19 pages,7 figures, contributed talk at Seventh European Workshop on Collisionless Shocks, Paris, 7- 9 November 2007. High resolution version can be obtained at http://gammaray.nsstc.nasa.gov/~nishikawa/shockws07.pdf

Relativistic electrons moving into a highly tangled magnetic field emit jitter radiation. We present a detailed computation of the jitter radiation spectrum, including self-absorption, for electrons inside Weibel-like shock generated magnetic fields. We apply our results to the case of the prompt and afterglow emission of gamma-ray bursts. We show that jitter emission can reproduce most of the observed features with some important differences with respect to standard synchrotron, especially in the frequency range between the self-absorption and the peak frequency. We discuss the similarities and differences between jitter and synchrotron and discuss experiments that can disentangle the two mechanisms., Comment: 12 pages, 7 postscript figures. Figures, discussion, and references updated. Accepted for publication in MNRAS

The standard model of GRB afterglows assumes that the radiation observed as a delayed emission is of synchrotron origin, which requires the shock magnetic field to be relatively homogeneous on small scales. An alternative mechanism -- jitter radiation, which traditionally has been applied to the prompt emission -- substitutes synchrotron when the magnetic field is tangled on a microscopic scale. Such fields are produced at relativistic shocks by the Weibel instability. Here we explore the possibility that small-scale fields populate afterglow shocks. We derive the spectrum of jitter radiation under the afterglow conditions. We also derive the afterglow lightcurves for the ISM and Wind profiles of the ambient density. Jitter self-absorption is calculated here for the first time. We find that jitter radiation can produce afterglows similar to synchrotron-generated ones, but with some important differences. We compare the predictions of the two emission mechanisms. By fitting observational data to the synchrotron and jitter afterglow lightcurves, it can be possible to discriminate between the small-scale vs large-scale magnetic field models in afterglow shocks., Comment: 16 pages, 1 figure

Resent observations of a number of galaxy clusters using the Sunyaev-Zel'dovich effect indicate that about 1/3 of baryonic mass is missing from the hot intracluster medium (ICM), which is significantly larger than the fraction of stars and cool gas, which account for only about 10%. Here we address the question whether the remaining $22\pm10%$ can be accounted for by thermal evaporation of gas from clusters. We have found that evaporation can occur only from the cluster ``surface'', $r\sim r_{\rm vir}$, and not from it's interior. We evaluated particle diffusion through the magnetized ICM for several scenarios of ISM turbulence and found that diffusivity is suppressed by at least a factor of 100 or more, compared to the Spitzer value. Thus, only particles from radii $r\ga0.9r_{\rm vir}$ can evaporate. Diffusion of particles from inside the cluster, $r\la0.9r_{\rm vir}$, takes longer than the Hubble time. This lowers the cluster-averaged fraction of the evaporated hot gas to few percent or less. However, if the missing hot component {\it is indeed} due to evaporation, this strongly constrains the magnetic field structure in the cluster envelope, namely either (i) the gas is completely unmagnetized ($B\le10^{-21}$ gauss) in the cluster halo or (ii) the magnetic fields in the ICM are rather homogeneous and non-turbulent., Comment: ApJL, accepted version; 4 pages, 2 figures

The shock model of gamma-ray bursts (GRBs) contains two equipartition parameters: the magnetic energy density and the kinetic energy density of the electrons relative to the total energy density of the shock, "epsilon_B" and "epsilon_e", respectively. These are free parameters within the model. Whereas the Weibel shock theory and numerical simulations fix "epsilon_B" at the level of ~few times(10^{-3}...10^{-4}), no understanding of "epsilon_e" exists so far. Here we demonstrate that it inevitably follows from the theory that "epsilon_e"~(epsilon_B)^(1/2). The GRB afteglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. The "epsilon_e"-"epsilon_B" relation can potentially be used to reduce the number of free parameters in afterglow models., Comment: 4 pages, 2 figs; ApJL accepted

Gamma-Ray Bursts (GRBs) directed at Earth from within a few kpc may have damaged the biosphere, primarily though changes in atmospheric chemistry which admit greatly increased Solar UV. However, GRBs are highly variable in spectrum and duration. Recent observations indicate that short (~0.1 s) burst GRBs, which have harder spectra, may be sufficiently abundant at low redshift that they may offer an additional significant effect. A much longer timescale is associated with shock breakout luminosity observed in the soft X-ray (~10^3 s) and UV (~10^5 s) emission, and radioactive decay gamma-ray line radiation emitted during the light curve phase of supernovae (~10^7 s). Here we generalize our atmospheric computations to include a broad range of peak photon energies and investigate the effect of burst duration while holding total fluence and other parameters constant. The results can be used to estimate the probable impact of various kinds of ionizing events (such as short GRBs, X-ray flashes, supernovae) upon the terrestrial atmosphere. We find that the ultimate intensity of atmospheric effects varies only slightly with burst duration from 10^-1 s to 10^8 s. Therefore, the effect of many astrophysical events causing atmospheric ionization can be approximated without including time development. Detailed modeling requires specification of the season and latitude of the event. Harder photon spectra produce greater atmospheric effects for spectra with peaks up to about 20 MeV, because of greater penetration into the stratosphere., Comment: 30 pages, to be published in ApJ. Replaced for conformity with published version, including correction of minor typos and updated references

Recent work has revealed a 62 (+/-) 3-million-year cycle in the fossil diversity in the past 542 My, however no plausible mechanism has been found. We propose that the cycle may be caused by modulation of cosmic ray (CR) flux by the Solar system vertical oscillation (64 My period) in the galaxy, the galactic north-south anisotropy of CR production in the galactic halo/wind/termination shock (due to the galactic motion toward the Virgo cluster), and the shielding by galactic magnetic fields. We revisit the mechanism of CR propagation and show that CR flux can vary by a factor of about 4.6 and reach a maximum at north-most displacement of the Sun. The very high statistical significance of (i) the phase agreement between Solar north-ward excursions and the diversity minima and (ii) the correlation of the magnitude of diversity drops with CR amplitudes through all cycles provide solid support for our model. Various observational predictions which can be used to confirm or falsify our hypothesis are presented., Comment: ApJ accepted. We added a number of observational predictions, which can be used to falsify the CR hypothesis. Total: 31 pages, 8 figures

We demonstrate that the rapid spectral variability of prompt GRBs is an inherent property of radiation emitted from shock-generated, highly anisotropic small-scale magnetic fields. We interpret the hard-to-soft evolution and the correlation of the soft index $\alpha$ with the photon flux observed in GRBs as a combined effect of temporal variation of the shock viewing angle and relativistic aberration of an individual thin, instantaneously illuminated shell. The model predicts that about a quarter of time-resolved spectra should have hard spectra, violating the synchrotron $\alpha=-2/3$ limit. The model also naturally explains why the peak of the distribution of $\alpha$ is at $\alpha\sim-1$. The presence of a low-energy break in the jitter spectrum at oblique angles also explains the appearance of a soft X-ray component in some GRBs and their paucity. We emphasize that our theory is based solely on the first principles and contains no ad hoc (phenomenological) assumptions., Comment: 4 pages, Proceedings of "Swift-05" meeting, Washington, DC

The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma, in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from tens of nano-Gauss in the intercluster medium to a few micro-Gauss inside galaxy clusters., Comment: 4 pages, 2 color figures

We develop the theory of jitter radiation from GRB shocks containing small-scale magnetic fields and propagating at an angle with respect to the line of sight. We demonstrate that the spectra vary considerably: the low-energy photon index, $\alpha$, ranges from 0 to -1 as the apparent viewing angle goes from 0 to $\pi/2$. Thus, we interpret the hard-to-soft evolution and the correlation of $\alpha$ with the photon flux observed in GRBs as a combined effect of temporal variation of the viewing angle and relativistic aberration of an individual thin, instantaneously illuminated shell. The model predicts that about a quarter of time-resolved spectra should have hard spectra, violating the synchrotron $\alpha=-2/3$ line of death. The model also naturally explains why the peak of the distribution of $\alpha$ is at $\alpha\approx-1$. The presence of a low-energy break in the jitter spectrum at oblique angles also explains the appearance of a soft X-ray component in some GRBs and a relatively small number of them. We emphasize that our theory is based solely on the first principles and contains no {\it ad hoc} (phenomenological) assumptions., Comment: 5 pages, 3 figures, accepted to ApJ

Self-gravitating systems with nonlocal, long-range interactions are described by nonextensive statistics. Recently, Leubner demonstrated that the nonextensivity parameter $\kappa$ should be negative for self-gravitating, pressureless systems, such as dark matter halos. The equation for the spherically symmetric nonextensive dark matter halos has also been derived. Here we demonstrate that this equation is identical to the classical Lane-Emden equation describing the structure of self-gravitating polytropic spheres. This establishes an intimate connection between self-gravitating polytropes and nonextensive thermostatistics. Moreover, based on this fact and observational data, we put a stronger constraint on $\kappa$, namely $\kappa<-3.4$., Comment: 2 pages, submitted to ApJL

Gamma-Ray Bursts (GRBs) are likely to have made a number of significant impacts on the Earth during the last billion years. We have used a two-dimensional atmospheric model to investigate the effects on the Earth's atmosphere of GRBs delivering a range of fluences, at various latitudes, at the equinoxes and solstices, and at different times of day. We have estimated DNA damage levels caused by increased solar UVB radiation, reduction in solar visible light due to $\mathrm{NO_2}$ opacity, and deposition of nitrates through rainout of $\mathrm{HNO_3}$. For the ``typical'' nearest burst in the last billion years, we find globally averaged ozone depletion up to 38%. Localized depletion reaches as much as 74%. Significant global depletion (at least 10%) persists up to about 7 years after the burst. Our results depend strongly on time of year and latitude over which the burst occurs. We find DNA damage of up to 16 times the normal annual global average, well above lethal levels for simple life forms such as phytoplankton. The greatest damage occurs at low to mid latitudes. We find reductions in visible sunlight of a few percent, primarily in the polar regions. Nitrate deposition similar to or slightly greater than that currently caused by lightning is also observed, lasting several years. We discuss how these results support the hypothesis that the Late Ordovician mass extinction may have been initiated by a GRB., Comment: Minor revisions, includes more discussion of reaction rates; 1 new figure. Accepted for publication in ApJ. 68 pages, 21 figures, 12 tables

We strongly disagree with the criticism by G. Fleishman, which has been published on the astro-ph arXiv (astro-ph/0502245), of our paper on jitter radiation from GRB shocks (Medvedev 2000, ApJ, 540, 704). In this note, we present the rebuttal of all critical points raised, demonstrating that our original paper (i) contains no errors, and (ii) correctly describes prompt GRB emission spectra., Comment: AAS-TeX, 5 pages

A substantial fraction of prompt GRB spectra have soft spectral indexes exceeding the maximum allowed by the synchrotron model alpha_{max}=-2/3. Some spectra also exhibit very sharp break at E_p, inconsistent with the smooth synchrotron spectra. These facts pose a serious problem for the ``optically thin synchrotron'' interpretation of the prompt emission. We review various models suggested in order to resolve this puzzle., Comment: 6 pages, 5 figures. Review talk at GRB-03 conference (Santa Fe)

We investigate the long-time evolution of magnetic fields generated by the two-stream instability at ultra- and sub-relativistic astrophysical collisionless shocks. Based on 3D PIC simulation results, we introduce a 2D toy model of interacting current filaments. Within the framework of this model, we demonstrate that the field correlation scale in the region far downstream the shock grows nearly as the light crossing time, lambda(t) ~ ct, thus making the diffusive field dissipation inefficient. The obtained theoretical scaling is tested using numerical PIC simulations. This result extends our understanding of the structure of collisionless shocks in gamma-ray bursts and other astrophysical objects., Comment: 5 pages. 2 figures. Submitted to ApJL

(Abridged) A new type of self-similar hot viscous radiative accretion flow onto a rapidly spinning neutron star has recently been discovered. This ``hot brake'' flow forms in the two-temperature zone (close to a central object), but at a sufficiently low accretion rate and a high spin it may extend in the radial direction beyond ~300 Schwarzchild radii into a one-temperature zone. When the spin of the star is small enough, the flow transforms smoothly to an advection-dominated accretion flow. All gas parameters (density, angular velocity, temperature, luminosity, angular momentum flux) except for the radial velocity are independent of the mass accretion rate. The radiative efficiency may be arbitrarily large as M-dot -> 0. The gas angular momentum is transported outward under most conditions, hence the central star is nearly always spun-down. The flow is convectively stable. We also find that themal conduction in the flow is strong enough to make the flow thermally stable. The very fact that the density, temperature and angular velocity of the gas at any radius in the hot brake flow are completely independent of the outer and inner (except for the star spin) boundary conditions implies that the flow cannot be smoothly matched to a general external medium as well as to general conditions on the star surface. We demonstrate that there are two extra self-similar solutions: one bridges the gap between the original solution and the external medium, and another represents a boundary layer between the bulk of the flow and the star surface, in which the gas temperature rapidly drops while the density builds up. Finally, we briefly discuss that a hot brake flow may form around other rapidly spinning compact objects: white dwarfs and black holes., Comment: Review to be published by NovaScience (45 pages, 5 figures)

We consider hot accretion onto a rapidly spinning neutron star (or any other compact object with a surface). A radiative hot settling flow has been discovered at low accretion rates in the early work by Medvedev & Narayan (2001) and analytical solution has been presented. It was shown later that this flow can match external medium smoothly, thus enforcing its physical feasibility. Here we complete the study of the global structure of such hot accretion by presenting the analytical solution for the boundary later, which forms between the bulk of the flow and the stellar surface. We confirm our results via full numerical solution of height-integrated two-temperature hydrodynamic equations., Comment: 14 pages, 1 figure. accepted for publication in ApJ

Charged plasma particles form an anisotropic counter-streaming distribution at the front of a shock. In the near- and ultra-relativistic regimes, Weibel (or two-stream) instability produces near-equipartition, chaotic, small-scale magnetic fields. The fields introduce effective collisions and "thermalize" the plasma particles via pitch-angle scattering. The properties of jitter radiation emitted by accelerated electrons from these small-scale fields are markedly different from synchrotron spectra. Here we theoretical summary and the results of recent numerical 3D PIC plasma kinetic simulations. The relation of the obtained results to the theory of gamma-ray bursts is outlined., Comment: To be published in proceedings of the conference "Astrophysical particle acceleration in geospace and beyond", Chattanooga, 2002. (11 pages, 6 figures, AGU style)

Gamma-ray bursts (hereafter GRB) produce a flux of radiation detectable across the observable Universe, and at least some of them are associated with galaxies. A GRB within our own Ggalaxy could do considerable damage to the Earth's biosphere; rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. At least five times in the history of life, the Earth experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and GRB may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB. A special feature of GRB in terms of terrestrial effects is a nearly impulsive energy input of order 10 s. Due to expected severe depletion of the ozone layer, intense solar ultraviolet radiation would result from a nearby GRB, and some of the patterns of extinction and survivorship at this time may be attributable to elevated levels of UV radiation reaching the Earth. In addition a GRB could trigger the global cooling which occurs at the end of the Ordovician period that follows an interval of relatively warm climate. Intense rapid cooling and glaciation at that time, previously identified as the probable cause of this mass extinction, may have resulted from a GRB., Comment: Accepted for publication in the International Journal of Astrobiology. 24 pages, moderate revisions, including more quantitative detail on GRB rates, atmospheric effects, and more description of correlations of extinction rates with habitat and lifestyle of fossil fauna

We report Chandra observations of the dwarf nova WX Hyi in quiescence. The X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S and Fe. The various ionization states implied by the lines suggest that the emission is produced within a flow spanning a wide temperature range, from T ~ 10^6 K to T >~ 10^8 K. Line diagnostics indicate that most of the radiation originates from a very dense region, with n ~ 10^{13}-10^{14} cm^{-3}. The Chandra data allow the first tests of specific models proposed in the literature for the X-ray emission in quiescent dwarf novae. We have computed the spectra for a set of models ranging from hot boundary layers, to hot settling flows solutions, to X-ray emitting coronae. WX Hyi differs from other dwarf novae observed at minimum in having much stronger low temperature lines, which prove difficult to fit with existing models, and possibly a very strong, broad O VII line, perhaps produced in a wind moving at a few x 10^3 km/s. The accretion rate inferred from the X-rays is lower than the value inferred from the UV. The presence of high-velocity mass ejection could account for this discrepancy while at the same time explaining the presence of the broad O VII line. If this interpretation is correct, it would provide the first detection of a wind from a dwarf nova in quiescence., Comment: accepted to ApJ; 19 pages, 3 figures, 1 table

We present the first three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations of the collision of two interpenetrating plasma shells. The highly accurate plasma-kinetic "particle-in-cell" (with the total of $10^8$ particles) parallel code OSIRIS has been used. Our simulations show: (i) the generation of long-lived near-equipartition (electro)magnetic fields, (ii) non-thermal particle acceleration, and (iii) short-scale to long-scale magnetic field evolution, in the collision region. Our results provide new insights into the magnetic field generation and particle acceleration in relativistic and sub-relativistic colliding streams of particles, which are present in gamma-ray bursters, supernova remnants, relativistic jets, pulsar winds, etc.., Comment: 9 pages, 4 figures (jpg), accepted for publication in ApJLett

Medvedev & Narayan have described a hot accretion flow onto a spinning neutron star in which the gas viscously brakes the spin of the star. Their self-similar solution has the surprising property that the density, temperature and angular velocity of the gas at any radius are completely independent of the outer boundary conditions. Hence, the solution cannot be matched to a general external medium. We resolve this paradoxical situation by showing that there is a second self-similar solution which bridges the gap between the original solution and the external medium. This new solution has an extra degree of freedom which permits it to match general outer boundary conditions. We confirm the main features of the analytical results with a full numerical solution., Comment: 6 pages, 1 figure, MNRAS accepted

We have calculated lower limits for the global effective thermal conductivity in a sample of 165 Abell clusters. We assumed that cluster X-ray luminosity is compensated by a conductive heat flux which we then compare with an upper limit to the temperature gradient inferred from the cluster temperature and radius. This gives a lower limit to the thermal conductivity and therefore to the relative suppression from the Spitzer conductivity, kappa_{Sp}. Not a single cluster requires super-Spitzer values of thermal conduction to balance the observed X-ray luminosity. The suppression coefficient f=kappa/kappa_{Sp} is clustered in a range 10^{-2} < f < 0.4. A weak dependence of $f$ versus $z$ is observed over $ 0 < z <0.41$. Possible biases and/or selection effects are discussed., Comment: 7 pages, 1 figure

The energetics of electromagnetic acceleration of ultra-high-energy cosmic rays (UHECRs) is constrained both by confinement of a particle within an acceleration site and by radiative energy losses of the particle in the confining magnetic fields. We demonstrate that the detection of ~ 3 x 10^{20} eV events is inconsistent with the hypothesis that compact cosmic accelerators with high magnetic fields can be the sources of UHECRs. This rules out the most popular candidates, namely spinning neutron stars, active galactic nuclei (AGNs), and gamma-ray burst blast waves. Galaxy clusters and, perhaps, AGN radio lobes remain the only possible (although not very strong) candidates for UHECR acceleration sites. Our analysis places no limit on linear accelerators. With the data from the future Auger experiment one should be able to answer whether a conventional theory works or some new physics is required to explain the origin of UHECRs., Comment: 5 pages, 2 figures. Accepted for publication in PRE

We study the structure and properties of hot MHD accretion onto a Kerr black hole. In such a system, the hole is magnetically coupled to the inflowing gas and exerts a torque onto the accretion flow. A hot settling flow can form around the hole and transport the angular momentum outward, to the outer edge of the flow. Unlike other hot flows, such as advection- and convection-dominated flows and inflow-outflow solutions (ADAFs, CDAFs, and ADIOS), the properties of the hot settling flow are determined by the spin of the central black hole, but are insensitive to the mass accretion rate. Therefore, it may be possible to identify rapidly spinning BHs simply from their broad-band spectra. Observationally, the hot settling flow around a Kerr hole is somewhat similar to other hot flows in that they all have hard, power-law spectra and relatively low luminosities. Thus, most black hole candidates in the low/hard and, perhaps, intermediate X-ray state may potentially accrete via the hot settling flow. However, a settling flow will be somewhat more luminous than ADAFs/CDAFs/ADIOS, will exhibit high variability in X-rays, and may have relativistic jets. This suggests that galactic microquasars and active galactic nuclei may be powered by hot settling flows. We identify several galactic X-ray sources as the best candidates., Comment: 7 pages, 1 figure. Submitted to ApJ

We present dynamically consistent solutions for hot accretion onto unmagnetized, rotating white dwarfs (WDs) in five quiescent dwarf novae. The measured WD rotation rates (and other system parameters) in RX And, SS Cyg, U Gem, VW Hyi and WZ Sge imply spindown of the WD by an extended hot flow emitting most of its X-rays in the vicinity of the stellar surface. In general, energy advection is absent and the flow is stable to convection and hydrodynamical outflows. In rapidly rotating systems, the X-ray luminosity provides only an upper limit on the quiescent accretion rate because of substantial stellar spindown luminosity. We suggest that the presence of hot flows in quiescent dwarf novae may limit the long-term WD rotation rates to significantly sub-Keplerian values., Comment: 7 pages, 2 eps figures. Accepted to ApJL

We estimate the thermal conductivity of a weakly collisional magnetized plasma with chaotic magnetic field fluctuations. When the fluctuation spectrum extends over two or more decades in wave-vector, we find that thermal conduction is very efficient; the conduction coefficient is only a factor ~5 below the classical Spitzer estimate. We suggest that conduction could play a significant role in cooling flows in clusters of galaxies., Comment: 4 pages, 2 figures. Accepted for publication in ApJ Letters

We present the first results of time-resolved spectroscopic observations of little-studied dwarf nova BZ Ursae Majoris in quiescence., Comment: 2 pages, 1 figure, Latex, uses newpasp.sty; to appear in `The Physics of Cataclysmic Variables and Related Objects', ASP Conf. Ser., eds. B.T. Gaensicke, K. Beuermann, K. Reinsch

Recently, Medvedev & Narayan (2001) discovered a new type of accretion flow, a hot settling flow around a rapidly rotating neutron star. The flow is cooling-dominated and energetically similar to the Shapiro, Lightman, & Eardley (1976, SLE) solution. Since the SLE solution is known to be thermally unstable, one might suspect that the new solution would also be unstable. However, due to the very high temperature of the accreting gas, thermal conduction is very strong and could suppress the thermal instability. We analyze the role of thermal conduction in both the hot settling flow and the SLE solution. In the hot settling flow collisions are very rare. Therefore, thermal transport occurs via free streaming of electrons along tangled magnetic field lines. We find that conduction is strong enough to make the flow marginally stable. In contrast, in the cooler SLE solution, conduction is via collisional, Spitzer-type transport. In this case, conduction is weaker, and we find that the SLE solution is thermally unstable even in the presence of conduction., Comment: 12 pages, 2 figures; submitted to ApJ. For related papers see http://cita.utoronto.ca/~medvedev/

We present a theory of ultrarelativistic collisionless shocks based on the relativistic kinetic two-stream instability. We demonstrate that the shock front is unstable to the generation of small-scale, randomly tangled magnetic fields. These fields are strong enough to scatter the energetic incoming (in the shock frame) protons and electrons over pitch angle and, therefore, to convert their kinetic energy of bulk motion into heat with very high efficiency. This validates the use of MHD approximation and the shock jump conditions in particular. The effective collisions are also necessary for the diffusive Fermi acceleration of electrons to operate and produce an observed power-law. Finally, these strong (sub-equipartition) magnetic fields are also required for the efficient synchrotron-type radiation emission from the shocks. The predicted magnetic fields have an impact on polarization properties of the observed radiation (e.g., a linear polarization from a jet-like ejecta and polarization scintillations in radio for a spherical one) and on its spectrum. We present an analytical theory of jitter radiation, which is emitted when the magnetic field is correlated on scales smaller then the gyration (Larmor) radius of the accelerated electrons. A composite jitter+synchrotron model of GRB $\gamma$-ray emission from internal shocks is capable of resolving many puzzles of GRB spectra, such as the violation of the ``line of death'', sharp spectral breaks, and multiple spectral components seen in some bursts (good examples are GRB910503, GRB910402, etc.). We stress that simultaneous detection of both spectral components opens a way to a precise diagnostics of the conditions in GRB shocks. We also discuss the relation of our results to other systems, such as internal shocks in blazars, radio lobes, and supernova shocks., Comment: 5 pages, 4 figures. To appear in proceedings of the Gamma 2001 symposium

We present analytical and numerical solutions which describe a hot, viscous, two-temperature accretion flow onto a rotating neutron star or any other rotating compact star with a surface. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. Outside a thin boundary layer, where the accretion flow meets the star, we show that there is an extended settling region which is well-described by two self-similar solutions: (i) a two-temperature solution which is valid in an inner zone $r\le10^{2.5}$ ($r$ is in Schwarzchild units), and (ii) a one-temperature solution at larger radii. In both zones, $\rho\propto r^{-2}, \Omega\propto r^{-3/2}, v\propto r^0, T_p\propto r^{-1}$; in the two-temperature zone, $T_e\propto r^{-1/2}$. The luminosity of the settling zone arises from the rotational energy of the star as the star is braked by viscosity. Hence the luminosity and the flow parameters (density, temperature, angular velocity) are independent of $\dot M$. The settling solution described here is not advection-dominated, and is thus different from the self-similar ADAF found around black holes. When the spin of the star is small enough, however, the present solution transforms smoothly to a (settling) ADAF. We carried out a stability analysis of the settling flow. The flow is convectively and viscously stable and is unlikely to have strong winds or outflows. Unlike another cooling-dominated system --- the SLE disk, --- the settling flow is thermally stable provided that thermal conduction is taken into account. This strong saturated-like thermoconduction does not change the structure of the flow., Comment: 5 pages, 2 figures. To appear in proceedings of the Gamma 2001 symposium

The self-interacting dark matter (SIDM) model with flavor mixing (astro-ph/0010616) was proposed to resolve problems of the CDM model on small scales by keeping attractive features of both SIDM and annihilating dark matter, and simultaneously avoid their drawbacks. A dark particle produced in a flavor eigenstate will separate into two mass eigenstates because they propagate with different velocities and, in a gravitational filed, along different geodesics, see Fig. 1. Thus, in the flavor-mixed SIDM, dark halos are made of heavy eigenstates, whereas light eigenstates may leave the halo. Collisions (elastic scattering) of mass states results in eigenstate conversion, see Fig. 2, which leads to the gradual decrease of the halo mass in high-density cusps. On the other hand, in the early Universe, one may expect a problem of over-production of light (hot) particles over heavy (cold) when the temperature of dark matter falls below the mass of the heavy component. We show how this problem is avoided., Comment: 8 pages, 2 figures. To be published in proceedings of the Dark Universe symposium, 2001

The crisis of the cold dark matter and problems of the self-interacting dark matter models is resolved by postulating flavor mixing of dark matter particles. Flavor-mixed particles segregate in the gravitational field to form dark halos composed of heavy mass eigenstates. Since these particles are mixed in the interaction basis, elastic collisions convert some of heavy eigenstates into light ones which leave dense central regions of the halo. This annihilation-like process will soften dense central cusps of halos. The proposed model accumulates most of the attractive features of self-interacting and annihilating dark matter models, but does not suffer from their severe drawbacks. This model is natural; it does not require fine tuning., Comment: 3 pages with 2 eps figures, aipproc.sty. To appear in Proceedings of the 20th Texas Symposium on Relativistic Astrophysics, Austin, Texas, 2000, edited by J. Craig Wheeler and Hugo Martel (American Institute of Physics)

We present a theory of generation of strong (sub-equipartition) magnetic fields in relativistic collisionless GRB shocks. These fields produced by the kinetic two-stream instability are tangled on very small spatial scales. This has a clear signature in the otherwise synchrotron(-self-Compton) $\gamma$-ray spectrum. Second, we present an analytical theory of jitter radiation, which is emitted when the correlation length of the magnetic field is smaller then the gyration (Larmor) radius of the accelerated electrons. We demonstrate that the spectral power $P(\nu)$ for pure jitter radiation is well-described by a sharply broken power-law: $P(\nu)\propto\nu^1$ for $\nu<\nu_j$ and $P(\nu)\propto\nu^{-(p-1)/2}$ for $\nu>\nu_j$, where $p$ is the electron power-law index and $\nu_j$ is the jitter break, which is independent of the magnetic field strength and depends on the shock energetics and kinematics. Here we mostly focus on the first problem. The radiation theory and comparison with observations will be discussed in the forthcoming publications., Comment: 3 pages with 2 eps figures, aipproc.sty. To appear in Proceedings of the 20th Texas Symposium on Relativistic Astrophysics, Austin, Texas, 2000, edited by J. Craig Wheeler and Hugo Martel (American Institute of Physics)

We present analytical and numerical solutions which describe a hot, viscous, two-temperature accretion flow onto a neutron star or any other compact star with a surface. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. Outside a thin boundary layer, where the accretion flow meets the star, we show that there is an extended settling region which is well-described by two self-similar solutions: (1) a two-temperature solution which is valid in an inner zone $r\le10^{2.5}$ ($r$ is in Schwarzchild units), and (2) a one-temperature solution at larger radii. In both zones, $\rho\propto r^{-2}, \Omega\propto r^{-3/2}, v\propto r^0,\ T_p\propto r^{-1}$; in the two-temperature zone, $T_e\propto r^{-1/2}$. The luminosity of the settling zone arises from the rotational energy of the star as the star is braked by viscosity; hence the luminosity is independent of $\dot M$. The settling solution is convectively and viscously stable and is unlikely to have strong winds or outflows. The flow is thermally unstable, but the instability may be stabilized by thermal conduction. The settling solution described here is not advection-dominated, and is thus different from the self-similar ADAF found around black holes. When the spin of the star is small enough, however, the present solution transforms smoothly to a (settling) ADAF., Comment: 3 pages with 1 eps figure, aipproc.cls. To appear in Proceedings of the 20th Texas Symposium on Relativistic Astrophysics, Austin, Texas, 2000, edited by J. Craig Wheeler and Hugo Martel (American Institute of Physics)

We investigate the structure of self-gravitating polytropic stellar systems. We present a method which allows to obtain approximate analytical solutions, $\psi_{n+\epsilon}({\bf x})$, of the nonlinear Poisson equation with the polytropic index $n+\epsilon$, given the solution $\psi_n({\bf x})$ with the polytropic index n, for any positive or negative $\epsilon$ such that $|\epsilon|\ll1$. Application of this method to the spherically symmetric stellar polytropes with $n\simeq5$ yields the solutions which describe spatially bound systems if n<5 and the formation of a second core if n>5. A heuristic approximate expressions for the radial profiles are also presented. Due to the duality between stellar and gas polytropes, our results are valid for gaseous, self-gravitating polytropic systems (e.g., molecular clouds) with the index $\gamma\simeq 6/5$. Stability of such systems and observational consequences for both stellar and gaseous systems are discussed., Comment: LaTeX, emulateapj, 5 pages, 2 PS figures. ApJ,accepted version. Other works are at http://www.cita.utoronto.ca/~medvedev/

The crisis of both cold and collisional dark matter (DM) models is resolved by postulating flavor mixing of DM particles. Flavor-mixed particles segregate in the gravitational field to form dark halos composed of heavy mass eigenstates. Since these particles are mixed in the interaction basis, elastic collisions convert some of heavy eigenstates into light ones which leave the halo. This annihilation-like process will soften dense central cusps of halos. The proposed model accumulates most of the attractive features of self-interacting and annihilating DM models, but does not suffer from their severe drawbacks. This model is natural does not require fine tuning., Comment: TeX, 4 pages. Submitted to PRL. Other works are at http://www.cita.utoronto.ca/~medvedev/

We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling betweenthe protons and electrons, and free-free cooling from the electrons. We show that the accretion flow has an extended settling region which can be described by means of two analytical self-similar solutions: a two-temperature solution which is valid in an inner zone, r<10^{2.5}, where r is the radius in Schwarzchild units; and a one-temperature solution which is valid in an outer zone, r>10^{2.5}. In both zones the density varies as \rho ~ r^{-2} and the angular velocity as \Omega ~ r^{-3/2}. We solve the flow equations numerically and confirm that the analytical solutions are accurate. The settling flow radiates the energy dissipated by viscosity; so it is not advection-dominated. Except for the radial velocity, all other gas properties - density, angular velocity, temperature, luminosity, angular momentum flux - are independent of the mass accretion rate; these quantities do depend sensitively on the spin of the neutron star. The angular momentum flux is outward under most conditions; therefore, the central star is nearly always spun-down. The luminosity of the settling zone arises from the rotational energy that is released as the star is braked by viscosity, and the contribution from gravity is small; hence the radiative efficiency can be arbitrarily large at low $\mdot$. For reasonable values of the gas adiabatic index, the Bernoulli parameter is negative; therefore, a strong outflow or wind is not expected. The flow is convectively stable, but may be thermally unstable. The described solution is not advection-dominated; however, when the spin of the star is small enough, it transforms smoothly to an advection-dominated branch of solution., Comment: LaTeX, 26 pages, 4 figures. Reubmitted to ApJ. Sec 3.3 changed, sec. 5, fig. 5, and more references added. Related works may be found at http://www.cita.utoronto.ca/~medvedev/

Particle heating in advection-dominated accretion flows (ADAFs) by nonlinear MHD (Alfvenic) turbulence is investigated. Such turbulence with highly-fluctuating magnetic fields, $\tilde B\sim B_0$, is believed to be naturally produced by the magnetic shearing instability near the nonlinear saturation. It is shown that the energy is dissipated in the parallel cascade, which occurs due to nonlinear compressibility of high-amplitude turbulence, and predominantly heats protons, but not electrons. The conservative limit on the electron--to--proton heating fraction is $\delta\lesssim{\rm few}\times10^{-2}$., Comment: 10 pages, 3 figures. Related works may be found at http://cfa-www.harvard.edu/~mmedvede/

Abridged.-- We demonstrate that the radiation emitted by ultrarelativistic electrons in highly nonuniform, small-scale magnetic fields is different from synchrotron radiation if the electron's transverse deflections in these fields are much smaller than the beaming angle. A quantitative analytical theory of this radiation, which we refer to as jitter radiation, is developed. It is shown that the emergent spectrum is determined by statistical properties of the magnetic field. As an example,we then use the model of a magnetic field in internal shocks of GRBs. The spectral power distribution of radiation produced by the power-law electrons is well described by a sharply broken power-law with indices 1 and -(p-1)/2 and the jitter break frequency is independent of the field strength but depends on the electron density in the ejecta. Since large-scale fields may also be present in the ejecta, we construct a two-component, jitter+synchrotron spectral model of the prompt $\gamma$-ray emission. Quite surprisingly, this model seems to be readily capable of explaining several properties of time-resolved spectra of some GRBs, such as (i) the violation of the constraint on the low-energy spectral index called the synchrotron ``line of death'', (ii) the sharp spectral break at the peak frequency, inconsistent with the broad synchrotron bump, (iii) the evidence for two spectral sub-components, and (iv) possible existence of emission features called ``GRB lines''. We believe these facts strongly support both the existence of small-scale magnetic fields and the proposed radiation mechanism from GRB shocks. As an example, we use the composite model to analyze GRB 910503 which has two spectral peaks., Comment: 12 pages (emulateapj), 11 figures (EPS), ApJ, accepted. For related work, see http://cfa-www.harvard.edu/~mmedvede

We demostrate that the measurement of fluctuations of polarization due to the galactic interstellar scintillations may be used to study the structure of the radiation field at compact radio sources. We develop a mathematical formalism and demonstrate it on a simple analytical model in which the scale of the polarization variation through the source is comparable to the source size. The predicted amplitude of modulation of the polarized radiation flux is ~20% x (pi_s) x (m_sc), where (pi_s) is the characteristic degree of polarization of radiation at the source and (m_sc) is the typical modulation index due to scattering, i.e., (m_sc)~1 for diffractive scintillations and (m_sc)<1 for refractive scintillations., Comment: 5 pages, 2 figures, emilateapj.sty. Submitted to ApJL

All axisymmetric self-similar equilibria of self-gravitating, rotating, isothermal systems are identified by solving the nonlinear Poisson equation analytically. There are two families of equilibria: (1) Cylindrically symmetric solutions in which the density varies with cylindrical radius as R^(-alpha), with 0 <= alpha <= 2. (2) Axially symmetric solutions in which the density varies as f(theta)/r^2, where `r' is the spherical radius and `theta' is the co-latitude. The singular isothermal sphere is a special case of the latter class with f(theta)=constant. The axially symmetric equilibrium configurations form a two-parameter family of solutions and include equilibria which are surprisingly asymmetric with respect to the equatorial plane. The asymmetric equilibria are, however, not force-free at the singular points r=0, infinity, and their relevance to real systems is unclear. For each hydrodynamic equilibrium, we determine the phase-space distribution of the collisionless analog., Comment: 13 pages, 7 figures, uses emulateapj.sty. Submitted to ApJ

We show that the relativistic two-stream instability can naturally generate strong magnetic fields with 1e-5 - 1e-1 of the equipartition energy density, in the collisionless shocks of Gamma-Ray-Burst (GRB) sources. The generated fields are parallel to the shock front and fluctuate on the very short scale of the plasma skin depth. The synchrotron radiation emitted from the limb-brightened source image is linearly polarized in the radial direction relative to the source center. Although the net polarization vanishes under circular symmetry, GRB sources should exhibit polarization scintillations as their radio afterglow radiation gets scattered by the Galactic interstellar medium. Detection of polarization scintillations could therefore test the above mechanism for magnetic field generation., Comment: 12 pages, 2 figures included. Submitted to ApJ

Thermodynamic and spectral properties of a photon gas in $e^+e^-\gamma$ plasmas are studied. The effect of a finite effective mass of a photon, associated with the plasma frequency cutoff, is self-consistently included. In the ultra-relativistic plasma, the photon spectrum turns out to be universal with the temperature normalized plasma frequency cutoff being a fundamental constant independent of plasma parameters. Such a universality does not hold in the non-relativistic plasma., Comment: 4 pages (REVTeX) with one EPS figure. Submitted to Phys. Rev. E