153 research outputs found

    Accretion onto Black Holes and Neutron Stars: Differences and Similarities

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    Accreting black holes and neutron stars at luminosities above 0.01 of the critical Eddington luminosity have a lot of similarities, but also drastic differences in their radiation and power density spectra. The efficiency of energy release due to accretion onto a rotating neutron star usually is higher than in the case of a black hole. The theory of the spreading layer on the surface of an accreting neutron star is discussed. It predicts the appearance of two bright belts equidistant from the equator. This layer is unstable and its radiation flux must vary with high frequencies.Comment: 12 pages, 10 figures, invited talk, to appear in Proc. of ESO Workshop Held in Garching 6-8 Sept. 1999 in Honour of Riaccardo Giacconi (Springer, eds. L. Kaper et al.

    Preheating of the Universe by cosmic rays from primordial supernovae at the beginning of cosmic reionization

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    The 21-cm signal from the cosmic reionization epoch can shed light on the history of heating of the primordial intergalactic medium (IGM) at z~30-10. It has been suggested that X-rays from the first accreting black holes could significantly heat the Universe at these early epochs. Here we propose another IGM heating mechanism associated with the first stars. As known from previous work, the remnants of powerful supernovae (SNe) ending the lives of massive Population III stars could readily expand out of their host dark matter minihalos into the surrounding IGM, aided by the preceeding photoevaporation of the halo's gas by the UV radiation from the progenitor star. We argue that during the evolution of such a remnant a significant fraction of the SN kinetic energy can be put into low-energy (E<30 MeV) cosmic rays that will eventually escape into the IGM. These subrelativistic cosmic rays could propagate through the Universe and heat the IGM by ~10-100 K by z~15, before more powerful reionization/heating mechanisms associated with the first galaxies and quasars came into play. Future 21-cm observations could thus constrain the energetics of the first supernovae and provide information on the magnetic fields in the primordial IGM.Comment: 10 pages, 1 figure, accepted for publication in MNRA

    Close stars and an inactive accretion disk in Sgr A*: Eclipses and flares

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    A cold neutral and extremely dim accretion disk may be present as a remnant of a past vigorous activity around the black hole in our Galactic Center (GC). Here we discuss ways to detect such a disk through its interaction with numerous stars present in the central ~0.1 parsec of the Galaxy. The first major effect expected is X-ray and near infrared (NIR) flares arising when stars pass through the disk. The second is eclipses of the stars by the disk. We point out conditions under which the properties of the expected X-ray flares are similar to those recently discovered by Chandra. Since orbits of bright stars are now being precisely measured, the combination of the expected flares and eclipses offers an invaluable tool for constraining the disk density, size, plane and even direction of rotation. The winds of the O-type stars are optically thick to free-free absorption in radio frequencies. If present near Sgr A* core, such powerful stellar winds can modulate and even occult the radio source.Comment: typo in eq. 3 correcte

    The "missing" YSOs in the central parsec of the Galaxy: evidence for star formation in a massive accretion disk and a top-heavy IMF

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    Few dozens of young high mass stars orbit Sgr A* at distances as short as 0.1 parsec, where star formation should be quenched by the strong tidal shear from Sgr A*. The puzzling young stellar population is believed to come into existence in one of the two ways: (i) "normal" star formation at several tens of parsec in a very massive star cluster that then spiraled in, or (ii) star formation in situ in a massive self-gravitating disk. We propose to constrain these two scenarios via the expected X-ray emission from young low mass stars that should have formed alongside the massive stars. To this end we compare the recent Chandra observations of X-ray emission from young stars in the Orion Nebula, and the Chandra observations of Sgr A* field. We show that the cluster inspiral model is ruled out irrespectively of the initial mass function (IMF) of the young stars. In addition, for the in situ model, we find that no more than few thousand low-mass stars could have formed alongside the massive stars. This is more than a factor of ten fewer than expected if these stars were formed with the standard IMF as elsewhere in the Galaxy. The young stars in the GC are thus the first solid observational evidence for star formation in AGN disks and also require the IMF of these stars to be top-heavy. We briefly consider implication of these results for AGN in general.Comment: submitted to MNRAS, 5 pages, no figure

    Relative velocity of dark matter and barions in clusters of galaxies and measurements of their peculiar velocities

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    The increasing sensitivity of current experiments, which nowadays routinely measure the thermal SZ effect within galaxy clusters, provide the hope that peculiar velocities of individual clusters of galaxies will be measured rather soon using the kinematic SZ effect. Also next generation of X-ray telescopes with microcalorimeters, promise first detections of the motion of the intra cluster medium (ICM) within clusters. We used a large set of cosmological, hydrodynamical simulations, which cover very large cosmological volume, hosting a large number of rich clusters of galaxies, as well as moderate volumes where the internal structures of individual galaxy clusters can be resolved with very high resolution to investigate, how the presence of baryons and their associated physical processes like cooling and star-formation are affecting the systematic difference between mass averaged velocities of dark matter and the ICM inside a cluster. We, for the first time, quantify the peculiar motion of galaxy clusters as function of the large scale environment. We also demonstrate that especially in very massive systems, the relative velocity of the ICM compared to the cluster peculiar velocity add significant scatter onto the inferred peculiar velocity, especially when measurements are limited to the central regions of the cluster. Depending on the aperture used, this scatter varies between 50% and 20%, when going from the core (e.g. ten percent of the virial radius) to the full cluster (e.g. the virial radius).Comment: 17 pages, 18 figures, submitted to MNRA

    Time of primordial Be-7 conversion into Li-7, energy release and doublet of narrow cosmological neutrino lines

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    One of the important light elements created during the big bang nucleosynthesis is Be-7 which then decays to Li-7 by electron capture when recombination becomes effective but well before the Saha equilibrium recombination is reached. This means that Be-7 should wait until its recombination epoch even though the half-life of the hydrogenic beryllium atom is only 106.4 days. We calculate when the conversion from primordial Be-7 to Li-7 occurs taking into account the population of the hyperfine structure sublevels and solving the kinetic equations for recombination, photoionization and conversion rate. We also calculate the energies and the spectrum of narrow neutrino doublet lines resulting from Be-7 decay.Comment: Minor typos correcte
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