3,834,988 research outputs found

    The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer

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    We show that the 1996 outburst of the X-ray binary transient system GRO J1655-40 can be explained by the standard dwarf-nova type disc instability, followed by an episode of enhanced mass transfer from the secondary if the mass transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit. We argue that irradiation of the secondary during the onset of the outburst driven by the thermal instability in the outer disc can increase the mass transfer rate above the minimum value required for stable accretion. This will then produce the period of near-constant X-ray emission seen in this system. This scenario can also explain the observed anti-correlation between the optical and X-ray fluxes. It is generally accepted that optical emission in low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays. There is also strong circumstantial evidence that in order for the outer disc to see the irradiating flux, it must be warped. Depending on the warp propagation mechanism, either a burst of mass from the secondary or viscous decay are likely to decrease the degree of warping, thereby causing the decrease in the observed optical flux while the X-ray flux remains constant or even increases, exactly as observed in GRO J1655-40. Finally, the decrease of the disc warping and, therefore, irradiation will cause the disc to become unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres

    Vorticity generation in large-scale structure caustics

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    A fundamental hypothesis for the interpretation of the measured large-scale line-of-sight peculiar velocities of galaxies is that the large-scale cosmic flows are irrotational. In order to assess the validity of this assumption, we estimate, within the frame of the gravitational instability scenario, the amount of vorticity generated after the first shell crossings in large-scale caustics. In the Zel'dovich approximation the first emerging singularities form sheet like structures. Here we compute the expectation profile of an initial overdensity under the constraint that it goes through its first shell crossing at the present time. We find that this profile corresponds to rather oblate structures in Lagrangian space. Assuming the Zel'dovich approximation is still adequate not only at the first stages of the evolution but also slightly after the first shell crossing, we calculate the size and shape of those caustics and their vorticity content as a function of time and for different cosmologies. The average vorticity created in these caustics is small: of the order of one (in units of the Hubble constant). To illustrate this point we compute the contribution of such caustics to the probability distribution function of the filtered vorticity at large scales. We find that this contribution that this yields a negligible contribution at the 10 to 15 h−1h^{-1}Mpc scales. It becomes significant only at the scales of 3 to 4 h−1h^{-1}Mpc, that is, slightly above the galaxy cluster scales.Comment: 25 pages 16 figures; accepted for publication by A&A vol 342 (1999

    X-ray observations and mass determinations in the cluster of galaxies Cl0024+17

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    We present a detailed analysis of the mass distribution in the rich and distant cluster of galaxies Cl0024+17. X-ray data come from both a deep ROSAT/HRI image of the field (Bohringer et al. 1999) and ASCA spectral data. Using a wide field CCD image of the cluster, we optically identify all the faint X-ray sources, whose counts are compatible with deep X-ray number counts. In addition we marginally detect the X-ray counter-part of the gravitational shear perturbation detected by Bonnet et al. (1994) at a 2.5 σ\sigma level. A careful spectral analysis of ASCA data is also presented. In particular, we extract a low resolution spectrum of the cluster free from the contamination by a nearby point source located 1.2 arcmin from the center. The X-ray temperature deduced from this analysis is TX=5.7−2.1+4.9T_X = 5.7 ^{+4.9}_{-2.1} keV at the 90% confidence level. The comparison between the mass derived from a standard X-ray analysis and from other methods such as the Virial Theorem or the gravitational lensing effect lead to a mass discrepancy of a factor 1.5 to 3. We discuss all the possible sources of uncertainties in each method of mass determination and give some indications on the way to reduce them. A complementary study of optical data is in progress and may solve the X-ray/optical discrepancy through a better understanding of the dynamics of the cluster.Comment: Revised version, accepted in Astronomy and Astrophysics (Main Journal). Few changes in the discussio

    Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255

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    Results of a careful analysis of the absorption systems with zabs = zem seen toward the bright, z_em ~ 3.91, gravitationally lensed quasar APM 08279+5255 are presented. Two of the narrow-line systems, at z_abs = 3.8931 and z_abs = 3.9135, show absorptions from singly ionized species with weak or no NV and O V absorptions at the same redshift. Absorption due to fine structure transitions of C II and Si II (excitation energies corresponding to, respectively, 156ÎŒ\mum and 34ÎŒ\mum) are detected at z_abs = 3.8931. Excitation by IR radiation is favored as the column density ratios are consistent with the shape of APM 08279+5255 IR spectrum. The low-ionization state of the system favors a picture where the cloud is closer to the IR source than to the UV source, supporting the idea that the extension of the IR source is larger than ~ 200 pc. The absence of fine structure lines at z_abs = 3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are ~ 0.01 and 1Z⊙Z_{\odot} at z_abs = 3.913 and 3.8931 and aluminum could be over-abundant with respect to silicon and carbon by at least a factor of two and five. All this suggests that whereas the \zabs = 3.8931 system is probably located within 200 pc from the QSO and ejected at a velocity larger than 1000 kms^{-1}, the \zabs = 3.9135 system is farther away and part of the host-galaxy. (abridged)Comment: 15 pages with 15 figures (psfiles), To appear in A&

    Galaxy Modelling - II. Multi-Wavelength Faint Counts from a Semi-Analytic Model of Galaxy Formation

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    (Abridged) This paper predicts self-consistent faint galaxy counts from the UV to the submm wavelength range. The STARDUST spectral energy distributions described in Devriendt et al. (1999) are embedded within the explicit cosmological framework of a simple semi-analytic model of galaxy formation and evolution. We build a class of models which capture the luminosity budget of the universe through faint galaxy counts and redshift distributions in the whole wavelength range spanned by our spectra. In contrast with a rather stable behaviour in the optical and even in the far-IR, the submm counts are dramatically sensitive to variations in the cosmological parameters and changes in the star formation history. Faint submm counts are more easily accommodated within an open universe with a low value of Ω0\Omega_0, or a flat universe with a non-zero cosmological constant. This study illustrates the implementation of multi-wavelength spectra into a semi-analytic model. In spite of its simplicity, it already provides fair fits of the current data of faint counts, and a physically motivated way of interpolating and extrapolating these data to other wavelengths and fainter flux levels.Comment: 13 pages, 10 figures, to appear in A&

    A direct view of the AGN powering IRAS12393+3520

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    We report the first direct X-ray evidence that an AGN is hidden in the center of IRAS12393+3520. An ASCA observation of this target unveiled a bright (0.5-10 keV luminosity 3.9 x 10^42 erg/s) and variable source, with minimum observed doubling/halving time scale comprised in the range 30-75 ks. A model composed by a simple power-law, with photon index ~1.8 and an absorption edge, whose threshold energy is consistent with K-shell photoionization of OVII, provides an adequate fit of the spectrum. This suggests that we are observing the emission from the nuclear region through a warm absorber of N_H a few 10^{21}/cm/cm. If it has internal dust with Galactic gas-to-dust ratio, it could explain the lack of broad Hbeta emission, even in the episodic presence of a broad Halpha emission line. Optical spectra obtained over several years show indeed variations in the strength of this broad Halpha component. A distribution of dusty, optically thick matter on spatial scales a few hundreds parsec, which does not intercept the line of sight towards the nucleus, is probably required to account simultaneously for the relative [OIII] luminosity deficit in comparison to the X-rays. The high IR to X-ray luminosity ratio is most likely due to intense star formation in the circumnuclear region. IRAS12393+3520 might thus exhibit simultaneously nuclear activity and remarkable star formation.Comment: 9 Latex pages, 8 figures, Accepted for publication in Astronomy & Astrophysic

    pH Protocol

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    The purpose of this resource is to measure the pH of water. Students use either a pH meter or pH paper to measure the pH. If using the pH meter, the meter needs to be calibrated with buffer solutions that have pH values of 4, 7, and 10. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school

    pH Game

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    The purpose of this resource is to teach students about the acidity levels of liquids and other substances around their school so they understand what pH levels tell us about the environment. Students will create mixtures of water samples, soil samples, plants and other natural materials to better understand the importance of pH levels. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school

    Galactic Disk Warps

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    This review addresses recent developments in the field of disk galaxy warps. Both results from a new HI survey of edgeon disk galaxies, and of simulations of the interaction between a disk+halo and an orbiting satelite, will be discussed.Comment: paper presented at ``Galaxy Disks and Disk Galaxies'', Rome, June 200