180,153 research outputs found

    The long-term optical behavior of MRK421

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    All data available in B band for the BL Lac object MRK421 from 22 publications are used to construct a historical light curve, dating back to 1900. It is found that the light curve is very complicated and consists of a set of outbursts with very large duration. The brightness of MRK421 varies from 11.6 magnitude to more than 16 magnitude. Analyses with Jurkevich method of computing period of cyclic phenomena reveal in the light curve two kinds of behaviors. The first one is non-periodic with rapid, violent variations in intensity on time scales of hours to days. The second one is periodic with a possible period of 23.1±1.123.1\pm 1.1 years. Another possible period of 15.3±0.715.3\pm 0.7 years is not very significant. We have tested the robustness of the Jurkevich method. The period of about one year found in the light curves of MRK421 and of other objects is a spurious period due to the method and the observing window. We try to explain the period of 23.1±1.123.1 \pm1.1 years under the thermal instability of a slim accretion disk around a massive black hole of mass of 2106M2 *10^6 M_\odot.Comment: Tex, 14 pages, 5 Postscript figures. Accepted for publication in A&A Supplement Serie

    The Spectral Features of Disk and Corona with Mass Evaporation in the Low/Hard State

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    We investigate the spectral features of accretion flows composed of an outer cool, optically thick disk and inner hot, optically thin, advection dominated accretion flows (ADAF) within the framework of disk and corona with mass evaporation (Liu et al. 2002a). In this work, both the magnetic field and Compton scattering of soft photons from the disk by electrons in the corona are included to calculate the evaporation rates at different distances. The disk is truncated at the distance where the evaporation rate equals to the accretion rate (m˙evap(rtr)=m˙\dot m_{\rm evap}(r_{\rm tr})=\dot m). For a series of accretion rates, the corresponding truncation radii are calculated out, with which we are able to calculate the emergent spectra from the inner ADAF + outer disk + corona. At very low accretion rates, the spectra are similar to that of a pure ADAF because the disk is truncated at large distances. The disk component becomes important at high accretion rates since the truncation occurs at small distances. An anti-correlation between the Eddington ratio ξL0.525keV/LEdd\xi \equiv L_{\rm 0.5-25 \,keV}/L_{\rm Edd} and the hard X-ray photon index Γ325keV\Gamma_{\rm 3-25 \,keV} at low/hard states is predicted by the model. Comparing the theoretical results with observations, we find that our model can reproduce the anti-correlation between the Eddington ratio ξ\xi and the hard X-ray photon index observed for the X-ray binary XTE J1118+480.Comment: 11 pages and 7 figures, published by PAS

    The Dependence of Spectral State Transition and Disk Truncation on Viscosity Parameter $\alpha

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    A wealth of Galactic accreting X-ray binaries have been observed both in low/hard state and high/soft state. The transition between these two states was often detected. Observation shows that the transition luminosity between these two states is different for different sources, ranging from 1% to 4% of the Eddington luminosity. Even for the same source the transition luminosity at different outbursts is also different. The transition can occur from 0.0069 to 0.15 Eddington luminosity. To investigate the underlying physics, we study the influence of viscosity parameter α\alpha on the transition luminosity on the basis of the disk-corona model for black holes. We calculate the mass evaporation rate for a wide range of viscosity parameter, 0.1α0.90.1\le \alpha\le 0.9. By fitting the numerical results, we obtain fitting formulae for both the transition accretion rate and the corresponding radius as a function of α\alpha. We find that the transition luminosity is very sensitive to the value of α\alpha, L/LEddα2.34L/L_{\rm Edd}\propto\alpha^{2.34}. For 0.1α0.60.1\le\alpha\le 0.6, the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2 Eddington luminosity. Comparing with observations we find that the transition luminosity can be fitted by adjusting the value of α\alpha, and the model determined values of α\alpha are mostly in the range of observationally inferred value. Meanwhile we investigate the truncation of the disk in the low/hard state for some luminous sources. Our results are roughly in agreement with the observations.Comment: 8 pages, 3 figures,accepted by PAS

    On integrable natural Hamiltonian systems on the suspensions of toric automorphism

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    We point out a mistake in the main statement of \cite{liu} and suggest and proof a correct statement.Comment: 5 pages, no figure

    Assessment of traffic impact on future cooperative driving systems: challenges and considerations

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    Connect & Drive is a start-up project to develop a cooperative driving system and improve the traffic performance on Dutch highways. It consists of two interactive subsystems: cooperative adaptive cruise control (CACC) and connected cruise control (CCC). To assess the traffic performance, a traffic simulation model will be established for large-scale evaluation and providing feedbacks to system designs. This paper studies the factors determining the traffic performance and discusses challenges and difficulties to establish such a traffic simulation model
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