46,617 research outputs found

    Star Formation in Bulges from GALEX

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    Early-type galaxies, considered as large bulges, have been found to have had a much-more-than-boring star formation history in recent years by the UV satellite GALEX. The most massive bulges, brightest cluster galaxies, appear to be relatively free of young stars. But smaller bulges, normal ellipticals and lenticulars, often show unambiguous sign of recent star formation in their UV flux. The fraction of such UV-bright bulges in the volume-limited sample climbs up to the staggering 30%. The bulges of spirals follow similar trends but a larger fraction showing signs of current and recent star formation. The implication on the bulge formation and evolution is discussed.Comment: 7pages 4figures IAU symposium 245 (Oxford) Reference mistake fixe

    A new approach to the GeV flare of PSR B1259-63/LS2883

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    PSR B1259-63/LS2883 is a binary system composed of a pulsar and a Be star. The Be star has an equatorial circumstellar disk (CD). The {\it Fermi} satellite discovered unexpected gamma-ray flares around 30 days after the last two periastron passages. The origin of the flares remain puzzling. In this work, we explore the possibility that, the GeV flares are consequences of inverse Compton-scattering of soft photons by the pulsar wind. The soft photons are from an accretion disk around the pulsar, which is composed by the matter from CD captured by the pulsar's gravity at disk-crossing before the periastron. At the other disk-crossing after the periastron, the density of the CD is not high enough so that accretion is prevented by the pulsar wind shock. This model can reproduce the observed SEDs and light curves satisfactorily.Comment: 14 pages, 8 figures, 1 table. Accepted for publication in Ap

    UV Upturn in Elliptical Galaxies: Theory

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    The UV upturn is the rising flux with decreasing wavelength between the Lyman limit and 2500\AA found virtually in all bright spheroidal galaxies. It has been a mystery ever since it was first detected by the OAO-2 space telescope (Code & Welch 1979) because such old metal-rich populations were not expected to contain any substantial number of hot stars. It was confirmed by following space missions, ANS (de Boer 1982), IUE (Bertola et al. 1982) and HUT (Brown et al. 1997). The positive correlation between the UV-to-optical colour (i.e., the strength of the UV upturn) and the Mg2 line strength found by Burstein et al. (1987) through IUE observations has urged theorists to construct novel scenarios in which metal-rich (Z\gtrsim Z_{\odot}) old (\gtrsim a few Gyr) stars become UV bright (Greggio & Renzini 1990; Horch et al. 1992). Also interesting was to find using HUT that, regardless of the UV strength, the UV spectral slopes at 1000--2000\AA in the six UV bright galaxies were nearly identical suggesting a very small range of temperatures of the UV sources in these galaxies (Brown et al. 1997), which corresponds to Teff20,000±3,000T_{\rm eff} \approx 20,000 \pm 3,000 K. This, together with other evidence, effectively ruled out young stars as the main driver of the UV upturn. A good review on the observational side of the story is given in the next article by Tom Brown, as well as in the recent articles of Greggio & Renzini (1999) and O'Connell (1999).Comment: 6 figures; belated paper from Keele Conferenc

    Statistical and fuzzy approach for database security

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    A new type of database anomaly is described by addressing the concept of Cumulated Anomaly in this paper. Dubiety-Determining Model (DDM), which is a detection model basing on statistical and fuzzy set theories for Cumulated Anomaly, is proposed. DDM can measure the dubiety degree of each database transaction quantitatively. Software system architecture to support the DDM for monitoring database transactions is designed. We also implemented the system and tested it. Our experimental results show that the DDM method is feasible and effective

    Probing the properties of the pulsar wind via studying the dispersive effects in the pulses from the pulsar companion in a double neutron-star binary system

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    The velocity and density distribution of e±e^\pm in the pulsar wind are crucial distinction among magnetosphere models, and contains key parameters determining the high energy emission of pulsar binaries. In this work, a direct method is proposed, which might probe the properties of the wind from one pulsar in a double-pulsar binary. When the radio signals from the first-formed pulsar travel through the relativistic e±e^\pm flow in the pulsar wind from the younger companion, the components of different radio frequencies will be dispersed. It will introduce an additional frequency-dependent time-of-arrival delay of pulses, which is function of the orbital phase. In this paper, we formulate the above-mentioned dispersive delay with the properties of the pulsar wind. As examples, we apply the formula to the double pulsar system PSR J0737-3039A/B and the pulsar-neutron star binary PSR B1913+16. For PSR J0737-3039A/B, the time delay in 300\,MHz is 10μ\lesssim10\mus near the superior-conjunction, under the optimal pulsar wind parameters, which is \sim half of the current timing accuracy. For PSR B1913+16, with the assumption that the neutron star companion has a typical spin down luminosity of 103310^{33}\,ergs/s, the time delay is as large as 1020μ10\sim20\mus in 300\,MHz. The best timing precision of this pulsar is 5μ\sim5\mus in 1400\,MHz. Therefore, it is possible that we can find this signal in archival data. Otherwise, we can set an upper-limit on the spin down luminosity. Similar analysis can be apply to other eleven known pulsar-neutron star binariesComment: 6 pages, 6 figures, accepted for publication in MNRA