47,275 research outputs found
Star Formation in Bulges from GALEX
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
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A dubiety-determining based model for database cumulated anomaly intrusion
The concept of Cumulated Anomaly (CA), which describes a new type of database anomalies, is addressed. A
typical CA intrusion is that when a user who is authorized to modify data records under certain constraints deliberately
hides his/her intentions to change data beyond constraints in different operations and different transactions. It happens
when some appearing to be authorized and normal transactions lead to certain accumulated results out of given thresholds.
The existing intrusion techniques are unable to deal with CAs. This paper proposes a detection model,
Dubiety-Determining Model (DDM), for Cumulated Anomaly. This model is mainly based on statistical theories and fuzzy
set theories. It measures the dubiety degree, which is presented by a real number between 0 and 1, for each database
transaction, to show the likelihood of a transaction to be intrusive. The algorithms used in the DDM are introduced. A
DDM-based software architecture has been designed and implemented for monitoring database transactions. The
experimental results show that the DDM method is feasible and effective
A new approach to the GeV flare of PSR B1259-63/LS2883
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
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 () old ( 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 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
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
The velocity and density distribution of 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 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 s near the
superior-conjunction, under the optimal pulsar wind parameters, which is
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
\,ergs/s, the time delay is as large as s in 300\,MHz.
The best timing precision of this pulsar is s 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
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