1,669 research outputs found
Quantitative Estimates of Environmental Effects on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies
A simple model is constructed to evaluate the change of star formation rate
of a disk galaxy due to environmental effects in clusters of galaxies. Three
effects, (1) tidal force from the potential well of the cluster, (2) increase
of external pressure when the galaxy plows into the intracluster medium, (3)
high-speed encounters between galaxies, are investigated. General analysis
indicates that the star formation rate increases significantly when the
pressure of molecular clouds rises above in yr. The tidal force from the potential well of the cluster increases
pressures of molecular clouds in a disk galaxy infalling towards the cluster
center. Before the galaxy reaches the cluster center, the star formation rate
reaches a maximum. The peak is three to four times larger than the initial
value. If this is the main mechanism of the Butcher-Oemler effect, blue
galaxies are expected to be located within kpc from the center of
the cluster. However this prediction is inconsistent with the recent
observations. The increase of external pressure when the galaxy plows into the
intracluster medium does not change star formation rate of a disk galaxy
significantly. The velocity perturbation induced by a single high-speed
encounter between galaxies is too small to affect star formation rate of a disk
galaxy, while successive high-speed encounters (galaxy harassment) trigger star
formation activity because of the accumulation of gas in the galaxy center.
Therefore, the galaxy harassment remains as the candidate for a mechanism of
the Butcher-Oemler effect.Comment: 12 pages, 13 figures. To be published in Ap
Temperature dependence of the ohmic conductivity and activation energy of Pb1+y(Zr0.3Ti0.7)O3 thin films
The ohmic conductivity of the sol-gel derived Pb1+y(Zr0.3Ti0.7)O3 thin films
(with the excess lead y=0.0 to 0.4) are investigated using low frequency small
signal alternate current (AC) and direct current (DC) methods. Its temperature
dependence shows two activation energies of 0.26 and 0.12 eV depending on
temperature range and excess Pb levels. The former is associated with Pb3+
acceptor centers, while the latter could be due to a different defect level yet
to be identified.Comment: 13 pages, 3 figures, PostScript. Submitted to Applied Physics Letter
INTEGRAL High Energy Observations of 2S 0114+65
We report the first INTEGRAL timing and spectral analysis of the high mass
X-ray binary source 2S 0114+65 at high energies (5-100 keV). The pulse period
was found at 2.668 hr with a high pulsed fraction, ~80% in both the 20-40 keV
and 40-80 keV energy bands. The spin-up trend over ~8 years was measured to be
-8.9 * 10^{-7}. The hard X-ray spectrum obtained with JEM-X/ISGRI is well
described by a high energy exponential cut-off power law model where the
estimated luminosity is 1.8 * 10^{36} erg/s in the 5-100 keV energy band, for a
source distance of 7.2 kpc. We tentatively identify a cyclotron resonance
scattering feature at ~22 keV with one harmonic, implying a magnetic field of
2.5 * 10^{12} G.Comment: 4 pages, 5 figures, accepted to A&A Letter
V0332+53 in the outburst of 2004--2005: luminosity dependence of the cyclotron line and pulse profile
We present results of observations of the transient X-ray pulsar V0332+53
performed during a very powerful outburst in Dec, 2004 -- Feb, 2005 with the
INTEGRAL and RXTE observatories in a wide (3-100 keV) energy band. A cyclotron
resonance scattering line at an energy of ~26 keV has been detected in the
source spectrum together with its two higher harmonics at ~50 and ~73 keV,
respectively. We show that the energy of the line is not constant but linearly
changes with the source luminosity. Strong pulse profile variations, especially
near the cyclotron line, are revealed for different levels of the source
intensity. We discuss the obtained results in terms of the theoretical models
of X-ray pulsars.Comment: Accepted for publication in MNRAS. 10 pages, 9 figure
The broad band spectral properties of binary X-ray pulsars
The X-ray telescopes on board BeppoSAX are an optimal set of instruments to
observe bright galactic binary pulsars. These sources emit very hard and quite
complex X-ray spectra that can be accurately measured with BeppoSAX between 0.1
and 200 keV. A prototype of this complexity, the source Her X-1, shows at least
seven different components in its spectrum. A broad band measure is therefore
of paramount importance to have a thorough insight into the physics of the
emitting region. Moreover the detection of cyclotron features, when present,
allows a direct and highly significant measure of the magnetic field intensity
in the emission region. In this paper we briefly report the results obtained
with BeppoSAX on this class of sources, with emphasis on the detection and on
the measured properties of the cyclotron lines.Comment: 10 Latex pages, 4 figures, uses psfig.sty. Accepted for publication
in Advances in Space Research, in Proceedings of 32nd Scientific Assembly of
COSPAR - Symposium E1.1: "Broad-Band X-ray Spectroscopy of Cosmic Sources
Discovery of a QPO in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability
Our goal is to investigate the X-ray timing and spectral variability of the
high-mass X-ray binary 1A 1118-615 during a type-II outburst. We performed a
detailed color, spectral and timing analysis of a giant outburst from 1A
1118-615, using RXTE data. Results. We report the discovery of a variable
quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615,
with a centroid frequency of ~0.08 Hz. The centroid frequency of the QPO
correlates with the X-ray flux, as expected according to the most accredited
models for QPO production. For energies above ~4 keV, the QPO rms variability
decreases as the energy increases. Pulse profiles display energy dependence,
with a two-peak profile at lower energies, and a single peak at higher
energies. From spectral analysis, we confirm the presence of a cyclotron
absorption feature at ~60 keV, the highest value measured for an X-ray pulsar.
We find that the spectral parameters (photon index, cutoff energy, iron
fluorescence line strength) display a marked dependence with flux. We detect
two different levels of neutral hydrogen column density, possibly due to the Be
companion activity. We report for the first time a correlation between the
timing and spectral parameters in an X-ray pulsar. All the correlations found
between spectral/timing parameters and X-ray flux are present up to a flux of
~6x10^-9 erg cm^-2 s^-1, when a saturation level is reached. We propose that
the saturation observed corresponds to the minimum extent of the neutron star
magnetosphere. We estimate the magnetic field of the neutron star from two
independent ways, using results from spectral (cyclotron line energy) and
timing (QPO frequency) analysis, obtaining consistent values, of ~7-8x10^12 G.
Results from the comprehensive spectral and timing analysis are discussed in
comparison with other X-ray pulsars.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysic
The first outburst of the black hole candidate MAXI J1836-194 observed by INTEGRAL, Swift, and RXTE
MAXI J1836-194 is a transient black-hole candidate discovered in outburst by
MAXI on 30 August 2011. We report on the available INTEGRAL, Swift, and RXTE
observations performed in the direction of the source during this event before
55 864 MJD. Combining the broad band (0.6-200 keV) spectral and timing
information obtained from these data with the results of radio observations, we
show that the event displayed by MAXI J1836-194 is another example of "failed"
outburst. During the first ~20 days after the onset of the event, the source
underwent a transition from the canonical low/hard to the hard intermediate
state, while reaching the highest X-ray flux. In the ~40 days following the
peak of the outburst, the source displayed a progressive spectral hardening and
a decrease of the X-ray flux, thus it entered again the low/hard state and
began its return to quiescence.Comment: Accepted for A&A Letters on 22 Dec. 201
Influence of a dynamical gluon mass in the and forward scattering
We compute the tree level cross section for gluon-gluon elastic scattering
taking into account a dynamical gluon mass, and show that this mass scale is a
natural regulator for this subprocess cross section. Using an eikonal approach
in order to examine the relationship between this gluon-gluon scattering and
the elastic and channels, we found that the dynamical gluon
mass is of the same order of magnitude as the {\it ad hoc} infrared mass scale
underlying eikonalized QCD-inspired models. We argue that this
correspondence is not an accidental result, and that this dynamical scale
indeed represents the onset of non-perturbative contributions to the elastic
hadron-hadron scattering. We apply the eikonal model with a dynamical infrared
mass scale to obtain predictions for ,
, slope , and differential elastic
scattering cross section at Tevatron and CERN-LHC
energies.Comment: 20 pages, 5 figures; misprints corrected and comments added. To
appear in Phys. Rev.
Discovery of a flux-related change of the cyclotron line energy in Her X-1
We present the results of ten years of repeated measurements of the Cyclotron
Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar
Her X-1 and report the discovery of a positive correlation of the centroid
energy of this absorption feature in pulse phase averaged spectra with source
luminosity.Our results are based on a uniform analysis of observations bythe
RXTE satellite from 1996 to 2005, using sufficiently long observations of 12
individual 35-day Main-On states of the source. The mean centroid energy E_c of
the CRSF in pulse phase averaged spectra of Her X-1 during this time is around
40 keV, with significant variations from one Main-On state to the next. We find
that the centroid energy of the CRSF in Her X-1 changes by ~5% in energy for a
factor of 2 in luminosity. The correlation is positive, contrary to what is
observed in some high luminosity transient pulsars. Our finding is the first
significant measurement of a positive correlation between E_c and luminosity in
any X-ray pulsar. We suggest that this behaviour is expected in the case of
sub-Eddington accretion and present a calculation of a quantitative estimate,
which is very consistent with the effect observed in Her X-1.We urge that Her
X-1 is regularly monitored further and that other X-ray pulsars are
investigated for a similar behaviour.Comment: 4 pages, 2 figures, accepted by A&A Letter
Biochemical Discrimination between Selenium and Sulfur 2: Mechanistic Investigation of the Selenium Specificity of Human Selenocysteine Lyase
Selenium is an essential trace element incorporated into selenoproteins as selenocysteine. Selenocysteine (Sec) lyases (SCLs) and cysteine (Cys) desulfurases (CDs) catalyze the removal of selenium or sulfur from Sec or Cys, respectively, and generally accept both substrates. Intriguingly, human SCL (hSCL) is specific for Sec even though the only difference between Sec and Cys is a single chalcogen atom
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