3,370 research outputs found
X-ray Variability of the Magnetic Cataclysmic Variable V1432 Aql and the Seyfert Galaxy NGC 6814
V1432 Aquilae (=RX J1940.2-1025) is the X-ray bright, eclipsing magnetic
cataclysmic variable ~37' away from the Seyfert galaxy, NGC 6814. Due to a 0.3%
difference between the orbital (12116.3 s) and the spin (12150 s) periods, the
accretion geometry changes over the ~50 day beat period. Here we report the
results of an RXTE campaign to observe the eclipse 25 times, as well as of
archival observations with ASCA and BeppoSAX. Having confirmed that the eclipse
is indeed caused by the secondary, we use the eclipse timings and profiles to
map the accretion geometry as a function of the beat phase. We find that the
accretion region is compact, and that it moves relative to the center of white
dwarf on the beat period. The amplitude of this movement suggest a low-mass
white dwarf, in contrast to the high mass previously estimated from its X-ray
spectrum. The size of the X-ray emission region appears to be larger than in
other eclipsing magnetic CVs. We also report on the RXTE data as well as the
long-term behavior of NGC 6814, indicating flux variability by a factor of at
least 10 on time scales of years.Comment: 44 pages including 16 figures; ApJ, in pres
Large-scale analysis of human alternative protein isoforms: pattern classification and correlation with subcellular localization signals
We investigated human alternative protein isoforms of >2600 genes based on full-length cDNA clones and SwissProt. We classified the isoforms and examined their co-occurrence for each gene. Further, we investigated potential relationships between these changes and differential subcellular localization. The two most abundant patterns were the one with different C-terminal regions and the one with an internal insertion, which together account for 43% of the total. Although changes of the N-terminal region are less common than those of the C-terminal region, extension of the C-terminal region is much less common than that of the N-terminal region, probably because of the difficulty of removing stop codons in one isoform. We also found that there are some frequently used combinations of co-occurrence in alternative isoforms. We interpret this as evidence that there is some structural relationship which produces a repertoire of isoformal patterns. Finally, many terminal changes are predicted to cause differential subcellular localization, especially in targeting either peroxisomes or mitochondria. Our study sheds new light on the enrichment of the human proteome through alternative splicing and related events. Our database of alternative protein isoforms is available through the internet
Initial POLAR MFE observation of substorm signatures in the polar magnetosphere
This paper studies substorm influences in the polar magnetosphere using data from the POLAR magnetic field experiment (MFE). The POLAR spacecraft remains in the high altitude polar magnetosphere for extended periods around apogee. There it can stay at nearly constant altitude through all phases of a substorm, which was not possible on previous missions. We report such an event on March 28, 1996. Ground magnetometers monitored substorm activity, while the POLAR spacecraft, directly over the pole at (â0.8, â0.6, 8.5) RE in GSM coordinates, observed a corresponding perturbation in the total magnetic field strength. The total magnetic field first increased, then recovered toward quiet levels, consistent with erosion of magnetic flux from the dayside magnetosphere, followed by transport of that flux to the magnetotail, and eventual onset of tail reconnection and the return of that magnetic flux to the dayside magnetosphere
Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic Stars
The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary
stars is quite soft. Several symbiotic WDs, however, produce strong X-ray
emission at energies greater than ~20 keV. The Swift BAT instrument has
detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT
Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected
X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining
data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT
Cru, T CrB, and CD -57 3057 are well described by emission from a
single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9
keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright
soft component. For all 4 systems, the spectra suffer high levels of absorption
from material that both fully and partially covers the source of hard X-rays.
The XRT data did not show any of the rapid, periodic variations that one would
expect if the X-ray emission were due to accretion onto a rotating, highly
magnetized WD. The X-rays were thus more likely from the accretion-disk
boundary layer around a massive, non-magnetic WD in each binary. The X-ray
emission from RT Cru varied on timescales of a few days. This variability is
consistent with being due to changes in the absorber that partially covers the
source, suggesting localized absorption from a clumpy medium moving into the
line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during
the 9 months of Swift observations, in a manner that was also consistent with
variable absorption.Comment: Accepted for publication in ApJ. 9 pages, 6 figure
Broad-band characteristics of seven new hard X-ray selected cataclysmic variables
IndexaciĂłn: Web of Science; Scopus.We present timing and spectral analysis of a sample of seven hard X-ray selected cataclysmic
variable candidates based on simultaneous X-ray and optical observations collected
with XMMâNewton, complemented with Swift/BAT and INTEGRAL /IBIS hard X-ray data
and ground-based optical photometry. For six sources, X-ray pulsations are detected for the
first time in the range of âŒ296â6098 s, identifying them as members of the magnetic class.
Swift J0927.7â6945, Swift J0958.0â4208, Swift J1701.3â4304, Swift J2113.5+5422 and
possibly PBC J0801.2â4625 are intermediate polars (IPs), while Swift J0706.8+0325 is a
short (1.7 h) orbital period polar, the 11th hard X-ray-selected identified so far. X-ray orbital
modulation is also observed in Swift J0927.7â6945 (5.2 h) and Swift J2113.5+5422
(4.1 h). Swift J1701.3â4304 is discovered as the longest orbital period (12.8 h) deep eclipsing
IP. The spectra of the magnetic systems reveal optically thin multitemperature emission
between 0.2 and 60 keV. Energy-dependent spin pulses and the orbital modulation in
Swift J0927.7â6945 and Swift J2113.5+5422 are due to intervening local high-density absorbing
material (NH ⌠1022 â 23 cmâ2). In Swift J0958.0â4208 and Swift J1701.3â4304, a
soft X-ray blackbody (kT ⌠50 and âŒ80 eV) is detected, adding them to the growing group
of âsoftâ IPs. White dwarf masses are determined in the range of âŒ0.58â1.18 M, indicating
massive accreting primaries in five of them. Most sources accrete at rates lower than the
expected secular value for their orbital period. Formerly proposed as a long-period (9.4 h)
nova-like CV, Swift J0746.3â1608 shows peculiar spectrum and light curves suggesting either
an atypical low-luminosity CV or a low-mass X-ray binary.https://academic.oup.com/mnras/article/470/4/4815/390658
A Synoptic, Multiwavelength Analysis of a Large Quasar Sample
We present variability and multi-wavelength photometric information for the
933 known quasars in the QUEST Variability Survey. These quasars are grouped
into variable and non-variable populations based on measured variability
confidence levels. In a time-limited synoptic survey, we detect an
anti-correlation between redshift and the likelihood of variability. Our
comparison of variability likelihood to radio, IR, and X-ray data is consistent
with earlier quasar studies. Using already-known quasars as a template, we
introduce a light curve morphology algorithm that provides an efficient method
for discriminating variable quasars from periodic variable objects in the
absence of spectroscopic information. The establishment of statistically robust
trends and efficient, non-spectroscopic selection algorithms will aid in quasar
identification and categorization in upcoming massive synoptic surveys.
Finally, we report on three interesting variable quasars, including variability
confirmation of the BL Lac candidate PKS 1222+037.Comment: AJ, accepted for publication 15 Dec 200
Confocal microphotoluminescence of InGaN-based light-emitting diodes
Spatially resolved photoluminescence (PL) of InGaN/GaN/AlGaN-based quantum-well-structured light-emitting diodes (LEDs) with a yellow-green light (530 nm) and an amber light (600 nm) was measured by using confocal microscopy. Submicron-scale spatial inhomogeneities of both PL intensities and spectra were found in confocal micro-PL images. We also found clear correlations between PL intensities and peak wavelength for both LEDs. Such correlations for yellow-green and amber LEDs were different from the reported correlations for blue or green LEDs. This discrepancy should be due to different diffusion, localization, and recombination dynamics of electron-hole pairs generated in InGaN active layers, and should be a very important property for influencing the optical properties of LEDs. In order to explain the results, we proposed a possible carrier dynamics model based on the carrier localization and partial reduction of the quantum confinement Stark effect depending on an indium composition in InGaN active layers. By using this model, we also considered the origin of the reduction of the emission efficiencies with a longer emission wavelength of InGaN LEDs with high indium composition
A New Sample of Low-mass Black Holes in Active Galaxies
We present an expanded sample of low-mass black holes (BHs) found in galactic
nuclei. Using standard virial mass techniques to estimate BH masses, we select
from the Fourth Data Release of the Sloan Digital Sky Survey all broad-line
active galaxies with masses < 2 x 10^6 M_sun. BHs in this mass regime provide
unique tests of the relationship between BHs and galaxies, since their
late-type galaxy hosts do not necessarily contain classical bulges.
Furthermore, they provide observational analogs of primordial seed BHs and are
expected, when merging, to provide strong gravitational signals for future
detectors such as LISA. From our preliminary sample of 19, we have increased
the total sample by an order of magnitude to 174, as well as an additional 55
(less secure) candidates. The sample has a median BH mass of = 1.3 x
10^6 M_sun, and in general the objects are radiating at high fractions of their
Eddington limits. We investigate the broad spectral properties of the sample;
55 are detected by \rosat, with soft X-ray luminosities in the range 10^40 to 7
x 10^43 ergs/sec. Much like the preliminary sample, these objects are
predominantly radio-quiet (R = f_6cm/f_4400A < 10), but 11 objects are detected
at 20 cm, with radio powers (10^21-10^23 W/Hz) that may arise from either star
formation or nuclear activity; only 1% of the sample is radio-loud. We further
confirm that, with =-19.3 and = 0.7 mag, the host galaxies are
low-mass, late-type systems. At least 40% show disk-like morphologies, and the
combination of host galaxy colors and higher-order Balmer absorption lines
indicate intermediate-age stellar populations in a subset of the sample.Comment: to appear in ApJ; 13 pages, 8 figure
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