910 research outputs found
Detecting a gravitational-wave background with next-generation space interferometers
Future missions of gravitational-wave astronomy will be operated by
space-based interferometers, covering very wide range of frequency. Search for
stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for
such missions, and we here discuss the prospects for direct measurement of
isotropic and anisotropic components of (primordial) GWBs around the frequency
0.1-10 Hz. After extending the theoretical basis for correlation analysis, we
evaluate the sensitivity and the signal-to-noise ratio for the proposed future
space interferometer missions, like Big-Bang Observer (BBO), Deci-Hertz
Interferometer Gravitational-wave Observer (DECIGO) and recently proposed
Fabry-Perot type DECIGO. The astrophysical foregrounds which are expected at
low frequency may be a big obstacle and significantly reduce the
signal-to-noise ratio of GWBs. As a result, minimum detectable amplitude may
reach h^2 \ogw = 10^{-15} \sim 10^{-16}, as long as foreground point sources
are properly subtracted. Based on correlation analysis, we also discuss
measurement of anisotropies of GWBs. As an example, the sensitivity level
required for detecting the dipole moment of GWB induced by the proper motion of
our local system is closely examined.Comment: 19 pages, 6 figures, references added, typos correcte
Water Maser Emission from the Active Nucleus in M51
22 GHz water vapor `kilomaser' emission is reported from the central region
of the Whirlpool galaxy M 51 (NGC 5194). The red-shifted spectral features
(Vlsr ~ 560 km/s), flaring during most of the year 2000, originate from a
spatially unresolved maser spot of size < 30 mas (< 1.5 pc), displaced by < 250
mas from the nucleus. The data provide the first direct evidence for the
association of an H2O kilomaser with an active galactic nucleus (AGN). In early
2001, blue-shifted maser emission (Vlsr ~ 435 km/s) was also detected. Red- and
blue-shifted features bracket the systemic velocity asymmetrically. Within the
standard model of a rotating Keplerian torus, this may either suggest the
presence of a highly eccentric circumnuclear cloud or red- and blue-shifted
`high velocity' emission from a radially extended torus. Most consistent with
the measured H2O position is, however, an association of the red-shifted H2O
emission with the northern part of the bipolar radio jet. In this scenario, the
(weaker) northern jet is receding while the blue-shifted H2O emission is
associated with the approaching southern jet.Comment: 9 pages, 1 figur
Detecting a stochastic background of gravitational waves in the presence of non-Gaussian noise: A performance of generalized cross-correlation statistic
We discuss a robust data analysis method to detect a stochastic background of
gravitational waves in the presence of non-Gaussian noise. In contrast to the
standard cross-correlation (SCC) statistic frequently used in the stochastic
background searches, we consider a {\it generalized cross-correlation} (GCC)
statistic, which is nearly optimal even in the presence of non-Gaussian noise.
The detection efficiency of the GCC statistic is investigated analytically,
particularly focusing on the statistical relation between the false-alarm and
the false-dismissal probabilities, and the minimum detectable amplitude of
gravitational-wave signals. We derive simple analytic formulae for these
statistical quantities. The robustness of the GCC statistic is clarified based
on these formulae, and one finds that the detection efficiency of the GCC
statistic roughly corresponds to the one of the SCC statistic neglecting the
contribution of non-Gaussian tails. This remarkable property is checked by
performing the Monte Carlo simulations and successful agreement between
analytic and simulation results was found.Comment: 15 pages, 8 figures, presentation and some figures modified, final
version to be published in PR
Abelian dominance and the dual Meissner effect in local unitary gauges in SU(2) gluodynamics
Performing highly precise Monte-Carlo simulations of SU(2) gluodynamics, we
observe for the first time Abelian dominance in the confining part of the
static potential in local unitary gauges such as the F12 gauge. We also study
the flux-tube profile between the quark and antiquark in these local unitary
gauges and find a clear signal of the dual Meissner effect. The Abelian
electric field is found to be squeezed into a flux tube by the monopole
supercurrent. This feature is the same as that observed in the non-local
maximally Abelian gauge. These results suggest that the Abelian confinement
scenario is gauge independent. Observing the important role of space-like
monopoles in the Polyakov gauge also indicates that the monopoles defined on
the lattice do not necessarily correspond to those proposed by 't Hooft in the
context of Abelian projection.Comment: 4 pages, 7 figure
Submillimeter CO emission from shock-heated gas in the L1157 outflow
We present the CO J=6-5, 4-3, and 3-2 spectra from the blueshifted gas of the
outflow driven by the low-mass class 0 protostar in the L1157 dark cloud.
Strong submillimeter CO emission lines with T_mb > 30 K have been detected at
63" (~0.13 pc) south from the protostar. It is remarkable that the blue wings
in the submillimeter lines are stronger by a factor of 3-4 than that of the CO
J=1-0 emission line. The CO line ratios suggest that the blueshifted lobe of
this outflow consists of moderately dense gas of n(H_2) = (1-3)x10^4 cm^-3
heated to T_kin = 50-170 K.It is also suggested that the kinetic temperature of
the outflowing gas increases from ~80 K near the protostar to ~170 K at the
shocked region in the lobe center, toward which the largest velocity dispersion
of the CO emission is observed. A remarkable correlation between the kinetic
temperature and velocity dispersion of the CO emission along the lobe provides
us with direct evidence that the molecular gas at the head of the jet-driven
bow shock is indeed heated kinematically. The lower temperature of ~80 K
measured at the other shocked region near the end of the lobe is explained if
this shock is in a later evolutionary stage, in which the gas has been cooled
mainly through radiation of the CO rotational lines.Comment: 10 pages, 4 PDF figures, APJL in pres
SHG microscopic observations of polar state in Li-doped KTaO3 under electric field
Incipient ferroelectric KTaO3 with off-center Li impurity of the critical
concentration of 2.8 mol% was investigated in order to clarify the dipole glass
state under electric field. Using optical second-harmonic generation (SHG)
microscope, we observed a marked history dependence of SHG intensity through
zero-field cooling (ZFC), zero-field heating (ZFH), field heating after ZFC
(FH/ZFC) and FH after field cooling (FH/FC). These show different paths with
respect to temperature: In the ZFC/ZFH process, weak SHG was observed at low
temperature, while in the FH/ZFC process, relatively high SHG appears in a
limited temperature range below TF depending on the field strength, and in the
FC and FH/FC processes, the SHG exhibits ferroelectric-like temperature
dependence: it appears at the freezing temperature of 50K, increases with
decreasing temperature and has a tendency of saturation. These experimental
results strongly suggest that dipole glass state or polar nano-clusters which
gradually freezes with decreasing temperature is transformed into
semi-macroscopic polar state under the electric field. However at sufficiently
low temperature, the freezing is so strong that the electric field cannot
enlarge the polar clusters. These experimental results show that the polar
nano-cluster model similar to relaxors would be more relevant in KTaO3 doped
with the critical concentration of Li. Further experiments on the anisotropy of
SHG determine that the average symmetry of the field-induced polar phase is
tetragonal 4mm or 4, which is also confirmed by the X-ray diffraction
measurement.Comment: 26 pages, 8 figures, 1 tabl
Detection of Polarized Broad Emission in the Seyfert 2 Galaxy Mrk 573
We report the discovery of the scattered emission from a hidden broad-line
region (BLR) in a Seyfert 2 galaxy, Mrk 573, based on our recent
spectropolarimetric observation performed at the Subaru Telescope. This object
has been regarded as a type 2 AGN without a hidden BLR by the previous
observations. However, our high quality spectrum of the polarized flux of Mrk
573 shows prominent broad (~3000 km/s) H_alpha emission, broad weak H_beta
emission, and subtle Fe II multiplet emission. Our new detection of these
indications for the presence of the hidden BLR in the nucleus of Mrk 573 is
thought to be owing to the high signal-to-noise ratio of our data, but the
possibility of a time variation of the scattered BLR emission is also
mentioned. Some diagnostic quantities such as the IRAS color, the radio power,
and the line ratio of the emission from the narrow-line region of Mrk 573 are
consistent with the distributions of such quantities of type 2 AGNs with a
hidden BLR. Mrk 573 is thought to be an object whose level of the AGN activity
is the weakest among the type 2 AGNs with a hidden BLR. In terms of the
systematic differences between the type 2 AGNs with and without a hidden BLR,
we briefly comment on an interesting Seyfert 2 galaxy, Mrk 266SW, which may
possess a hidden BLR but has been treated as a type 2 AGNs without a hidden
BLR.Comment: 9 pages including 6 figures, to appear in The Astronomical Journa
Identification of a new small ubiquitin-like modifier (SUMO)-interacting motif in the E3 ligase PIASy
Small ubiquitin-like modifier (SUMO) conjugation is a reversible post-translational modification process implicated in the regulation of gene transcription, DNA repair, and cell cycle. SUMOylation depends on the sequential activities of E1 activating, E2 conjugating, and E3 ligating enzymes. SUMO E3 ligases enhance transfer of SUMO from the charged E2 enzyme to the substrate. We have previously identified PIASy, a member of the Siz/protein inhibitor of activated STAT (PIAS) RING family of SUMO E3 ligases, as essential for mitotic chromosomal SUMOylation in frog egg extracts and demonstrated that it can mediate effective SUMOylation. To address how PIASy catalyzes SUMOylation, we examined various truncations of PIASy for their ability to mediate SUMOylation. Using NMR chemical shift mapping and mutagenesis, we identified a new SUMO-interacting motif (SIM) in PIASy. The new SIM and the currently known SIM are both located at the C terminus of PIASy, and both are required for the full ligase activity of PIASy. Our results provide novel insights into the mechanism of PIASy-mediated SUMOylation. PIASy adds to the growing list of SUMO E3 ligases containing multiple SIMs that play important roles in the E3 ligase activity
Large-scale Filamentary Structure around the Protocluster at Redshift z=3.1
We report the discovery of a large-scale coherent filamentary structure of
Lyman alpha emitters in a redshift space at z=3.1. We carried out spectroscopic
observations to map the three dimensional structure of the belt-like feature of
the Lyman alpha emitters discovered by our previous narrow-band imaging
observations centered on the protocluster at z=3.1. The feature was found to
consist of at least three physical filaments connecting with each other. The
result is in qualitative agreement with the prediction of the 'biased'
galaxy-formation theories that galaxies preferentially formed in large-scale
filamentary or sheet-like mass overdensities in the early Universe. We also
found that the two known giant Lyman alpha emission-line nebulae showing high
star-formation activities are located near the intersection of these filaments,
which presumably evolves into a massive cluster of galaxies in the local
Universe. This may suggest that massive galaxy formation occurs at the
characteristic place in the surrounding large-scale structure at high redshift.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
VLBI study of water maser emission in the Seyfert 2 galaxy NGC5793. I: Imaging blueshifted emission and the parsec-scale jet
We present the first result of VLBI observations of the blueshifted water
maser emission from the type 2 Seyfert galaxy NGC5793, which we combine with
new and previous VLBI observations of continuum emission at 1.7, 5.0, 8.4, 15,
and 22 GHz. Maser emission was detected earlier in single-dish observations and
found to have both red- and blueshifted features relative to the systemic
velocity. We could image only the blueshifted emission, which is located 3.6 pc
southwest of the 22 GHz continuum peak. The blueshifted emission was found to
originate in two clusters that are separated by 0.7 milliarcsecond (0.16 pc).
No compact continuum emission was found within 3.6 pc of the maser spot. A
compact continuum source showing a marginally inverted spectrum between 1.7 and
5.0 GHz was found 4.2 pc southwest of the maser position. The spectral turnover
might be due to synchrotron self-absorption caused by a shock in the jet owing
to collision with dense gas, or it might be due to free-free absorption in an
ionized screen possibly the inner part of a disk, foreground to the jet.
The water maser may be part of a maser disk. If so, it would be rotating in
the opposite sense to the highly inclined galactic disk observed in CO
emission. We estimate a binding mass within 1 pc of the presumed nucleus to be
on the order of 10^7 Msun. Alternatively, the maser emission could result from
the amplification of a radio jet by foreground circumnuclear molecular gas. In
this case, the high blueshift of the maser emission might mean that the masing
region is moving outward away from the molecular gas surrounding an active
nucleus.Comment: 20 pages, 6 figures, to appear in ApJ, Oct. 200
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