4,502 research outputs found
The early reionization with the primordial magnetic fields
The early reionization of the intergalactic medium, which is favored from the
WMAP temperature-polarization cross-correlations, contests the validity of the
standard scenario of structure formation in the cold dark matter cosmogony. It
is difficult to achieve early enough star formation without rather extreme
assumptions such as very high escape fraction of ionizing photons from
proto-galaxies or a top-heavy initial mass function. Here we propose an
alternative scenario that is additional fluctuations on small scales induced by
primordial magnetic fields trigger the early structure formation. We found that
ionizing photons from Population III stars formed in dark haloes can easily
reionize the universe by if the strength of primordial magnetic
fields is larger than Gauss.Comment: 8 pages, 5 figures. accepted for publication in MNRA
Evidence of non-thermal X-ray emission from radio lobes of Cygnus A
Using deep Chandra ACIS observation data for Cygnus A, we report evidence of
non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster
medium (ICM). The diffuse X-ray emission, which are associated with the eastern
and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The
lobe spectra are reproduced with not only a single-temperature Mekal model,
such as that of the surrounding ICM component, but also an additional power-law
(PL) model. The X-ray flux densities of PL components for the eastern and
western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and
52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are
1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal
component is considered to be produced via the inverse Compton (IC) process, as
is often seen in the X-ray emission from radio lobes. From a re-analysis of
radio observation data, the multiwavelength spectra strongly suggest that the
seed photon source of the IC X-rays includes both cosmic microwave background
radiation and synchrotron radiation from the lobes. The derived parameters
indicate significant dominance of the electron energy density over the magnetic
field energy density in the Cygnus A lobes under the rich ICM environment.Comment: 8 pages, 5 figures, accepted for publication in Ap
Study of systematics effects on the Cross Power Spectrum of 21 cm Line and Cosmic Microwave Background using Murchison Widefield Array Data
Observation of the 21cm line signal from neutral hydrogen during the Epoch of
Reionization is challenging due to extremely bright Galactic and extragalactic
foregrounds and complicated instrumental calibration. A reasonable approach for
mitigating these problems is the cross correlation with other observables. In
this work, we present the first results of the cross power spectrum (CPS)
between radio images observed by the Murchison Widefield Array and the cosmic
microwave background (CMB), measured by the Planck experiment. We study the
systematics due to the ionospheric activity, the dependence of CPS on group of
pointings, and frequency. The resulting CPS is consistent with zero because the
error is dominated by the foregrounds in the 21cm observation. Additionally,
the variance of the signal indicates the presence of unexpected systematics
error at small scales. Furthermore, we reduce the error by one order of
magnitude with application of a foreground removal using a polynomial fitting
method. Based on the results, we find that the detection of the 21cm-CMB CPS
with the MWA Phase I requires more than 99.95% of the foreground signal
removed, 2000 hours of deep observation and 50% of the sky fraction coverage.Comment: 15 pages, 16 figures, accepted to MNRA
Self-Regulation of Star Formation in Low Metallicity Clouds
We investigate the process of self-regulated star formation via
photodissociation of hydrogen molecules in low metallicity clouds. We evaluate
the influence region's scale of a massive star in low metallicity gas clouds
whose temperatures are between 100 and 10000 Kelvin. A single O star can
photodissociate hydrogen molecules in the whole of the host cloud. If
metallicity is smaller than about 10^{-2.5} of the solar metallicity, the
depletion of coolant of the the host cloud is very serious so that the cloud
cannot cool in a free-fall time, and subsequent star formation is almost
quenched. On the contrary, if metallicity is larger than about 10^{-1.5} of the
solar metallicity, star formation regulation via photodissociation is not
efficient. The typical metallicity when this transition occurs is about 1/100
of the solar metallicity. This indicates that stars do not form efficiently
before the metallicity becomes larger than about 1/100 of the solar metallicity
and we considered that this value becomes the lower limit of the metallicity of
luminous objects such as galaxies.Comment: 14 pages, including 5 figures, To appear in ApJ, Vol. 53
Laser Phase and Frequency Stabilization Using Atomic Coherence
We present a novel and simple method of stabilizing the laser phase and
frequency by polarization spectroscopy of an atomic vapor. In analogy to the
Pound-Drever-Hall method, which uses a cavity as a memory of the laser phase,
this method uses atomic coherence (dipole oscillations) as a phase memory of
the transmitting laser field. A preliminary experiment using a distributed
feedback laser diode and a rubidium vapor cell demonstrates a
shot-noise-limited laser linewidth reduction (from 2 MHz to 20 kHz). This
method would improve the performance of gas-cell-based optical atomic clocks
and magnetometers and facilitate laser-cooling experiments using narrow
transitions.Comment: 7 pages, 6 figures, appendix on the derivation of Eq.(3) (transfer
function for a polarization-spectroscopy-based frequency discriminator) has
been adde
Electrical interference with pickup coil in induction magnetometer
2011 Fifth International Conference on Sensing Technology took place 28 November - 1 December 2011 in Palmerston North, New ZealandIn this paper, we consider electrical interference with pickup coil in an induction magnetometer. By using a dummy load in place the pickup coil, we confirm that there is no significant electrical interference with a differential-input type current-to-voltage converter. In order to reveal electrical interference with the pickup coil, we investigate the output voltage of the magnetometer as grounding condition parameters. From the number of experimental considerations, we formulate a suitable condition to solve the electrical interference problemArticleSensing Technology (ICST), 2011 Fifth International Conference on : 90-93 (2011)conference pape
Probing Primordial Magnetic Fields with the 21cm Fluctuations
Primordial magnetic fields possibly generated in the very early universe are
one of the candidates for the origin of magnetic fields observed in many
galaxies and galaxy clusters. After recombination, the dissipation process of
the primordial magnetic fields increases the baryon temperature. The Lorentz
force acts on the residual ions and electrons to generate density fluctuations.
These effects are imprinted on the cosmic microwave background (CMB) brightness
temperature fluctuations produced by the neutral hydrogen 21cm line. We
calculate the angular power spectrum of brightness temperature fluctuations for
the model with the primordial magnetic fields of a several nano Gauss strength
and a power-law spectrum. It is found that the overall amplitude and the shape
of the brightness temperature fluctuations depend on the strength and the
spectral index of the primordial magnetic fields. Therefore, it is expected
that the observations of the CMB brightness temperature fluctuations give us a
strong constraint on the primordial magnetic fields.Comment: 12 pages, submitted to MNRA
Nonlinear Evolution of Cosmic Magnetic Fields and Cosmic Microwave Background Anisotropies
In this work we investigate the effects of the primordial magnetic fields on
cosmic microwave background anisotropies (CMB). Based on cosmological
magnetohydrodynamic (MHD) simulations we calculate the CMB anisotropy spectra
and polarization induced by fluid fluctuations (Alfv\'en modes) generated by
primordial magnetic fields. The strongest effect on the CMB spectra comes from
the transition epoch from a turbulent regime to a viscous regime. The balance
between magnetic and kinetic energy until the onset of the viscous regime
provides a one to one relation between the comoving coherence length and
the comoving magnetic field strength , such as . The resulting CMB temperature and polarization anisotropies are
somewhat different from the ones previously obtained by using linear
perturbation theory. Our calculation gives a constraint on the magnetic field
strength in the intermediate scale of CMB observations. Upper limits are set by
WMAP and BOOMERANG results for comoving magnetic field strength of with a comoving coherence length of for the most extreme
case, or for the most conservative case.Comment: accepted for publication in Phys. Rev.
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