Hidekazu Hanayama

Cosmological Magnetic Field: A Fossil of Density Perturbations in the Early Universe

The origin of the substantial magnetic fields that are found in galaxies and on even larger scales, such as in clusters of galaxies, is yet unclear. If the second-order couplings between photons and electrons are considered, then cosmological density fluctuations, which explain the large-scale structure of the universe, can also produce magnetic fields on cosmological scales before the epoch of recombination. By evaluating the power spectrum of these cosmological magnetic fields on a range of scales, we show here that magnetic fields of 10–18.1 gauss are generated at a 1-megaparsec scale and can be even stronger at smaller scales (10–14.1 gauss at 10 kiloparsecs). These fields are large enough to seed magnetic fields in galaxies and may therefore have affected primordial star formation in the early universe.

Magnetic field generation from cosmological perturbations.

In this Letter, we discuss the generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton, and photon) evaluating the collision term between electrons and photons up to the second order. The collision term is shown to induce electric current, which then generates magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity, which is purely second order. We estimate the magnitudes of the former contributions and show that the amplitude of the produced magnetic field is about approximately 10(-19) G at 10 Mpc comoving scale at the decoupling. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

OBSERVATIONAL EVIDENCE FOR AN IMPACT ON THE MAIN-BELT ASTEROID (596) SCHEILA

An unexpected outburst was observed around (596) Scheila in 2010 December. We observed (596) Scheila soon after the impact using ground-based telescopes. We succeeded in the detection of a faint linear tail after 2011 February, which provides a clue to determine the dust ejection date. It is found that the dust particles ranging from 0.1-1 ?m to 100 ?m were ejected into the interplanetary space impulsively on December 3.5 ?1.0 day. The ejecta mass was estimated to be (1.5-4.9)?108?kg, suggesting that an equivalent mass of a 500-800?m diameter crater was excavated by the event. We also found that the shape of the light curve changed after the impact event probably because fresh material was excavated around the impact site. We conclude that a decameter-sized asteroid collided with (596) Scheila only eight days before the discovery.

Interpretation of (596) Scheila's Triple Dust Tails

Strange-looking dust cloud around asteroid (596) Scheila was discovered on 2010 December 11.44-11.47. Unlike normal cometary tails, it consisted of three tails and faded within two months. We constructed a model to reproduce the morphology of the dust cloud based on the laboratory measurement of high-velocity impacts and the dust dynamics. As a result, we succeeded in reproducing the peculiar dust cloud by an impact-driven ejecta plume consisting of an impact cone and downrange plume. Assuming an impact angle of 45 Degree-Sign , our model suggests that a decameter-sized asteroid collided with (596) Scheila from the direction of ({alpha}{sub im}, {delta}{sub im}) = (60 Degree-Sign , -40 Degree-Sign ) in J2000 coordinates on 2010 December 3. The maximum ejection velocity of the dust particles exceeded 100 m s{sup -1}. Our results suggest that the surface of (596) Scheila consists of materials with low tensile strength.

Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields

We study the generation of magnetic fields by the Biermann mechanism in the supernova explosions of the first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We perform a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the magnetic fields that are produced. We find that magnetic fields with amplitude 10-14 to 10-17 G are generated inside the bubble, although the amount of magnetic field generated depends on the specific values of the initial conditions. This corresponds to magnetic fields of 1028-1031 ergs for each supernova remnant, which is strong enough to be the seed magnetic field for a galactic and/or interstellar dynamo.

MINUTE-SCALE RAPID VARIABILITY OF THE OPTICAL POLARIZATION IN THE NARROW-LINE SEYFERT 1 GALAXY PMN J0948+0022

We report on optical photopolarimetric results of the radio-loud narrow-line Seyfert 1 (RL-NLSy1) galaxy PMN J0948+0022 on 2012 December to 2013 February triggered by flux enhancements in the near infrared and ?-ray bands. With the one-shot polarimetry of the Hiroshima One-shot Wide field Polarimeter installed on the Kanata Telescope, we detected very rapid variability in the polarized-flux (PF) light curve on MJD 56281 (2012 December 20). The rise and decay times were about 140?s and 180?s, respectively. The polarization degree (PD) reached 36% ? 3% at the peak of the short-duration pulse, while the polarization angle remained almost constant. In addition, temporal profiles of the total flux and PD showed highly variable but well correlated behavior and discrete correlation function analysis revealed that no significant time lag of more than 10?minutes was present. The high PD and minute-scale variability in PF provides clear evidence of synchrotron radiation from a very compact emission region of ~1014 cm size with a highly ordered magnetic field. Such micro-variability of polarization is also observed in several blazar jets, but its complex relation between total flux and PD are explained by a multi-zone model in several blazars. The implied single emission region in PMN J0948+0022 might reflect a difference of jets between RL-NLSy1s and blazars.

Long-Term Evolution of Supernova Remnants in Magnetized Interstellar Medium

The evolution of supernova remnants (SNRs) is studied, with particular attention to the effect of magnetic fields with axisymmetric two-dimensional magnetohydrodynamic simulations. We study the interaction of SNRs with a quiescent, magnetized ISM having uniform density, temperature, and magnetic field. The evolution of magnetic SNRs is the same as that of nonmagnetic ones in the adiabatic Sedov stage. After a thin shell is formed, the shell is driven by the pressure of the hot interior gas (bubble). Evolution in the pressure-driven snowplow phase is much affected by the magnetic field. The shell sweeping the magnetic field lines thickens owing to the magnetic pressure force. After 5 × 105 to 2 × 106 yr, the inner boundary of the thick shell begins to contract. This compresses the hot bubble radially and maintains its thermal pressure. Thus, the bubble forms a prolate spheroidal shape and becomes thinner and thinner, since it expands in a direction parallel to the magnetic field for B0 3 μG. Finally, the bubble contracts. The porosity of the hot low-density gas in the ISM is reduced, taking the effect of the magnetic field into account.

Effect of Primordial Magnetic Field on Seeds for Large Scale Structure

Magnetic field plays a very important role in many astronomical phenomena at various scales of the universe. It is no exception in the early universe. Since the energy density, pressure, and tension of the primordial magnetic field affect gravitational collapses of plasma, the formation of seeds for large-scale structures should be influenced by them. Here we numerically investigate the effects of stochastic primordial magnetic field on the seeds of large-scale structures in the universe in detail. We found that the amplitude ratio between the density spectra with and without PMF (

Variable optical polarization during high state in γ-ray loud, narrow-line Seyfert 1 galaxy 1H 0323+342

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Multiband Optical Observation of The P/2010 A2 Dust Tail

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