午餐會報 (2017)

Imaging Atmospheric Cherenkov Telescopes have revealed more than 100 TeV gamma-ray sources along the Galactic Plane, around 45% of them remain unidentified. Nevertheless, radio observations recently revealed that dense molecular clumps are associated with 67% of 18 unidentified TeV sources. We demonstrate that copious TeV gamma-rays can be emitted from rapidly rotating, stellar-mass black holes when they encounter gaseous clouds. Since the general-relativistic effect plays an essential role in this magnetospheric lepton accelerator scenario, the emissions take place in the direct vicinity of the event horizon, resulting in a point-like gamma-ray image. We demonstrate that their gamma-ray spectra have two peaks in MeV-10GeV and 10GeV-TeV, via curvature and inverse-Compton processes respectively, and that the accelerators become most luminous when the mass accretion rate becomes about 0.01% of the Eddington rate.

I will present the results of a study (Dib, Schmeja & Hony 2017) to investigate the occurence rates of single high-mass stars in clusters in order to put constraints on the IMF. The key idea is that the for small clusters (of a few hundred solar masses) that have trouble to make an O-type star this occurence rate will depend strongly on the IMF. We compare the fractions of single and lonely massive O stars in a recent catalogue of the Milky Way clusters with a library of simulated clusters built with various distribution functions of the IMF parameters. The synthetic clusters are corrected for the effects of the binary population, stellar evolution and sample incompleteness. Our findings indicate that significant cluster-to-cluster variations of the IMF parameters are required in order to reproduce the fractions of single and lonely O stars in Galactic clusters.

The sky distribution of Gamma-Ray Bursts (GRBs) has been intensively studied for more than two decades. Most of these studies, test the isotropy of GRBs based on the sky number density distribution. We propose a new method which inspects the isotropy of the properties of GRBs such as their duration, fluences and peak fluxes at various energy bands and different time scales. The method was applied on the Fermi / Gamma-ray Burst Monitor data. We found a relatively significant feature near the Galactic coordinates approximately l = 30 deg, b = 15 deg and radius r = 20 - 40 deg with the inferred probability for the occurrence of such signal (in a random isotropic sample) to be less than a percent. However, more comprehensive analysis using different statistical tests and different samples show that the detected feature can be due to statistical fluctuations. Investigations on the updated Fermi / GBM sample as well as on the data sets of other instruments can clarify on the issue.

I will talk about the discovery of a "plane" dictating dark-matter halo evolution from our gravitational lensing and X-ray observations. We show that high-mass galaxy clusters—cosmic giants formed in the universe—lie on a plane in the 3D logarithmic space of their characteristic halo radius "rs", mass "Ms" , and X-ray temperature "Tx" with a very small orthogonal scatter. The tight correlation indicates that the gas temperature was determined at a specific cluster formation time, which is encoded in (rs,Ms) and that the gas was heated when the clusters were in the fast-growing phase. Intriguingly, the plane is significantly tilted with respect to the canonical virial expectation, Tx ∝ Ms /rs. We show that the self-similar solution of Bertschinger (1985) for secondary infall and accretion can explain the observed relation, Tx ∝ Ms^1.5 /rs^2. Numerical simulations reproduce the observed plane and its angle. This result holds independently of the gas physics implemented in the code, revealing the fundamental origin of this plane.

The Magnetohydrodynamic (MHD) jet/wind model, which is the very popular theory of astrophysical jets (in such as YSOs and AGNs), is briefly discussed. The "magneto-centrifugal" (MC) acceleration (Blandford & Payne 1982, MNRAS, hereafter BP82) plays a primal role in sub-Alfvenic regime, but there are many misunderstandings in the literature. It is nice to consider the MC process in the inertial frame to see what is essential for the MC acceleration. The standard solution of BP82 includes the magnetic pressure gradient (MPG) acceleration; both the MC and the MPG process would take place in a single streamline of the MHD jet in trans-Alfvenic regime. The talk aims to share fundamentals of MHD jets with undergraduate knowledge, especially with those who are not experts in this field.

Non-research staff are welcome.

Beside the regular PI projects of ALMA, one can also utilize the public ALMA data for his/her own research. In this talk, I will present the motivation why we need an interval share data archive and the plan.

The cosmic microwave background (CMB) continues to reveal new aspects of the large scale universe. Technologies for ground-based and balloon-borne instruments measuring the polarization of the CMB have been well established and advanced in the last decade. Experience from the current generation of CMB projects indicates that the integration of the cryogenic detector array package, including its multiplexing and readout elements, is critical to the performance of the CMB instrument. The next generation of instruments needed to fully explore the potential of the CMB will require many more detectors than have currently been deployed. As the number of detectors per focal plane scales up, improvements and new technologies for large array packaging will be required. In this talk, I will present the concept from ACTPol and Advanced ACTPol for the assembly of large bolometer arrays. I will summarize the possible improvements and limitations anticipated in driving towards large numbers of even higher density detector arrays when comparing to the successes from other CMB projects, keeping in mind the need for faster production rates while maintaining good performance and high yield.

Dust is an essential component in understanding star formation properties of galaxies both observationally and theoretically. When we consider the absorption of UV light by dust, the dust abundance and the grain size distribution are both important. Dust is formed and destroyed in multiple processes, which depend on the local metallicity, density and temperature. We implement these processes of dust into hydrodynamic simulation code GADGET3-Osaka taking into account of the evolution of dust size distribution. In order to realize it, we include dust formation end destruction processes in ISM, which are not only generation and destruction by supernovae but also accretion, coagulation and shattering. We obtained the time evolution of the abundance $Omega_{\rm dust}(z)$, the mass function and the relation between dust-to-gas mass ratio to metallicity of each galaxy. They agree well with the corresponding observations.

GAMER, a parallel Graphic-processing-unit-accerelated Adaptive-MEsh-Refinement hydrodynamic code, has been extended to support magnetohydrodynamics (MHD), where the solver for MHD features the corner-transport-upwind (CTU) scheme with the constraint transport (CT) technique. The divergent preserving operator for adaptive mesh refinement (AMR) has been applied to reinforce the divergence-free constraint on the magnetic field. GAMER-MHD has fully exploited concurrent executions of the MHD solver of GPU computation and others, including AMR, of CPU computation, and nearly 100% overlap in GPU and CPU computation is observed. Various standard tests of one-dimensional, two-dimensional and three-dimesional problems have been performed, and these tests show that GAMER-MHD is robust producing results either demosntrating the expected second-order accuracy of our numerical scheme or as good as those given by high-resolution uniform-grid runs. A new 3D MHD test problem has been explored, where the magnetic field assumes the Arnold- Beltrami-Childress (ABC) configuration that can be unstable, temporarily becomes turbulent with thin-tube reconnection and finally settles to a lowest-energy equilibrium state. This 3D problem is adopted for the performance test of GAMER-MHD in a supercomputer equipped with top-end GPU and CPU. The single-GPU performance can reach 2 × 10^7 cell-update/sec, and this code also shows high efficiency of parallelization and scalability up to O(10^3) nodes.

In this talk, I will mention why we need and quartz windows. And why using LDPE to reduce the reflection. How we coated the LDPE ( Low-density polyethylene ) at the quartz surface. And preliminary result.

I will present ongoing simulations to study the interaction between protoplanetary disks and embedded protoplanets using a modified version of the PLUTO code. I will show how dusty disk-planet torques can differ from that in pure gas disks, which is considered in most previous studies.

In a black hole magnetosphere, when the plasma accretion rate is much lower than the Eddington limit, a strong electric field inevitably appears along the magnetic field lines due to charge deficit. In such a charge-starved region (or a gap), the electric field accelerates migratory electrons and positrons into ultra relativistic energies. These relativistic leptons emit copious gamma-rays via curvature and inverse-Compton (IC) processes. Some of such gamma-rays collide with the submillimeter-IR photons emitted from the radiatively inefficient accretion flow (RIAF) to materialize as pairs. The created pairs polarize to partially screen the original acceleration electric field. By solving this pair-creation cascade self-consistently for the first time, we find that the gap emissions from the direct vicinity of stellar-mass, intermediate-mass, and super-massive black holes are detectable with Fermi/LAT and Imaging Atmospheric Cherenkov Telescopes (e.g., MAGIC, HESS, VERITAS, and CTA) when the accretion rate is in a certain range. Moreover, such HE/VHE gamma-ray fluxes are predicted to exhibit anti-correlation with the RIAF submillimeter fluxes, which enables us to discriminate the gap emission from the standard, shock-in-jet model, e.g., by using ALMA and CTA.