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NTHU Institute of Astronomy
Welcome to the Institute of Astronomy at National Tsing Hua University!
Recent Colloquia

Time Coordinate: 3:30 pm 28th Oct. 2016 (Friday)

Space Coordinate: NTHU General Building II, R521

Speaker: Dr. Kouichi Hirotani (TIARA/ASIAA)

High-energy and very-high-energy emissions from the direct vicinity of rotating black holes

In a black hole magnetosphere, when the plasma accretion rate is low, the radiatively inefficient accretion flow (RIAF) can no longer sustain the force-free magnetosphere via two-photon collisions. In such a charge-starved region (or a gap), an electric field arises along the magnetic field lines to accelerate 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 RIAF to materialize as pairs. The created pairs polarize to partially screen the original acceleration electric field. It is found that the gap emissions from several nearby black hole transients are detectable with Fermi/LAT and and future ground-based atmospheric Cherenkov telescopes like CTA. Since their HE/VHE gamma-ray fluxes are predicted to exhibit anti-correlation with the IR/optical fluxes, it is possible to discriminate the gap emission from the jet.


Time Coordinate: 3:30 pm 21th Oct. 2016 (Friday)

Space Coordinate: NTHU General Building II, R521

Speaker: Dr. Eswaraiah Chakali (NTHU)

On the morphological correspondence among magnetic fields,filaments, bipolar bubbles in RCW57A

The influence of magnetic fields (B-fields) in the formation and evolution of bipolar bubbles due to the expanding ionization fronts driven by the HII regions that are formed and embedded in filamentary molecular clouds has not been well-studied yet. In addition to the anistropic expansion of ionization fronts in a filament, B-fields are expected to introduce an additional anisotropic pressure in favor of expansion and propagation of ionization fronts to form a bipolar bubble. 

I will present the results based on near-infrared polarimetric observations towards the central area of the star-forming region RCW57A hosting an HII region, a filament and a bipolar bubble. Polarization measurements of reddened background stars in the JHKs-bands reveal that B-fields, that are threaded perpendicular to the filament long axis, are configured into an hour-glass morphology thereby closely following the structure of the bipolar bubble which manifests one-to-one correspondence between them. B-field pressure is found to be dominated over thermal, turbulent and radiative pressures in the region, thereby reinforcing the fact that B-fields indeed playing an influenceable role. Based on the observed morphological correlations among the the B-fields, filament and bipolar bubble, we believe that the B-fields are important not only in cloud contraction to form a filament, gravitational collapse and star formation in it, but also in the formation and evolution of bipolar bubble in RCW57A. I also present few slides on my involvement on other projects related to optical/NIR/sub-mm polarimetry.

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