• Journal of the Korean Statistical Society
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• 제31권3호
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• pp.343-357
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• 2002

This paper proposes a method for producing smooth and positive estimates of the intensity function of an inhomogeneous Poisson process based on the shrinkage of wavelet coefficients of the observed counts. The information projection is used in conjunction with the level-dependent thresholds to yield smooth and positive estimates. This work is motivated by and demonstrated within the context of a problem involving gamma-ray burst data in astronomy. Simulation results are also presented in order to show the performance of the information projection estimators.

• Journal of Astronomy and Space Sciences
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• 제22권4호
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• pp.377-384
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• 2005

감마선 폭발체는 1973년 처음으로 알려진 후 현재까지 많은 과학자들에 의해 연구되고 있다. 짝은 지속 기간을 가진 감마선 폭발체에 비해 비교적 긴 시간 척도를 가진 잔유휘광의 분광 분석자료는 감마선 폭발체 생성 환경 연구에 중요한 정보를 제공한다. 그러나, 모든 감마선 폭발체에서 모든 영역의 잔유휘광이 관측되는 것은 아니다. 전파나 엑스선 영역의 잔유휘광 관측 불능은 검출기나 광대역 모니터의 한계로 인한 문제로 보고 있으며, 광학 잔유휘광 관측 불능은 광원내부 또는 소속 은하내의 먼지 그리고 성간 매질의 흡수에 의한 것으로 보고 있다. 우리는 이러한 잔유휘광이 관측되지 않은 경우에 대해 의문을 가지고, 광학 관측으로 거리가 정해진 감마선 폭발체의 거리에 따른 에너지 영역별 잔유휘광 개수 분포를 비교 분석해 보았다. 그 결과 우리는 엑스선 잔유휘광이 관측된 감마선 폭발체들이나 광학 잔유휘광이 관측된 감마선 폭발체들의 거리 분포가 같다는 것을 알 수 있었다. 이같은 결과로부터 우리는 광학 잔유휘광이 성간 물질에 의한 소광으로 관측되지 않을 수 있다는 이론은 타당치 못하다는 결론을 주장해 본다.

• 천문학회보
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• 제33권1호
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• pp.33.1-33.1
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• 2008

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 1999년도 한국우주과학회보 제8권1호
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• pp.29-29
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• 1999

No Abstract, See Full Text

• Journal of Astronomy and Space Sciences
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• 제33권2호
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• pp.109-117
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• 2016

We explain the results of Yu et al. (2015b) of the novel sharpness angle measurement to a large number of spectra obtained from the Fermi gamma-ray burst monitor. The sharpness angle is compared to the values obtained from various representative emission models: blackbody, single-electron synchrotron, synchrotron emission from a Maxwellian or power-law electron distribution. It is found that more than 91% of the high temporally and spectrally resolved spectra are inconsistent with any kind of optically thin synchrotron emission model alone. It is also found that the limiting case, a single temperature Maxwellian synchrotron function, can only contribute up to 58⁺²³₋₁₈% of the peak flux. These results show that even the sharpest but non-realistic case, the single-electron synchrotron function, cannot explain a large fraction of the observed spectra. Since any combination of physically possible synchrotron spectra added together will always further broaden the spectrum, emission mechanisms other than optically thin synchrotron radiation are likely required in a full explanation of the spectral peaks or breaks of the GRB prompt emission phase.

• 천문학회보
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• 제37권1호
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• pp.67.1-67.1
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• 2012

Shocks are evolved when the relativistic jets in active galactic nuclei (AGNs), black hole binaries, supernova remnants (SNR) and gamma-ray bursts (GRBs) interact with the surrounding medium. The high energy particles are believed to be accelerated by the diffusive shock acceleration and the strong magnetic field is generated by Weibel instability in the shock. When ultrarelativistic electrons with strong magnetic field cool by the synchrotron emission, the radiation is observed in gamma-ray burst and the near-equipartitioned magnetic field in the external shock delays the afterglow emission. In this paper, we performed the 3D particle-in-cell (PIC) simulations to understand the characteristics of these relativistic shock and particle acceleration. Forward and reverse shocks are shaped while the unmagnetized injecting jet interacts with the unmagnetized ambient medium. Both upstream and downstream become thermalized and the particle accelerations are shown in each transition region of the shock structures.

• 천문학회보
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• 제42권1호
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• pp.37.3-37.3
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• 2017

We present a novel GPU accelerated search algorithm for broadband extended gravitational-wave emission (BEGE) with better than real-time analyis of H1-L1 LIGO S6 data. It performs matched filtering with over 8 million one-second duration chirps. Parseval's Theorem is used to predict the standard deviation ${\sigma}$ of filter output, taking advantage of near-Gaussian LIGO (H1,L1)-data in the high frequency range of 350-2000 Hz. A multiple of ${\sigma}$ serves as a threshold to filter output back to the central processing unit. This algorithm attains 80% efficiency, normalized to the Fast Fourier Transform (FFT). We apply it to a blind, all-sky search for BEGE in LIGO data, such as may be produced by long gamma-ray bursts and superluminous supernovae. We report on mysterious features, that are excluded by exact simultaneous occurrance. Our results are consistent with no events within a radius of about 20 Mpc.

• 천문학회보
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• 제41권1호
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• pp.64.4-65
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• 2016

Observations of the early photons from evolution of optical afterglows or internal shock provides the crucial clues on the nature of the bursts and environments. Hundreds of GRBs afterglow observations in multi-wavelength region have been made mainly thanks to the fast (~ 60 seconds after the trigger) localisation GRB by Swift and its fast alert to the ground telescope. It helps to improve our understandings tremendously, however many enigmas still remain, such as burst mechanism, transition prompt emission to the afterglow, early optical flash, rise phase of the early optical light curve and some missing afterglows. They could be addressed by fast slewing and multi colour and IR follow-up by future telescopes. The primary aim of UFFO/Lomonosov is to follow up optical fast ever, within a couple of seconds after trigger by onboard X-ray telescope. Its optical FOV is $30{\times}30degrees$. As a key instrument, the Slewing Mirror to redirect the optical beam from GRBs rapidly to the Ritchey-Chretien telescope. The status and launch schedule of the UFFO/Lomonosov and its test performance will be reported and prospects for the next missions will be discussed.

• 천문학회보
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• 제37권1호
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• pp.32.2-32.2
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• 2012

Gamma Ray Bursts (GRBs) are the most energetic events in the universe, and are known to arise from the death of massive stars in many cases. Their extreme brightness makes it possible to detect them at very high redshift (z > 6.5), well into the epoch of re-ionization, providing us with an opportunity to investigate the deaths of the first stars when the universe was much younger than 1 Gyr. Here, we report the discovery of GRB 100905A, a GRB at $z$ - 7.5 (age of the universe at 700 Myr). Our observation revealed a strong spectral break between z and J band, allowing us to estimate its photometric redshift. Its gamma-ray light curve shows a very short duration of about 0.7 sec, the shortest duration event at z > 5. Investigation of this and three other known GRBs at z > 6.5 reveals that they are all short duration bursts. This is puzzling, considering that GRBs from death of massive stars do not show short duration. We suggest two possible explanations for this: (i) the BAT light curves of the high redshift GRBs suffered from observational selection effect where we are only observing the very tip of the light curve; (ii) the stars in the early universe had a peculiar nature that are different from ordinary stars at lower redshifts.

• 천문학회지
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• 제47권1호
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• pp.15-39
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• 2014

Polarization is a basic property of light and is fundamentally linked to the internal geometry of a source of radiation. Polarimetry complements photometric, spectroscopic, and imaging analyses of sources of radiation and has made possible multiple astrophysical discoveries. In this article I review (i) the physical basics of polarization: electromagnetic waves, photons, and parameterizations; (ii) astrophysical sources of polarization: scattering, synchrotron radiation, active media, and the Zeeman, Goldreich-Kylafis, and Hanle effects, as well as interactions between polarization and matter (like birefringence, Faraday rotation, or the Chandrasekhar-Fermi effect); (iii) observational methodology: on-sky geometry, influence of atmosphere and instrumental polarization, polarization statistics, and observational techniques for radio, optical, and $X/{\gamma}$ wavelengths; and (iv) science cases for astronomical polarimetry: solar and stellar physics, planetary system bodies, interstellar matter, astrobiology, astronomical masers, pulsars, galactic magnetic fields, gamma-ray bursts, active galactic nuclei, and cosmic microwave background radiation.