• 천문학회보
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• 제37권2호
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• pp.67-67
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• 2012

Exploring the distant universe by observing various astronomical objects and extending knowledge on the cosmos by applying human intuition and reasoning to observations are astronomers' professional activity. Astronomers are the people born under a lucky star since this elegant and beautiful job is their the only duty. Being in the 21st century we astronomers now know that galaxies are holding evolving stars and gas, and distribute in the infinite spacetime in an interesting way revealing the secrets of the beginning of the universe. Cosmic structures such as galaxies, large-scale structures, and cosmic microwave background fluctuations are also the tracers of the expansion of space and the invisible components of the energy contents of the universe. Unlike the past century we are in a situation where integral knowledge on various cosmic structures as well as that on a variety of observational and analysis tools are available to everyone and often required for our special mission. However, my experience made me think that accumulating critical questions on nature driven by curiosity is vital for researchers and far more important than absorbing knowledge from others and books. Transforming one's own question marks to acclamation marks is the reward of our life. That is THE fun.

• 천문학회지
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• 제37권5호
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• pp.597-600
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• 2004

A substantial number of processes have been suggested as possible contributors to the extragalactic $\gamma$-ray background (EGRB). Yet another contribution to this background will be emission produced in hadronic interactions of cosmic-ray protons with the cluster thermal gas; this class of cosmic rays (CRs) has been shown to be responsible for the EUV emission in the Coma Cluster of galaxies. In this paper we assume the CRs in the Coma Cluster is prototypic of all clusters and derive the contribution to the EGRB from all clusters over time. We examine two different possibilities for the scaling of the CR flux with cluster size: the number density of the CRs scale with the number density of the thermal plasma, and alternatively, the energy density of the CRs scale with the energy density of the plasma. We find that in all scenarios the EGRB produced by this process is sufficiently low that it will not be observable in comparison with other mechanisms that are likely to produce an EGRB.

• 천문학논총
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• 제22권4호
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• pp.113-132
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• 2007

On the framework of stochastic gravitational wave background(SGWB) by compact binary systems, we studied the strain spectra of SGWB produced by cosmological cataclysmic variables(CV). For this we reviewed the empirical properties of CVs by using newly published CV catalogue and calculated the cosmological densities of CVs considering the galaxy luminosity function and cosmic stellar birth rate function. Assuming the secular evolution of CVs, we calculated the time scale of CV gravitational wave(GW) radiation and derived formulae for the strain spectra of SGWB by cosmological CVs.

• 천문학논총
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• 제26권2호
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• pp.55-70
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• 2011

Cosmological linear perturbation theory has fundamental importance in securing the current cosmological paradigm by connecting theories with observations. Here we present an explanation of the method used in relativistic cosmological perturbation theory and show the derivation of basic perturbation equations.

• 천문학회지
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• 제37권5호
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• pp.295-297
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• 2004

Observations with EUVE, ROSAT, and BeepoSAX have shown that some clusters of galaxies produce intense EUV emission. These findings have produced considerable interest; over 100 papers have been published on this topic in the refereed literature. A notable suggestion as to the source of this radiation is that it is a 'warm' (106 K) intracluster medium which, if present, would constitute the major baryonic component of the universe. A more recent variation of this theme is that this material is 'warm-hot' intergalactic material condensing onto clusters. Alternatively, inverse Compton scattering of low energy cosmic rays against cosmic microwave background photons has been proposed as the source of this emission. Various origins of these particles have been posited, including an old (${\~}$Giga year) population of cluster cosmic rays; particles associated with relativistic jets in the cluster; and cascading particles produced by shocks from sub-cluster merging. The observational situation has been quite uncertain with many reports of detections which have been subsequently contradicted by analyses carried out by other groups. Evidence supporting a thermal and a non-thermal origin has been reported. The existing EUV, FUV, and optical data will be briefly reviewed and clarified. Direct observational evidence from a number of different satellites now rules out a thermal origin for this radiation. A new examination of subtle details of the EUV data suggests a new source mechanism: inverse Compton scattered emission from secondary electrons in the cluster. This suggestion will be discussed in the context of the data.

• 천문학회보
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• 제35권2호
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• pp.43.1-43.1
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• 2010

A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogen-ionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of high-redshift universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is found crucial for this analysis. Square Kilometer Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.

• Journal of Radiation Protection and Research
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• 제13권2호
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• pp.9-20
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• 1988

자연 방사선 피폭선량 중 우주선 전리 성분의 기여를 정량하기 위하여 LiF TLD를 이용하여 약 1년반에 걸쳐 3개월 간격의 주기적 측정을 수행하였다. TLD는 세 가지를 사용하였는데 그것은 칩과 PTFE원판형으로 된 두 가지의 $^{7}LiF$와 중성자 성분의 기여를 가려내기 위한 원판형의 $^{6}LiF$였다. 선량 측정은 뱃지에 넣은 TLD를 충남대학교 대덕 캠퍼스의 한4층 콘크리트 건물내 3층의 한 연구실에 설치한 10-15cm 두께의 납차폐 상자에 넣어서 90일 간격으로 다섯주기 동안 수행하였다. 비교 연구를 위하여 3'${\phi}\;{\times}\;3$'원통형NaI(Tl) 섬광 검출기와 1024채널 MCA를 이용하여 3MeV 이상의 우주선 경성분에 대한 분광분석을 병행하였는데 그 결과 옥내 차폐체를 이용한 TLD측정치는 옥외 우주선 전리 성분의 약 75%를 측정하고 있음이 밝혀졌다. 이와 같은 차폐 손실을 보정한 TLD측정 결과에 의하면 충남대 대덕 캠퍼스 옥외에서 우주선 전리 성분의 기여는 $34.3{\pm}1.1nGy-h^{-1}$로 나타났는데 이것은 이 측정 지점에서 예측할 수 있었던 선량 값과 매우 잘 일치하는 것이다.

• 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.

• 천문학회보
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• 제37권1호
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• pp.42-42
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• 2012

The First stars (Pop.III stars) in the universe are expected to be formed between the recombination era at z - 1100 and the most distant quasar (z - 8). They have never been directly detected due to its faintness so far, but can be observed as a background radiation at around 1${\mu}m$ which is called the Cosmic Near-Infrared Background (CNB). Main part of the CNB is thought to be redshifted Lyman-alpha from gas clouds surrounding the Pop.III stars. Until now, the COBE (COsmic Background Explorer) and the IRTS (Infrared Telescope in Space) observed excess emission over the background due to galaxies. To confirm the COBE and the IRTS results and pursue more observational evidences, we carried out the sounding rocket experiment named the Cosmic Infrared Background ExpeRiment (CIBER). The CIBER is successfully launched on July 10, 2010 at White Sands Missile Range, New Mexico, USA. It consists of three kinds of instruments. We report the results obtained by LRS (Low Resolution Spectrometer) which is developed to fill the uncovered spectrum around 1${\mu}m$. LRS is a refractive telescope of 5.5 cm aperture with spectral resolution of 20 - 30 and wavelength coverage of 0.7 to 2.0${\mu}m$. After subtracting foreground components (zodiacal light, integrated star light and diffuse galactic light) from the sky brightness of observed five fields, there remained significant residual emission (even for the lower limit case) consistent with the IRTS and the COBE results. In addition, there exists a clear gap at 0.7 - 0.8${\mu}m$ in the CNB spectrum over the background due to galaxies according to recent results (Matsuoka et al. 2011; Mattila et al. 2011). The origin of the excess emission could be ascribed to the Pop.III stars with its active era of z = 7 - 10.

• Journal of Radiation Protection and Research
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• 제15권2호
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• pp.87-100
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• 1990

자연환경 방사선의 주 요소의 하나인 지각 방사선에 의한 피폭선량을 정량적으로 측정 해석하기 위하여 24개월간에 걸친 TLD에 의한 연속 적산선량 측정과 주기적인 감마선 분광분석을 수행하였다. TLD로는 LiF PTFE disk형을 사용하였으며 감마선 분광분석에서는 $3&{\phi}{\times}3'$원주형 Nal(Tl) 섬광검출기와 휴대형 다중파고 분석기 (4096 ch)를 사용하였다. 측정한 선량은 모두 실효 선량당량으로 평가하였으며 부수적으로 우주선 전리 성분에 의한 선량당량도 평가할 수 있었다. 분산 가중 평균값을 취한 결과에 의하면 대전 지역의 측정점에서 지각 방사선과 우주선 전리 성부의 체외 피폭에 의한 년간 실효 선량당량은 각각 $564{\pm}4\;{\mu}Sv(64.8{\pm}0.5nSv{\cdot}h^{-1}$ 및 $300{\pm}2\;{\mu}Sv(34.3{\pm}0.2nSv{\cdot}h^{-1}$로 나타났다.