모달창 닫기
- 홈으로 이동
1968년 ~ 2023년까지 1,187 건의 한국천문학회지를 격월간 확인하실 수 있습니다.
- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,187건 (DB Construction : 1,187 Articles)
총 게시글 1,187건
페이지 21/119
201
- CHO, SE-HYUNG
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.267-275
- 2015
- 원문 바로보기
Simultaneous time monitoring observations of H<sub>2</sub>O and SiO maser lines were performed toward the D-type symbiotic binary system V407 Cyg with the Korean VLBI Network single dish radio telescope. These monitoring observations were carried out from March 2, 2010 (optical phase ϕ = 0.0), 8 days before the nova outburst on March 10, 2010 to June 5, 2014 (ϕ = 2.13). Eight days before the nova outburst, we detected the SiO v = 1, 2, J = 1–0 maser lines which exhibited values of 0.51 K (∼ 6.70 Jy) and 0.71 K (∼ 9.30 Jy), respectively, while after the outburst we could not detect them on April 2 (ϕ = 0.04), May 5 (ϕ = 0.09), May 8 (ϕ = 0.09), or on June 5, 2010 (ϕ = 0.13) within the upper limits of our KVN observations. After restarting our monitoring observations, we detected SiO v = 2, J = 1–0 masers starting on October 20, 2011 (ϕ = 0.83) and detected SiO v = 1, J = 1–0 masers starting on December 22, 2011 (ϕ = 0.92). These results provide clear evidence of the interaction between the shock from the nova outburst and the SiO maser regions of the Mira envelope. The peak emission of SiO v = 1, 2, J = 1–0 masers always occurred at blueshifted velocities with respect to the stellar velocity except for that of SiO v = 1 at one epoch. These phenomena may be related to the redistribution of SiO maser regions after the outburst. The peak velocity variations of SiO masers associated with stellar pulsation phases show an increasing blueshifted trend during our monitoring interval after the outburst.
202
- CHANG, HEON-YOUNG
- Journal of the Korean astronomical society = 천문학회지
- 48, n.6
- pp.325-331
- 2015
- 원문 바로보기
We study an association between the duration of solar activity and characteristics of the latitude distribution of sunspots by means of center-of-latitude (COL) of sunspots observed during the period from 1878 to 2008 spanning solar cycles 12 to 23. We first calculate COL by taking the area-weighted mean latitude of sunspots for each calendar month to determine the latitudinal distribution of COL of sunspots appearing in the long and short cycles separately. The data set for the long solar cycles consists of the solar cycles 12, 13, 14, 20, and 23. The short solar cycles include the solar cycles 15, 16, 17, 18, 19, 21, and 22. We then fit a double Gaussian function to compare properties of the latitudinal distribution resulting from the two data sets. Our main findings are as follows: (1) The main component of the double Gaussian function does not show any significant change in the central position and in the full-width-at-half-maximum (FWHM), except in the amplitude. They are all centered at ~ 11° with FWHM of ~ 5°. (2) The secondary component of the double Gaussian function at higher latitudes seems to differ in that even though their width remains fixed at ~ 4°, their central position peaks at ~ 22.1° for the short cycles and at ~ 20.7° for the long cycles with quite small errors. (3) No significant correlation could be established between the duration of an individual cycle and the parameters of the double Gaussian. Finally, we conclude by briefly discussing the implications of these findings on the issue of the cycle 4 concerning a lost cycle.
203
- ALGABA, JUAN-CARLOS
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.237-255
- 2015
- 원문 바로보기
The Interferometric Monitoring of Gamma–ray Bright Active galactic nuclei (iMOGABA) program provides not only simultaneous multifrequency observations of bright gamma–ray detected active galactic nuclei (AGN), but also covers the highest Very Large Baseline Interferometry (VLBI) frequencies ever being systematically monitored, up to 129 GHz. However, observation and imaging of weak sources at the highest observed frequencies is very challenging. In the second paper in this series, we evaluate the viability of the frequency phase transfer technique to iMOGABA in order to obtain larger coherence time at the higher frequencies of this program (86 and 129 GHz) and image additional sources that were not detected using standard techniques. We find that this method is applicable to the iMOGABA program even under non–optimal weather conditions.
204
- Kang, Hyesung
- Journal of the Korean astronomical society = 천문학회지
- 48, n.2
- pp.155-164
- 2015
- 원문 바로보기
In <xref>Kang (2015)</xref> we calculated the acceleration of cosmic-ray electrons at weak spherical shocks that are expected to form in the cluster outskirts, and estimated the diffuse synchrotron radiation emitted by those electrons. There we demonstrated that, at decelerating spherical shocks, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws predicted for constant planar shocks, because the shock compression ratio and the flux of inject electrons decrease in time. In this study, we consider spherical blast waves propagating through a constant density core surrounded by an isothermal halo with ρ ∝ r<sup>−n</sup> in order to explore how the deceleration of the shock affects the radio emission from accelerated electrons. The surface brightness profile and the volumeintegrated radio spectrum of the model shocks are calculated by assuming a ribbon-like shock surface on a spherical shell and the associated downstream region of relativistic electrons. If the postshock magnetic field strength is about 0.7 or 7 µG, at the shock age of ∼ 50 Myr, the volume-integrated radio spectrum steepens gradually with the spectral index from α<sub>inj</sub> to α<sub>inj</sub> + 0.5 over 0.1–10 GHz, where α<sub>inj</sub> is the injection index at the shock position expected from the diffusive shock acceleration theory. Such gradual steepening could explain the curved radio spectrum of the radio relic in cluster A2266, which was interpreted as a broken power-law by <xref>Trasatti et al. (2015)</xref>, if the relic shock is young enough so that the break frequency is around 1 GHz.
205
- Amin, S.M.
- Journal of the Korean astronomical society = 천문학회지
- 48, n.1
- pp.1-7
- 2015
- 원문 바로보기
The orbital period changes of the W UMa eclipsing binary AU Ser are studied using the (O-C) method. We conclude that the period variation is due to mass transfer from the primary star to the secondary one at a very low and decreasing rate <TEX>$dP/dt=-8.872{\times}10^{-8}$</TEX>, superimposed on the sinusoidal variation due to a third body orbiting the binary with period <TEX>$42.87{\pm}3.16$</TEX> years, orbital eccentricity <TEX>$e=0.52{\pm}0.12$</TEX> and a longitude of periastron passage <TEX>${\omega}=133^{\circ}.7{\pm}15$</TEX>. On studying the magnetic activity, we have concluded that the Applegate mechanism failed to describe the cycling variation of the (O-C) diagram of AU Ser.
206
- Kang, Hyesung
- Journal of the Korean astronomical society = 천문학회지
- 48, n.1
- pp.9-20
- 2015
- 원문 바로보기
We study the evolution of the energy spectrum of cosmic-ray electrons accelerated at spherically expanding shocks with low Mach numbers and the ensuing spectral signatures imprinted in radio synchrotron emission. Time-dependent simulations of diffusive shock acceleration (DSA) of electrons in the test-particle limit have been performed for spherical shocks with parameters relevant for typical shocks in the intracluster medium. The electron and radiation spectra at the shock location can be described properly by the test-particle DSA predictions with instantaneous shock parameters. However, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws, because the shock compression ratio and the flux of injected electrons at the shock gradually decrease as the shock slows down in time. So one needs to be cautious about interpreting observed radio spectra of evolving shocks based on simple DSA models in the test-particle regime.
207
- Cho, Se-Hyung
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.293-302
- 2014
- 원문 바로보기
Simultaneous time monitoring observations of <TEX>$H_2O$</TEX> <TEX>$6_{16}-5_{23}$</TEX>, SiO J = 1-0, 2-1, 3-2, and <TEX>$^{29}SiO$</TEX> <TEX>${\upsilon}=0$</TEX>, J = 1-0 lines are carried out in the direction of the Mira variable star TX Cam with the Korean VLBI Network single dish radio telescopes. For the first time, the <TEX>$H_2O$</TEX> maser emission from TX Cam is detected near the stellar velocity at five epochs from April 10, 2013 (<TEX>${\phi}=3.13$</TEX>) to June 4, 2014 (<TEX>${\phi}=3.89$</TEX>) including minimum optical phases. The intensities of <TEX>$H_2O$</TEX> masers are very weak compared to SiO masers. The variation of peak antenna temperature ratios among SiO <TEX>${\upsilon}=1$</TEX>, J = 1-0, J = 2-1, and J = 3-2 masers is investigated according to their phases. The shift of peak velocities of <TEX>$H_2O$</TEX> and SiO masers with respect to the stellar velocity is also investigated according to observed optical phases. The <TEX>$H_2O$</TEX> maser emission occurs around the stellar velocity during our monitoring interval. On the other hand, the peak velocities of SiO masers show a spread compared to the stellar velocity. The peak velocities of SiO J = 2-1, and J = 3-2 masers show a smaller spread with respect to the stellar velocity than those of SiO J = 1-0 masers. These simultaneous observations of multi-frequencies will provide a good constraint for maser pumping models and a good probe for investigating the stellar atmosphere and envelope according to their different excitation conditions.
208
- Lee, Sang-Sung
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.303-309
- 2014
- 원문 바로보기
We present results of our investigation of the radio intrinsic brightness temperatures of compact radio jets. The intrinsic brightness temperatures of about 100 compact radio jets at 2, 5, 8, 15, and 86 GHz are estimated based on large VLBI surveys conducted in 2001-2003 (or in 1996 for the 5 GHz sample). The multi-frequency intrinsic brightness temperatures of the sample of jets are determined by a statistical method relating the observed brightness temperatures with the maximal apparent jet speeds, assuming one representative intrinsic brightness temperature for a sample of jets at each observing frequency. By investigating the observed brightness temperatures at 15 GHz in multiple epochs, we find that the determination of the intrinsic brightness temperature for our sample is affected by the flux density variability of individual jets at time scales of a few years. This implies that it is important to use contemporaneous VLBI observations for the multi-frequency analysis of intrinsic brightness temperatures. Since our analysis is based on the VLBI observations conducted in 2001-2003, the results are not strongly affected by the flux density variability. We find that the intrinsic brightness temperature <TEX>$T_0$</TEX> increases as <TEX>$T_0{\propto}{\nu}^{\xi}_{obs}$</TEX> with <TEX>${\xi}=0.7$</TEX> below a critical frequency <TEX>${\nu}_c{\approx}9GHz$</TEX> where the energy loss begins to dominate the emission. Above <TEX>${\nu}_c$</TEX>, <TEX>$T_0$</TEX> decreases with <TEX>${\xi}=-1.2$</TEX>, supporting the decelerating jet model or particle cascade model. We also find that the peak value of <TEX>$T_0{\approx}3.4{\times}10^{10}$</TEX> K is close to the equipartition temperature, implying that the VLBI cores observable at 2-86 GHz may be representing jet regions where the magnetic field energy dominates the total energy in jets.
209
- Na, Sung-Ho
- Journal of the Korean astronomical society = 천문학회지
- 47, n.1
- pp.41-41
- 2014
- 원문 바로보기
The equivalent phase lag <TEX>${\varepsilon}_{lmpq}$</TEX> of Earth tide suggested by Lambeck have been found reasonable. Due to a mistake in his calculation, the author falsely claimed that those are too large. However, the main contents of the article are valid and not affected by this mistake.
210
- Hanna, Magdy
- Journal of the Korean astronomical society = 천문학회지
- 47, n.3
- pp.99-104
- 2014
- 원문 바로보기
We present the rst period variation study for the Algol eclipsing binary V346 Cyg by constructing the (O-C) residual diagram using all the available precise minima times. We conclude that the period variation can be explained by a sine-like variation due to the presence of a third body orbiting the binary in about <TEX>$68.89{\pm}4.69$</TEX> years, together with a long-term orbital period decrease (<TEX>$dP/dt=-1.23{\times}10^{-7}day/yr$</TEX>) that can be interpreted to be due to slow mass loss from the <TEX>${\delta}$</TEX>-Scuti primary component. The sinusoidal variation may also be explained by using the the Applegate (1992) mechanism involving cyclic magnetic activity due to star-spots on the secondary component. The present preliminary solution needs more precise photometric observations to be confirmed.