arXiv subjects

Chul-Sung Choi

Publications and source records attributed to Chul-Sung Choi.

14 recordsLinked to original sources

arXiv2016

Classifying Gamma-Ray Bursts with Gaussian Mixture Model

Using Gaussian Mixture Model (GMM) and Expectation Maximization Algorithm, we perform an analysis of time duration ($T_{90}$) for \textit{CGRO}/BATSE, \textit{Swift}/BAT and \textit{Fermi}/GBM Gamma-Ray Bursts. The $T_{90}$ distributions of 298 redshift-known \textit{Swift}/BAT GRBs have also been studied in both observer and rest frames. Bayesian Information Criterion has been used to compare between different GMM models. We find that two Gaussian components are better to describe the \textit{CGRO}/BATSE and \textit{Fermi}/GBM GRBs in the observer frame. Also, we caution that two groups are expected for the \textit{Swift}/BAT bursts in the rest frame, which is consistent with some previous results. However, \textit{Swift} GRBs in the observer frame seem to show a trimodal distribution, of which the superficial intermediate class may result from the selection effect of \textit{Swift}/BAT.

astro-ph.HE↗

arXiv2015

Radio Afterglows and Host Galaxies of Gamma-Ray Bursts

Considering the contribution of the emission from the host galaxies of gamma-ray bursts (GRBs) to the radio afterglows, we investigate the effect of host galaxies on observations statistically. For the three types of events, e.g. low-luminosity, standard and high-luminosity GRBs, it is found that a tight correlation exists between the ratio of the radio flux (RRF) of host galaxy to the total radio peak emission and the observational frequency. Especially, toward lower frequencies, the contribution from the host increases significantly. The correlation can be used to get a useful estimate for the radio brightness of those host galaxies which only have very limited radio afterglow data. Using this prediction, we re-considered the theoretical radio afterglow light curves for four kinds of events, i.e. high-luminosity, low-luminosity, standard and failed GRBs, taking into account the contribution from the host galaxies and aiming at exploring the detectability of these events by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Lying at a typical redshift of $z=1$, most of the events can be detected by FAST easily. For the less fierce low-luminosity GRBs, their radio afterglows are not strong enough to exceed the sensitivity limit of FAST at such distances. However, since a large number of low luminosity bursts actually happen very near to us, it is expected that FAST will still be able to detect many of them.

astro-ph.HE↗

arXiv2008

Resolving a Class I Protostar Binary System with Chandra

Using a sub-pixel event repositioning technique, we spatially resolved X-ray emission from the infrared double system IRS 5 in the R Corona Australis molecular cloud with ~0.8" separation. As far as we know, this result - obtained from 8 Chandra archival observations between 2000 and 2005 - is the first X-ray study of individual sources in a Class I protostar binary system with a projected separation of less than 200 AU. We extracted light curves and spectra of the individual sources using a two-dimensional image fitting method. IRS 5a at the south, the source which was brighter in the near-infrared, showed three X-ray flares lasting >20 ksec, reminiscent of X-ray flares from pre-main sequence stars, while the northern source (IRS 5b) was quiescent in X-rays in all the observations except for a 2005 August 9 observation with a factor of ~2 flux enhancement. In quiescence, these sources showed almost identical X-ray spectra, with NH ~4e22 cm-2, kT ~2 keV, and log Lx ~30.2-3 ergs s-1. IRS 5a showed plasma at temperatures up to kT ~5-6 keV during flares, while the column density of IRS 5b increased by a factor of 2 during an observation on 2005 August 9. We discuss the evolutionary stages and variation of the X-ray activity of these sources.

astro-ph↗

arXiv2008

An analysis of the durations of Swift Gamma-Ray Bursts

We report the systematic analysis of the durations for Swift gamma-ray bursts (GRBs) and compare the results with those of pre-Swift data. For 95 GRBs with known redshift, we show that the observed durations have two lognormal distributions that are clearly divided at $T_{90}\simeq2$ s. This is consistent with the earlier BATSE results. The intrinsic durations also show a bimodal distribution but shift systematically toward the smaller value and the distribution exhibits a narrower width compared with the observed one. We find that the intrinsic distributions of long GRBs between Swift and pre-Swift are significantly different particularly in the width and the median value. In addition, the Swift data exhibit a wider dynamic range of duration. Our present study not only confirms the spectra of short GRBs are in general harder than the long GRBs in the observer frame but also shows this trend becomes weaker in the source frame.

astro-ph↗

arXiv2007

The X-ray Pulse Profile of BG CMi

We present an analysis of the X-ray data of a magnetic cataclysmic variable, BG CMi, obtained with ROSAT in March 1992 and with ASCA in April 1996. We show that four peaks clearly exist in the X-ray pulse profile, unlike a single peak found in the optical profile. The fluxes of two major pulses are $\sim 2 - 5$ times larger than those of two minor pulses. The fraction of the total pulsed flux increases from 51% to 85% with increasing energy in 0.1 $-$ 2.0 keV, whereas it decreases from 96% to 22% in 0.8 $-$ 10 keV. We discuss the implications of our findings for the origin of the pulse profile and its energy dependence.

astro-ph↗

arXiv2006

A Flare of AE Aquarii Observed with XMM-Newton

We present the results of analyzing the XMM-Newton data obtained in 2001 November 7 - 8. A flare is observed simultaneously in X-ray and UV together with a quiescence. We find that during the flare event X-ray flux varies with UV with no significant time lag, indicating a close correlation of flux variation for X-ray and UV flares. An upper limit of the lag is estimated to be \~1 min. From a timing analysis for X-ray data, we find that both pulsed and unpulsed flux increase clearly as the flare advances in the entire energy band 0.15 - 10 keV. The net increase of pulsed flux to the quiescence is, however, small and corresponds to about 3 - 4% of the increase in unpulsed flux, confirming that a flux variation of flare in AE Aqr is dominated by unpulsed X-rays. A spectral analysis reveals that the energy spectrum is similar to that of the quiescence at the beginning of the flare, but the spectrum becomes harder as the flare advances. Based on these results, we discuss the current issues that need to be clarified, e.g., the possible flaring site and the mass accretion problem of the white dwarf. We also discuss the flare properties obtained in this study.

astro-ph↗

arXiv2003

Radio Variability of the Sagittarius $ {\rm A^*}$ due to an Orbiting Star

Recently, unprecedentedly accurate data on the orbital motion of stars in the vicinity of the Sagittarius $ {\rm A^*}$ have been available. Such information can be used not only to constrain the mass of the supermassive black hole (SMBH) in the Galactic center but also to study the source of the radio emission. Two major competing explanations of the radio spectrum of the Sagittarius $ {\rm A^*}$ are based on two different models, that is, hot accretion disk and jet. Hence, independent observational constraints are required to resolve related issues. We explore the possibility of using the observational data of the star S2, currently closest to the Galactic center, to distinguish physical models for the radio emission of the Sagittarius $ {\rm A^*}$, by applying the stellar cooling model to the Sagittarius $ {\rm A^*}$ with the orbital parameters derived from the observation. The relative difference in the electron temperature due to the stellar cooling from the star S2 at the pericenter is a few parts of a thousand and the consequent relative radio luminosity difference is order of $10^{-4}$. Therefore, one could possibly expect to observe the radio flux variation with a periodic or quasi-periodic modulation in the frequency range at $ν\la {\rm 100 MHz}$ if the radiatively inefficient hot accretion flows are indeed responsible for the radio emission, contrary to the case of a jet. According to our findings, even though no periodic radio flux variations have been reported up to date a radiatively inefficient hot accretion disk model cannot be conclusively ruled out.

astro-ph↗

arXiv2002

Radio/X-Ray Luminosity Relation for X-Ray Bright Galactic Nuclei: Implications on Weighing Supermassive Black Hole

Optically thin and geometrically thick accretion flows are known to be responsible for the observed radio/X-ray luminosity relation of the X-ray bright galactic nuclei. It has also been suggested that supermassive black hole masses can be estimated from measurements of the core radio luminosity and the X-ray luminosity by using the advection-dominated accretion flow (ADAF) model. In this study we increase the number of data available by compiling the radio/X-ray fluxes and the mass in published literatures, and compare the observed ratio of the luminosities with predictions from various models of optically thin accretion flows. Semi-analytically derived relations of the luminosities are presented in cases of the standard ADAF model and modified ADAF models, in which a truncation of inner parts of the flows and winds causing a reduction of the infalling matter are included. We show that the observed relation can be used indeed to estimate the supermassive black hole mass, provided that properties of such accretion flows are known. Having investigated sensitivities of the method on modifications of the 'standard' ADAF model, we find that a general trend of model predictions from the 'standard' ADAF, the truncated ADAF and the 'windy' ADAF are somewhat indistinguishable. We also find, however, that the extreme case of the windy model is inconsistent with currently available observational data, unless microphysics parameters are to be substantially changed. High resolution radio observations, however, are required to avoid the contamination of non-disk components, such as, a jet component, which, otherwise, results in the over-estimated SMBH mass.

astro-ph↗

arXiv2002

Long-Term X-Ray Variabilities of the Seyfert Galaxy MCG-2-58-22 : Secular Flux Decrease and Flares

We have studied the long-term X-ray light curve (2$-$10 keV) of the luminous Seyfert 1 galaxy MCG-2-58-22 by compiling data, from various X-ray satellites, which together cover more than 20 years. We have found two distinct types of time variations in the light curve. One is a gradual and secular decrease of the X-ray flux, and the other is the episodic increase of X-ray flux (or flare) by a factor of 2$-$4 compared with the level expected from the secular variation. We detected 3 such flares in total; a representative duration for the flares is $\sim $2 years, with intervening quiescent intervals lasting $\sim 6-8$ years. We discuss a few possible origins for these variabilities. Though a standard disk instability theory may explain the displayed time variability in the X-ray light curve, the subsequent accretions of stellar debris, from a tidal disruption event caused by a supermassive black hole in MCG-2-58-22, cannot be ruled out as an alternative explanation.

astro-ph↗

arXiv2001

A New Distance Estimator and Spatial Distribution of GRBS Observed by BATSE

We propose a redshift estimator for the long ($T_{90} > 20$) gamma-ray bursts (GRBs) observed by the BATSE. It is based on an empirical relation between the redshift and the power-law index of power density spectra (PDSs) of the observed GRBs. This relation is constructed by using the fact that the power-law index is dependent upon a characteristic timescale of GRB light curves which are inevitably stretched by cosmological time dilation. We construct the empirical relation using both individual PDSs and averaged PDSs. An error estimates of $z$ are 1.09 and 1.11 for the empirical relation by individual PDS fits, 1.72 and 1.56 by averaged PDS fits, for the least squares fit and the maximum likelihood fit, respectively. We attempt to determine the spatial distribution of the GRBs observed by the BATSE as a function of redshifts on the basis of the resulting redshift estimator. We find that the obtained spatial distribution of the observed GRBs seems consistent with that of the GRBs whose redshifts are reported, even though the estimated errors are not very accurate. The GRBs observed by the BATSE seem distributed within $z \sim 5-6$. This result has implications on theoretical calculations of stellar formations at high redshifts and beaming geometry via a statistical study of the observed GRBs involving beaming-induced luminosity functions. We discuss such implications, and possible uncertainties of the suggested method.

astro-ph↗

arXiv2000

Estimating Supermassive Black Hole Mass Through Radio/X-Ray Luminosity Relation of X-Ray Bright Galactic Nuclei

It has been suggested that optically thin and geometrically thick accretion flows are responsible for the observed radio/X-ray luminosity relation of the X-ray bright galactic nuclei. If this is the case then central supermassive black hole masses can be estimated directly from measurements of the core radio luminosity and the X-ray luminosity, provided that properties of such accretion flows are known. Calculated ratios of the luminosities are presented in cases of the standard ADAF model and modified ADAF models, in which a truncation of inner parts of the flows and winds causing a reduction of the infalling matter are included. We compare the observed ratio of the luminosities with predictions from models of optically thin accretion flows. We also discuss the possible effects of the convection in ADAFs. We confirm that the supermassive black hole (SMBH) mass estimate is possible with the radio/X-ray luminosity relation due to ADAF models in the absence of a radio jet. We find that observational data are insufficient to distinguish the standard ADAF model from its modified models. However, the ADAF model with convection is inconsistent with observations, unless microphysics parameters are to be substantially changed. High resolution radio observations are required to avoid the contamination of other components, such as, a jet component. Otherwise, the SMBH mass is inclined to be over-estimated.

astro-ph↗

arXiv2000

Observations of Spectral and Time Variabilities from MCG-2-58-22

We present results from analysis of the X-ray archive data of MCG-2-58-22, acquired with ROSAT from 1991 to 1993 and with ASCA from 1993 to 1997. By analyzing light curves, we find that MCG-2-58-22 shows a clear time variability in X-ray flux. The time scales of the variations range widely from about 1000 s to more than years. Among the variations, a flare-like event overlaid on the gradual flux decrease from 1979 to 1993 is detected. We also find clear time variability of the spectra in the energy range of 0.1 - 2.0 keV. However, the flux variation does not influences on their spectra in the energy range of 2 - 10 keV. The implications of these observational results are discussed in terms of a supermassive black hole model and accretion flow dynamics near the central black hole.

astro-ph↗

arXiv1999

A Study of the X-Ray Emission of Magnetic Cataclysmic Variable Ae Aquarii

We report results from analysis of the X-ray observations of AE Aqr, made with Ginga in June 1988 and with ASCA in October 1995. Pulsations are detected clearly with a sinusoidal pulse profile with periods of $33.076\pm0.001$ s (Ginga) and $33.077\pm0.003$ s (ASCA)\@. The pulse amplitude is relatively small and the modulated flux remains nearly constant despite a factor of 3 change in the average flux during the flare. We reproduce the time-averaged spectrum in the 0.4 -- 10 keV energy band by a thermal emission model with a combination of two different temperatures: kT$_1 = 0.68^{+0.01}_{-0.02}$ keV and kT$_2 = 2.9^{+0.3}_{-0.2}$ keV\@. There is no significant difference between the quiescent and flare energy spectra, although a hint of spectral hardening is recognized during the flare. We interpret these observational results with a model in which AE Aqr is in a propeller stage. Based on this propeller scenario, we suggest that the X-ray emission is originated from magnetospheric radiation.

astro-ph↗

arXiv1999

On the Rapid Spin-down and Low Luminosity Pulsed Emission from AE Aquarii

AE Aqr is an unusual close binary system with a very short white dwarf spin period, a high spin-down rate, a relatively low quiescent luminosity, and clear pulse signals. The exact nature of the large spin-down power has not been well explained mainly due to the fact that the observed luminosities in various energy ranges are much lower than the spin-down power. We consider an unconventional picture of AE Aqr in which an accreting white dwarf, modeled as a magnetic dipole whose axis is misaligned with the spin axis, is rapidly spun-down via gravitational radiation emission and therefore the spin-down power is not directly connected to any observable electromagnetic emission.

astro-ph↗