arXiv subjects

Heon-Young Chang

Publications and source records attributed to Heon-Young Chang.

29 records · Page 2Linked to original sources

arXiv2002

Variation of Spot-induced Anomalies in Caustic-crossing Binary Microlensing Event Light Curves

We investigate the pattern of anomalies in the light curves of caustic-crossing binary microlensing events induced by spot(s) on the lensed source star. For this purpose, we perform simulations of events with various models of spots. From these simulations, we find that the spot-induced anomalies take various forms depending on the physical state of spots, which is characterized by the surface brightness contrast, the size, the number, the umbra/penumbra structure, the shape, and the orientation with respect to the sweeping caustic. We also examine the feasibility of distinguishing the two possibly degenerate types of anomalies caused by a spot and a transiting planet and find that the degeneracy in many cases can be broken from the characteristic multiple deviation feature in the spot-induced anomaly pattern caused by the multiplicity of spots.

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arXiv2001

Afterglow Light Curve Modulated by a Highly Magnetized Millisecond Pulsar

We investigate consequences of a continuously energy-injecting central engine of gamma-ray burst (GRB) afterglow emission, assuming that a highly magnetized pulsar is left beaming in the core of a GRB progenitor. Beaming and continuous energy-injection are natural consequences of the pulsar origin of GRB afterglows. Whereas previous studies have considered continuous energy-injection from a new-born pulsar to interpret the deviation of afterglow light curves of GRBs from those with the simple power law behavior, a beaming effect, which is one of the most important aspects of pulsar emissions, is ignored in earlier investigations. We explicitly include the beaming effect and consider a change of the beaming with time due to a dynamical evolution of a new-born pulsar. We show that the magnitude of the afterglow from this fireball indeed first decreases with time, subsequently rises, and declines again. One of the most peculiar optical afterglows light curve of GRB 970508 can be accounted for by continuous energy injection with beaming due to a highly magnetized new-born pulsar. We discuss implications on such observational evidence for a pulsar.

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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.

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arXiv2001

Properties of Microlensing Light Curve Anomalies Induced by Multiple Planets

In this paper, we show that the pattern of microlensing light curve anomalies induced by multiple planets are well described by the superposition of those of the single-planet systems where the individual planet-primary binary pairs act as independent lens systems. Since the outer deviation regions around the planetary caustics of the individual planets occur in general at different locations, we find that the pattern of anomalies in these regions are hardly affected by the existence of other planet(s). This implies that even if an event is caused by a multiple planetary system, a simple single-planet lensing model is good enough for the description of most anomalies caused by the source passage of the outer deviation regions. Detection of the anomalies resulting from the source trajectory passing both the outer deviation regions caused by more than two planets will provide a new channel of detecting multiple planets.

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arXiv2001

Fourier Analysis of Gamma-Ray Burst Light Curves: Searching for Direct Signature of Cosmological Time Dilation

We study the power density spectrum (PDS) of light curves of the observed gamma-ray bursts (GRBs) to search for a direct signature for cosmological time dilation in the PDS statistics with the GRBs whose redshifts $z$'s are known. The anticorrelation of a timescale measure and a brightness measure is indirect evidence of its effect. On the other hand, we directly demonstrate that a time dilation effect can be seen in GRB light curves. We find that timescales tend to be shorter in bursts with small redshift, as expected from cosmological time-dilation effects, and we also find that there may be non-cosmological effects constituting to this correlation. We discuss its implication on interpretations of the PDS analysis results. We put forward another caution to this kind of analysis when we statistically exercise with GRBs whose $z$ is unknown.

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arXiv2001

Another Channel to detect Close-in Binary Companions via Gravitational Microlensing

Gaudi & Gould (1997) showed that close companions of remote binary systems can be efficiently detected by using gravitational microlensing via the deviations in the lensing light curves induced by the existence of the lens companions. In this paper, we introduce another channel to detect faint close-in binary companions by using microlensing. This method utilizes a caustic-crossing binary lens event with a source also composed of binary stars, where the companion is a faint star. Detection of the companion is possible because the flux of the companion can be highly amplified when it crosses the lens caustic. The detection is facilitated since the companion is more amplified than the primary because it, in general, has a smaller size than the primary, and thus experiences less finite source effect. The method is extension of the previous one suggested to detect close-in giant planets by Graff & Gaudi (2000) and Lewis & Ibata (2000) and further developed by Ashton & Lewis (2001). From the simulations of realistic Galactic bulge events, we find that companions of K-type main sequence or brighter can be efficiently detected from the current type microlensing followup observations by using the proposed method. We also find that compared to the method of detecting lens companions for which the efficiency drops significantly for binaries with separations $\lesssim 0.2$ of the angular Einstein ring radius, the proposed method has an important advantage of being able to detect companions with substantially smaller separations down to order of 0.01 θ_E.

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arXiv2001

Power Density Spectra of Gamma-Ray Burst Light Curves: Implications on Theory and Observation

We study the power density spectrum (PDS) of artificial light curves of observed gamma-ray bursts (GRBs). We investigate statistical properties of GRB light curves by comparing the reported characteristics in the PDSs of the observed GRBs with those that we model, and discuss implications on interpretations of the PDS analysis results. Results of PDS analysis of observed GRBs suggest that the averaged PDS of GRBs follows a power law over about two decades of frequency with the power law index, -5/3, and the distribution of individual power follows an exponential distribution. Though an attempt to identify the most sensitive physical parameter has been made on the basis of the internal shock model, we demonstrate that conclusions of this kind of approach should be derived with due care. It is indicative that the physical information extracted from the slope can be misleading. We show that the reported slope and the distribution can be reproduced by adjusting the sampling interval in the time domain for a given decaying timescale of individual pulse in a specific form of GRB light curves. In particular, given that the temporal feature is modeled by a two-sided exponential function, the power law behavior with the index of -5/3 and the exponential distribution of the observed PDS is recovered at the 64 ms trigger time scale when the decaying timescale of individual pulse is $\sim 1$ second, provided that the pulse sharply rises. Another way of using the PDS analysis is an application of the same method to individual long bursts in order to examine a possible evolution of the decaying timescale in a single burst.

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arXiv2001

Effects of Luminosity Functions Induced by Relativistic Beaming on Statistics of Cosmological Gamma-Ray Bursts

We study the effects of the beaming-induced luminosity function on statistics of observed GRBs, assuming the cosmological scenario. We select and divide the BATSE 4B data into 588 long bursts (T$_{90}>2.5$ sec) and 149 short bursts (T$_{90}<2.5$ sec), and compare the statistics calculated in each subgroup. The $ $ of the long bursts is $ 0.2901\pm 0.0113$, and that of the short bursts is $0.4178\pm 0.0239$, which is a Euclidean value. For luminosity function models, we consider a cylindrical-beam and a conic-beam. We take into account the spatial distribution of GRB sources as well. A broad luminosity function is naturally produced when one introduces beaming of GRBs. We calculate the maximum detectable redshift of GRBs, $z_{\rm max}$. The estimated $z_{\rm max}$ for the cylindrical-beam case is as high as $\sim 14$ for the long bursts and $\sim 3$ for the short bursts. The large $z_{\rm max}$ value for the short bursts is rather surprising in that the $ $ for this subgroup is close to the so-called Euclidean value, 0.5. We calculate the fraction of bursts whose redshifts are larger than a certain redshift $z'$, i.e. $f_{\rm > z'}$. When we take $z'=3.42$ and apply the luminosity function derived for the cylindrical-beam, the expected $f_{\rm > z'}$ is $\sim 75 %$ for long bursts. When we increase the opening angle of the conic beam to $Δθ=3^\circ.0$, $f_{\rm > z'}$ decreases to $\sim 20 %$ at $ {\rm z'=3.42}$. We conclude that the beaming-induced luminosity functions are compatible with the redshift distribution of observed GRBs and that the apparent Euclidean value of $ $ may not be due to the Euclidean space distribution but to the luminosity distribution.

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arXiv2000

Cooling a Hot Disk Around a Supermassive Black Hole by a Star

If a supermassive black hole resides in the centers of galaxies, there are several effects expected to be observed. It is likely that accretion disks are around the supermassive black hole. Stellar interactions with the accretion flows around the supermassive black hole play a role in that an flying-by star may cool a hot accretion disk as a result of Comptonization. It is shown that the Comptonization of the stellar emission will take place in a hot accretion disk such as the ADAFs around the supermassive black hole and become a relatively important source of the accretion disk cooling when the mass accretion rate is low, and stars are passing outer parts of the hot accretion disk. We suggest that such a stellar cooling can be observed in the radio frequency regime since synchrotron luminosity of the ADAFs depends strongly on the electron temperature and occurs much more frequently than a tidal disruption event.

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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.

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arXiv2000

Relativistic Conic Beams and Spatial Distribution of Gamma-Ray Bursts

We study the statistics of gamma-ray bursts, assuming that gamma-ray bursts are cosmological and they are beamed in the form of a conical jet with a large bulk Lorentz factor $\sim 100$. In such a conic beam, the relativistic ejecta may have a spatial variation in the bulk Lorentz factor and the density distribution of gamma-ray emitting jet material. An apparent luminosity function arises because the axis of the cone is randomly oriented with respect to the observer's line of sight. The width and the shape of the luminosity function are determined by the ratio of the beam opening angle of the conical jet to the inverse of the bulk Lorentz factor, when the bulk Lorentz factor and the jet material density is uniform on the photon emitting jet surface. We calculate effects of spatial variation of the Lorentz factor and the spatial density fluctuations within the cone on the luminosity function and the statistics of gamma-ray bursts. In particular, we focus on the redshift distribution of the observed gamma-ray bursts. The maximum distance to and the average redshift of the gamma-ray bursts are strongly affected by the beaming-induced luminosity function. The bursts with the angle-dependent Lorentz factor which peaks at the center of the cone have substantially higher average gamma-ray burst redshifts. When both the jet material density and the Lorentz factor are inhomogeneous in the conical beam, the average redshift of the bursts could be 5 times higher than that of the case in which relativistic jet is completely homogeneous and structureless. Even the simplest models for the gamma-ray burst jets and their apparent luminosity distributions have a significant effect on the redshift distribution of the gamma-ray bursts.

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