arXiv subjects

Insu Yi

Publications and source records attributed to Insu Yi.

At least 19 recordsLinked to original sources

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.

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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.

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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

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

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

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.

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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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arXiv2000

Cosmological Evolution Models for QSO/AGN Luminosity Functions: Effects of Spectrum-Luminosity Correlation and Massive Black Hole Remnants

We investigate a large number of cosmological evolution models for QSOs and Active Galactic Nuclei (AGN). We introduce a spectrum-luminosity correlation as a new input parameter and adopt the estimated mass function (MF) of massive black holes in centers of nearby galactic nuclei as a constraint to distinguish among different QSO/AGN models. We explore three basic types of phenomenological scenarios; (i) Models with multiple short-lived ($\sim$ a few $10^{6-8}$ yrs) populations, (ii) Models with a single long-lived ($\sim 10^9$ yr) QSO population, and (iii) Models with recurrent QSO/AGN activities which are driven by long-term variabilities of the disk instability type. In each model, we derive the expected theoretical luminosity function (LF) and the MF of black holes which grow through mass accretion. We assess the plausibility of each model based on whether each model's LF and MF are compatible with the observed data. We find that the best fits to the observed LFs are obtained in the model with multiple short-lived populations and without any significant spectral evolution. This finding suggests that the QSO populations may be composed of many short-lived generations ($\sim$ a few 10$^8$ yrs) and that there is no significant spectral evolution within each generation. On the other hand, we also show that there is no satisfactory model which can simultaneously account for the observed LF and the estimated MF. We speculate that some of the present-day black holes (BHs) found in galactic nuclei may have formed without undergoing the QSO/AGN phase.

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arXiv2000

On Companion-Induced Off-Center Supernova-Like Explosions

We suggest that a neutron star with a strong magnetic field, spiraling into the envelope of a companion star, can generate a ``companion induced SN-like off-center explosion". The strongly magnetized neutron star ("magnetar") is born in a supernova explosion before entering into an expanding envelope of a supergiant companion. If the neutron star collapses into a black hole via the hypercritical accretion during the spiral-in phase, a rapidly rotating black hole with a strong magnetic field at the horizon results. The Blandford-Znajek power is sufficient to power a supernova-like event with the center of explosion displaced from the companion core. The companion core, after explosion, evolves into a C/O-white dwarf or a neutron star with a second explosion. The detection of highly eccentric black-hole, C/O-white dwarf binaries or the double explosion structures in the supernova remnants could be an evidence of the proposed scenario.

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arXiv2000

Asymmetric Supernovae, Pulsars, Magnetars, and Gamma-Ray Bursts

We outline the possible physical processes, associated timescales, and energetics that could lead to the production of pulsars, jets, asymmetric supernovae, and weak gamma-ray bursts in routine circumstances and to a magnetar and perhaps stronger gamma-ray burst in more extreme circumstances in the collapse of the bare core of a massive star. The production of a LeBlanc-Wilson MHD jet could provide an asymmetric supernova and result in a weak gamma-ray burst when the jet accelerates down the stellar density gradient of a hydrogen-poor photosphere. The matter-dominated jet would be formed promptly, but requires 5 to 10 s to reach the surface of the progenitor of a Type Ib/c supernova. During this time, the newly-born neutron star could contract, spin up, and wind up field lines or turn on an alpha-Omega dynamo. In addition, the light cylinder will contract from a radius large compared to the Alfven radius to a size comparable to that of the neutron star. This will disrupt the structure of any organized dipole field and promote the generation of ultrarelativistic MHD waves (UMHDW) at high density and Large Amplitude Electromagnetic Waves (LAEMW) at low density. The generation of the these waves would be delayed by the cooling time of the neutron star about 5 to 10 seconds, but the propagation time is short so the UMHDW could arrive at the surface at about the same time as the matter jet. In the density gradient of the star and the matter jet, the intense flux of UMHDW and LAEMW could drive shocks, generate pions by proton-proton collision, or create electron/positron pairs depending on the circumstances. The UMHDW and LAEMW could influence the dynamics of the explosion and might also tend to flow out the rotation axis to produce a collimated gamma-ray burst.

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arXiv1999

The Type Ib/c Supernova, Gamma-Ray Burst, Soft Gamma-Ray Repeater, Magnetar Connection

The polarization of core-collapse supernovae shows that many if not all of these explosions must be strongly bi-polar. The most obvious way to produce this axial symmetry is by the imposition of a jet as an intrinsic part of the explosion process. These jets could arise by MHD processes in the formation of pulsars and be especially strong in the case of magnetars. The jets will blow iron-peak material out along the axes and other elements from the progenitor along the equator, a very different composition structure than pictured in simple spherical "onion skin" models. In extreme cases, these processes could lead to the production of gamma-ray bursts powered by strong Poynting flux.

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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.

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arXiv1999

Advection-Dominated Accetion Flows

We review basic properties of advection-dominated accretion flows (ADAFs) and their applications to astrophysical systems ranging from Galactic binary systems to galactic nuclei. A new classification scheme for low-luminosity, X-ray bright galactic nuclei is highlighted. Some outstanding unresolved issues are discussed.

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arXiv1999

On Radio and X-ray Emission Mechanisms in Nearby, X-ray Bright Galactic Nuclei

It has been suggested that advection-dominated accretion flows (ADAFs) are responsible for the X-ray activity in nearby galactic nuclei. These X-ray bright galactic nuclei (XBGN) are a heterogeneous group which includes LINERs, low to moderate luminosity Seyferts, and narrow-line X-ray galaxies with 2-10 keV X-ray luminosities in the range ~10**39 to ~10**43 erg/s. In the absence of a radio jet, the core 15 GHz radio luminosity of an ADAF is relatively low and roughly proportional to the mass of the central black hole. The predicted radio luminosity depends primarily on the black hole mass and for XBGN typically falls in the range 10**35-10**39 erg/s. We designate these as ``radio quiet'' XBGN. However, some level of jet activity seems to be present in most sources and the radio emission can be considerably larger than that from the ADAF core. We discuss connections between radio-bright XBGN and radio-loud, powerful active galactic nuclei (AGN) and suggest that the radio activities are directly correlated with black hole spins in both cases. Even in the presence of a radio jet, high resolution, high frequency radio imaging of nearby XBGN could identify compact, inverted spectrum ADAF radio sources. The unique radio/X-ray luminosity relation is confirmed in a few cases where black hole masses are known and could be used as a tool to estimate unknown black hole masses. For radio-dim (L_<10**39 erg/s), X-ray bright (L_x>10**43 erg/s) sources, which are primarily Seyferts, the X-ray emission mechanism is not accounted for by pure ADAFs and radio activities are likely to be similar to those of the radio-quiet AGN.

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arXiv1999

Signatures of Energetic Protons in Hot Accretion Flows: Synchrotron Cooling of Protons in Strongly Magnetized Pulsars

The existence of hot, two-temperature accretion flows is essential to the recent discussions of the low luminosity, hard X-ray emission from accreting neutron stars and black holes in Galactic binaries and massive black holes in low luminosity galactic nuclei. In these flows, protons are essentially virialized and relativistic energies for non-thermal protons are likely. Observational confirmation of the energetic protons' presence could further support the two-temperature accretion flow models. We point out that synchrotron emission from nonthermal relativistic protons could provide an observational signature in strongly magnetized neutron star systems. The self-absorbed synchrotron emission from an accreting neutron star with the magnetic moment $\sim 10^{30} G cm^3$ is expected to exhibit a spectrum νI_ν~ ν**2 with the luminosity ~ a few x 10^**33 (L_x/10**36 erg/s)**0.4 erg/s at ν~10**15 Hz where L_x is the X-ray luminosity from the neutron star surface. The detection of the expected synchrotron signature in optical and UV bands during the low luminosity state of the pulsar systems such as 4U 1626-67 and GX 1+4 could prove the existence of the hot, two-temperature accretion flows during their spin-down episodes. The detected optical emission in 4U 1626-67 has a spectral shape and luminosity level very close to our predictions.

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arXiv1999

Spectral Transition and Torque Reversal in X-ray Pulsar 4U 1626-67

The accretion-powered, X-ray pulsar 4U 1626-67 has recently shown an abrupt torque reversal accompanied by a dramatic spectral transition and a relatively small luminosity change. The time-averaged X-ray spectrum during spin-down is considerably harder than during spin-up. The observed torque reversal can be explained by an accretion flow transition triggered by a gradual change in the mass accretion rate. The sudden transition to spin-down is caused by a change in the accretion flow rotation from Keplerian to sub-Keplerian. 4U 1626-67 is estimated to be near spin equilibrium with a mass accretion rate Mdot~2x10**16 g/s, Mdot decreasing at a rate ~6x10**14 g/s/yr, and a polar surface magnetic field of ~2b_p**{-1/2} 10^**12G where b_p is the magnetic pitch. During spin-up, the Keplerian flow remains geometrically thin and cool. During spin-down, the sub-Keplerian flow becomes geometrically thick and hot. Soft photons from near the stellar surface are Compton up-scattered by the hot accretion flow during spin-down while during spin-up such scattering is unlikely due to the small scale-height and low temperature of the flow. This mechanism accounts for the observed spectral hardening and small luminosity change. The scattering occurs in a hot radially falling column of material with a scattering depth ~0.3 and a temperature ~10^9K. The X-ray luminosity at energies >5keV could be a poor indicator of the mass accretion rate. We briefly discuss the possible application of this mechanism to GX 1+4, although there are indications that this system is significantly different from other torque-reversal systems.

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arXiv1999

Cosmological Luminosity Evolution of QSO/AGN Population

We apply the observed optical/X-ray spectral states of the Galactic black hole candidates (GBHCs) to the cosmological QSO luminosity evolution under the assumptions that QSOs and GBHCs are powered by similar accretion processes and that their emission mechanisms are also similar. The QSO luminosity function (LF) evolution in various energy bands is strongly affected by the spectral evolution which is tightly correlated with the luminosity evolution. We generate a random sample of QSOs born nearly synchronously by allowing the QSOs to have redshifts in a narrow range around an initial high redshift, black hole masses according to a power-law, and mass accretion rates near Eddington rates. The QSOs evolve as a single long-lived population on the cosmological time scale. The pure luminosity evolution results in distinct luminosity evolution features due to the strong spectral evolution. Most notably, different energy bands (optical/UV, soft X-ray, and hard X-ray) show different evolutionary trends and the hard X-ray LF in particular shows an apparent reversal of the luminosity evolution (from decreasing to increasing luminosity) at low redshifts, which is not seen in the conventional pure luminosity evolution scenario without spectral evolution. The resulting mass function of black holes (BHs), which is qualitatively consistent with the observed QSO LF evolution, shows that QSO remnants are likely to be found as BHs with masses in the range 10**8-5x10**10 solar masses. The long-lived single population of QSOs are expected to leave their remnants as supermassive BHs residing in rare, giant elliptical galaxies.

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