Chul-Sung Choi

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

We report results from analysis of the X-ray observations of AE Aqr, made with Ginga in 1988 June and with ASCA in 1995 October. Pulsations are detected clearly with a sinusoidal pulse profile with periods of 33.076 ± 0.001 s (Ginga) and 33.077 ± 0.003 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: kT1 = 0.68 keV and kT2 = 2.9 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 originates from magnetospheric radiation.

An analysis of the durations of Swift gamma-ray bursts

Context. Swift detectors are found to be more sensitive to long-soft bursts than pre-Swift missions. This may largely bias the distribution of durations and thus classification of gamma-ray bursts. Aims. We systematically investigate the duration distribution of gamma-ray bursts in the Swift era vs. that of pre-Swift bursts. Methods. For the purpose of this study, statistical methods such as the K-S test and linear/non-linear fitting analysis have been used to examine the duration properties of Swift bursts in both observer and source frames. Results. For 95 GRBs with known redshift, we show that two log-normal distributions of duration are clearly divided at T(90) similar or equal to 2 s. The intrinsic durations also show a bimodal distribution but shift systematically toward the smaller value and the distribution exhibits a narrower width than the observed one. Swift long bursts exhibit a wider duration dynamic range in both observer and source frames in comparison to pre-Swift long bursts. Conclusions. We find that Swift bursts and pre-Swift ones can share the same criterion of classification in terms of duration at 2 s, although both monitors have large differences with respect to sensitivity of a given energy band.

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 because 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. The rapid spin-down is caused by the nonaxisymmetric polar mounds of accreted material slowly spreading away from the magnetic poles over the surface of the star. The effectiveness of the spin-down driven by the gravitational radiation depends on complex diffusion and thermonuclear burning of the accretion mound material. The accretion proceeds at high altitudes toward the magnetic poles of the white dwarf, while a large fraction of the inflowing material is ejected in a propeller-like manner. Based on the observed quiescent X-ray and UV emission, the magnetic field strength is estimated as ~1 × 105η G and the mass accretion rate as ~1 × 1015η g s-1, where ηX < 1 is the X-ray radiative efficiency. A large fraction of the accreted mass is flung out by the propeller action, and ~50% of the accreted material arrives at the magnetic poles. The electromagnetic dipole emission is expected at the level of ~1 × 1029η ergs s-1, which suggests that, for ηX ~ 0.1, the observed radio luminosity could be well accounted for by dipole radiation.

ANTI-CORRELATED TIME LAGS IN THE Z SOURCE GX 5-1: POSSIBLE EVIDENCE FOR A TRUNCATED ACCRETION DISK

We investigate the nature of the inner accretion disk in the neutron star source GX 5-1 by making a detailed study of time lags between X-rays of different energies. Using the cross-correlation analysis, we found anti-correlated hard and soft time lags of the order of a few tens to a few hundred seconds and the corresponding intensity states were mostly the horizontal branch (HB) and upper normal branch (NB). The model independent and dependent spectral analysis showed that during these time lags the structure of accretion disk significantly varied. Both eastern and western approaches were used to unfold the X-ray continuum and systematic changes were observed in soft and hard spectral components. These changes along with a systematic shift in the frequency of quasi-periodic oscillations (QPOs) made it substantially evident that the geometry of the accretion disk is truncated. Simultaneous energy spectral and power density spectral study shows that the production of the horizontal branch oscillations (HBOs) are closely related to the Comptonizing region rather than the disk component in the accretion disk. We found that as the HBO frequency decreases from the hard apex to upper HB, the disk temperature increases along with an increase in the coronal temperature which is in sharp contrast with the changes found in black hole binaries where the decrease in QPO frequency is accompanied by a decrease in the disk temperature and a simultaneous increase in the coronal temperature. We discuss the results in the context of re-condensation of coronal material in the inner region of the disk.

ANTI-CORRELATED SOFT LAGS IN THE INTERMEDIATE STATE OF BLACK HOLE SOURCE GX 339-4

We report the few hundred second anti-correlated soft lags between soft and hard energy bands in the source GX 339–4 using RXTE observations. In one observation, anti-correlated soft lags were observed using the ISGRI/INTEGRAL hard energy band and the PCA/RXTE soft energy band light curves. The lags were observed when the source was in hard and soft intermediate states, i.e., in a steep power-law state. We found that the temporal and spectral properties were changed during the lag timescale. The anti-correlated soft lags are associated with spectral variability during which the geometry of the accretion disk is changed. The observed temporal and spectral variations are explained using the framework of truncated disk geometry. We found that during the lag timescale, the centroid frequency of quasi-periodic oscillation is decreased, the soft flux is decreased along with an increase in the hard flux, and the power-law index steepens together with a decrease in the disk normalization parameter. We argue that these changes could be explained if we assume that the hot corona condenses and forms a disk in the inner region of the accretion disk. The overall spectral and temporal changes support the truncated geometry of the accretion disk in the steep power-law state or in the intermediate state.

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 minute. 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.

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 2 years, with intervening quiescent intervals lasting 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.

A SPECTRAL STUDY OF THE RAPID TRANSITIONS OF TYPE-B QUASI-PERIODIC OSCILLATIONS IN THE BLACK HOLE TRANSIENT XTE J1859+226

The fast transitions of type-B and type-A quasi-periodic oscillations (QPOs) are rarely found, and they are observed at the peak of the outburst in black hole transient (BHT) sources. The associated spectral variations during such events are crucial to understand the origin and location of such QPOs in the accretion disk. During the 1999 outburst of XTE J1859+226, on four occasions a rapid transition of type-B/A QPOs was noted. We performed broadband spectral analysis on these four observations to unveil the responsible spectral parameter causing the rapid transitions. After invoking simple spectral models, it was observed that disk parameters were consistently varying along with disk and power-law fluxes, and almost no change was noted in the power-law index parameter. Though using a complex physical model showed consistent results, the spectral parameter variations across the transitions were not significant. It was observed that the type-B QPO was always associated with an inner disk front which is closer to the BH. In one observation, a type-A QPO appeared as the source count rate suddenly dropped, and the power-law index as well as disk normalization parameter considerably changed during this transition. The spectral changes in this particular observation were similar to the changes observed in XTE J1817-330, indicating a common underlying mechanism. We have also examined a similar observation of BHT source GX 339-4, where a sudden transition of a type-A/B QPO was noted. Similar spectral study again revealed that the disk parameters were changing. We discuss the results in the framework of a truncated disk model and conclude that the movement of the coupled inner disk-corona region is responsible for such rapid transitions of type-B QPOs.Context. The evolution of di fferent types of quasi-periodic oscillations (QPOs) and the c oupled radiative /physical changes in the accretion disk are still poorly understood. In a few black ho le binaries it was found that fast evolution of QPOs is associ ated with spectral variations. Such studies in other black hole binar ies are important to understand the QPO phenomenon. Aims. For the black hole transient XTE J1817-330, we study fast QPO transitions and accompanying spectral variations to inves tigate what causes the spectral variation during the QPO transitio . Methods. Roy et al. (2011) found QPOs in ten RXTE observations of XTE J1 817-330. We found that, among the ten observations, only one observation shows erratic dips in its X-ray light cu rve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of th e lig t curve. Results. We found that type-B∼6 Hz QPO changes into type-A QPO in a few tens of seconds along w ith a flux decrease. This transient evolution is accompanied with a significant spect ral variation. Conclusions. We report a transient QPO feature and accompanying spectral vari tion in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral var iations.

Orbital phase spectroscopy of four high mass X-ray binary pulsars to study the stellar wind of the companion

Our work focuses on a comprehensive orbital phase-dependent spectroscopy of the four High Mass X-ray Binary Pulsars (HMXBPs) 4U 1538-52, GX 301-2, OAO 1657-415 and Vela X-1. We hereby report the measurements of the variation of the absorption column density and iron-line flux along with other spectral parameters over the binary orbit for the above-mentioned HMXBPs in elliptical orbits, as observed with the Rossi X-ray Timing Explorer (RXTE) and the BeppoSAX satellites. A spherically symmetric wind profile was used as a model to compare the observed column density variations. Out of the four pulsars, only in 4U 1538-52, we find the model having a reasonable corroboration with the observations, whereas in the remaining three the stellar wind seems to be clumpy and a smooth symmetric stellar wind model appears to be quite inadequate in explaining the data. Moreover, in GX 301-2, neither the presence of a disk nor a gas stream from the companion was validated. Furthermore, the spectral results obtained in the case of OAO 1657-415 and Vela X-1 were more or less similar to that of GX 301-2.

RADIO/X-RAY LUMINOSITY RELATION FOR X-RAY-BRIGHT GALACTIC NUCLEI: IMPLICATIONS FOR WEIGHING SUPERMASSIVE BLACK HOLES

Optically thin and geometrically thick accretion flows are known to be responsible for the observed radio/X-ray luminosity relation of 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 amount 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. Semianalytically 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 indeed be used 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 nondisk components such as a jet component, which otherwise results in an overestimation of SMBH mass.