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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 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.

We report results from analysis of the X-ray observations of AE Aqr, made 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.

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.

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We report the few hundred second anti-correlated soft lags between soft and hard energy bands in the source GX 3394 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.

Context. The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is associated with spectral variations. These studies in other black hole binaries are important to understand the QPO phenomenon. Aims. We study fast QPO transitions and accompanying spectral variations of the black hole transient XTE J1817-330 to investigate the causes of the spectral variation during the QPO transition. Methods. Recently, QPOs in ten RXTE observations of XTE J1817-330 were found. We found that among these, only one observation shows erratic dips in its X-ray light curve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of the light curve. Results. We found that type-B ∼6 Hz QPO changes into type-A QPO in a few tens of seconds along with a flux decrease. This transient evolution is accompanied by a significant spectral variation. Conclusions. We report a transient QPO feature and accompanying spectral variation in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral variations.

We analyze X-ray archive data of the W UMa-type binary VW Cep taken with ASCA on 1993 November 5 { 6. By analyzing the light curve, we nd a long-duration are of 7:5 hrs with the peak luminosity of 1:2 10 30 ergs s ?1 (0.4{3.0 keV) for the assumed distance of 23.2 pc. A ux dip is detected in the light curve at the orbital phase of 0:5, and it is identi ed as an eclipse by the secondary star. We determine the time scale of the eclipse egress to be 30 minutes from a model t to the light curve. With this time scale, we estimate the linear size of the aring region is 5:5 10 10 cm regardless of the are models. From the spectral analysis of the data, we nd that the spectrum can be well reproduced by the variable abundance plasma model with a combination of two di erent temperatures, kT = 0.64 keV and kT = 1.91 keV. The hotter component is considered to be associated with the are. The results are interpreted in terms of two-ribbon are model, in which we also discuss possible enhancement of element abundances.