M. T. Ozkan

Solar-type cycles of the secondary stars in cataclysmic variables

We found cyclical variations in the quiescent magnitudes and outburst intervals of 21 dwarf novae, 1 nova and 1 nova-like star produced by the solar-type cycles of the late-type secondary components. More than 70% of individual systems in this work are rst studied by Fourier periodogram analysis. It seems that the mean cycle amplitudes of the quiescent magnitudes are considerably larger for SU UMa stars. Cyclical modulations of quiescent magnitudes found for the stars, which have orbital periods below the period gap, support the hypothesis that even almost fully convective secondaries may possess activity cycles. The estimated probability density functions show peaks at 9.7, 7.9 and 8.6 yrs for cataclysmic variables (CVs), single main-sequence stars (MS) and all stars (CVs+MS), respectively. Using a larger set of data, we conrm that there is no correlation of the cycle period with the rotational regime of the secondary star. It is also conrmed that cycle periods of CVs are in the range of those of single main-sequence stars. We note that the observational cycle parameters (i.e. cycle period, Pcyc, and cycle amplitude of quiescent magnitudes, m) do not seem in relation with any other parameter such as masses of primary and secondary stars, mean outburst interval, mean outburst duration, mean decline and rise rate of outbursts, absolute magnitudes in quiescent and outburst states etc.

Statistical analysis of the long-term visual light curve parameters of dwarf novae

The long-term visual light curve parameters of 36 dwarf novae are measured. The data is anayzed in search of likely relationships among the light curve parameters and system parameters. New relations of the orbital period with the mean light curve parameters are given. Our ndings for the correlation analysis of individual systems agree with some of the results in previous studies. Almost all of the systems in this study have a bimodal frequency distribution of the outburst duration. By comparing the observational correlations with their theoretical counterparts, we nd that the value of the viscosity parameter of the disk instability model is hot 0:2.

X-ray spectral variations of U Gem from quiescence to outburst

In this paper, we report the discovery of a high-energy component of the X-ray spectra of U Gem, which can be observed while the source is in outburst. We used Chandra and XMM Newton observations to compare the quiescence and outburst X-ray spectra of the source. The additional component may be the result of the reflection of X-rays emitted from an optically thin plasma close to the white dwarf, from the optically thick boundary layer during the outburst. Another possible explanation is that some magnetically channelled accretion may occur on to the equatorial belt of the primary causing shocks similar to the ones in the intermediate polars as it was suggested by Warner and Woudt. We have also found a timing structure at about 73 mHz (∼13.7 s) in the RXTE observation, resembling dwarf novae oscillations.

Accretion Disk Winds in Dwarf Novae

It is well known that the International Ultraviolet Explorer observations of high-excitation P Cygni profiles of C IV and S IV lines provide direct evidence for winds in cataclysmic variables which reach speeds of ~5000 km s -1. We present in this paper our analysis of P Cygni profile for C IV line in seven dwarf novae based on 195 low-resolution IUE spectra. This work aims to do two things: i) to examine them in terms of wind properties, and ii) to seek time dependence of the lines indicating departure from axial symmetry in the outflow and/or disk and changes in the wind ionisation and/or mass loss rate. We infer the optical depth of C IV, the column density and the number density of absorbing and scattering ions in the region where the wind ionized.

IUE Spectra of Three Z Cam Type Dwarf Novae: A Study of P Cygni Profiles

We study the P Cygni profiles in three dwarf novae. In a study of Z Cam by Szkody and Mateo (1986), it was claimed that the mass loss continues for 1 month after the start of a standstill and ceases within 6 months. This results of the work depends on only one standstill spectrum. On the other hand, in another work carried out by Ozkan and Esenoglu (1994) it was found that the case may not be true for Z Cam, because two ultraviolet spectra obtained during the sixth month of another standstill configuration shows the P Cygni profile having a stronger absorption component and a weaker emission component which implies that the mass loss may continue during the standstill. Following these works in the present study, we aimed to investigate this situation for Z Cam and two other Z Cam type dwarf novae, RX And and HL CMa. considering all ultraviolet spectra obtained with IUE and AAVSO observations (Mattei, 1994).

Time dependence of the UV resonance lines in the cataclysmic variable star VW Hyi

In this work, we study orbital variations in the ultraviolet resonance lines of the SU UMa type dwarf nova VW Hyi examining spectra in the data archive of the International Ultraviolet Explorer (IUE). We show that VW Hyi exhibits an orbital and long term variability in terms of UV line profiles and continuum flux at 1450 A during superoutburst.