19 research outputs found
On the accretion disc properties in eclipsing dwarf nova EM Cyg
In this paper we analyzed the behavior of the unusual dwarf nova EM Cyg using
the data obtained in April-October, 2007 in Vyhorlat observatory (Slovak
Republic) and in September, 2006 in Crimean Astrophysical Observatory
(Ukraine). During our observations EM Cyg has shown outbursts in every 15-40
days. Because on the light curves of EM Cyg the partial eclipse of an accretion
disc is observed we applied the eclipse mapping technique to reconstruct the
temperature distribution in eclipsed parts of the disc. Calculations of the
accretion rate in the system were made for the quiescent and the outburst
states of activity for different distances.Comment: 6 pages, 3 figures, accepted in Astrophysics and Space Scienc
Wavelets with Ridges: A High-Resolution Representation of Cataclysmic Variable Time-Series
Quasi-periodic oscillations and dwarf nova oscillations occur in dwarf novae
and nova-like variables during outburst and occasionally during quiescence, and
have analogues in high-mass X-ray binaries and black-hole candidates. The
frequent low coherence of quasi-period oscillations and dwarf nova oscillations
can make detection with standard time-series tools such as periodograms
problematic. This paper develops tools to analyse quasi-periodic brightness
oscillations. We review the use of time-frequency representations in the
astronomical literature, and show that representations such as the
Choi-Williams Distribution and Zhao-Atlas-Marks Representation, which are best
suited to high signal-to-noise data, cannot be assumed a priori to be the best
techniques for our data, which have a much higher noise level and lower
coherence. This leads us to a detailed analysis of the time-frequency
resolution and statistical properties of six time-frequency representations. We
conclude that the wavelet scalogram, with the addition of wavelet ridges and
maxima points, is the most effective time-frequency representation for
analysing quasi-periodicities in low signal-to-noise data, as it has high
time-frequency resolution, and is a minimum variance estimator.
We use the wavelet ridges method to re-analyse archival data from VW Hyi, and
find 62 new QPOs and 7 new long-period DNOs. Relative to previous analyses, our
method substantially improves the detection rate for QPOs.Comment: 62 pages,35 figure
Constraints on the black hole spin in the quasar SDSS J094533.99+100950.1
The spin of the black hole is an important parameter which may be responsible
for the properties of the inflow and outflow of the material surrounding a
black hole. Broad band IR/optical/UV spectrum of the quasar SDSS
J094533.99+100950.1 is clearly disk-dominated, with the spectrum peaking up in
the observed frequency range. Therefore, disk fitting method usually used for
Galactic black holes can be used in this object to determine the black hole
spin. We develop the numerical code for computing disk properties, including
radius-dependent hardening factor, and we apply the ray-tracing method to
incorporate all general relativity effects in light propagation. We show that
the simple multicolor disk model gives a good fit, without any other component
required, and the disk extends down to the marginally stable orbit. The best
fit accretion rate is 0.13, well below the Eddington limit, and the black hole
spin is moderate, 0.3. The contour error for the fit combined with the
constraints for the black hole mass and the disk inclination gives a constraint
that the spin is lower than 0.8. We discuss the sources of possible systematic
errors in the parameter determinations
GENETIC ALGORITM ECLIPSE MAPPING
In this paper we analyse capabilities of eclipse mapping technique, based on genetic algorithm optimization. To model of accretion disk we used the "fire-flies" conception. This model allows us to reconstruct the distribution of radiating medium in the disk using less number of free parameters than in other methods. Test models show that we can achieve good approximation without optimizing techniques