Inner Disk Structure of Dwarf Novae in the Light of X-ray Observations

Diversity of the X-ray observations of dwarf nova are still not fully understood. I review the X-ray spectral characteristics of dwarf novae during the quiescence in general explained by cooling flow models and the outburst spectra that show hard X-ray emission dominantly with few sources that reveal soft X-ray/EUV blackbody emission. The nature of aperiodic time variability of brightness of dwarf novae shows band limited noise, which can be adequately described in the framework of the model of propagating fluctuations. The frequency of the break (1-6 mHz) indicates inner disk truncation of the optically thick disk with a range of radii (3.0-10.0)$\times$10$^{9}$ cm. The RXTE and optical (RTT150) data of SS Cyg in outburst and quiescence reveal that the inner disk radius moves towards the white dwarf and receeds as the outburst declines to quiescence. A preliminary analysis of SU UMa indicates a similar behaviour. In addition, I find that the outburst spectra of WZ Sge shows two component spectrum of only hard X-ray emission, one of which may be fitted with a power law suggesting thermal Comptonization occuring in the system. Cross-correlations between the simultaneous UV and X-ray light curves (XMM-Newton) of five DNe in quiescence show time lags in the X-rays of 96-181 sec consistent with travel time of matter from a truncated inner disk to the white dwarf surface. All this suggests that dwarf novae and other plausible nonmagnetic systems have truncated accretion disks indicating that the disks may be partially evaporated and the accretion may occur through hot (coronal) flows in the disk.Comment: 7 pages and 10 figures, accepted for publication in Acta Polytechnica as proceedings of the 2013 workshop on "The Golden Age of Cataclysmic Variables and Related Objects - II", Franco Giovannelli & Lola Sabau-Graziati (eds.

A search for periodicities from a ULX in the LINER galaxy NGC 4736

We report our findings on a new quasi-periodic oscillation (QPO) and a long period from the ultraluminous X-ray source (ULX) X-2 in nearby galaxy NGC 4736 based on the Chandra and XMM-Newton archival data. To examine the timing properties, power density spectra of the source have been obtained using Fast Fourier Transform. Also the spectral parameters of the source have been calculated by obtaining and fitting the energy spectra. Power density spectrum of this source reveals a QPO peak at $0.73_{-0.14}^{+0.16}$ mHz with an fractional rms variability of 16% using the Chandra data (in the year 2000-lower state of the source). The XMM-Newton data analysis indicates a peak at $0.53_{-0.35}^{+0.09}$ mHz with a fractional rms variation of 5% (in the year 2006-higher state of the source). These recovered QPOs overlap within errors and may be the same oscillation. In addition, we detect a long periodicity or a QPO in the Chandra data of about $(5.2\pm2.0)\times10^{-5}$ Hz ($\sim$ 5.4 hrs) over 3 $\sigma$ confidence level. If this is a QPO, it is the lowest QPO detected from a ULX. The mass of the compact object in ULX X-2 is estimated using the Eddington luminosity and a disk blackbody model in the range (10$-$80) M_{\sun}.Comment: 6 pages, 4 Figures; Accepted for publication in Astrophysics and Space Scienc

The Chandra Observation of the Shell of Nova Persei 1901 (GK Persei): Detection of localized Non-thermal X-ray Emission from a Miniature Supernova Remnant

I present the data of the shell of classical Nova Persei (1901) obtained by the Advanced CCD Imaging Spectrometer S3 detector on-board \cha Observatory. The X-ray nebula is affected mostly by the complex interstellar medium around the nova and has not developed a regular shell with bulk of emission coming from the southwestern quadrant. The part of the bright zone that is co-spatial with the brightest non-thermal radio emission region, is found to be a source of non-thermal X-ray emission with a power law photon index of 2.3$^{+1.5}_{-0.9}$ and alpha=0.68^${+0.03}_{-0.15}$ at about a flux of 1.7x10^{-13} erg cm^{-2} s^{-1}. There are strong indications for nonlinear diffusive shock acceleration occurring in the forward shock/transition zone with an upper limit on the non-thermal X-ray flux of 1.0\times 10^{-14} erg cm^{-2} s^{-1}.The total X-ray spectrum of has two components of emission. The component dominant below 2 keV is most likely a non-equilibrium ionization thermal plasma of kT_s=0.1-0.3 keV with an X-ray flux of 1.6x10^{-11} erg cm^{-2} s^{-1}. There is also a higher temperature, kT_s=0.5-2.6 keV, embedded, N_H=(4.0-22.0)x10^{22} cm^{-2}, emission component prominent above 2 keV. The unabsorbed X-ray flux from this component is 1.5x10^{-10} erg cm^{-2} s^{-1}. The X-ray emitting plasma is of solar composition except for enhancement in the elemental abundances (mean abundances over the remnant)of Ne/Ne$_{\odot}$ and N/N$_{\odot}$ in a range 13-21 and 1-5, respectively. A distinct emission line of neon, He-like Ne IX, is detected which reveals a distribution of several emission knots/blobs and shows a cone-like structure with wings extending toward NW and SE at expansion velocities about 2600 km s^{-1} in the X-rays.Comment: 56 pages and 16 figures; Accepted to be published in the Astrophysical Journal (Part 1) as it stand