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

A Search For Supernova Remnants in The Nearby Spiral Galaxy M74 (NGC 628)

We have identified nine new SNR candidates in M74 with [S II]/H$\alpha$ $\geq$ 0.4 as the basic criterion. We obtain [S II]/H$\alpha$ ratio in the range from 0.40 to 0.91 and H$\alpha$ intensities from 2.8 $\times$ $10^{-15}$ erg cm$^{-2}$ s$^{-1}$ to 1.7 $\times$ $10^{-14}$ erg cm$^{-2}$ s$^{-1}$. We also present spectral follow-up observations of the SNR candidates and can confirm only three of them (SNR2, SNR3, and SNR5). The lack of confirmation for the rest might be due to the contamination by the nearby H II emission regions as well as due to the inaccurate positioning of the long slit on these objects. In addition, we search the $Chandra$ Observatory archival data for the X-ray counterparts to the optically identified candidates. We find positional coincidence with only three SNR candidates, SNR1, SNR2, and SNR8. The spectrum of SNR2 yields a shock temperature of 10.8 keV with an ionization timescale of 1.6 $\times$ 10$^{10}$ s cm$^{-3}$ indicating a relatively young remnant in an early Sedov phase which is not supported by our optical wavelength analysis. Given the high luminosity of 10$^{39}$ erg s$^{-1}$ and the characteristics of the X-ray spectrum, we favor an Ultra Luminous X-ray Source interpretation for this source associated with an SNR. We calculate an X-ray flux upper limit of 9.0 $\times$ $10^{-15}$ erg cm$^{-2}$ s$^{-1}$ for the rest of the SNRs including spectroscopically identified SNR3 and SNR5.Comment: 10 pages, 8 figures, accepted to be published in A&

The Detection of a 3.5-h Period in the Classical Nova Velorum 1999 (V382 Vel) and the Long Term Behavior of the Nova Light Curve

We present CCD photometry, light curve and time series analysis of the classical nova V382 Vel (N Vel 1999). The source was observed for 2 nights in 2000, 21 nights in 2001 and 7 nights in 2002 using clear filters. We report the detection of a distinct period in the light curve of the nova P=0.146126(18) d (3.5 h). The period is evident in all data sets, and we interpret it as the binary period of the system. We also measured an increase in the amplitude modulation of the optical light (in magnitude) by more than 55% from 2000 to 2001 and about 64% from 2001 to 2002. The pulse profiles in 2001 show deviations from a pure sinusoidal shape which progressively become more sinusoidal by 2002. The main cause of the variations in 2001 and 2002 can be explained with the occultation of the accretion disk by the secondary star. We interpret the observed deviations from a pure sinusoidal shape as additional flux resulting from the aspect variations of the irradiated face of the secondary star.Comment: 16 pages and 4 figures, accepted as it stands to be published in the Astronomical Journal (AJ

New X-ray observations of the old nova CP Puppis and of the more recent nova V351 Pup

We present X-ray observations of the field containing Nova Puppis 1942 (CP Pup) and Nova Puppis 1991 (V351 Pup), done with ASCA in 1998, and with XMM-Newton in 2005. The X-ray and UV luminosity of CP Pup seem to have remained approximately constant since the last X-ray observations of the 1980'ies, while the optical luminosity has decreased. The X-ray properties of this nova are explained by a high mass white dwarf accreting at low rate, in agreement with the nova theory given the large amplitude and other characteristics of the 1942 outburst. Assuming a distance of 1600 pc, the X-ray luminosity of CP Pup is L=2.2 x 10(33) erg/s in the 0.15-10 keV range covered with EPIC, compatible with a magnetic system. The RGS grating spectrum shows a few prominent emission lines, and it is fitted with a cooling flow with mass accretion rate mdot <= 1.6 x 10(-10) msol/year. We detected also the O VII complex at 21.6-21.8 A that does not arise in the cooling flow. Most likely this feature originates in a wind or in the nova shell. The RGS and EPIC spectra are fitted only with thermal models with a very high shock temperature, T>60 keV, indicating a white dwarf with M>1.1 M(sun). The X-ray flux is modulated with the spectroscopic period of 1.47 hours detected in the optical. Since CP Pup is not an eclipsing system, this is better understood if magnetic accretion occurs: we discuss this possibility and its implications in detail. V351 Pup (N Pup 1991) was detected with XMM-Newton, but not with ASCA. It is a faint, non-super-soft X-ray source with luminosity L(x) =~ 3 x 10(31) erg/s, a factor of 50 less than measured with ROSAT in 1993.Comment: in press on the Astrophysical Journa

Astrophysical Fluids of Novae: High Resolution Pre-decay X-ray spectrum of V4743 Sagittarii

Eight X-ray observations of V4743 Sgr (2002), observed with Chandra and XMM-Newton are presented. The nova turned off some time between days 301.9 and 371, and the X-ray flux subsequently decreased from day 301.9 to 526 following an exponential decline time scale of $(96 \pm 3)$ days. We use the absorption lines present in the SSS spectrum for diagnostic purposes, and characterize the physics and the dynamics of the expanding atmosphere during the explosion of the nova. The information extracted from this first stage is then used as input for computing full photoionization models of the ejecta in V4743 Sgr. The SSS spectrum is modeled with a simple black-body and multiplicative Gaussian lines, which provides us of a general kinematical picture of the system, before it decays to its faint phase (Ness et al. 2003). In the grating spectra taken between days 180.4 and 370, we can resolve the line profiles of absorption lines arising from H-like and He-like C, N, and O, including transitions involving higher principal quantum numbers. Except for a few interstellar lines, all lines are significantly blue-shifted, yielding velocities between 1000 and 6000 km/s which implies an ongoing mass loss. It is shown that significant expansion and mass loss occur during this phase of the explosion, at a rate $\dot{M} \approx (3-5) \times 10^{-4} ~ (\frac{L}{L_{38}}) ~ M_{\odot}/yr$. Our measurements show that the efficiency of the amount of energy used for the motion of the ejecta, defined as the ratio between the kinetic luminosity $L_{\rm kin}$ and the radiated luminosity $L_{\rm rad}$, is of the order of one.Comment: 25 pages, 9 figures. Accepted in book: Recent Advances in Fluid Dynamics with Environmental Applications, pp.365-39