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Register a new userThe Relationship Between X-ray Luminosity and Duty Cycle for Dwarf Novae and their Specific Frequency in the Inner Galaxy
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We measure the duty cycles for an existing sample of well observed, nearby dwarf novae using data from AAVSO, and present a quantitative empirical relation between the duty cycle of dwarf novae outbursts and the X-ray luminosity of the system in quiescence. We have found that $log DC=0.63(pm0.21)times(log L_{X}({rm erg,s^{-1}})-31.3)-0.95(pm0.1)$, where DC stands for duty cycle. We note that there is intrinsic scatter in this relation greater than what is expected from purely statistical errors. Using the dwarf nova X-ray luminosity functions from citet{Pretorius12} and citet{Byckling10}, we compare this relation to the number of dwarf novae in the Galactic Bulge Survey which were identified through optical outbursts during an 8-day long monitoring campaign. We find a specific frequency of X-ray bright ($L_{X}>10^{31},{rm erg,s^{-1}}$) Cataclysmic Variables undergoing Dwarf Novae outbursts in the direction of the Galactic Bulge of $6.6pm4.7times10^{-5},M_{odot}^{-1}$. Such a specific frequency would give a Solar neighborhood space density of long period CVs of $rho=5.6pm3.9times10^{-6},$pc$^{-3}$. We advocate the use of specific frequency in future work, given that projects like LSST will detect DNe well outside the distance range over which $rhoapprox{textrm const}$.
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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.
We quantify to what extent tidal erosion of globular clusters (GCs) has contributed to the observed u-shaped relation between GC specific frequencies S_N and host galaxy luminosity M_V. We used our MUESLI code to calculate GC survival rates for typical early-type galaxy potentials covering a wide range of observed galaxy properties. We do this for isotropic and radially anisotropic GC velocity distributions. We find that the calculated GC survival fraction, f_s, depends linearly on the logarithm of the 3D mass density, rho_3D, within the galaxys half light radius, with f_s proportional to (rho_3D)^(-0.17). For a given galaxy, survival rates are lower for radially anisotropic configurations than for the isotropic GC cases. We apply these relations to a literature sample of 219 early-type galaxies from Harris et al. (2013) in the range M_V=[-24.5:-15.5] mag. The expected GC survival fraction ranges from ~50% for the most massive galaxies with the largest radii to ~10% for the most compact galaxies. We find that intermediate luminosity galaxies M_V=[-20.5:-17.5] mag have the strongest expected GC erosion. Within the considered literature sample, the predicted GC survival fraction therefore defines a u-shaped relation with M_V, similar to the relation between specific frequency S_N and M_V. As a consequence, the u-shape of S_N vs. M_V gets erased almost entirely when correcting the S_N values for the effect of GC erosion. We conclude that tidal erosion is an important contributor to the u-shaped relation between GC specific frequency and host galaxy luminosity. It must be taken into account when inferring primordial star cluster formation efficiencies from observations of GC systems in the nearby universe.
An unexpectedly slow evolution in the pre-optical-maximum phase was suggested in the very short recurrence period nova M31N 2008-12a. To obtain reasonable nova light curves we have improved our calculation method by consistently combining optically thick wind solutions of hydrogen-rich envelopes with white dwarf (WD) structures calculated by a Henyey-type evolution code. The wind mass loss rate is properly determined with high accuracy. We have calculated light curve models for 1.2 and 1.38 M_sun WDs with mass accretion rates corresponding to recurrence periods of 10 and 1 yr, respectively. The outburst lasts 590/29 days in which the pre-optical-maximum phase is 82/16 days, for 1.2/1.38 M_sun, respectively. Optically thick winds start at the end of X-ray flash and cease at the beginning of supersoft X-ray phase. We also present supersoft X-ray light curves including a prompt X-ray flash and later supersoft X-ray phase.
We performed simulations of a large number of so-called very faint X-ray transient sources from surveys obtained using the X-ray telescope aboard the Neil Gehrels emph{Swift} Observatory on two Galactic globular clusters, and the Galactic Center. We calculated the ratio between the duty cycle we input in our simulations and the one we measure after the simulations. We found that fluctuations in outburst duration and recurrence times affect our estimation of the duty cycle more than non detected outbursts. This biases our measures to overestimate the simulated duty cycle of sources. Moreover, we determined that compact surveys are necessary to detect outbursts with short duration because they could fall in gaps between observations, if such gaps are longer than their duration. On the other hand, long surveys are necessary to detect sources with low duty cycle because the smallest duty cycle a survey can observe is given by the ratio between the shortest outburst duration and the total length of the survey. If one has a limited amount of observing time, these two effects are competing, and a compromise is required which is set by the goals of the proposed survey. We have also performed simulations with several artificial survey strategies in order to evaluate the optimal observing campaign aimed at detecting transients as well as at having the most accurate estimates of the duty cycle. As expected, the best campaign would be a regular and dense monitoring that extends for a very long period. The closest real example of such a dataset is the monitoring of the Galactic Centre.
X-ray grating spectra have opened a new window on the nova physics. High signal-to-noise spectra have been obtained for 12 novae after the outburst in the last 13 years with the Chandra and XMM-Newton gratings. They offer the only way to probe the temperature, effective gravity and chemical composition of the hydrogen burning white dwarf before it turns off. These spectra also allow an analysis of the ejecta, which can be photoionized by the hot white dwarf, but more often seem to undergo collisional ionization. The long observations required for the gratings have revealed semi-regular and irregular variability in X-ray flux and spectra. Large short term variability is especially evident in the first weeks after the ejecta have become transparent to the central supersoft X-ray source. Thanks to Chandra and XMM-Newton, we have discovered violent phenomena in the ejecta, discrete shell ejection, and clumpy emission regions. As expected, we have also unveiled the white dwarf characteristics. The peak white dwarf effective temperature in the targets of our samples varies between ~400,000 K and over a million K, with most cases closer to the upper end, although for two novae only upper limits around 200,000 K were obtained. A combination of results from different X-ray satellites and instruments, including Swift and ROSAT, shows that the shorter is the supersoft X-ray phase, the lower is the white dwarf peak effective temperature, consistently with theoretical predictions. The peak temperature is also inversely correlated with t(2) the time for a decay by 2 mag in optical. I strongly advocate the use of white dwarf atmospheric models to obtain a coherent physical picture of the hydrogen burning process and of the surrounding ejecta.
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