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Register a new userAlternation of the flickering morphology between the high and low state in MV Lyr
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AIMS: We studied unique data of a nova-like system MV Lyr during transition from the high to low state and vice versa taken by the Kepler space telescope. We were interested in evolution of frequency components found previously by Scaringi et al. in different data also obtained by Kepler. METHODS: We divided the light curve into 10 day segments and investigated the corresponding power density spectra. We searched for individual frequency components by fitting with Lorentzian functions. Additionally, we investigated the variability using averaged shot profiles calculated from the light curve divided into 10 equally spaces subsamples. RESULTS: We found very complex changes of the power density spectra. We focused our study onto three frequency components. Strong activity increase is seen at low frequencies. Contrariwise, the high frequency part of the spectrum strongly decreases in power with specific rise in characteristic frequencies of the individual components. We discuss various scenarios of this phenomenology as reprocessing of X-rays in a receding accretion disc or a radiation from a more active region at the outer disc. Finally, we show that various cataclysmic variables show similar characteristic frequencies in their power density spectra. These are dependent on activity stage, making the situation similar to X-ray binaries.
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The cataclysmic variable MV Lyr was present in the Kepler field yielding a light curve with the duration of almost 1500 days with 60 second cadence. Such high quality data of this nova-like system with obvious fast optical variability show multicomponent power density spectra. Our goal is to study the light curve from different point of view, and perform a shot profile analysis. We search for characteristics not discovered with standard power density spectrum based methods. The shot profile method identifies individual shots in the light curve, and averages them in order to get all substructures with typical time scales. We also tested the robustness of our analysis using simple shot noise model. We obtained mean profiles with multicomponent features. The shot profile method distinguishes substructures with similar time scales which appear as a single degenerate feature in power density spectra. Furthermore, this method yields the identification of another high frequency component in the power density spectra of Kepler and XMM-Newton data not detected so far. Moreover, we found side-lobes accompanied with the central spike, making the profile very similar to another Kepler data of blazar W2R 1926+42, and Ginga data of Cyg X-1. All three objects show similar time scale ratios of the rising vs. declining part of the central spikes, while the two binaries have also similar rising profiles of the shots described by a power-law function. The similarity of both binary shot profiles suggests that the shots originate from the same origin, e.g. aperiodic mass accretion in the accretion disc. Moreover, the similarity with the blazar may imply that the ejection fluctuations in the blazar jet are connected to accretion fluctuations driving the variability in binaries. This points out to connection between jet and the accretion disc.
Four VY Scl-type nova-like systems were observed in X-rays during both the low and the high optical states. We examined Chandra, ROSAT, Swift and Suzaku archival observations of BZ Cam, MV Lyr, TT Ari, and V794 Aql. The X-ray flux of BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) as hypothesized in previous articles. No SSS was detected in the low state of the any of the other systems, with the X-ray flux decreasing by a factor between 2 and 50. The best fit to the Swift X-ray spectra is obtained with a multi-component model of plasma in collisional ionization equilibrium. The high state high resolution spectra of TT Ari taken with Chandra ACIS-S and the HETG gratings show a rich emission line spectrum, with prominent lines of in Mg, Si, Ne, and S. The complexity of this spectrum seems to have origin in more than one region, or more than one single physical mechanism. While several emission lines are consistent with a cooling flow in an accretion stream, there is at least an additional component. We discuss the origin of this component, which is probably arising in a wind from the system. We also examine the possibility that the VY Scl systems may be intermediate polars, and that while the boundary layer of the accretion disk emits only in the extreme ultraviolet, part of the X-ray flux may be due to magnetically driven accretion.
AIMS: A few well studied cataclysmic variables (CVs) have shown discrete characteristic frequencies of fast variability; the most prominent ones are around log(f/Hz) $simeq$ -3. Because we still have only small number statistics, we obtained a new observation to test whether this is a general characteristic of CVs, especially if mass transfer occurs at a high rate typical for dwarf nova in outbursts, in the so called high state. METHODS: We analyzed optical Kepler data of the quiescent nova and intermediate polar V4743 Sgr. This system hosts a white dwarf accreting through a disk in the high state. We calculated the power density spectra, and searched for break or characteristic frequencies. Our goal is to assess whether the mHz frequency of the flickering is a general characteristic. RESULTS: V4743 Sgr has a clear break frequency at log(f/Hz) $simeq$ -3. This detection increases the probability that the mHz characteristic frequency is a general feature of CVs in the high state, from 69% to 91%. Furthermore, we propose the possibility that the variability is generated by similar mechanism as in the nova-like system MV Lyr, which would make V4743 Sgr unique.
We present a broad-band timing analysis of the accreting white dwarf system MV Lyrae based on data obtained with the Kepler satellite. The observations span 633 days at a cadence of 58.8 seconds and allow us to probe 4 orders of magnitude in temporal frequency. The modelling of the observed broad-band noise components is based on the superposition of multiple Lorentzian components, similar to the empirical modelling adopted for X-ray binary systems. We also present the detection of a frequency varying Lorentzian component in the lightcurve of MV Lyrae, where the Lorentzian characteristic frequency is inversely correlated with the mean source flux. Because in the literature similar broad-band noise components have been associated to either the viscous or dynamical timescale for different source types (accreting black holes or neutron stars), we here systematically explore both scenarios and place constraints on the accretion disk structure. In the viscous case we employ the fluctuating accretion disk model to infer parameters for the viscosity and disk scale height, and infer uncomfortably high parameters to be accommodated by the standard thin disk, whilst in the dynamical case we infer a large accretion disk truncation radius of ~10 white dwarf radii. More importantly however, the phenomenological properties between the broad-band variability observed here and in X-ray binaries and Active Galactic Nuclei are very similar, potentially suggesting a common origin for the broad-band variability.
We report on a 40ks Chandra observation of the TeV emitting high mass X-ray binary HESS J0632+057 performed in February 2011 during a high-state of X-ray and TeV activity. We have used the ACIS-S camera in Continuos Clocking mode to search for a possible X-ray pulsar in this system. Furthermore, we compare the emission of the source during this high state, with its X-ray properties during a low state of emission, caught by a 47ks XMM-Newton observation on September 2007. We did not find any periodic or quasi-periodic signal in any of the two observations. We derived an average pulsed fraction 3sigma upper limit for the presence of a periodic signal of ~35% and 25% during the low and high emission state, respectively (although this limit is strongly dependent on the frequency and the energy band). Using the best X-ray spectra derived to date for HESS J0632+057, we found evidence for a significant spectral change between the low and high X-ray emission states, with the absorption value and the photon index varying between Nh ~ 2.1-4.3x10^{21} cm^{-2} and Gamma ~ 1.18-1.61. At variance with what observed in other TeV binaries, it seems that in this source the higher the flux the softer the X-ray spectrum.
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