arXiv subjects

Matthias R. Schreiber

Publications and source records attributed to Matthias R. Schreiber.

76 records · Page 5Linked to original sources

arXiv2007

The dwarf nova SS Cygni: what is wrong?

Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was used to measure the distance to SS Cyg to be $166\pm12$ pc, it became apparent that at this distance the disc instability model fails to explain the absolute magnitude during outburst. It remained, however, an open question whether the model or the distance have to be revised. Recent observations led to a revision of the system parameters of SS Cyg and seem to be consistent with a distance of $d\gta 140$ pc. We re-discuss the problem taking into account the new binary and stellar parameters measured for SS Cyg. We confront not only the observations with the predictions of the disc instability model but also compare SS Cyg with other dwarf novae and nova-like systems. We assume the disc during outburst to be in a quasi stationary state and use the black-body approximation to estimate the accretion rate during outburst as a function of distance. Using published analysis of the long term light curve we determine the mean mass transfer rate of SS Cyg as a function of distance and compare the result with mass transfer rates derived for other dwarf novae and nova-like systems. At a distance of $d\gta 140$ pc, both the accretion rate during outburst as well as the mean mass transfer rate of SS Cyg contradict the disc instability model. More important, at such distances we find the mean mass transfer rate of SS Cyg to be higher or comparable to those derived for nova-like systems. Our findings show that a distance to SS Cyg $\gta 140$ pc contradicts the main concepts developed for accretion discs in cataclysmic variables during the last 30 years. Either our current picture of disc accretion in these systems must be revised or the distance to SS Cyg is $\sim 100$ pc

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arXiv2007

The statistical significance of the superhump signal in U Gem

Although its well determined mass ratio of $q=\Msec/\Mwd=0.357\pm0.007$ should avoid superoutbursts according to the thermal tidal instability model, the prototypical dwarf nova U Gem experienced in 1985 an extraordinary long outburst resembling very much superoutbursts observed in SU UMa systems. Recently, the situation for the model became even worse as superhump detections have been reported for the 1985 outburst of U Gem. The superhump signal is noisy and the evidence provided by simple periodograms seems to be weak. Therefore and because of the importance for our understanding of superoutbursts and superhumps, we determine the statistical significance of the recently published detection of superhumps in the AAVSO light curve of the famous long 1985 outburst of U Gem. Using Lomb-Scargle periodograms, analysis of variance (AoV), and Monte-Carlo methods we analyse the 160 visual magnitudes obtained by the AAVSO during the outburst and relate our analyse to previous superhump detections. The 160 data points of the outburst alone do not contain a statistically significant period. However, using additionally the characteristics of superhumps detected previously in other SU UMa systems and searching only for signals that are consistent with these, we derive a $2σ$ significance for the superhump signal. The alleged appearance of an additional superhump at the end of the outbursts appears to be statistically insignificant. Although of weak statistical significance, the superhump signal of the long 1985 outburst of U Gem can be interpreted as further indication for the SU UMa nature of this outburst. This further contradicts the tidal instability model as the explanation for the superhump phenomenon.

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arXiv2004

Delays in dwarf novae: VW Hyi, the tidal instability and enhanced mass transfer models

We discuss the multi-wavelength predictions of the two models proposed for SU UMa stars, i.e the enhanced mass transfer (EMT) and the tidal thermal instability (TTI) models. We focus on the systematic differences of the suggested scenarios before discussing the model predictions together with the observations of the best-studied SU UMa system, VW Hyi. We find that assuming the standard form of the viscosity parameter alpha, both models predict only outbursts being triggered at the inner edge of the accretion disc. In the TTI model the superoutbursts are triggered when the outer radius of the disc reaches a certain value, i.e. the 3:1 resonance radius. In contrast, the EMT scenario predicts superoutbursts when the disc mass exceeds a critical value. This causes the EMT model to be much more sensitive to mass transfer variations than the TTI model. In both models we find the predicted UV and EUV delays in agreement with the observations of VW Hyi for alpha_hot/alpha_cold<=4. In addition both models can generate precursor outbursts which are more pronounced at short wavelengths, in agreement with observations. Variations found in the observed light curve of single systems (e.g. VW Hyi) as well as the difference between ordinary SU UMa stars and ER UMa systems are a natural outcome of the EMT model while the TTI model fails to explain them.

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arXiv2003

Delays in dwarf novae I: The case of SS Cygni

Using the disc instability model and a simple but physically reasonable model for the X-ray, extreme UV, UV and optical emission of dwarf novae we investigate the time lags observed between the rise to outburst at different wavelengths. We find that for ``normal'', i.e. fast-rise outbursts, there is good agreement between the model and observations provided that the disc is truncated at a few white dwarf radii in quiescence, and that the viscosity parameter alpha is ~0.02 in quiescence and ~0.1 in outburst. In particular, the increased X-ray flux between the optical and EUV rise and at the end of an outburst, is a natural outcome of the model. We cannot explain, however, the EUV delay observed in anomalous outbursts because the disc instability model in its standard alpha-prescription form is unable to produce such outbursts. We also find that the UV delay is, contrary to common belief, slightly longer for inside-out than for outside-in outbursts, and that it is not a good indicator of the outburst type.

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