scholarly journals Dwarf Nova Outbursts and Superoutbursts

1996 ◽  
Vol 158 ◽  
pp. 45-46
Author(s):  
J. Smak
Keyword(s):  
Accretion Disks ◽  
Time Scale ◽  
Strong Dependence ◽  
Light Curves ◽  
Instability Wave ◽  
Dwarf Nova ◽  
Rate Of Decline ◽  
Nova Outbursts ◽  
Nova Outburst ◽  
Insight Into

Dwarf nova outbursts provide an almost unique opportunity of getting an insight into the nature of viscosity in accretion disks or, within the α- disk approach, of putting some constraints on α. In particular, the strong dependence of the viscous time-scale on viscosity itself permits us to estimate a almost directly from the observed time-scales. In the case of the hot branch of the ∑ — Te relation, the most reliable estimates (αhot) are based on the rate of decline following the dwarf nova outburst. From a comparison with model light curves calculated with different αs one gets: αhot ≈ 0.2(e.g. Smak 1984b). An independent, but much cruder, estimate can be obtained from the widths of normal outbursts, by assuming that the duration of an outburst represents the travel time of an instability wave across the disk. The result is similar: αhot ≈ 0.2 (Gicger 1987).

scholarly journals Thermal-viscous instability in tilted accretion disks: A possible application to IW Andromeda-type dwarf novae

2020 ◽  
Vol 72 (2) ◽  
Author(s):  
Mariko Kimura ◽  
Yoji Osaki ◽  
Taichi Kato ◽  
Shin Mineshige
Keyword(s):  
Accretion Disks ◽  
Outer Edge ◽  
Light Curves ◽  
Dwarf Novae ◽  
Transfer Rates ◽  
Secondary Star ◽  
Viscous Instability ◽  
Simplifying Assumptions ◽  
Nova Outbursts

Abstract IW And stars are a subgroup of dwarf novae characterized by repetition of the intermediate brightness state with oscillatory variations terminated by brightening. This group of dwarf novae is also known to exhibit a wide variety even within one system in long-term light curves, including the usual dwarf-nova outbursts, Z Cam-type standstills, and so on, besides the typical IW And-type variations mentioned above. Following recent observations suggesting that some IW And stars seem to have tilted disks, we have investigated how the thermal-viscous instability works in tilted accretion disks in dwarf novae and whether it could reproduce the essential features of the light curves in IW And stars. By adopting various simplifying assumptions for tilted disks, we have performed time-dependent one-dimensional numerical simulations of a viscous disk by taking into account various mass supply patterns to the disk; that is, the gas stream from the secondary star flows not only to the outer edge of the disk but also to the inner portions of the disk. We find that tilted disks can achieve a new kind of accretion cycle, in which the inner disk almost always stays in the hot state while the outer disk repeats outbursts, thereby reproducing alternating mid-brightness intervals with dips and brightening, which are quite reminiscent of the most characteristic observational light variations of IW And stars. Further, we have found that our simulations produce diverse light variations, depending on different mass supply patterns even without time variations in mass transfer rates. This could explain the wide variety in long-term light curves of IW And stars.

scholarly journals On the Time-Scale for Turn-Off of a Nova After the Outburst

1979 ◽  
Vol 53 ◽  
pp. 274-279
Author(s):  
Sumner Starrfield
Keyword(s):  
Chemical Composition ◽  
White Dwarf ◽  
Time Scale ◽  
Light Curves ◽  
Substantial Agreement ◽  
The Past ◽  
Thermonuclear Runaway ◽  
Mass Ejection ◽  
Nova Outburst

It is now generally accepted that a nova outburst is caused by a thermonuclear runaway (TR) in the accreted hydrogen rich envelope of a carbon white dwarf. Over the past few years we have studied the evolution of such runaways and have shown that the calculated evolutionary sequences are in substantial agreement with the observations (Starrfield, et. al. 1978; Sparks, et. al. 1978). In the published work we have varied the white dwarf mass, the envelope mass, the accreted envelope mass, and the chemical composition in the envelope (Starrfield, et. al. 1976; Gallagher and Starrfield 1978). In all cases we find that a TR results in mass ejection and the luminosity variations of this ejected material can reproduce the observed light curves of the fast and slow novae.

scholarly journals Flickering around the outburst cycle in Kepler dwarf novae

2021 ◽  
Author(s):  
Albert Bruch
Keyword(s):  
Spectral Index ◽  
High Frequency ◽  
Power Spectra ◽  
Light Curves ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
New Feature ◽  
Nova Outbursts ◽  
High Cadence

Abstract Taking advantage of the unparallel quantity and quality of high cadence Kepler light curves of several dwarf novae, the strength of the flickering and the high frequency spectral index of their power spectra are investigated as a function of magnitude around the outburst cycle of these systems. Previous work suggesting that the flickering strength (on a magnitude scale) is practically constant above a given brightness threshold and only rises at fainter magnitudes is confirmed for most of the investigated systems. As a new feature, a hysteresis in the flickering strength is seen in the sense that at the same magnitude level flickering is stronger during decline from outburst than during the rise. A similar hysteresis is also seen in the spectral index. In both cases, it can qualitatively be explained under plausible assumptions within the DIM model for dwarf nova outbursts.

scholarly journals Do We See Magnetic Effects in Dwarf Nova Outbursts?

1995 ◽  
Vol 151 ◽  
pp. 285-285
Author(s):  
N. Vogt ◽  
E. Meyer-Hofmeister ◽  
F. Meyer
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Accretion Disk ◽  
White Dwarf ◽  
Magnetic Activity ◽  
Light Curves ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Cyclic Behaviour ◽  
Nova Outbursts

Many observations indicate that fast rotating late type stars show magnetic activity. We therefore argue that some of the secondary stars in cataclysmic binaries might also have magnetic fields. Such magnetic fields would reach over the accretion disk around the white dwarf primary. We investigate their effect on dwarf nova outbursts. The magnetic field lines will penetrate the disk and remove angular momentum. This shifts the accumulation of mass towards the inner disk, closer to the white dwarf, and therefore leads to a different outburst behaviour, which can be recognized in observed light curves of dwarf novae. If a magnetic field of the order 50 - 100 gauss is acting on the accretion disk, we expect narrower and more frequent outbursts as compared to the non-magnetic case. Outburst records of three dwarf novae above the period gap (P > 3h), whose long-term light curves are well covered, were used to search for traces of magnetic activity. All three cases display a pronounced bimodality in the distribution of their outburst width, i.e. either narrow or wide outbursts occur. We found evidence for a cyclic behaviour in one case, SS Aur: possible “magnetic episodes” repeating every 18 years reveal epochs with abnormally frequent narrow outbursts and nearly or totally missing wide ones. There are also indications for a similar behaviour of SS Cyg with a 7-years cycle, but with less pronounced periodicity. The third case, U Gem, does not show clear evidence of magnetic activity although we found some indications for a transitory oscillation of the width of wide outbursts after 1926, with a period of 13.6 years. The behaviour of SS Aur and SS Cyg resembles the theoretical predictions, there is, however, an important difference: Throughout the entire cycle, marked by the “magnetic episodes”, neither wide nor narrow outbursts alter their mean light curves: magnetic fields seem to affect only the observed proportion in the frequency of both outburst types without altering the light curves of individual outbursts. Remarkably, most of the “anomalous outbursts” (which are characterized by an abnormally slow rising branch to an outburst) occur also at or near the “magnetic episodes”.

scholarly journals Instabilities in Accretion Disks of Cataclysmic Variables: A Unification Model for Dwarf Nova Outburst

1997 ◽  
Vol 163 ◽  
pp. 269-278
Author(s):  
Yoji Osaki
Keyword(s):  
Accretion Disks ◽  
Thermal Instability ◽  
Mass Transfer Rate ◽  
Cataclysmic Variables ◽  
Variable Stars ◽  
Limit Cycle Oscillation ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Magnetic Cataclysmic Variables ◽  
Nova Outbursts

AbstractInstabilities of accretion disks in cataclysmic variable stars are reviewed in relation to dwarf nova outbursts. Two different kinds of instabilities of accretion disks are now known: the thermal instability and the tidal instability. The thermal instability is produced by hydrogen ionization-recombination transition, which gives rises to a thermal limit-cycle oscillation in accretion disks and it is thought to be responsible for outbursts of U Gem-type dwarf novae. The tidal instability is produced by the tidal effects of the secondary star on accretion disks, by which the disk is deformed to eccentric form and it slowly precesses in the inertial frame of reference. The tidal instability is thought to be responsible for the superhump phenomenon observed during superoutbursts of SU UMa-type dwarf novae. There is a rich variety in outburst behaviors of non-magnetic cataclysmic variables, starting from non-outbursting nova-like stars to various sub-classes of dwarf novae. A unification model for dwarf nova outbursts is then proposed based on these two instabilities. In this model, the non-magnetic cataclysmic variables are classified in the orbital-period versus mass-transfer-rate diagram into four regions depending on different combination to these two instabilities, and their observed outburst behaviors are basically understood on this diagram.

Convective accretion disks and the onset of dwarf nova outbursts

1982 ◽  
Vol 260 ◽  
pp. L83 ◽  
Author(s):  
J. K. Cannizzo ◽  
P. Ghosh ◽  
J. C. Wheeler
Keyword(s):  
Accretion Disks ◽  
Dwarf Nova ◽  
Nova Outbursts

scholarly journals EUVE Photometry of SS Cygni: Dwarf Nova Outbursts and Oscillations

1996 ◽  
Vol 152 ◽  
pp. 317-324
Author(s):  
Christopher W. Mauche
Keyword(s):  
Accretion Disk ◽  
Outer Edge ◽  
Light Curves ◽  
Optical Measurements ◽  
Dwarf Nova ◽  
Sinusoidal Oscillations ◽  
Deep Survey ◽  
Different Response ◽  
June July ◽  
Nova Outbursts

I present EUVE Deep Survey photometry and AAVSO optical measurements of the 1993 August and 1994 June/July outbursts of the dwarf nova SS Cygni. The EUV and optical light curves are used to illustrate the different response of the accretion disk to outbursts which begin at the inner edge and propagate outward, and those which begin at the outer edge and propagate inward. Furthermore, we describe the properties of the quasi-coherent 7-9 s sinusoidal oscillations in the EUV flux detected during the rise and plateau stages of these outbursts.

scholarly journals Accretion Disks with Convective Viscosity

1997 ◽  
Vol 163 ◽  
pp. 173-176
Author(s):  
J. Smak
Keyword(s):  
Accretion Disks ◽  
Critical Points ◽  
Effective Temperature ◽  
Surface Density ◽  
Vertical Structure ◽  
Turbulent Viscosity ◽  
Dwarf Novae ◽  
Dwarf Nova ◽  
Nova Outbursts

AbstractModels of vertical structure are calculated for a range of parameters applicable to dwarf novae with two types of viscosity being included: the standard α-disk viscosity and the additional turbulent viscosity in the convective regions. The resulting surface density (∑) υs. effective temperature (Te) relations, compared to those without convective viscosity, show larger separation between the two critical points of the ∑ – Te relation, ∑min and ∑max. This could suggest that with such a modification the dwarf nova outbursts could be reproduced with a single value of α. (Note that in the standard α approach two different α’s are needed on the hot and cool branch of the ∑ – Te relation, with αcool ≈ αhot/4). It turns out, however, that this is not the case. This is due to the fact that additional convective viscosity makes also ∑max smaller than in the pure α case.

scholarly journals Monte Carlo simulations of fast Newtonian and mildly relativistic shock breakout from a stellar wind

2020 ◽  
Vol 499 (4) ◽  
pp. 4961-4971
Author(s):  
Hirotaka Ito ◽  
Amir Levinson ◽  
Ehud Nakar
Keyword(s):  
Monte Carlo ◽  
Monte Carlo Simulations ◽  
Stellar Wind ◽  
Compact Star ◽  
Strong Dependence ◽  
Shock Velocity ◽  
Light Curves ◽  
Ultraviolet Emission ◽  
Relativistic Shocks ◽  
Strong Explosion

ABSTRACT Strong explosion of a compact star surrounded by a thick stellar wind drives a fast (>0.1c) radiation mediated shock (RMS) that propagates in the wind, and ultimately breaks out gradually once photons start escaping from the shock transition layer. In exceptionally strong or aspherical explosions, the shock velocity may even be relativistic. The properties of the breakout signal depend on the dynamics and structure of the shock during the breakout phase. Here we present, for the first time, spectra and light curves of the breakout emission of fast Newtonian and mildly relativistic shocks, that were calculated using self-consistent Monte Carlo simulations of finite RMS with radiative losses. We find a strong dependence of the νFν peak on shock velocity, ranging from ∼1 keV for vs/c = 0.1 to ∼100 keV for vs/c = 0.5, with a shift to lower energies as losses increase. For all cases studied the spectrum below the peak exhibits a nearly flat component (Fν ∼ ν0) that extends down to the break frequency below which absorption becomes important. This implies much bright optical/ultraviolet emission than hitherto expected. The computed light curves show a gradual rise over tens to hundreds of seconds for representative conditions. The application to SN 2008D/XRT 080109 and the detectability limits are also discussed. We predict a detection rate of about one per year with eROSITA.

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