Thermal Limit Cycle Research Articles

OAO/MITSuME Photometry of Dwarf Novae. I. SU Ursae Majoris

Abstract We report on simultaneous $g', R_{\rm c}$, and $I_{\rm c}$ photometry of SU Ursae Majoris during 2011 December–2012 February using OAO/MITSuME. Our photometry revealed that quiescence is divided into three types based on the magnitude and color. Quiescent light curves showed complicated profiles with various amplitudes and time scales. Although no superoutbursts were observed during our run, five normal outbursts occurred with intervals of 11–21 d. The shapes of the normal outbursts were characteristic of the outside-in type. During the rising phase of a normal outburst, the light curve showed periodic modulations with a period of ${\sim}$ 0.048111(354) d, but the origin of this peirod was unclear. We examined daily averaged color–color diagrams, and found that two cycles exist. This implies that the thermal limit cycle in SU UMa is complicated. We newly discovered that $g' - R_{\rm c}$ becomes red about 3 days prior to an outburst. Although the working mechanism on this reddening is unclear, we propose two possibilities: one is that the inner portion of the accretion disk is filled by matter and obscures the central white dwarf; the other is that the stagnation effect works in the outer region of the accretion disk.

TheV−EUV Delay for Dwarf Nova Outbursts: A Case Study for VW Hydri, U Geminorum, and SS Cygni

We present a parameter study using time dependent calculations of the thermal limit cycle model for dwarf nova outbursts. Our goal is to delineate the dependence of the delay between the initial rapid rise of the visual and EUV fluxes during the start of an outburst on model parameters, concentrating on three bright, nearby systems for which complete optical and EUV observations exist - VW Hyi, U Gem, and SS Cyg. For these systems we infer that the outburst must be triggered at ~0.7-0.8 of the outer disk radius to produce delays which are in accord with observations. We also see a stagnation or period of slow warming (first noted by Mineshige) during the early stages of thermal instability, but find it does not influence the V-EUV delay since it precedes the rapid rise in V at the start of the outburst. We find that if one decreases the surface density in the inner disk, interior to the instigation radius, the V-EUV delay can be lengthened by as much as a factor of two. In addition, we find there to be a weak relation between the V-EUV delay and the value of the alpha viscosity parameter in quiescence.

Ionization, Magnetorotational, and Gravitational Instabilities in Thin Accretion Disks around Supermassive Black Holes

We consider the combined role of the thermal ionization, magnetorotational, and gravitational instabilities in thin accretion disks around supermassive black holes. We find that in the portions of the disk unstable to the ionization instability, the gas remains well coupled to the magnetic field even on the cold, neutral branch of the thermal limit cycle. This suggests that the ionization instability is not a significant source of large-amplitude time-dependent accretion in active galactic nuclei (AGNs). We also argue that, for 10-2 M☉ yr-1, the gravitationally unstable and magnetorotationally unstable regions of the accretion disk overlap; for lower accretion rates they may not. Some low-luminosity AGN, e.g., NGC 4258, may thus be in a transient phase in which mass is building up in a nonaccreting gravitationally and magnetorotationally stable dead zone. We comment on possible implications of these findings.

Are Z Camelopardalis–Type Dwarf Novae Brighter at Standstill?

ABSTRACTAverage V‐band brightnesses over multiple outbursting and standstill intervals are computed for the light curves of five Z Cam–type dwarf novae: RX And, Z Cam, SY Cnc, AH Her, and HX Peg. Both CCD photometry from RoboScope and AAVSO visual estimates are used in the analysis, with consistent results from the two databases for the stars in common. In four of the five stars, the standstills are as bright as or brighter than the mean V‐band brightness during outbursting intervals. This is consistent with Z Cam stars being near the upper stability boundary in M for operation of the thermal limit cycle that is thought to be responsible for dwarf nova outbursts. However, the unusual standstills or hesitations observed in SY Cnc are sometimes fainter than the mean brightness during outbursting intervals. From several well‐observed examples we describe the light curves of Z Cam stars as they enter standstill. During this characteristic damped oscillation the amplitude decreases with a timescale approximately equal to the mean outburst recurrence interval, and the period of the oscillation decreases with P≃ -0.4 as standstill is approached. The exits from standstill usually change abruptly into outbursting mode by emerging from standstill into a decline from outburst.

TheS-shaped limit cycle of accretion disk and quasi-periodic variation of BL Lac object on231

If we adopt the analytical approximation of the full intergrations of thin disk, we obtainS-shaped limit cycle around the supermassive black holes in BL Lac objects. On the basis of this, we explain quasi-periodic outburst of the BL Lac object ON+231. Conclusion is drawn that the accretion of black holes is the possible energy source of ON+231 and its variability is caused by the thermal limit cycle with alternating higher and lower accretion rates onto the black hole.

THEORY OF ACTIVE GALACTIC NUCLEI

ABSTRACT The involvement of accretion disks around supermassive black holes in the theory of active galactic nuclei (AGN) is discussed. The physics of thin and thick accretion disks is discussed and the partition between thermal and nonthermal energy production in supermassive disks is seen as uncertain. The thermal limit cycle may operate in supermassive disks (Shields, 1985), with accumulation of gas in the disk for periods of 10 to the 4th to 10 to the 7th years, punctuated by briefer outbursts during which the mass is rapidly transferred to smaller radii. An extended X-ray source in AGN is consistent with observations (Tennant and Mushotsky, 1983), and a large wind mass loss rate exceeding the central accretion rate means that only a fraction of the mass entering the disk will reach the central object; the rest being lost to the wind. Controversy in the relationship between the broad lines and the disk is also discussed.

On the evolution of accretion disc flow in cataclysmic variables - I. The prospect of a limit cycle in dwarf nova systems

We present the basic physics of a mechanism which explains dwarf nova outbursts as accretion disc instabilities. Briefly put, the strong and oppositely directed temperature dependences of opacity in hydrogen ionization regions drive a thermal instability which modulates mass transfer throughout the disc. We also show that the conventional thin disc treatment breaks down in cool regions, where the surface boundary condition dominates the structure. One major purpose of the paper is to modify the conventional treatment, to produce analytical approximations for many of the relationships of interest. When these results are applied to a schematic local treatment of disc flows, the prospect of a thermal limit cycle is revealed. Finally, these results will be particularly useful for time-dependent treatments of global accretion disc flow.

Thermal limit cycle oscillations on the surface of accreting neutron stars X-ray bursters

With the help of a very simple two-zone model we deomonstrate the possiblity of periodic thermal relaxation (limit cycle) oscillations in the helium burning envelope of accreting neutron stars. Physically reasonable model parameters can be chosen which yield agreement with the observed features of X-ray bursts. We suggest that this limit cycle is operative in neutron stars which have an accretion rate in a specific range. For hydrogen burning a similar cycle is possible, but it operates at such high temperatures that an unrealistically large accretion rate would be required.

Thermal relaxation oscillations in a two-zone model

We simulate the behavior of stars in those advanced evolutionary phases in which thermal cycles have been reported. To this end we consider a two-zone stellar model in hydrostatic equilibrium, consisting of a thin energy-generating shell and a buffer zone on top of it with the rest of the star acting as boundary conditions. Under the conditions apposite to thin-shell helium burning we show that this model can exhibit rigorous thermal limit cycle oscillations. By the same token our result implies that the two-time asymptotic method may be used to bridge such otherwise inaccessible evolutionary phases.