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Register a new userExploring the tilted accretion disc of AQ Men with TESS
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AQ Men is a nova-like variable which is presumed to have a tilted, precessing accretion disc. Grazing eclipses in this system have been speculated to be useful in exploring the geometry of its accretion disc. In this work we analysed TESS observations of AQ Men, which provide the best light curve of this object thus far. We show that the depths of the eclipses are changing with the orientation of the accretion disc, which means that they can serve as a direct test of the tilted accretion disc models. The precession period of the accretion disc is increasing during the TESS observations. However, it is still shorter than the period determined in the previous studies. The amplitude of the variability related to the precession of the accretion disc varies, and so does the shape of this variability. Moreover, we have detected a positive superhump that was previously unseen in AQ Men. Interestingly, the positive superhump has a strongly non-sinusoidal shape, which is not expected for a nova-like variable.
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FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low-state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low-state, the mass-transfer rate is in principle too low for the disc to be fully ionized and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of 2 magnitudes in the optical band without showing outbursts. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). We show that although it is marginally possible for the accretion disc in the low-state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 10$^{19}$ gs$^{-1}$ or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularization radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still needs investigations.
V341 Ara was recently recognised as one of the closest (d ~ 150 pc) and brightest (V~ 10) nova-like cataclysmic variables. This unique system is surrounded by a bright emission nebula, likely to be the remnant of a recent nova eruption. Embedded within this nebula is a prominent bow-shock, where the systems accretion disc wind runs into its own nova shell. In order to establish its fundamental properties, we present the first comprehensive multi-wavelength study of the system. Long-term photometry reveals quasi-periodic, super-orbital variations with a characteristic time-scale of 10-16 days and typical amplitude of ~1 mag. High-cadence photometry from TESS reveals for the first time both the orbital period and a negative superhump period. The latter is usually interpreted as the signature of a tilted accretion disc. We propose a recently developed disc instability model as a plausible explanation for the photometric behaviour. In our spectroscopic data, we clearly detect anti-phased absorption and emission line components. Their radial velocities suggest a high mass ratio, which in turn implies an unusually low white dwarf mass. We also constrain the wind mass-loss rate of the system from the spatially resolved [O iii] emission produced in the bow-shock; this can be used to test and calibrate accretion disc wind models. We suggest a possible association between V341 Ara and a guest star mentioned in Chinese historical records in AD1240. If this marks the date of the systems nova eruption, V341 Ara would be the oldest recovered nova of its class and an excellent laboratory for testing nova theory.
We report the analysis of time-series of infrared $JHK_s$ photometry of the dwarf nova V2051 Oph in quiescence with eclipse mapping techniques to investigate structures and the spectrum of its accretion disc. The light curves after removal of the ellipsoidal variations caused by the mass-donor star show a double-wave modulation signalling the presence of two asymmetric light sources in the accretion disc. Eclipse maps reveal two spiral arms on top of the disc emission, one at $R_1= 0.28pm 0.02 ,R_mathrm{L1}$ and the other at $R_2= 0.42pm 0.02 ,R_mathrm{L1}$ (where $R_mathrm{L1}$ is the distance from disc centre to the inner Lagrangian point), which are seen face-on at binary phases consistent with the maxima of the double-wave modulation. The wide open angle inferred for the spiral arms ($theta_s= 21^o pm 4^o$) suggests the quiescent accretion disc of V2051 Oph has high viscosity. The accretion disc is hot and optically thin in its inner regions ($T_mathrm{gas}sim 10-12 times 10^3,K$ and surface densities $sim 10^{-3}-10^{-2},g,cm^{-2}$), and becomes cool and opaque in its outer regions.
IW And stars are a subgroup of dwarf novae characterized by repetitive light variations of the intermediate-brightness state with oscillations, which is 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 usual dwarf-nova outbursts, Z Cam-type standstills, and so on, besides the typical IW And-type variations mentioned above. Following the 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 interval sometimes 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.
Luminous active galactic nuclei (AGN) and X-Ray binaries (XRBs) tend to be surrounded by geometrically thin, radiatively cooled accretion discs. According to both theory and observations, these are -- in many cases -- highly misaligned with the black hole spin axis. In this work we present the first general relativistic magnetohydrodynamic simulations of very thin ($h/r sim 0.015-0.05$) accretion discs around rapidly spinning ($a sim 0.9$) black holes and tilted by 45-65 degrees. We show that the inner regions of the discs with $h/r lesssim 0.03$ align with the black hole equator, though at smaller radii than predicted by theoretical work. The inner aligned and outer misaligned disc regions are separated by a sharp break in tilt angle accompanied by a sharp drop in density. We find that frame-dragging by the spinning black hole overpowers the disc viscosity, which is self-consistently produced by magnetized turbulence, tearing the disc apart and forming a rapidly precessing inner sub-disc surrounded by a slowly precessing outer sub-disc. We find that at all tilt values the system produces a pair of relativistic jets. At small distances the jets precess rapidly together with the inner sub-disc, whereas at large distances they partially align with the outer sub-disc and precess more slowly. If the tearing radius can be modeled accurately in future work, emission model independent measurements of black hole spin based on precession-driven quasi-periodic oscillations may become possible.
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