THE 3-D NUMERICAL SIMULATIONS OF THE SMALL RADIUS ACCRETION DISK FORMATION IN MICROQUASAR CYG X-1. THE CASE OF THE HIGH RESOLUTION GRID IN THE VERTICAL DIRECTION

The present paper is devoted to small radius accretion disk formation in microquasar CYG X-1. The results show that in the case of the strong wind action on a disk the disk radius is about of 20 ÷ 30 per sent of accretor’s Roche lobe radius (it is about of 0.08 of orbital separation) instead of the standard disk radius equal to 80 ÷ 85 per sent of accretor’s Roche lobe radius (the last magnitude is a disk radius equal to 0.22 of orbital separation). In the present paper we try to resolve the problem that is arising in the case of microquasars when we investigate the accretion disk formation in these objects. Indeed, since the microquasars are the massive close binary systems (MCBS) in which the donor is massive stars of O-B class the strong wind is blowing from these stars. In this case the problem is arising: what is the situation in which an accretion disk in microqausars is formed. By the other words, it means what are the processes and the matter that are responsible for an accretion disk formation in microquasars: is this matter from one-point stream only or a disk is formed from the donor’s wind in essential or one is formed from both processes simul- taneously. This question is not idle since one is strong affects on ON-OFF state generations in the precession mechanism model. Since this mechanism is strong depending from the magnitude of the disk centre density and all the parameters affecting on it are very important for calculations. The matter configuration in the vicinity of one-point is one of these parameters that strong affects on ON-OFF state production and disk structure and the central disk density. By this reason we have investigated in the present paper how the disk structure is depending from the wind configuration in the vicinity of one-point.

Unexpected Short-period Variability in Dwarf Carbon Stars from the Zwicky Transient Facility

Dwarf carbon (dC) stars, main-sequence stars showing carbon molecular bands, are enriched by mass transfer from a previous asymptotic-giant-branch (AGB) companion, which has since evolved to a white dwarf. While previous studies have found radial-velocity variations for large samples of dCs, there are still relatively few dC orbital periods in the literature and no dC eclipsing binaries have yet been found. Here, we analyze photometric light curves from DR5 of the Zwicky Transient Facility for a sample of 944 dC stars. From these light curves, we identify 34 periodically variable dC stars. Remarkably, of the periodic dCs, 82% have periods less than two days. We also provide spectroscopic follow-up for four of these periodic systems, measuring radial velocity variations in three of them. Short-period dCs are almost certainly post-common-envelope binary systems, because the periodicity is most likely related to the orbital period, with tidally locked rotation and photometric modulation on the dC either from spots or from ellipsoidal variations. We discuss evolutionary scenarios that these binaries may have taken to accrete sufficient C-rich material while avoiding truncation of the thermally pulsing AGB phase needed to provide such material in the first place. We compare these dCs to common-envelope models to show that dC stars probably cannot accrete enough C-rich material during the common-envelope phase, suggesting another mechanism like wind-Roche lobe overflow is necessary. The periodic dCs in this paper represent a prime sample for spectroscopic follow-up and for comparison to future models of wind-Roche lobe overflow mass transfer.

Orbital evolution of neutron-star–white-dwarf binaries by Roche lobe overflow and gravitational wave radiation

ABSTRACT We investigate the effects of mass transfer and gravitational wave (GW) radiation on the orbital evolution of contact neutron-star–white-dwarf (NS–WD) binaries, and the detectability of these binaries by space GW detectors (e.g. Laser Interferometer Space Antenna, LISA; Taiji; Tianqin). A NS–WD binary becomes contact when the WD component fills its Roche lobe, at which the GW frequency ranges from ∼0.0023 to 0.72 Hz for WD with masses ∼0.05–1.4 M⊙. We find that some high-mass NS–WD binaries may undergo direct coalescence after unstable mass transfer. However, the majority of NS–WD binaries can avoid direct coalescence because mass transfer after contact can lead to a reversal of the orbital evolution. Our model can well interpret the orbital evolution of the ultra-compact X-ray source 4U 1820–30. For a 4-yr observation of 4U 1820–30, the expected signal-to-noise-ratio (SNR) in GW characteristic strain is ∼11.0/10.4/2.2 (LISA/Taiji/Tianqin). The evolution of GW frequencies of NS–WD binaries depends on the WD masses. NS–WD binaries with masses larger than 4U 1820–30 are expected to be detected with significantly larger SNRs. For a $(1.4+0.5) \, {\rm M}_{\odot }$ NS–WD binary close to contact, the expected SNR for a one week observation is ∼27/40/28 (LISA/Taiji/Tianqin). For NS–WD binaries with masses of $(1.4+\gtrsim 1.1) \, {\rm M}_{\odot }$, the significant change of GW frequencies and amplitudes can be measured, and thus it is possible to determine the binary evolution stage. At distances up to the edge of the Galaxy (∼100 kpc), high-mass NS–WD binaries will be still detectable with SNR ≳ 1.

Conditions for accretion disc formation and observability of wind-accreting X-ray binaries

We explore the effect of anisotropic wind driving on the properties of accretion onto black holes (BHs) in close binaries. We specifically focus on line-driven winds, which are common in high-mass X-ray binaries (HMXBs). In close binary systems, the tidal force from the companion star can modify the wind structure in two different ways. One is the reduction of wind terminal velocity due to the weaker effective surface gravity. The other is the reduction in mass flux due to gravity darkening (GD). We incorporate these effects into the so-called CAK theory in a simple way and investigate the wind flow around the accretor on the orbital scale. We find that a focused accretion stream is naturally formed when the Roche lobe filling factor is ${\gtrsim}0.8$ –0.9, analogous to that of wind Roche lobe overflow, but only when the velocity reduction is taken into account. The formation of a stream is necessary to bring in sufficient angular momentum to form an accretion disc around the BH. GD effects reduce the amount of accreted angular momentum, but not enough to prevent the formation of a disc. Based on these results, we expect there to be a discrete step in the observability of HMXBs depending on whether the donor Roche lobe filling factor is below or above ${\sim}$ 0.8–0.9.

X-ray variability of the HMXB Cen X-3: evidence for inhomogeneous accretion flows.

Cen X-3 is a compact high mass X-ray binary likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of two pointed XMM-Newton observations. The first one took place during a normal state of the source, when it has a luminosity LX ∼ 1036 erg s−1. This observation covered orbital phases φ = 0.00 − 0.37, i.e. the egress from the eclipse. The egress lightcurve is highly structured, showing distinctive intervals. We argue that different intervals correspond to the emergence of different emitting structures. The lightcurve analysis enables us to estimate the size of such structures around the compact star, the most conspicuous of which has a size ∼0.3R*, of the order of the Roche lobe radius. During the egress, the equivalent width of Fe emission lines, from highly ionized species, decreases as the X-ray continuum grows. On the other hand, the equivalent width of the Fe Kα line, from near neutral Fe, strengthens. This line is likely formed due to the X-ray illumination of the accretion stream. The second observation was taken when the source was 10 times X-ray brighter and covered the orbital phases φ = 0.36 − 0.80. The X-ray lightcurve in the high state shows dips. These dips are not caused by absorption but can be due to instabilities in the accretion stream. The typical dip duration, of about 1000 s, is much longer than the timescale attributed to the accretion of the clumpy stellar wind of the massive donor star, but is similar to the viscous timescale at the inner radius of the accretion disk.

Black widow formation by pulsar irradiation and sustained magnetic braking

ABSTRACT Black widows are millisecond pulsars with low-mass companions, a few per cent the mass of the sun, on orbits of several hours. These companions are presumably the remnants of main-sequence stars that lost their mass through a combination of Roche lobe overflow and ablation by the host pulsar’s high-energy radiation. While ablation itself is too weak to significantly reduce the mass of the companion star, the ablated wind couples to its magnetic field, removes orbital angular momentum, and thus maintains stable Roche lobe overflow. We use the mesa stellar evolution code, complemented by analytical estimates, to track initially main-sequence companions as they are reduced to a fraction of their original mass by this ablation-driven magnetic braking. We argue that magnetic braking remains effective even for low-mass companions. A key ingredient of our model is that the irradiating luminosity of the pulsar Lirr deposits energy in the companion’s atmosphere and thereby slows down its Kelvin–Helmholtz cooling. We find that the high-energy luminosities measured by Fermi $L_{\rm irr}=0.1\rm {-}3$ L⊙ can explain the span of black widow orbital periods. The same Lirr range reproduces the companions’ night-side temperatures, which cluster around 3000 K, as inferred from optical light curves.