arXiv subjects
Publications and source records attributed to Ulrich Kolb.
Relativistic jets can extract mass-energy from a black hole. In semi-detached black hole binaries the jet ejection process constitutes a `consequential angular momentum loss' (CAML) process. The effect of this jet-induced CAML is to lower the transfer rate below the value set by systemic driving and to stabilize otherwise unstable systems. Implications of jet-induced CAML for GRO J1655-40 are discussed.
We consider the formation of low-mass X-ray binaries containing accreting neutron stars via the helium-star supernova channel. The predicted relative number of short-period transients provides a sensitive test of the input physics in this process. We investigate the effect of varying mean kick velocities, orbital angular momentum loss efficiencies, and common envelope ejection efficiencies on the subpopulation of short-period systems, both transient and persistent. Guided by the thermal-viscous disk instability model in irradiation-dominated disks, we posit that short-period transients have donors close to the end of core-hydrogen burning. We find that with increasing mean kick velocity the overall short-period fraction, s, grows, while the fraction, r, of systems with evolved donors among short-period systems drops. This effect, acting in opposite directions on these two fractions, allows us to constrain models of LMXB formation through comparison with observational estimates of s and r. Without fine tuning or extreme assumptions about evolutionary parameters, consistency between models and current observations is achieved for a regime of intermediate average kick magnitudes of about 100-200 km/s, provided that (i) orbital braking for systems with donor masses in the range 1-1.5 solar masses is weak, i.e., much less effective than a simple extrapolation of standard magnetic braking beyond 1.0 solar mass would suggest, and (ii) the efficiency of common envelope ejection is low.
We investigate the effects of discontinuous mass loss in recurrent outburst events on the long-term evolution of cataclysmic variables (CVs). Similarly we consider the effects of frictional angular momentum loss (FAML), i.e. interaction of the expanding nova envelope with the secondary. Numerical calculations of CV evolution over a wide range of parameters demon- strate the equivalence of a discontinuous sequence of nova cycles and the corresponding mean evolution (replacing envelope ejection by a continuous wind), even close to mass transfer instability. A formal stability analysis of discontinuous mass transfer confirms this, independent of details of the FAML model. FAML is a consequential angular momentum loss which amplifies the mass transfer rate driven by systemic angular momentum losses such as magnetic braking. We show that for a given v_exp and white dwarf mass the amplification increases with secondary mass and is significant only close to the largest secondary mass consistent with mass transfer stability. The amplification factor is independent of the envelope mass ejected during the outburst, whereas the mass transfer amplitude induced by individual nova outbursts is proportional to it.
We apply the disc instability model for soft X-ray transients to identify system parameters along evolutionary sequences of black hole X-ray binaries (BHXBs) that are consistent with transient behaviour. In particular, we focus on the hitherto neglected group of BHXBs with intermediate-mass giant donor stars. These spend a significant fraction of their X-ray active phase crossing the Hertzsprung gap. Three Case B binary sequences with a black hole accretor and 2.5 M_sun initial donor mass are presented in detail. We formulate rules which summarize the behaviour of these sequences and provide an approximate description for Case B mass transfer in intermediate-mass BHXBs. Chiefly, the timescale of the overall radius expansion is given by the initial donor mass, while the surface appearance is determined by the current donor mass. With these rules we obtain a general overview of transient and persistent behaviour of all intermediate-mass BHXBs by just considering single star sequences of different mass. We find that although systems in the process of crossing the Hertzsprung gap are in general persistently bright, with Eddington- or super-Eddington transfer rates, there is a narrow instability strip where transient behaviour is possible. This strip extends over a secondary mass range 2.0-3.5 M_sun. GRO J1655-40 might be such a system. We predict that there are no BHXB transients with donors more massive than 3.5 M_sun, and no neutron star transients in the Hertzsprung gap.
I discuss population synthesis methods in the context of low-mass compact binaries. Two examples, both constraining the largely unknown strength of orbital angular momentum losses, illustrate the power and problems of such studies. For CVs, the ``standard'' disrupted magnetic braking model predicts that systems below the period gap are on average older than systems above the gap. The corresponding difference in the space velocity dispersion is testable by observations, independent of brightness-dependent selection effects. For LMXBs, the fraction of transients among short-period neutron star systems turns out to be an important diagnostic quantity constraining not only the angular momentum loss rate but also the kick velocity imparted to the neutron star at birth and the common envelope efficiency. Small kicks (less than 100 km/s), low efficiencies and weak magnetic braking are strongly favoured.
We consider the evolutionary state of the black-hole X-ray source GRO J1655-40 in the context of its transient nature. Recent optical observations show that the donor in GRO J1655-40 is an intermediate-mass star (~ 2.3 solar masses) crossing the Hertzsprung gap. Usually in such systems the donor's radius expansion drives a near-Eddington or super-Eddington mass transfer rate which would sustain a persistently bright accretion disk. We show that GRO J1655-40 is close to a narrow parameter range where disk instabilities can occur. This range corresponds to a short-lived evolutionary stage where the secondary's radius expansion stalls (or reverses), with a correspondingly lower mass transfer rate. If GRO J1655-40 belongs to this class of transients the predicted accretion rates imply large populations of luminous persistent and transient sources, which are not seen in X-rays. The transient nature of the related system GRS 1915+105 may reflect spectral variations in a bolometrically persistent source rather than a genuine luminosity increase.
We consider transient behavior in low-mass X-ray binaries. In short-period neutron-star systems (orbital period less than ~ 1d) irradiation of the accretion disk by the central source suppresses this except at very low mass transfer rates. Formation constraints however imply that a significant fraction of these neutron star systems have nuclear-evolved main-sequence secondaries and thus mass transfer rates low enough to be transient. But most short-period low-mass black-hole systems will form with unevolved main-sequence companions and have much higher mass transfer rates. The fact that essentially all of them are nevertheless transient shows that irradiation is weaker, as a direct consequence of the fundamental black-hole property - the lack of a hard stellar surface.