What if i ask you, how X-ray binary system lose its angular momentum and change its orbital period? Some of you may answer, through mass loss by the system, magnetic braking or either through gravitational waves. But, recent study done by Avakyan and colleagues suggested a new possible mechanism: mass loss in the form of a wind from an accretion disk. Their study recently appeared in Arxiv.
In this scenario, the matter ejected by the wind from the accretion disc, or escaping through the second Lagrange point (which is located behind less massive component in case of X-ray binary), can form a toroidal/circumbinary disc or ring of matter around the binary system. Due to the viscosity processes, the circumbinary disc can drain the moment of tidal forces acting on it, and thereby, reduce the angular momentum of the binary system (therefore decreasing its orbital period).
“The characteristic viscous time of evolution of a circumbinary disc significantly exceeds the viscous time of the hot disc around a compact object, which allows it to remove the angular momentum on a longer time scale.”
You may have heard of XTE J1118+480 (GH12), if not let me tell you, it is the first low mass X-ray binary (LMXB)¹ in which we observed, decrease in orbital period. Later, we have found a decrease in the period for A0620−00 and GRS 1124−68 (also called GH14 and GH17, respectively). For these systems, Avakyan and colleagues suggested that, the decrease in the period in all three systems is due to the gravitational radiation (CP19) which is 2 − 3 orders of magnitude less than the observed rate, and estimates for the magnetic braking are less by an order of magnitude, even with very favorable parameters (see Table 2 below). Of all the mechanisms they have discussed like impact of accretion disc wind, flow of matter into accretion disc and outflow from the Lagrange point L2, they found that, only the outflow of matter from point second Lagrange point (L2) with an excessively high rate could explain the observed rates (see Fig. 2, 3, and 4 below).
“It is necessary to involve either extremely strong magnetic braking, or the drain of the orbital angular momentum by a ring of matter surrounding a binary system, in order to explain the decreasing orbital period of LMXB systems with binary black holes.”— concluded authors of the study
1) In low mass X-ray binary systems, matter flows from the optical star to the compact component, accompanied by the formation of a hot accretion disc, which produces X-ray radiation.
A. L. Avakyan, G. V. Lipunova, K. L. Malanchev, N.I. Shakura, “Change in the orbital period of a binary system due to an outburst in a windy accretion disc”, Arxiv, pp. 1-10, 2021. https://arxiv.org/abs/2105.11974
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