Institute of Astronomy

On the Velocities that BH- and NS-XRBs Receive at Formation

SpeakerTalk DateTalk Series
Serena Repetto28 July 2016Binary Stars Talks


The similarity between the Galactic distribution of black hole (BH) and neutron star (NS) X-ray binaries (XRBs) led Jonker \& Nelemans 2004 to suggest that BHs could potentially receive high natal kick (NK) at birth, similarly to NSs. Motivated by this idea, we investigate whether different assumptions on compact object formation (such as a different NK distribution and/or a different amount of mass ejected in the supernova, SN) have an imprint on the Galactic distribution of BH- and NS-XRBs, and we quantify these effects. We build synthetic populations of BH- and NS-XRBs and we model their binary evolution and their kinematics in the Galaxy. We find that the root mean square of the height above the Galactic plane of these binaries is a powerful proxy to discriminate among different formation scenarios, in particular when it comes to quantify how similar are the conditions at formation between NSs and BHs. Furthermore, we conclude that binary evolution following the BH/NS formation does not significantly affect the Galactic distributions of the binaries. When comparing our population synthesis results with the observed BH-XRBs, we find that a population model in which at least some BHs receive a high (or relatively high) NK fits the observed BH-XRBs best. This is an agreement with the results of Repetto et al. 2012, while now making use of the most recent measurements of BH-XRB distances and of a detailed modeling of the binary evolution of the sources. In the second part of the talk, we study the {\emph{minimal}} velocities of BH-XRBs rather than their {\emph{expected}} velocities. We analyze the simple method we previously used to estimate the minimal peculiar velocity of an individual BH-XRB at birth (Repetto et al. 2015), which was recently challenged by Mandel 2016. We find that this method may be less reliable in the bulge of the Galaxy and for certain models for the Galactic potential, but that our estimate is excellent for most of the BH-XRBs.