Novel modelling of ultracompact X-ray binary evolution – stable mass transfer from white dwarfs to neutron stars

Abstract

Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually "merge" due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultra-compact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-LMXB systems, and evolved these to detached MSP+WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 minutes. We followed the subsequent widening of the systems until the donor stars become planets with a mass of ∼ 0.005 M ⊙ after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate vs. orbital period.

A feasibility study on the photometric detection of quiescent black hole X-ray binaries

“…A combination of hydrogen deficiency, together with the presence of He i emission lines, places these systems in a vertical stream at negative EWs. The region will also contain ultra-compact X-ray binaries with accreting NSs (Sengar et al 2017). Ultra-compact binaries are very faint and rare, with a space density comparable to that of BHXBs, and thus only a few tens are currently known (Carter et al 2013).…”

Effects of Irradiation on the Evolution of Ultracompact X-Ray Binaries

“…They found that the UCXBs with orbital periods longer than about 50 minutes are much brighter than theoretically estimated values in van Haaften et al (2012a). Very recently, Sengar et al (2017) obtained similar results. A possible explanation is that these pulsars irradiate their low-mass companion stars (van Haaften et al 2012b).…”

“…Sengar et al (2017) also computed this threshold. It is about 22 minutes for the helium accretion disk under X-ray heating.…”

“…van Haaften et al (2012b) suggested that the WDs in these UCXBs are heated by the irradiation from X-ray emitted by accreting NS. Sengar et al (2017) considered that these systems may be LMXBs which are evolving into UCXBs. However, these LMXBs have masstransfer rates lower than these of UCXBs (See Figure 2 in Sengar et al (2017)).…”

“…Sengar et al (2017) considered that these systems may be LMXBs which are evolving into UCXBs. However, these LMXBs have masstransfer rates lower than these of UCXBs (See Figure 2 in Sengar et al (2017)). …”