Black widow evolution: magnetic braking by an ablated wind

Abstract

Black widows are close binary systems in which a millisecond pulsar is orbited by a companion, a few per cent the mass of the sun. It has been suggested that the pulsar’s rotationally powered γ-ray luminosity gradually evaporates the companion, eventually leaving behind an isolated millisecond pulsar. The evaporation efficiency is determined by the temperature Tch ∝ F2/3 to which the outflow is heated by the flux F on a dynamical time-scale. Evaporation is most efficient for companions that fill their Roche lobes. In this case, the outflow is dominated by a cap around the L1 point with an angle θg ∼ (Tch/Tg)1/2, and the evaporation time is tevap = 0.46(Tch/Tg)−2 Gyr, where Tg > Tch is the companion’s virial temperature. We apply our model to the observed black widow population, which has increased substantially over the last decade, considering each system’s orbital period, companion mass, and pulsar spin-down power. While the original black widow (PSR B1957+20) evaporates its companion on a few Gyr time-scale, direct evaporation on its own is too weak to explain the overall population. We propose instead that the evaporative wind couples to the companion’s magnetic field, removes angular momentum from the binary, and maintains stable Roche lobe overflow. While a stronger wind carries more mass, it also reduces the Alfvén radius, making this indirect magnetic braking mechanism less dependent on the flux $t_{\rm mag}\propto t_{\rm evap}^{1/3}$. This reduces the scatter in evolution times of observed systems, thus better explaining the combined black widow and isolated millisecond pulsar populations.

NICER Discovery of Millisecond X-Ray Pulsations and an Ultracompact Orbit in IGR J17494-3030

“…As an example, we consider the case of gamma-ray ablation. If we assume the gamma-ray luminosity is 10% of the spin-down luminosity ( 3×10 35 erg s −1 based on our long-termν measurement) as typically seen in black widow systems (Abdo et al 2013), this would imply a companion mass loss rate of ∼ 10 −11 M /yr (Ginzburg & Quataert 2020). For a source distance of 8 kpc and assuming that gravitational wave losses dominate in Equation 4, this implies β ≈ 0.04 and α ≈ 0.4, suggesting that the mass ejection occurs somewhere between the pulsar and the L 1 point (α ≈ 0.8).…”

“…These are motivated by the study of rotation-powered radio millisecond pulsars in detached (non-accreting) binaries: the so-called "black widow" (M c 0.05M ) and "redback" (M c 0.1M ) systems, where M c is the companion mass (see, e.g., Romani et al 2016, and references therein). One possibility is black-widow-like ablation of the companion, driven by rotation-powered gamma-ray emission from the pulsar (Ginzburg & Quataert 2020). Such ablation could also be driven by particle heating via the rotation-powered pulsar wind (see Harding & Gaisser 1990, and references therein).…”

“…For a source distance of 8 kpc and assuming that gravitational wave losses dominate in Equation 4, this implies β ≈ 0.04 and α ≈ 0.4, suggesting that the mass ejection occurs somewhere between the pulsar and the L 1 point (α ≈ 0.8). However, Ginzburg & Quataert (2020) argue that magnetic braking of the donor (through magnetic coupling to the ablated wind) likely dominates gravitational radiation as an angular momentum sink in black widow systems. If so, then that could both decrease β and increase α even further in our case.…”

Compact Object Modeling in the Globular Cluster 47 Tucanae

“…Thus it is reasonable to assume that a captured, puffedup MS star can transfer mass to a NS and spin it up to a MSP. However, CMC currently lacks detailed treatments of MS stars in tidal capture interactions, and COSMIC does not include the latest pulsar physics such as pulsar irradiation (e.g., Chen et al 2013;Ginzburg & Quataert 2020, and references therein). We will consider these details and study model MSPs and tidal-captured NS-MS star binaries that overlap with the observed pulsar binaries in future works.…”

Long-term coherent timing of the accreting millisecond pulsar IGR J17062-6143

“…Spinorbit coupling relies on the presence of a variable mass quadrupole in the companion star (Applegate & Shaham 1994;Richman et al 1994), however, a low-mass companion likely does not have a sufficient energy budget for this mechanism (Brinkworth et al 2006), which is only exacerbated for IGR J17062, which has the smallest known mass function among stellar binaries (Strohmayer et al 2018). Enhanced magnetic braking might play a role (Justham et al 2006;Ginzburg & Quataert 2020), but only if the companion star can sustain a ∼ 1 kG magnetic field and is also losing mass through a stellar wind. In the following, we therefore limit our discussion to just the mass loss model.…”