B-ducted Heating of Black Widow Companions

Abstract

The companions of evaporating binary pulsars (black widows and related systems) show optical emission suggesting strong heating. In a number of cases large observed temperatures and asymmetries are inconsistent with direct radiative heating for the observed pulsar spindown power and expected distance. Here we describe a heating model in which the pulsar wind sets up an intrabinary shock (IBS) against the companion wind and magnetic field, and a portion of the shock particles duct along this field to the companion magnetic poles. We show that a variety of heating patterns, and improved fits to the observed light curves, can be obtained at expected pulsar distances and luminosities, at the expense of a handful of model parameters. We test this 'IBS-B' model against three well observed binaries and comment on the implications for system masses.

The Synchrotron Emission Pattern of Intrabinary Shocks

“…To model the IBS emission, we estimate the magnetic field strength by assuming a toroidal field outside of the light-cylinder and I ∼ 10 45 g cm 2 together with the orbital parameters, which give B 0 ∼ 20 G. Fitting this lightcurve with our model (for a sin 2 θ * wind) gives i ∼ 75.8 • ± 5.9 • (see Table 2), which is higher than that inferred for the direct heating model (e.g. van Kerkwijk et al 2011, Sanchez & Romani 2017). With such inclination, the mass of neutron star would be ∼ 1.86M , a value much smaller than inferred through optical fit.…”

NuSTAR and Parkes observations of the transitional millisecond pulsar binary XSS J12270–4859 in the rotation-powered state

“…The NuSTAR spectrum does not show a break up to 70 keV and thus we conservatively assume Ex,max at this energy. Thus γ ∼ 2.5 × 10 5 (Ex,max/B2) 1/2 ∼ (5 − 10) × 10 5 for high and low magnetizations, respectively (Rybicki & Lightman 1979). The electron population has a power law energy spectrum with index p related to the X-ray power-law index Γ as p ∼ 2Γ − 1 ∼ 1.3 below the maximum energy, extending at least down to ∼0.1-0.3 keV.…”

Atmospheric Circulation on Black Widow Companions

“…But any effect that redirects heating flux to these zones (e.g. magnetically ducted IBS particles (Sanchez & Romani 2017) or other star-spot inducing effects, see van Staden & Antoniadis 2016) will produce similar RV curve distortions. Accordingly, when showing application to two spiders in §4 we compare WH effects with those from a simple hot spot (HS).…”

“…High precision photometry near orbital minimum can detect the characteristic asymmetry imposed by wind flow, while more detail measurements of blue colors near maximum are sensitive to hot spots. For example, the small M NS ≈ 1.7M allowed by the magnetically ducted heating model of Sanchez & Romani (2017) resulted from a very small particle-heated region needed to allow d well under 2kpc. High precision B or u photometry should reveal such structure, which is not indicated in the present analysis.…”

“…While rough measurements of the optical modulation are broadly consistent with this picture, more detailed light curves often show significant peak asymmetries (Stappers et al 2001) and phase-shifts (Schroeder & Halpern 2014). Several elaborations to the heating model have been introduced to explain such effects, including companion irradiation by an asymmetric intrabinary shock (IBS; Romani & Sanchez 2016) or propagation of shock particles to heat a companion magnetic pole (Sanchez & Romani 2017). Here we discuss another effect, companion super-winds, that can advect the pulsar heat and affect the light curves and phaseresolved spectroscopy.…”