Global Kinetic Modeling of the Intrabinary Shock in Spider Pulsars

Abstract

Spider pulsars are compact binary systems composed of a millisecond pulsar and a low-mass companion. The relativistic magnetically dominated pulsar wind impacts onto the companion, ablating it and slowly consuming its atmosphere. The interaction forms an intrabinary shock, a proposed site of particle acceleration. We perform global fully kinetic particle-in-cell simulations of the intrabinary shock, assuming that the pulsar wind consists of plane-parallel stripes of alternating polarity and that the shock wraps around the companion. We find that particles are efficiently accelerated via shock-driven reconnection. We extract first-principles synchrotron spectra and light curves, which are in good agreement with X-ray observations: (1) the synchrotron spectrum is nearly flat, F ν ∝ const; (2) when the pulsar spin axis is nearly aligned with the orbital angular momentum, the light curve displays two peaks, just before and after the pulsar eclipse (pulsar superior conjunction), separated in phase by ∼0.8 rad; (3) the peak flux exceeds the one at the inferior conjunction by a factor of 10.

Luminous Optical and X-Ray Flaring of the Putative Redback Millisecond Pulsar 1FGL J0523.5–2529

“…Of principal interest is whether the flares are ultimately powered by the rotational energy of the pulsar, and if they require the release of magnetic energy, e.g., via reconnection of striped magnetic field compressed in the shocked pulsar wind (Sironi & Spitkovsky 2011;Cortés & Sironi 2022). Alternatively, the reservoir of magnetic energy in the companion may be greater, renewable, and capable of channeling the pulsar wind (Sanchez & Romani 2017).…”

“…The winds are sometimes seen in optical emission lines (Halpern et al 2017b;Strader et al 2019), while nonthermal emission, likely due to shocks from the collision of the pulsar wind and the stellar wind, is ubiquitous in X-rays (Bogdanov et al 2005(Bogdanov et al , 2011(Bogdanov et al , 2014Al Noori et al 2018). When good photon statistics are available, the X-ray flux is often seen to be modulated around the orbit, which has been interpreted in terms of the geometry of the intrabinary shock and relativistic beaming of synchrotron radiation by the emitting particles (Romani & Sanchez 2016;Wadiasingh et al 2017;Cortés & Sironi 2022).…”

Green Bank Telescope Discovery of the Redback Binary Millisecond Pulsar PSR J0212+5321

“…Such X-ray characteristics can be attributed to an intrabinary shock formed at the interface of the winds from the two stars. These data can provide important input for validating and improving recently developed semianalytic models of the wind interaction in redbacks and black widows (Romani & Sanchez 2016;Wadiasingh et al 2017;Kandel et al 2019;van der Merwe et al 2020;Cortés & Sironi 2022). The implied X-ray luminosity is one of the highest recorded among redbacks in the pulsar state and black widow MSPs in the Galactic field (Linares 2014;Linares et al 2017), followed by tMSPs PSR J1023+0038 and XSS J12270 −4859 (Archibald et al 2009;Bogdanov et al 2011Bogdanov et al , 2014bBogdanov et al , 2021Roy et al 2014).…”

Polarization of Intrabinary Shock Emission in Spider Pulsars

“…At the shock, magnetic reconnection likely occurs, accelerating shocked particles; a residual magnetic field remains in the IBS after the stripes are annihilated (Lyubarsky 2003;Pétri & Lyubarsky 2007;Sironi & Spitkovsky 2011). The very hard spectra observed in spiders (i.e., Γ x ≈ 1; Bogdanov et al 2014;Kandel et al 2021;Perez et al 2023) support this picture (Cortés & Sironi 2022;Zhang et al 2023). The toroidal field remaining after the annihilation of the pulsar wind represents a promising candidate for the magnetic field geometry, but postshock dynamics such as turbulence may produce differently ordered fields (e.g., Goldsmith & Pittard 2016).…”