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1. Particle acceleration and non-thermal emission at the intrabinary shock of spider pulsars – I. Non-radiative simulations

   https://doi.org/10.1093/mnras/stae2278  

   Cortés, Jorge; Sironi, Lorenzo (October 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Spider pulsars are compact binary systems composed of a millisecond pulsar and a low-mass companion. Their X-ray emission – modulated on the orbital period – is interpreted as synchrotron radiation from high-energy electrons accelerated at the intrabinary shock. We perform global two-dimensional particle-in-cell simulations of the intrabinary shock, assuming that the shock wraps around the companion star. When the pulsar spin axis is nearly aligned with the orbital angular momentum, we find that the magnetic energy of the relativistic pulsar wind – composed of magnetic stripes of alternating field polarity – efficiently converts to particle energy at the intrabinary shock, via shock-driven reconnection. The highest energy particles accelerated by reconnection can stream ahead of the shock and be further accelerated by the upstream motional electric field. In the downstream, further energization is governed by stochastic interactions with the plasmoids/magnetic islands generated by reconnection. We also extend our earlier work by performing simulations that have a larger (and more realistic) companion size and a more strongly magnetized pulsar wind. We confirm that our first-principles synchrotron spectra and light curves are in good agreement with X-ray observations. 

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2. Particle acceleration and nonthermal emission at the intrabinary shock of spider pulsars. II: Fast-cooling simulations

   https://doi.org/10.1093/mnras/staf284  

   Cortés, Jorge; Sironi, Lorenzo (February 2025, Monthly Notices of the Royal Astronomical Society)

   Abstract Spider pulsars are binary systems composed of a millisecond pulsar and a low-mass companion. Their X-ray emission, varying with orbital phase, originates from synchrotron radiation produced by high-energy electrons accelerated at the intrabinary shock. For fast-spinning pulsars in compact binary systems, the intrabinary shock emission occurs in the fast cooling regime. Using global two-dimensional particle-in-cell simulations, we investigate the effect of synchrotron losses on the shock structure and the resulting emission, assuming that the pulsar wind is stronger than the companion wind (so, the shock wraps around the companion), as expected in black widows. We find that the shock opening angle gets narrower for greater losses; the lightcurve shows a more prominent double-peaked signature (with two peaks just before and after the pulsar eclipse) for stronger cooling; below the cooling frequency, the synchrotron spectrum displays a hard power-law range, consistent with X-ray observations. 

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3. Multiwavelength Observation Campaign of the TeV Gamma-Ray Binary HESS J0632 + 057 with NuSTAR, VERITAS, MDM, and Swift

   https://doi.org/10.3847/1538-4357/ac2c6a  

   Tokayer, Y. M.; An, H.; Halpern, J. P.; Kim, J.; Mori, K.; Hailey, C. J.; Adams, C. B.; Benbow, W.; Brill, A.; Buckley, J. H.; et al (December 2021, The Astrophysical Journal)

   Abstract HESS J0632+057 belongs to a rare subclass of binary systems that emit gamma rays above 100 GeV. It stands out for its distinctive high-energy light curve, which features a sharp “primary” peak and broader “secondary” peak. We present the results of contemporaneous observations by NuSTAR and VERITAS during the secondary peak between 2019 December and 2020 February, when the orbital phase ( ϕ ) is between 0.55 and 0.75. NuSTAR detected X-ray spectral evolution, while VERITAS detected TeV emission. We fit a leptonic wind-collision model to the multiwavelength spectra data obtained over the four NuSTAR and VERITAS observations, constraining the pulsar spin-down luminosity and the magnetization parameter at the shock. Despite long-term monitoring of the source from 2019 October to 2020 March, the MDM observatory did not detect significant variation in H α and H β line equivalent widths, an expected signature of Be-disk interaction with the pulsar. Furthermore, fitting folded Swift-XRT light-curve data with an intrabinary shock model constrained the orbital parameters, suggesting two orbital phases (at ϕ D = 0.13 and 0.37), where the pulsar crosses the Be-disk, as well as phases for the periastron ( ϕ 0 = 0.30) and inferior conjunction ( ϕ IFC = 0.75). The broadband X-ray spectra with Swift-XRT and NuSTAR allowed us to measure a higher neutral hydrogen column density at one of the predicted disk-passing phases. 

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4. A New Flaring Black Widow Candidate and Demographics of Black Widow Millisecond Pulsars in the Galactic Field

   https://doi.org/10.3847/1538-4357/aca2ac  

   Swihart, Samuel J.; Strader, Jay; Chomiuk, Laura; Aydi, Elias; Sokolovsky, Kirill V.; Ray, Paul S.; Kerr, Matthew (December 2022, The Astrophysical Journal)

   Abstract We present the discovery of a new optical/X-ray source likely associated with the Fermi γ -ray source 4FGL J1408.6–2917. Its high-amplitude periodic optical variability, large spectroscopic radial-velocity semiamplitude, evidence for optical emission lines and flaring, and X-ray properties together imply the source is probably a new black widow millisecond pulsar binary. We compile the properties of the 41 confirmed and suspected field black widows, finding a median secondary mass of 0.027 ± 0.003 M ⊙ . Considered jointly with the more massive redback millisecond pulsar binaries, we find that the “spider” companion mass distribution remains strongly bimodal, with essentially zero systems having companion masses of between ∼0.07 and 0.1 M ⊙ . X-ray emission from black widows is typically softer and less luminous than in redbacks, consistent with less efficient particle acceleration in the intrabinary shock in black widows, excepting a few systems that appear to have more efficient “redback-like” shocks. Together black widows and redbacks dominate the census of the fastest spinning field millisecond pulsars in binaries with known companion types, making up ≳80% of systems with P spin < 2 ms. Similar to redbacks, the neutron star masses in black widows appear on average significantly larger than the canonical 1.4 M ⊙ , and many of the highest-mass neutron stars claimed to date are black widows with M NS ≳ 2.1 M ⊙ . Both of these observations are consistent with an evolutionary picture where spider millisecond pulsars emerge from short orbital period progenitors that had a lengthy period of mass transfer initiated while the companion was on the main sequence, leading to fast spins and high masses. 

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5. Shock-powered radio precursors of neutron star mergers from accelerating relativistic binary winds

   https://doi.org/10.1093/mnras/staa3794  

   Sridhar, Navin; Zrake, Jonathan; Metzger, Brian D; Sironi, Lorenzo; Giannios, Dimitrios (January 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT During the final stages of a compact object merger, if at least one of the binary components is a magnetized neutron star (NS), then its orbital motion substantially expands the NS’s open magnetic flux – and hence increases its wind luminosity – relative to that of an isolated pulsar. As the binary orbit shrinks due to gravitational radiation, the power and speed of this binary-induced inspiral wind may (depending on pair loading) secularly increase, leading to self-interaction and internal shocks in the outflow beyond the binary orbit. The magnetized forward shock can generate coherent radio emission via the synchrotron maser process, resulting in an observable radio precursor to binary NS merger. We perform 1D relativistic hydrodynamical simulations of shock interaction in the accelerating binary NS wind, assuming that the inspiral wind efficiently converts its Poynting flux into bulk kinetic energy prior to the shock radius. This is combined with the shock maser spectrum from particle-in-cell simulations, to generate synthetic radio light curves. The precursor burst with a fluence of ∼1 Jy·ms at ∼GHz frequencies lasts ∼1–500 ms following the merger for a source at ∼3 Gpc (Bd/1012 G)8/9, where Bd is the dipole field strength of the more strongly magnetized star. Given an outflow geometry concentrated along the binary equatorial plane, the signal may be preferentially observable for high-inclination systems, that is, those least likely to produce a detectable gamma-ray burst. 

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