On the Expected Orbitally-modulated TeV Signatures of Spider Binaries: The Effect of Intrabinary Shock Geometry

‘Spider’ binary systems - black widow and redback compact binaries differentiated by their companion’s mass and nature - are an important type of pulsar system exhibiting a rich empirical phenomenology, including radio eclipses, optical light curves from a heated companion, as well as non-thermal X-ray and GeV orbital light curves and spectra. Multi-wavelength observations have now resulted in the detection of >~50 of these systems in which a millisecond pulsar heats and ablates its low-mass companion via its intense pulsar wind. Broadband observations have established the presence of relativistic leptons that have been accelerated in the pulsar magnetosphere and near the intrabinary shock, as well as a hot companion, presenting an ideal environment for the creation of orbitally-modulated inverse Compton fluxes that should be within reach of current and future Cherenkov telescopes. We have included an updated synchrotron kernel, different parametric injection spectral shapes, and several intrabinary shock geometries in our emission code to improve our predictions of the expected TeV signatures from spider binaries. Our updated phase-dependent spectral and energy-dependent light curve outputs may aid in constraining particle energetics, wind properties, shock geometry, and system inclination of several spider binaries.

💡 Research Summary

Spider binaries—compact systems comprising a millisecond pulsar and a low‑mass companion (black widows or redbacks)—are prolific laboratories for high‑energy astrophysics. The pulsar wind collides with the companion’s outflow, forming an intrabinary shock that accelerates electrons and positrons to ultra‑relativistic energies. These particles emit synchrotron radiation (SR) from radio to X‑rays and up‑scatter the intense thermal photon field of the heated companion via inverse Compton (IC) processes, potentially producing orbitally‑modulated TeV γ‑rays detectable with current and next‑generation Cherenkov telescopes.

Building on the earlier UMBRELA code (van der Merwe et al. 2020), the authors introduce several substantial upgrades. First, the SR kernel is replaced by the MacLeod (2000) Chebyshev‑polynomial method, eliminating the need for logarithmic table interpolation of the modified Bessel function K5/3 and yielding smoother, more accurate high‑energy cut‑offs. Second, the particle injection spectrum is generalized from a single power‑law to three parametric families: (i) a pure power‑law Q∝E⁻Γ, (ii) a broken power‑law with a smooth transition controlled by a smoothing factor f, and (iii) an exponentially, sub‑exponentially, or super‑exponentially cut‑off power‑law Q∝E⁻Γ exp