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

TL;DR

This work addresses the challenge of predicting orbitally modulated TeV signatures from spider binaries by enhancing the UMBRELA emission code. It implements a smoother synchrotron kernel via Chebyshev evaluation, introduces flexible injection spectra, extends the photon-energy grid with careful lab-frame interpolation, and incorporates multiple intrabinary shock geometries. The results demonstrate that shock geometry can significantly alter phase- and inclination-dependent SEDs and light curves, with important implications for interpreting Cherenkov-telescope observations and constraining particle energetics and wind properties. The improvements lay the groundwork for more comprehensive predictions, including skymap projections, polarization signals, and broader multi-wavelength tests of spider-binary environments.

Abstract

'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.

Figures (2)

• Figure 1: Sample SEDs at an orbital phase of $\phi_{\rm b}=180^\circ$ for several shock shapes in the case of the redback PSR J2339$-$0533 (with the shock wrapped around the pulsar). We used a millisecond pulsar mass of $M_{\rm NS}= 1.48M_\odot$, radius $R_{\rm NS}= 1.2\times 10^6$ cm, orbital period $P_{\rm b} = 4.63$ h, mass ratio $q = 4.61$, $R_0 = 0.25a$, $B$-field at the shock of $B_{\rm sh}=0.2$ G, companion surface temperature $T_{\rm comp }= 6.9\times 10^3$ K, $M_{\rm pair}= 500$, $\eta_{\rm p,max}= 0.8$, acceleration efficiency $\eta_{\rm acc}= 0.01$, power-law injection spectral index $\Gamma=1.7$, distance $d$ = 0.45 kpc, $\theta_{\rm max}=55^\circ$, maximum bulk flow $\beta\gamma_{\rm max}=7$, inclination angle $i=57^\circ$, and pulsar moment of inertia $I=1.7\times10^{45}$ g cm$^2$.
• Figure 2: Same as Figure \ref{['fig:SED_shocks']}, but for orbital light curves. The top panel is for 1 keV and the bottom panel for 100 GeV.