Very high-energy gamma-ray and neutrino emission from hadronic interaction in compact binary millisecond pulsars

TL;DR

This work investigates hadronic emission from spider pulsar binaries by examining two proton-acceleration pathways—the pulsar wind (PW) and the intrabinary shock (IBS)—and two interaction sites (CW and CS). It computes the resulting gamma-ray and neutrino fluxes for $0.1-10^{3}$ TeV, assessing detectability with CTA, LHAASO, and the future TRIDENT detector, and then builds a synthetic Galactic spider population to estimate the cumulative neutrino contribution to IceCube. The results indicate that detectable gamma-ray signals require favorable combinations of spin-down power, companion magnetic field, and low pair multiplicity, while neutrino signals are generally below current detector thresholds except possibly for TRIDENT in PW-CS scenarios. Across the population, spiders contribute negligibly to the Galactic diffuse neutrino flux, underscoring the challenge of uncovering hadronic spider emission with present neutrino observatories.

Abstract

Blackwidow and redback systems are millisecond pulsars in compact orbits with ultra-light and low-mass companions, respectively, collectively known as ``spider pulsars". In such systems, an intrabinary shock can form between the pulsar and the companion winds, serving as a site for particle acceleration and associated non-thermal emission. Assuming that protons can be extracted from the neutron star surface and accelerated at the intrabinary shock and/or within the pulsar wind, we model the very high-energy gamma-ray and neutrino emissions ($0.1-10^3$~TeV) produced through interactions with the companion wind and the companion star. We first calculate the high-energy emissions using an optimistic combination of parameters to maximize the gamma-ray and neutrino fluxes. We find that, for energetic spider pulsars with a spin-down power $\gtrsim 10^{35}\rm erg\, s^{-1}$ and a magnetic field of $\sim 10^{3}\, \rm G$ in the companion region, the gamma-ray emission could be detectable as point sources by CTA and LHAASO, while the neutrino emission could be detectable by the future TRIDENT detector. Finally, we build a synthetic population of these systems, compute the cumulative neutrino flux expected from spider pulsars, and compare it with the Galactic neutrino diffuse emission measured by IceCube. We find that, under realistic assumptions on the fraction of the spin-down power converted into protons, the contribution of spiders to the diffuse Galactic neutrino flux is negligible.

Figures (10)

• Figure 1: Sketch of a spider system. The millisecond pulsar is represented as a blue sphere on the left (not to scale), while the companion star is shown as a yellow sphere on the right. The pulsar (companion) wind is indicated with blue (yellow) arrows. The interaction between the winds forms the IBS, depicted as a blue semicircle. Panel (a): The green hollow cylinder around the companion represents the interaction region of protons with the CW. Panel (b): The green cylinder around the companion represents the interaction region of protons with the CS.
• Figure 2: Timescales for the interaction of protons with the CW (panel (a)) and with the CS (panel (b)). The diffusion, pp, and ballistic timescales are displayed with colored-shadowed bands, and dashed, dash-dotted lines, respectively. Timescales for typical RBs (BWs) are displayed in red (black). The uncertainty in $\tau_{\rm diff}$ is due to the uncertainty on the magnetic field in the companion region. The lower part of the band is obtained assuming $B_{\rm c}=0.1$ G, while the upper part is obtained for $B_{\rm c}=10^3$ G.
• Figure 3: Histogram of the spin-down power for RBs (red), BWs (black), and RBs plus BWs (white) in Tab.\ref{['tab:Red Backs']} and Tab.\ref{['tab: Black widows']} with known spin-down power.
• Figure 4: Scenario 1: PW$\rightarrow$CW($\gamma$) Maximum (panel (a)) and minimum (panel (b)) gamma-ray flux from a typical RB (red bands) and BW (black bands). All the curves are calculated assuming that protons are injected as a delta function. The solid (dashed) lines are obtained for $\dot{E}=10^{35}\,\rm erg\, s^{-1}$ ($\dot{E}=10^{33}\,\rm erg\, s^{-1}$). The bands include the uncertainty on the magnetic field in the companion region $B_{\rm c}$. The upper (lower) bound is obtained assuming $B_{\rm c}=10^3$ G ($B_{\rm c}=0.1$ G). The maximum is obtained by calculating the multiplicity according to Eq. \ref{['eq: multiplicity']} The minimum is obtained assuming a fixed multiplicity of $10^4$. The dashed and dot-dashed orange lines represent the $50$ h CTA sensitivity in the north and south hemispheres, respectively Celli:2024cny. The blue solid line represents the $1$ year LHAASO sensitivity Celli:2024cny.
• Figure 5: Scenario 1: PW$\rightarrow$CS($\nu$) Same as in Fig. \ref{['fig:Gamma rays']} but for the all-flavor neutrino flux. The $90\%$ confidence-level median sensitivities of the IceCube (blue band) IceCube:2019cia, KM3Net-ARCA (orange band) KM3NeT:2018wnd, and TRIDENT (green band) TRIDENT:2022hql detectors, based on 10, 6, and 10 years of data taking, respectively, are shown assuming a power-law source spectrum with an index of 2. The exact sensitivity value will depend on the declination of the considered source.