X-ray Binaries: a potential dominant contributor to the cosmic ray spectral knee structure

TL;DR

The paper investigates whether X-ray binaries can dominate Galactic cosmic rays near the knee by integrating a statistically inferred XRB distribution with a spatially dependent diffusion model in DRAGON. It combines SNR contributions at sub-TeV energies, a nearby source to capture local CR features, and XRBs with a $\sim2.5$ PeV cutoff to explain the knee region, achieving consistency with CR spectra, anisotropy, and diffuse gamma-ray observations. The findings suggest XRBs as a plausible primary driver of knee CRs and make testable predictions for future high-energy observations by LHAASO and HAWC. This approach provides a coherent link between micquasar PeVatrons, CR propagation, and multi-mavelength gamma-ray data, with potential implications for understanding the Galactic CR budget.

Abstract

``PeVatrons" refer to astrophysical sources capable of accelerating particles to energies $\sim$PeV and higher, potentially contributing to the cosmic ray spectrum in the knee region. Recently, HAWC and LHAASO have discovered a new type PeVatrons -- X-ray binaries, allowing us to investigate in greater depth of the contributions of these sources to cosmic rays around the knee region. There are hundreds of X-ray binaries in our galaxy observed, which are potential PeVatrons.In this work, we derive the radial distribution of X-ray binaries in the Galaxy. Then we use the DRAGON package to calculate energy spectrum, anisotropy of cosmic rays as well as the resulting diffuse gamma ray emissions, after considering them as factories of cosmic rays in the knee energy bands. Our findings show that the contributions from X-ray binary PeVatrons may be dominant. More X-ray binary PeVatrons can be observed by LHAASO and HAWC in the future, and will confirm the contribution of X-ray binaries to high energy cosmic rays.

Figures (7)

• Figure 1: Projection of the sample X-ray binaries (XRBs) on to the Galactic plane. High-mass X-ray binaries (HMXBs) and low-mass X-ray binaries (LMXBs) are marked as red dots and blue pluses, respectively. The GC is the origin of the coordinate system and the coordinate of the Sun is (-8.5, 0.0). Other lines and symbols are described in the context.
• Figure 2: The average observed surface density of XRBs across the Galactic plane. $\rm R$ is the distance from the Galactic Center and $\rm r$ is the distance from the Sun.
• Figure 3: Normalized radial distribution of surface density of XRBs. The error bars are derived from Equation \ref{['error']}. Here the data points (in blue) are fitted by the equation \ref{['HMXBs_fit']} (in red), and the chi-square is $\rm \chi^{2}/n.d.f={24.04/17}$. The data points and the fitted curve in the figure have been normalized to the value at the Sun's location.
• Figure 4: The ratio of boron-to-carbon. The data points are taken from the AMS-02 2016PhRvL.117w1102A. $\rm E_{k}$ refers to the kinetic energy per nucleon of cosmic rays. Cosmic rays with energies below 10 GeV are influenced by solar modulation, but our theoretical calculations do not take this solar modulation effect into account.
• Figure 5: The calculated proton spectrum compared with observed data. The black solid lines is the fluxes from background SNRs, the green solid line is the contribution from the XRBs, and the blue dashed line is the contribution from the local SNR. The data points are taken from the AMS-02 (green) 2015PhRvL.114q1103A, DAMPE (red) 2019SciA....5.3793A, GRAPES 2024PhRvL.132e1002V, KASCADE 2005APh....24....1A.