The exotic black hole-neutron star binaries in our Galaxy

TL;DR

The paper tackles the missing galactic BH-NS binaries by proposing exotic BH-NS systems in which primordial BHs with surrounding DM density spikes heat the companion neutron stars to $T_infty\sim10^6$ K through DM capture, enabling a novel observational channel. It models DM capture and heating in NSs, deriving the heating rate and an equilibrium temperature under both optically thick and thin scattering, with a cooling balance yielding a timescale of ~$5\times10^4$ years. It Further shows that plausible spike densities can yield detectable $T_infty$ with JWST and XMM-Newton up to ~1 kpc, and in the optically thin regime the measurements could tightly constrain the DM-nucleon cross-section $\sigma_{hi N}$ far below the XENON1T limit, offering competitive new constraints on DM properties. The approach provides a complementary pathway to discover missing BH-NS binaries and to probe DM density spikes around black holes, with implications for relativistic gravity tests and DM particle physics.

Abstract

It has been suggested that there are $\sim 10^5$ black hole-neutron star (BH-NS) binaries in our Galaxy. However, despite the effort of intensive radio search for decades, none of these binaries has been found to date. These binaries are regarded as a holy grail of astronomy because they can greatly improve our understanding about relativistic systems of compact objects and fundamental physics. In this article, we propose the existence of exotic BH-NS binaries which can open a new way in searching the missing BH-NS binaries in our Galaxy. By considering the possible dark matter density spikes formed around the primordial black holes in the BH-NS binaries, we show that extremely high temperature ($\sim 10^6$ K) could be maintained on the surface of the neutron stars due to effective dark matter capture. This interesting feature can also help reveal the nature of dark matter and possibly further improve the upper limit of dark matter scattering cross section well below $10^{-47}$ cm$^2$.

Figures (5)

• Figure 2: The redshifted surface temperature $T_{\infty}$ against time in the optically thick regime. The red, black, and green solid lines respectively represent the cooling curves involving annihilating dark matter heating, non-annihilating dark matter heating, and without dark matter heating. Here, we have assumed $\rho_{\chi}=5.7 \times 10^{-13}$ g cm$^{-3}$.
• Figure 3: The upper limits of $\sigma_{\chi N}$ for annihilating dark matter heating scenario (red) and non-annihilating dark matter heating scenario (black) in the optically thin regime. Here, we have assumed $\rho_{\chi}=5.7 \times 10^{-13}$ g cm$^{-3}$.
• Figure 4: The shaded area bounded by the black line indicates the forbidden parameter space of $\sigma_{\chi N}-m_{\chi}$ (spin-independent) obtained from the XENON1T experiment Xenon. The red, green, and blue solid lines represent the upper limits of $\sigma_{\chi N}$ for the equilibrium redshifted surface temperature $T_{\infty}=10^6$ K, $8\times 10^5$ K and $3\times 10^5$ K respectively in the optically thin regime (heated by annihilating dark matter). Here, we have assumed $\rho_{\chi}=5.7 \times 10^{-13}$ g cm$^{-3}$.
• Figure 5: $T_{\infty}$ against $\rho_{\chi}$ in the optically thick regime.
• Figure 6: The minimum $T_{\infty}$ that can be observed by the Swift UVOT (black line), XMM-Newton (blue line) and JWST (red line) against the distance $d$ from us. The green dotted line indicates the order of magnitude of $T_{\infty}$ predicted. Here, the observing frequency ranges of the Swift UVOT, XMM-Newton and JWST are $(5.22-8.26) \times 10^{14}$ Hz, $(3.63-283) \times 10^{16}$ Hz and $(1.15-1.85) \times 10^{14}$ Hz respectively.