Constraining Fermionic Dark Matter with Galactic Neutron Stars

TL;DR

The paper addresses how fermionic dark matter can be captured, thermalized, and potentially collapse inside neutron stars, considering the motion of NSs in the Galaxy and the possibility of DM self-annihilation. It extends previous bosonic analyses by incorporating a fermionic Chandrasekhar limit and comparing annihilating versus non-annihilating DM across DM density profiles beyond the standard NFW form. The authors solve the DM accumulation equation with time-dependent capture and annihilation terms, delineating the parameter space in $(m_X,\sigma_{nX})$ where NS destruction is possible; they find that annihilation and degeneracy pressure significantly soften the constraints, while NS motion and different DM profiles moderately shape the excluded regions. By leveraging observations of pulsars, including PSR J2129−5721 and a sample of 413 NSs, the work offers robust astrophysical constraints on fermionic DM interactions with neutrons, providing complementary limits to terrestrial direct-detection searches and guiding DM model-building in high-mass and strongly interacting regimes.

Abstract

Dark matter (DM) remains one of the most significant open questions in modern physics, with its nature and interactions largely unexplored. In this study, we investigate the behavior of massive fermionic DM particles in the context of neutron stars (NSs), extending prior studies which focused on the bosonic DM. By incorporating the motion of NSs in the Galaxy and considering scenarios with and without DM self-annihilation, we demonstrate their impact on the DM capture rate and the accumulation process inside NSs. Observational data from pulsars in the Milky Way are used to place constraints on DM properties, including the mass and the DM-nucleon scattering cross-section, offering a more comprehensive picture in probing DM interactions in astrophysical environments.

Figures (7)

• Figure 1: Trajectory of PSR J2129$-$5721 in the past 500 Myr. The red dot represents its current position. The radial velocity $v_r$ is set manually and three cases are considered where $v_r = -100\,\mathrm{km/s}$, $0\,\mathrm{km/s}$ and $100\,\mathrm{km/s}$.
• Figure 2: The capture rate $F_{\mathrm c}$(top) and the accumulated particle number $N_X$(bottom) as functions of time for PSR J2129$-$5721, assuming $R_{\mathrm NS}=10\, \mathrm {km}$, $M_{\mathrm {NS}}=1.5\, M_{\odot}$, $T_{\mathrm {NS}}=10^6\,\mathrm K$, and $\langle\sigma_{\mathrm{a}}v\rangle=10^{-48}\,\mathrm{cm^3/s}$. In the bottom panel, blue lines correspond to non-annihilation cases, while yellow lines show annihilation cases; solid lines incorporate NS motion, while dashed lines assume a static NS. $F_{\mathrm c}$ changes periodically and $N(t)$ increases wavily as the NS orbits around the Galaxy. Parameters $m_X=10^1\,\mathrm{GeV}$, $\sigma_{\mathrm{nX}}=10^{-46}\,\mathrm{cm^2}$, and $v_r=0\,\mathrm{km/s}$ are used in this example.
• Figure 3: Time evolutions of the accumulated particle number $N_X$ for PSR J2129$-$5721. The same settings of parameters are used as in Fig. \ref{['fig:2']}.
• Figure 4: Constraints on the DM-nucleon cross section $\sigma_{\mathrm{nX}}$ from PSR J2129$-$5721. The areas above the orange and blue lines satisfy the Collapse Condition with and without DM self-annihilation, respectively (top). The areas above the green lines satisfy the Thermalization Condition (bottom). The Growth Condition is fulfilled by constraining $m_X \lesssim 10^{10}\,\mathrm{GeV}$. Since the green lines lie far below the blue and orange lines, the feasible regions for NS destruction are actually constrained by the Collapse Condition. Overall, the blue and orange shaded regions represent the parameter space where neutron star destruction is possible for non-annihilating and annihilating dark matter, respectively.
• Figure 5: Constraint on the DM-nucleon cross section $\sigma_{nX}$ from PSR J2129$-$5721 for bosonic DM particles. Same labels are used as in Fig. \ref{['fig:4']}, except that the red line now represents the Growth Condition. The blue and orange shaded regions represent the feasible regions where the star can be destroyed for the cases of non-annihilation and annihilation, respectively.