Hawking heating of neutron stars by dark matter

TL;DR

This work proposes Hawking heating, a novel mechanism where non-annihilating dark matter captured by a neutron star forms a small black hole that rapidly Hawking-evaporates, depositing energy and heating the star. It develops a detailed model of DM capture at rate $C_\chi$, thermalization on timescale $t_{\rm th}$, collapse when $M_{\rm BH}^0=\max[M_\chi^{\rm self},M_\chi^{\rm Ch}]$, and subsequent BH growth/evaporation governed by $\dot M_{\rm BH}=\dot M_{\rm Bondi}-\dot M_{\rm evap}$ with a Page factor $P(M_{\rm BH})$, defining $t_{\rm Bondi}$, $t_{\rm evap}$, and the heating cadence $\Delta t$. The resulting near-steady heating can keep NSs at detectable temperatures (e.g., $T_{\rm NS}$ around $1000$ K), enabling infrared observations and yielding cross-section limits $\sigma_{\chi n}$ that exceed purely kinetic heating for broad ranges of DM mass, notably $m_\chi \gtrsim 10^4$ GeV (spin-0) and $m_\chi \gtrsim 10^{10}$ GeV (spin-1/2). The limits are complementary to direct-detection and pulsar bounds, with caveats on NS core composition, DM clumping, and potential quantum/non-spherical effects, pointing to future observational and theoretical explorations in infrared astrophysics. $M_{\rm BH}^0$, $m_\chi$, $m_\chi^{\rm Bondi}$, and related timescales are central to the analysis.

Abstract

Interactions with particle dark matter could brighten old, isolated neutron stars to thermal luminosities detectable at current and next-generation telescopes. We present a novel mechanism for such signals. Non-annihilating (e.g., asymmetric) dark matter capturing in a neutron star could form a small black hole in its core, which could then rapidly evaporate away. If black holes form and evaporate within the cooling timescale of the neutron star, periodic episodes of black hole evaporation could impart a steady-state stellar luminosity, providing a source of heat additional to the kinetic energy of dark matter during capture. Consequently, we obtain sensitivities to dark matter-nucleon cross sections that are stronger than that from dark kinetic heating by a factor of a few for > $10^4$ GeV (> $10^{10}$ GeV) mass of spin-0 (spin-1/2) dark matter.

Figures (3)

• Figure 1: Illustration (not to scale) of our premise. Non-annihilating DM captures in, thermalizes with, and collapses to form a rapidly evaporating BH in the NS. The evaporation products heat the NS core, and the cycle continues. The resulting near-steady-state luminosity of the NS can be measured by infrared telescopes.
• Figure 2: Timescales for various processes in a 1000 K NS as a function of DM mass (and the corresponding $M_{\rm BH}^0$ in the top frame ticks). Across the DM masses for which $t_{\rm evap} < t_{\rm Bondi}$, the time for a black hole for form and decay ($t_{\rm th} + t_{\rm BH} + t_{\rm evap}$) is seen to be generally smaller than the NS cooling timescale. In these regions the NS may be observed to glow at a near-constant temperature. See Sec. \ref{['subsec:signatures']} for further particulars.
• Figure 3: Sensitivities of Hawking heating from the (non-)observation of a 1000 K neutron star to DM-neutron scattering cross section as a function of DM mass (and the corresponding $M_{\rm BH}^0$ in the top frame ticks). Also shown for comparison are sensitivities from just kinetic heating; the region that would be probed by observing a 1000 K NS from its not having turned into a black hole and current analogous limits from the existence of the $2 \times 10^6$ K PSR J0437$-$4715 as taken from Ref. Bramante:2015dfaBramante:2017ulk; spin-independent direct detection limits from LZ LZ:SS:2024zvo and sensitivities to the impending neutrino background Billard:2013qyaOHare:2021utq. See Sec. \ref{['sec:results']} for further details.