Heating up neutron stars with inelastic dark matter

Abstract

Neutron stars can provide new insight into dark matter properties, as these dense objects capture dark matter particles very efficiently. It has recently been shown that the energy transfer in the dark matter capture process can lead to appreciable heating of neutron stars, which may be observable with forthcoming infra-red telescopes. We examine this heating in the context of inelastic dark matter, for which signals in conventional nuclear-recoil based direct detection experiments are highly suppressed when the momentum transfer is small compared to the mass splitting between dark matter states. Neutron stars permit inelastic scattering for much greater mass splittings, because dark matter particles are accelerated to velocities close to the speed of light during infall. Using an effective operator approach for fermionic DM that scatters inelastically, we show that the observation of a very cold neutron star would lead to very stringent limits on the interaction strengths that, in most cases, are much stronger than any present, or future, direct detection experiment on Earth. This holds both for elastic scattering and for inelastic scattering with mass splittings up to ∼ 300 MeV.

Capture of leptophilic dark matter in neutron stars

“…Considering that DM scatters off a single SM particle within the NS core and the scattering cross section, σ, is sufficiently large that all DM particles are captured as they transit a NS, σ σ th , neglecting thermal effects, the capture rate tends to the geometric limit [21],…”

“…where E R is the energy transfer given by [21] 11) and θ cm is the scattering angle in the centre of mass frame. For m χ m e , we haveq…”

“…If we instead consider a neutron star (NS), we can constrain the type and strength of dark matter interactions via the requirements that the resultant neutron star heating is not too large [18][19][20][21][22], that the star does not accumulate so much dark matter that collapse to a black hole is triggered [23][24][25][26][27][28][29][30], and that the neutron star structure is not perturbed to the extent that the gravitational wave signatures from binary neutron star mergers are inconsistent with observations [31][32][33].…”

“…Provided NSs are nearby, faint and sufficiently isolated, they are likely to be discovered by existing radio telescopes such as the Fivehundred-meter Aperture Spherical radio Telescope (FAST) [37], or the future Square Kilometer Array (SKA) [38]. Their thermal emission can then be measured by infrared telescopes such as the James Webb Space Telescope (JWST), the Thirty Meter Telescope (TMT), or the European Extremely Large Telescope (E-ELT) [18].The projected NS kinetic heating limits on the DM-nucleon interaction strength were recently calculated in [18,19,21], where they were found to be comparable to, or in many cases much stronger than, existing or projected limits from direct detection experiments. Specifically, xenon-based DD experiments can provide superior limits only for unsuppressed SI scattering (i.e., for scalar or vector interactions) and only in the mass range 10 GeV < m χ < 1 TeV.…”

Dark matter interactions with muons in neutron stars

“…Assuming a large enough muon-DM cross section to capture a significant number of DM particles, the infalling DM unavoidably transfers heat to the NS, see Refs. [27][28][29][30][31]. This can increase the temperature of old NS from O(100 K) to O(2000 K), leading to an infrared blackbody spectrum that is in principle within range of future telescopes such as the James Webb Space Telescope, the Thirty Meter Telescope, or the European Extremely Large Telescope [27].…”