SKA sensitivity to sub-GeV dark matter decay: Synchrotron radio emissions in white dwarf magnetospheres

We investigate the potential of the Square Kilometre Array (SKA) in detecting synchrotron radiation emitted from the decay of sub-GeV dark matter (dark matter with masses below the GeV scale) in the presence of strong magnetic fields. As a concrete setup, we consider scenarios where the magnetosphere of a magnetic white dwarf overlaps with dense dark matter environments, such as those surrounding a primordial black hole. Our study reveals that the encounters of compact objects such as white dwarfs and black holes offer a promising avenue for upcoming radio telescopes to probe the properties of light dark matter, which has been less explored compared with more conventional heavier (masses above the GeV scale) dark matter.

Figure 1

The enhanced dark matter density around a primordial black hole for different black hole masses $(M={10}^{-4},1,{10}^4{M}_{\odot })$ and dark matter masses $(m_{\chi }={10}^{-2},{10}^4\mathrm{GeV}$). The curve is truncated at Schwarzschild radius. Two curves for $M={10}^4{M}_{\odot }$ with $m_{\chi }={10}^{-2}$ and $m_{\chi }={10}^4\mathrm{GeV}$ are indistinguishable.

Figure 2

The energy spectrum distribution $\nu S_{\nu }$ for the synchrotron radiation as a function of the frequency. The SKA sensitivity assuming 100 and 1000 hours of observations are shown for reference. The magnetic white dwarf is assumed to be 100 pc away from Earth. We assume that the magnetic field follows the dipole profile $B(r)=B_0(r/r_0{)}^{-3}$ and the magnetosphere spans from $r_0=0.01R_{\odot }$ to $10r_0$. The dark matter decay rate ${\mathrm{\Gamma }}_{\chi }={10}^{-25}[{\mathrm{s}}^{-1}]$ and the density ${\rho }_{\chi }=1[\mathrm{g}/{\mathrm{cm}}^3]$ are used. The top panel displays the cases for dark matter masses $m_{\chi }=5$, 50, 500 MeV with a magnetic field coefficient ($B_0={10}^5\mathrm{G}$). The bottom panel illustrates various magnetic field amplitudes with ($B_0={10}^6,{10}^5,{10}^4\mathrm{G}$) for a constant dark matter mass of $m_{\chi }=50\mathrm{MeV}$.

Figure 3

The lower bounds on the dark matter decay rate ${\mathrm{\Gamma }}_{\chi }$ to be detectable by the SKA assuming 100 hours of observation. The dipole magnetic field profile $B(r)=B_0(r/r_0{)}^{-3}$ is assumed and the magnetosphere spans from $r_0=0.01R_{\odot }$ to $10r_0$. The bounds for $B_0={10}^3–{10}^7\mathrm{G}$ are shown. The constant dark matter density ${\rho }_{\chi }=1\mathrm{g}/{\mathrm{cm}}^3$ is assumed. The upper bounds from the CMB and Voyager data are also plotted for reference.

Figure 4

The energy spectrum distribution $\nu S_{\nu }$ for the synchrotron radiation as a function of the frequency. The magnetic white dwarf is assumed to be 100 pc away and the dark matter density ${\rho }_{\chi }=1\mathrm{g}/{\mathrm{cm}}^3$ is assumed. The magnetic field follows the dipole profile $B(r)=B_0(r/r_0{)}^{-3}$ with $B_0={10}^6\mathrm{G}$ and the magnetosphere region is from $r_0=0.01R_{\odot }$ to $10r_0$. The dark matter masses for $m_{\chi }=5\mathrm{MeV}$, 1 GeV, 1 TeV are shown. The sensitivity of different experiments are also shown covering the radio (SKA assuming 100 and 1000 hours of observations), x-ray (Chandra, NuSTAR), and $\gamma$-ray (Fermi-LAT) frequencies. One hundred hours of observations are assumed for Chandra, NuSTAR, and Fermi-LAT sensitivity curves.