Search for the light dark matter with an X-ray spectrometer

TL;DR

This work assesses keV-scale sterile neutrinos as warm DM candidates and their radiative decay producing a monoenergetic X-ray line at $E_\gamma = M_s/2$, with decay width $\Gamma$ tied to the mixing angle $\sin^2(2\theta)$. It analyzes how existing and proposed X-ray missions could detect or constrain this line, highlighting the pivotal role of high spectral resolution and large grasp, and proposing two design paradigms (Type A wide FoV, Type B focusing optics) to achieve up to a 100-fold sensitivity improvement over current bounds. A concrete result with McCammon et al.'s microcalorimeter data demonstrates competitive, model-independent bounds on $\sin^2(2\theta)$ versus $M_s$, despite a short exposure. The study emphasizes that both designs are complementary for exploring the Milky Way halo and nearby structures, with Type B extending coverage to multiple keV energies and Type A enabling strong high-resolution searches in a broad field.

Abstract

Sterile neutrinos with the mass in the keV range are interesting warm dark matter (WDM) candidates. The restrictions on their parameters (mass and mixing angle) obtained by current X-ray missions (XMM-Newton or Chandra) can only be improved by less than an order of magnitude in the near future. Therefore the new strategy of search is needed. We compare the sensitivities of existing and planned X-ray missions for the detection of WDM particles with the mass ~1-20 keV. We show that existing technology allows an improvement in sensitivity by a factor of 100. Namely, two different designs can achieve such an improvement: [A] a spectrometer with the high spectral resolving power of 0.1%, wide (steradian) field of view, with small effective area of about cm^2 (which can be achieved without focusing optics) or [B] the same type of spectrometer with a smaller (degree) field of view but with a much larger effective area of 10^3 cm^2 (achieved with the help of focusing optics). To illustrate the use of the "type A" design we present the bounds on parameters of the sterile neutrino obtained from analysis of the data taken by an X-ray microcalorimeter. In spite of the very short exposure time (100 sec) the derived bound is comparable to the one found from long XMM-Newton observation.

Figures (5)

• Figure 1: Comparison of sensitivities of existing and proposed/planned X-ray missions for the detection of the DM decay line from the Milky Way DM halo. The plot shows the characteristics of different telescopes in the two parameter space relevant for the diffuse DM line detection: "Energy resolution" vs. "Grasp". The sensitivity of XMM-Newton is taken as a reference. Solid lines limit from below the regions of parameter space in which the improvement of sensitivity by factors of 1, 100 and 100 can be achieved (each line is marked by the corresponding numerical factor). Dashed lines show the improvement of conservative "total flux" bounds on neutrino mixing angle (see text for explanations) in the case of non-detection of the DM decay line.
• Figure 2: Same as in Fig. \ref{['fig:sensitivity']}, but for the sensitivity for the detection of DM decay line from a nearby dwarf galaxy of the angular size of 2 degrees.
• Figure 3: Same as in Fig. \ref{['fig:sensitivity']}, but for the sensitivity for the detection of DM decay line from a galaxy cluster of the angular size of 2 degrees.
• Figure 4: Red thin line: an upper limit on the flux in the dark matter decay line as a function of energy. Blue thick line: average level (line features smoothed) of soft X-ray spectrum in the 0.1-1 keV energy range.
• Figure 5: Exclusion plot in the $(m_s,\sin^22\theta)$ parameter space obtained from the analysis of the X-ray spectrometer data of McCammon:02 (solid thick red line). For comparison, the upper limit on the mixing angle obtained from the XMM-Newton observation of the Large Magellanic Cloud obtained in Boyarsky:06c is shown by the solid thin black line.