Dark Matter in X-rays: Revised XMM-Newton Limits and New Constraints from eROSITA

TL;DR

The paper investigates sub-GeV DM and asteroid-mass PBHs as DM candidates and forecasts their diffuse X-ray signatures from $e^±$ via bremsstrahlung and inverse Compton processes. It leverages the first eROSITA all-sky data release (eRASS1), models DM-induced continuum emission with DRAGON2/HERMES for various DM profiles, and computes Hawking-emission signals for a monochromatic, non-rotating PBH population to set upper limits on $igracevert igracevert angle$, $ au$, and $f_ ext{PBH}$. Through a ring-based chi-squared analysis incorporating propagation uncertainties, the study finds that eROSITA provides the strongest constraints below $\,\sim 30$ MeV for annihilation and decay into $e^+e^-$ and yields updated bounds for PBHs, while reanalyzing XMM-Newton data shows previous limits were overstated due to solid-angle treatment. The results emphasize the complementary role of all-sky X-ray surveys in probing the MeV gap and highlight that ongoing and future data releases could tighten these bounds further, whereas Voyager and 511 keV studies remain more constraining for PBHs in parts of parameter space.

Abstract

We investigate two classes of dark matter (DM) candidates, sub-GeV particles and primordial black holes (PBHs), that can inject low-energy electrons and positrons into the Milky Way and leave observable signatures in the X-ray sky. In the case of sub-GeV DM, annihilation or decay into $e^+e^-$ contributes to the diffuse sea of cosmic-ray (CR) leptons, which can generate bremsstrahlung and inverse Compton (IC) emission on Galactic photon fields, producing a broad spectrum from X-rays to $γ$-rays detectable by instruments such as eROSITA and XMM-Newton. For PBHs with masses below $\sim10^{17}$ g, Hawking evaporation similarly yields low-energy $e^\pm$, leading to comparable diffuse emission. Using the first data release from eROSITA and incorporating up-to-date CR propagation and diffusion parameters, we derive new constraints on both scenarios. For sub-GeV DM, we exclude thermally averaged annihilation cross sections in the range $\sim 10^{-27}-10^{-25} \ \mathrm{cm^3/s}$ and decay lifetimes of $\sim 10^{24}-10^{25}$ s for masses between 1 MeV and 1 GeV, with eROSITA outperforming previous X-ray constraints below $\sim$ 30 MeV. For asteroid-mass PBHs, we set new bounds on the DM fraction based on their Hawking-induced emission. Finally, we revisit earlier constraints from XMM-Newton, finding that they were approximately four orders of magnitude too stringent due to the use of the instrument's geometric solid angle rather than its exposure-weighted solid angle. Upon using the exposure-weighted solid angle, we show that the revised XMM-Newton limits are slightly weaker than those from eROSITA.

Figures (7)

• Figure 1: Mollweide projections of the X-ray sky observed by eROSITA. The left panel shows the map with the Galactic plane mask applied ($|b| < 5^\circ$), while the right panel displays the concentric $6^\circ$ ringed regions used for subdivision of the Galaxy that will be used to derive constraints on dark matter, indicating the observed count rate as a color bar.
• Figure 2: Comparison of eROSITA data (black points) in Ring 3 to the predicted particle DM or PBH evaporation induced X-ray signal for annihilation with $\langle\sigma v\rangle = 2.4 \times 10^{-26} \ \mathrm{cm^3/s}$ (top left) and decay with $\tau = 1 \times 10^{24} \ \mathrm{s}$ (top right) and for PBHs with $f_{\mathrm{PBH}} = 1$ (bottom). We show the predicted flux for different particle DM and PBH masses, as specified in the legends.
• Figure 3: Comparison of the $95\%$ confidence bounds on annihilating (top left panel) and decaying (top right panel) DM with previous CR constraints. The bottom left panel shows constraints on the fraction of PBHs as DM derived in this work. Finally, in the bottom right panel, we show a comparison of the eROSITA decay constraints with other probes. In all the cases we report the results using the best-fit propagation parameters for eROSITA (orange line), as well as uncertainties arising from using pessimistic and optimistic propagation parameters (reddish band), alongside other existing constraints (green lines).
• Figure 4: Comparison of the $95\%$ confidence bounds on annihilating (left panel) and decaying (right panel) DM with bands indicating the uncertainty from the DM density. The central line shows constraints from choosing the standard NFW profile and assuming a local DM density of $0.4$ GeV/cm$^3$. Shaded bands show uncertainty arising from considering a local DM density ranging from $0.3$ to $0.6$ GeV/cm$^3$ (darker bands) and varying the density profile from cNFW to isothermal distributions (lighter bands).
• Figure 5: Comparison of ring 3 flux for eROSITA (orange points) and XMM-Newton MOS with the use of exposure-weighted solid angle (black points) consistent with observational coverage of the dataset and with direct use of the geometric solid angle (grey points). In order to enable a viable comparison, all fluxes shown here use a 2$^\circ$ mask of the Galactic plane, as provided for the XMM-Newton data.