Search for a photon peak from keV-scale dark matter annihilation with NuSTAR: Constraints on $\langle σv \rangle$ after 11 years of observations

TL;DR

This study targets keV-scale dark matter annihilation into photon pairs, searching for a sharp X-ray line at $E_\gamma=m_\chi$ in 3–20 keV using 11 years of NuSTAR stray-light data. The authors model the diffuse X-ray background with a solar component and CXB, and perform a line-search with a fixed Doppler-broadened width $\sigma_{\text{Gauss}}=0.001$, translating line upper limits into constraints on $\langle\sigma v\rangle$ via the Galactic J-factor. No line is detected, yielding the strongest current X-ray limits in this mass range (down to $\sim5\times10^{-33}$–$10^{-34}$ cm$^3$ s$^{-1}$ for standard halo profiles) and demonstrating the utility of NuSTAR’s stray-light observations for diffuse DM searches. The results complement bounds from other X-ray facilities and motivate future high-resolution missions to probe this dark matter window further.

Abstract

We report new constraints on the velocity-independent annihilation cross section $\langle σv \rangle$ of keV-scale dark matter particles based on 11 years of observations with the NuSTAR X-ray telescope. Using the unfocused stray light mode of the instrument, which provides a wide field-of-view and a stable instrumental background, we perform a sensitive search for photon signatures from dark matter annihilation in the Galactic halo. We model the resulting diffuse X-ray spectrum over the 3-20 keV energy range and search for line-like spectral features that may arise from the annihilation of dark matter particles into photons. No statistically significant excess over the expected astrophysical background is found. We therefore place upper limits on $\langle σv \rangle$ as a function of dark matter mass, assuming a velocity-independent s-wave annihilation and several Galactic dark matter profiles. Across most of the explored mass range our results provide the strongest X-ray constraints to date, reaching the level of $\langleσv\rangle \lesssim 10^{-33}$ -- $10^{-34}$ cm$^3$ s$^{-1}$, and they are complementary to the most recent bounds derived from SRG/ART-XC observations.

Figures (8)

• Figure 1: The distribution of 3248 (FPMA) and 3139 (FPMB) NuSTAR observations on the sky in Galactic coordinates. Cyan and magenta points show the NuSTAR observations at $|b|<3^{\circ}$ and $|b|>3^{\circ}$, respectively.
• Figure 2: The NuSTAR detector count rate as a function of time, MJD stands for Modified Julian Date. Each point represents individual NuSTAR observation. A solid red line is a cubic polynomial outlier-resistant approximation used to describe long-term variation. A dashed red line shows the same approximation scaled by an arbitrary-chosen factor of 2, that is a threshold: the observations above it are excluded from the analysis, since the high count rate is due to the remaining contribution of the GRXE at slightly higher Galactic latitudes than $|b| \sim 3$ deg.
• Figure 3: Demonstration of the performance of a wavelet-based algorithm for the removal of focused X-rays. The left panel shows an image of the FPMB detector before the focused X-ray removal procedure was applied. The right panel shows the detector after. The right panel clearly shows the characteristic pattern from the SL (see right panel of Fig. \ref{['fig:solidAngle']}).
• Figure 4: Image of the NuSTAR FPMA (left) and FPMB (right) in physical detector pixels, showing the open portion of the sky for each detector pixel in squared degrees. The image is taken from Krivonos:2020qvl.
• Figure 5: The stacked spectrum of FPMA and FPMB for SL, mesured at $|b| \geq 3^\circ$.