Resurrecting Hitomi for Decaying Dark Matter and Forecasting Leading Sensitivity for XRISM.

The Hitomi x-ray satellite mission carried unique high-resolution spectrometers that were set to revolutionize the search for sterile neutrino dark matter (DM) by looking for narrow x-ray lines arising from DM decays. Unfortunately, the satellite was lost shortly after launch, and to date the only analysis using Hitomi for DM decay used data taken towards the Perseus cluster. In this work we present a significantly more sensitive search from an analysis of archival Hitomi data towards blank sky locations, searching for DM decaying in our own Milky Way. The recently launched XRISM satellite has nearly identical soft-x-ray spectral capabilities to Hitomi; we project the full-mission sensitivity of XRISM for analyses of their future blank-sky data, and we find that XRISM will have the leading sensitivity to decaying DM for masses between roughly 1 to 18 keV, with important implications for sterile neutrino and heavy axionlike particle DM scenarios.

Higgsino Dark Matter Confronts 14 Years of Fermi γ-Ray Data.

Thermal Higgsino dark matter (DM), with mass around 1 TeV, is a well-motivated, minimal DM scenario that arises in supersymmetric extensions of the standard model. Higgsinos may naturally be the lightest superpartners in split-supersymmetry models that decouple the scalar superpartners while keeping Higgsinos and gauginos close to the TeV scale. Higgsino DM may annihilate today to give continuum γ-ray emission at energies less than a TeV in addition to a linelike signature at energies equal to the mass. Previous searches for Higgsino DM, for example with the H.E.S.S. γ-ray telescope, have not reached the necessary sensitivity to probe the Higgsino annihilation cross section. In this work we make use of 14 years of data from the Fermi Large Area Telescope at energies above ∼10 GeV to search for the continuum emission near the Galactic Center from Higgsino annihilation. We interpret our results using DM profiles from Milky Way analog galaxies in the FIRE-2 hydrodynamic cosmological simulations. We set the strongest constraints to date on Higgsino-like DM. Our results show a mild, ∼2σ preference for Higgsino DM with a mass near the thermal Higgsino mass and, depending on the DM density profile, the expected cross section.

Upper Limit on the QCD Axion Mass from Isolated Neutron Star Cooling.

The quantum chromodynamics (QCD) axion may modify the cooling rates of neutron stars (NSs). The axions are produced within the NS cores from nucleon bremsstrahlung and, when the nucleons are in superfluid states, Cooper pair breaking and formation processes. We show that four of the nearby isolated magnificent seven NSs along with PSR J0659 are prime candidates for axion cooling studies because they are coeval, with ages of a few hundred thousand years known from kinematic considerations, and they have well-measured surface luminosities. We compare these data to dedicated NS cooling simulations incorporating axions, profiling over uncertainties related to the equation of state, NS masses, surface compositions, and superfluidity. Our calculations of the axion and neutrino emissivities include high-density suppression factors that also affect SN 1987A and previous NS cooling limits on axions. We find no evidence for axions in the isolated NS data, and within the context of the Kim-Shifman-Vainshtein-Zakharov QCD axion model, we constrain m_{a}≲16 meV at 95% confidence level. An improved understanding of NS cooling and nucleon superfluidity could further improve these limits or lead to the discovery of the axion at weaker couplings.

No Evidence for Axions from Chandra Observation of the Magnetic White Dwarf RE J0317-853.

Axions with couplings g_{aγγ}∼few×10^{-11} GeV^{-1} to electromagnetism may resolve a number of astrophysical anomalies, such as unexpected ∼TeV transparency, anomalous stellar cooling, and x-ray excesses from nearby neutron stars. We show, however, that such axions are severely constrained by the nonobservation of x rays from the magnetic white dwarf (MWD) RE J0317-853 using ∼40 ks of data acquired from a dedicated observation with the Chandra X-ray Observatory. Axions may be produced in the core of the MWD through electron bremsstrahlung and then convert to x rays in the magnetosphere. The nonobservation of x rays constrains the axion-photon coupling to g_{aγγ}≲5.5×10^{-13}sqrt[C_{aγγ}/C_{aee}] GeV^{-1} at 95% confidence for axion masses m_{a}≲5×10^{-6} eV, with C_{aee} and C_{aγγ} the dimensionless coupling constants to electrons and photons. Considering that C_{aee} is generated from the renormalization group, our results robustly disfavor g_{aγγ}≳4.4×10^{-11} GeV^{-1} even for models with no ultraviolet contribution to C_{aee}.

Dark matter from axion strings with adaptive mesh refinement.

Axions are hypothetical particles that may explain the observed dark matter density and the non-observation of a neutron electric dipole moment. An increasing number of axion laboratory searches are underway worldwide, but these efforts are made difficult by the fact that the axion mass is largely unconstrained. If the axion is generated after inflation there is a unique mass that gives rise to the observed dark matter abundance; due to nonlinearities and topological defects known as strings, computing this mass accurately has been a challenge for four decades. Recent works, making use of large static lattice simulations, have led to largely disparate predictions for the axion mass, spanning the range from 25 microelectronvolts to over 500 microelectronvolts. In this work we show that adaptive mesh refinement simulations are better suited for axion cosmology than the previously-used static lattice simulations because only the string cores require high spatial resolution. Using dedicated adaptive mesh refinement simulations we obtain an over three order of magnitude leap in dynamic range and provide evidence that axion strings radiate their energy with a scale-invariant spectrum, to within ~5% precision, leading to a mass prediction in the range (40,180) microelectronvolts.

Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey.

Axion dark matter (DM) may efficiently convert to photons in the magnetospheres of neutron stars (NSs), producing nearly monochromatic radio emission. This process is resonantly triggered when the plasma frequency induced by the underlying charge distribution approximately matches the axion mass. We search for evidence of this process using archival Green Bank Telescope data collected in a survey of the Galactic Center in the C band by the Breakthrough Listen project. While Breakthrough Listen aims to find signatures of extraterrestrial life in the radio band, we show that their high-frequency resolution spectral data of the Galactic Center region is ideal for searching for axion-photon transitions generated by the population of NSs in the inner pc of the Galaxy. We use data-driven models to capture the distributions and properties of NSs in the inner Galaxy and compute the expected radio flux from each NS using state-of-the-art ray tracing simulations. We find no evidence for axion DM and set leading constraints on the axion-photon coupling, excluding values down to the level g_{aγγ}∼10^{-11} GeV^{-1} for DM axions for masses between 15 and 35 µeV.

Galactic Potential and Dark Matter Density from Angular Stellar Accelerations.

We present an approach to measure the Milky Way (MW) potential using the angular accelerations of stars in aggregate as measured by astrometric surveys like Gaia. Accelerations directly probe the gradient of the MW potential, as opposed to indirect methods using, e.g., stellar velocities. We show that end-of-mission Gaia stellar acceleration data may be used to measure the potential of the MW disk at approximately 3σ significance and, if recent measurements of the solar acceleration are included, the local dark matter density at ∼2σ significance. Since the significance of detection scales steeply as t^{5/2} for observing time t, future surveys that include angular accelerations in the astrometric solutions may be combined with Gaia to precisely measure the local dark matter density and shape of the density profile.

Search for Low-Mass Axion Dark Matter with ABRACADABRA-10 cm.

Two of the most pressing questions in physics are the microscopic nature of the dark matter that comprises 84% of the mass in the Universe and the absence of a neutron electric dipole moment. These questions would be resolved by the existence of a hypothetical particle known as the quantum chromodynamics (QCD) axion. In this work, we probe the hypothesis that axions constitute dark matter, using the ABRACADABRA-10 cm experiment in a broadband configuration, with world-leading sensitivity. We find no significant evidence for axions, and we present 95% upper limits on the axion-photon coupling down to the world-leading level g_{aγγ}<3.2×10^{-11} GeV^{-1}, representing one of the most sensitive searches for axions in the 0.41-8.27 neV mass range. Our work paves a direct path for future experiments capable of confirming or excluding the hypothesis that dark matter is a QCD axion in the mass range motivated by string theory and grand unified theories.

Deep Search for Decaying Dark Matter with XMM-Newton Blank-Sky Observations.

Sterile neutrinos with masses in the keV range are well-motivated extensions to the Standard Model that could explain the observed neutrino masses while also making up the dark matter (DM) of the universe. If sterile neutrinos are DM then they may slowly decay into active neutrinos and photons, giving rise to the possibility of their detection through narrow spectral features in astrophysical x-ray data sets. In this Letter, we perform the most sensitive search to date for this and other decaying DM scenarios across the mass range from 5 to 16 keV using archival XMM-Newton data. We reduce 547 Ms of data from both the MOS and PN instruments using observations taken across the full sky and then use this data to search for evidence of DM decay in the ambient halo of the Milky Way. We determine the instrumental and astrophysical baselines with data taken far away from the Galactic Center, and use Gaussian process modeling to capture additional continuum background contributions. No evidence is found for unassociated x-ray lines, leading us to produce the strongest constraints to date on decaying DM in this mass range.

Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars.

Axions may be produced thermally inside the cores of neutron stars (NSs), escape the stars due to their feeble interactions with matter, and subsequently convert into x rays in the magnetic fields surrounding the stars. We show that a recently discovered excess of hard x-ray emission in the 2-8 keV energy range from the nearby magnificent seven isolated NSs could be explained by this emission mechanism. These NSs are unique in that they had previously been expected to only produce observable flux in the UV and soft x-ray bands from thermal surface emission at temperatures ∼100 eV. No conventional astrophysical explanation of the magnificent seven hard x-ray excess exists at present. We show that the hard x-ray excess may be consistently explained by an axionlike particle with mass m_{a}≲2×10^{-5} eV and g_{aγγ}×g_{ann}∈(2×10^{-21},10^{-18}) GeV^{-1} at 95% confidence, accounting for both statistical and theoretical uncertainties, where g_{aγγ} (g_{ann}) is the axion-photon (axion-neutron) coupling constant.

Green Bank and Effelsberg Radio Telescope Searches for Axion Dark Matter Conversion in Neutron Star Magnetospheres.

Axion dark matter (DM) may convert to radio-frequency electromagnetic radiation in the strong magnetic fields around neutron stars. The radio signature of such a process would be an ultranarrow spectral peak at a frequency determined by the mass of the axion particle. We analyze data we collected from the Robert C. Byrd Green Bank Telescope in the L band and the Effelsberg 100-m Telescope in the L band and S band from a number of sources expected to produce bright signals of axion-photon conversion, including the Galactic center of the Milky Way and the nearby isolated neutron stars RX J0720.4-3125 and RX J0806.4-4123. We find no evidence for axion DM and are able to set constraints on the existence of axion DM in the highly motivated mass range between ∼5 and 11 µeV with the strongest constraints to date on axions in the ∼10-11 µeV range.

Early-Universe Simulations of the Cosmological Axion.

Ultracompact dark matter (DM) minihalos at masses at and below 10^{-12} M_{⊙} arise in axion DM models where the Peccei-Quinn (PQ) symmetry is broken after inflation. The minihalos arise from density perturbations that are generated from the nontrivial axion self-interactions during and shortly after the collapse of the axion-string and domain-wall network. We perform high-resolution simulations of this scenario starting at the epoch before the PQ phase transition and ending at matter-radiation equality. We characterize the spectrum of primordial perturbations that are generated and comment on implications for efforts to detect axion DM. We also measure the DM density at different simulated masses and argue that the correct DM density is obtained for m_{a}=25.2±11.0 µeV.

The dark matter interpretation of the 3.5-keV line is inconsistent with blank-sky observations.

Observations of nearby galaxies and galaxy clusters have reported an unexpected x-ray emission line around 3.5 kilo-electron volts (keV). Proposals to explain this line include decaying dark matter-in particular, that the decay of sterile neutrinos with a mass around 7 keV could match the available data. If this interpretation is correct, the 3.5-keV line should also be emitted by dark matter in the halo of the Milky Way. We used more than 30 megaseconds of XMM-Newton (X-ray Multi-Mirror Mission) blank-sky observations to test this hypothesis, finding no evidence of the 3.5-keV line emission from the Milky Way halo. We set an upper limit on the decay rate of dark matter in this mass range, which is inconsistent with the possibility that the 3.5-keV line originates from dark matter decay.

X-Ray Searches for Axions from Super Star Clusters.

Axions may be produced in abundance inside stellar cores and then convert into observable x rays in the Galactic magnetic fields. We focus on the Quintuplet and Westerlund 1 super star clusters, which host large numbers of hot, young stars including Wolf-Rayet stars; these stars produce axions efficiently through the axion-photon coupling. We use Galactic magnetic field models to calculate the expected x-ray flux locally from axions emitted from these clusters. We then combine the axion model predictions with archival Nuclear Spectroscopic Telescope Array (NuSTAR) data from 10-80 keV to search for evidence of axions. We find no significant evidence for axions and constrain the axion-photon coupling g_{aγγ}≲3.6×10^{-12} GeV^{-1} for masses m_{a}≲5×10^{-11} eV at 95% confidence.

X-Ray Signatures of Axion Conversion in Magnetic White Dwarf Stars.

White dwarf (WD) stars may radiate keV-energy axions produced in their stellar cores. This has been extensively studied as an extra channel by which WDs may cool, with some analyses even suggesting that axions can help explain the observed WD luminosity function. We show that the radiated axions may convert into x rays in the strong magnetic fields surrounding the WDs, leading to observable x-ray signatures. We use Suzaku observations of the WD RE J0317-853 to set the strongest constraints to date on the combination of the axion-electron (g_{aee}) times axion-photon (g_{aγγ}) couplings, and we show that dedicated observations of magnetic WDs by telescopes such as Chandra, XMM-Newton, and NuSTAR could increase the sensitivity to |g_{aee}g_{aγγ}| by over an order of magnitude, allowing for a definitive test of the axionlike-particle explanation of the stellar cooling anomalies.

Probing Dark Matter Using Precision Measurements of Stellar Accelerations.

Dark matter comprises the bulk of the matter in the Universe, but its particle nature and cosmological origin remain mysterious. Knowledge of the dark matter density distribution in the Milky Way Galaxy is crucial both to our understanding of the standard cosmological model and for grounding direct and indirect searches for the particles comprising dark matter. Current measurements of Galactic dark matter content rely on model assumptions to infer the forces acting upon stars from the distribution of observed velocities. Here, we propose to apply the precision radial velocity method, optimized in recent years for exoplanet astronomy, to measure the change in the velocity of stars over time, thereby providing a direct probe of the local gravitational potential in the Galaxy. Using numerical simulations, we develop a realistic strategy to observe the differential accelerations of stars in our Galactic neighborhood with next-generation telescopes, at the level of 10^{-8} cm/s^{2}. Our simulations show that detecting accelerations at this level with an ensemble of 10^{3} stars requires the effect of stellar noise on radial velocity measurements to be reduced to <10 cm/s. The measured stellar accelerations may then be used to extract the local dark matter density and morphological parameters of the density profile.

First Results from ABRACADABRA-10 cm: A Search for Sub-µeV Axion Dark Matter.

The axion is a promising dark matter candidate, which was originally proposed to solve the strong-CP problem in particle physics. To date, the available parameter space for axion and axionlike particle dark matter is relatively unexplored, particularly at masses m_{a}≲1 µeV. ABRACADABRA is a new experimental program to search for axion dark matter over a broad range of masses, 10^{-12}≲m_{a}≲10^{-6} eV. ABRACADABRA-10 cm is a small-scale prototype for a future detector that could be sensitive to the QCD axion. In this Letter, we present the first results from a 1 month search for axions with ABRACADABRA-10 cm. We find no evidence for axionlike cosmic dark matter and set 95% C.L. upper limits on the axion-photon coupling between g_{aγγ}<1.4×10^{-10} and g_{aγγ}<3.3×10^{-9} GeV^{-1} over the mass range 3.1×10^{-10}-8.3×10^{-9} eV. These results are competitive with the most stringent astrophysical constraints in this mass range.

Constraining a Thin Dark Matter Disk with Gaia.

If a component of the dark matter has dissipative interactions, it could collapse to form a thin dark disk in our Galaxy that is coplanar with the baryonic disk. It has been suggested that dark disks could explain a variety of observed phenomena, including periodic comet impacts. Using the first data release from the Gaia space observatory, we search for a dark disk via its effect on stellar kinematics in the Milky Way. Our new limits disfavor the presence of a thin dark matter disk, and we present updated measurements on the total matter density in the Solar neighborhood.

Stellar Wakes from Dark Matter Subhalos.

We propose a novel method utilizing stellar kinematic data to detect low-mass substructure in the Milky Way's dark matter halo. By probing characteristic wakes that a passing dark matter subhalo leaves in the phase-space distribution of ambient halo stars, we estimate sensitivities down to subhalo masses of ∼10^{7} M_{⊙} or below. The detection of such subhalos would have implications for dark matter and cosmological models that predict modifications to the halo-mass function at low halo masses. We develop an analytic formalism for describing the perturbed stellar phase-space distributions, and we demonstrate through idealized simulations the ability to detect subhalos using the phase-space model and a likelihood framework. Our method complements existing methods for low-mass subhalo searches, such as searches for gaps in stellar streams, in that we can localize the positions and velocities of the subhalos today.

Search for Dark Matter Annihilation in Galaxy Groups.

We use 413 weeks of publicly available Fermi Pass 8 gamma-ray data combined with recently developed galaxy group catalogs to search for evidence of dark matter annihilation in extragalactic halos. In our study, we use luminosity-based mass estimates and mass-to-concentration relations to infer the J factors and associated uncertainties for hundreds of galaxy groups within a redshift range z≲0.03. We employ a conservative substructure boost factor model, which only enhances the sensitivity by an O(1) factor. No significant evidence for dark matter annihilation is found, and we exclude thermal relic cross sections for dark matter masses below ∼30 GeV to 95% confidence in the bb[over ¯] annihilation channel. These bounds are comparable to those from Milky Way dwarf spheroidal satellite galaxies. The results of our analysis increase the tension but do not rule out the dark matter interpretation of the Galactic Center excess. We provide a catalog of the galaxy groups used in this study and their inferred properties, which can be broadly applied to searches for extragalactic dark matter.