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

Limits on High-Frequency Gravitational Waves in Planetary Magnetospheres

High-frequency gravitational waves (HFGWs) carry a wealth of information on the early Universe with a tiny comoving horizon and astronomical objects of small scale but with dense energy. We demonstrate that the nearby planets, such as Earth and Jupiter, can be utilized as a laboratory for detecting the HFGWs. These GWs are then expected to convert to signal photons in the planetary magnetosphere, across the frequency band of astronomical observation. As a proof of concept, we present the first limits from the existing low-Earth-orbit satellite for specific frequency bands and project the sensitivities for the future more-dedicated detections. The first limits from Juno, the latest mission orbiting Jupiter, are also presented. Attributed to the long path of effective GW-photon conversion and the wide angular distribution of signal flux, we find that these limits are highly encouraging, for a broad frequency range including a large portion unexplored before.

Searching for ultralight dark matter conversion in solar corona using Low Frequency Array data

Ultralight dark photons and axions are well-motivated hypothetical dark matter candidates. Both dark photon dark matter and axion dark matter can resonantly convert into electromagnetic waves in the solar corona when their mass is equal to the solar plasma frequency. The resultant electromagnetic waves appear as monochromatic signals within the radio-frequency range with an energy equal to the dark matter mass, which can be detected via radio telescopes for solar observations. Here we show our search for converted monochromatic signals in the observational data collected by the high-sensitivity Low Frequency Array (LOFAR) telescope and establish an upper limit on the kinetic mixing coupling between dark photon dark matter and photon, which can reach values as low as 10-13 within the frequency range of 30 - 80 MHz. This limit represents an improvement of approximately one order of magnitude better than the existing constraint from the cosmic microwave background observation. Additionally, we derive an upper limit on the axion-photon coupling within the same frequency range, which is better than the constraints from Light-Shining-through-a-Wall experiments while not exceeding the CERN Axion Solar Telescope (CAST) experiment or other astrophysical bounds.

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}.

Astroparticle Physics with Compact Objects

Probing the existence of hypothetical particles beyond the Standard model often deals with extreme parameters: large energies, tiny cross-sections, large time scales, etc. Sometimes, laboratory experiments can test required regions of parameter space, but more often natural limitations lead to poorly restrictive upper limits. In such cases, astrophysical studies can help to expand the range of values significantly. Among astronomical sources, used in interests of fundamental physics, compact objects—neutron stars and white dwarfs—play a leading role. We review several aspects of astroparticle physics studies related to observations and properties of these celestial bodies. Dark matter particles can be collected inside compact objects resulting in additional heating or collapse. We summarize regimes and rates of particle capturing as well as possible astrophysical consequences. Then, we focus on a particular type of hypothetical particles—axions. Their existence can be uncovered due to observations of emission originated due to the Primakoff process in magnetospheres of neutron stars or white dwarfs. Alternatively, they can contribute to the cooling of these compact objects. We present results in these areas, including upper limits based on recent observations.

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.

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.

Reexamining the Solar Axion Explanation for the XENON1T Excess

The XENON1T collaboration has observed an excess in electronic recoil events below 5 keV over the known background, which could originate from beyond-the-standard-model physics. The solar axion is a well-motivated model that has been proposed to explain the excess, though it has tension with astrophysical observations. The axions traveling from the Sun can be absorbed by the electrons in the xenon atoms via the axion-electron coupling. Meanwhile, they can also scatter with the atoms through the inverse Primakoff process via the axion-photon coupling, which emits a photon and mimics the electronic recoil signals. We found that the latter process cannot be neglected. After including the keV photon produced via the inverse Primakoff process in the detection, the tension with the astrophysical constraints can be significantly reduced. We also explore scenarios involving additional new physics to further alleviate the tension with the astrophysical bounds.