Axions constituting dark matter (DM) are often considered to form a non-relativistic oscillating field. We explore bursts of relativistic axions from transient astrophysical sources, such as axion star explosions, where the sources are initially non-
relativistic. For the QCD axion, bursts from collapsing axion stars lead to potentially detectable signals over a wide range of axion masses $10^{-15} , textrm{eV} lesssim m lesssim 10^{-7} , textrm{eV}$ in future experiments, such as ABRACADABRA, DMRadio and SHAFT. Unlike conventional cold axion DM searches, the sensitivity to axion bursts is not necessarily suppressed as $1/f$ for large decay constants $f$. The detection of axion bursts could provide new insights into the fundamental axion potential, which is challenging to probe otherwise. An ensemble of bursts in the distant past, in direct analogy with neutrinos, would give rise to a diffuse axion background distinct from the usual cold axion DM. Coincidence with other signatures, such as electromagnetic and gravitational-wave emission, would provide a new beyond-the-standard-model window into multi-messenger astronomy.
We study luminous dark matter signals in models with inelastic scattering. Dark matter $chi_1$ that scatters inelastically off elements in the Earth is kicked into an excited state $chi_2$ that can subsequently decay into a monoenergetic photon insid
e a detector. The photon signal exhibits large sidereal-daily modulation due to the daily rotation of the Earth and anisotropies in the problem: the dark matter wind comes from the direction of Cygnus due to the Suns motion relative to the galaxy, and the rock overburden is anisotropic, as is the dark matter scattering angle. This allows outstanding separation of signal from backgrounds. We investigate the sensitivity of two classes of large underground detectors to this modulating photon line signal: large liquid scintillator neutrino experiments, including Borexino and JUNO, and the proposed large gaseous scintillator directional detection experiment CYGNUS. Borexinos (JUNOs) sensitivity exceeds the bounds from xenon experiments on inelastic nuclear recoil for mass splittings $delta gtrsim 240 (180)$ keV, and is the only probe of inelastic dark matter for ${350 text{ keV} lesssim delta lesssim 600 text{ keV}}$. CYGNUSs sensitivity is at least comparable to xenon experiments with $sim 10 ; {rm m}^3$ volume detector for $delta lesssim 150$ keV, and could be substantially better with larger volumes and improved background rejection. Such improvements lead to the unusual situation that the inelastic signal becomes the superior way to search for dark matter even if the elastic and inelastic scattering cross sections are comparable.
Analytic arguments and numerical simulations show that bosonic ultra-light dark matter (ULDM) would form cored density distributions (`solitons) at the center of galaxies. ULDM solitons offer a promising way to exclude or detect ULDM by looking for a
distinctive feature in the central region of galactic rotation curves. Baryonic contributions to the gravitational potential pose an obstacle to such analyses, being (i) dynamically important in the inner galaxy and (ii) highly non-spherical in rotation-supported galaxies, resulting in non-spherical solitons. We present an algorithm for finding the ground state soliton solution in the presence of stationary non-spherical background baryonic mass distribution. We quantify the impact of baryons on the predicted ULDM soliton in the Milky Way and in low surface-brightness galaxies from the SPARC database.
