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Register a new userHydrogen Axion Star: Metallic Hydrogen Bound to a QCD Axion BEC
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As a cold dark matter candidate, the QCD axion may form Bose-Einstein condensates, called axion stars, with masses around $10^{-11},M_{odot}$. In this paper, we point out that a brand new astrophysical object, a Hydrogen Axion Star (HAS), may well be formed by ordinary baryonic matter becoming gravitationally bound to an axion star. We study the properties of the HAS and find that the hydrogen cloud has a high pressure and temperature in the center and is likely in the liquid metallic hydrogen state. Because of the high particle number densities for both the axion star and the hydrogen cloud, the feeble interaction between axion and hydrogen can still generate enough internal power, around $10^{13}~mbox{W}times(m_a/5~mbox{meV})^4$, to make these objects luminous point sources. High resolution ultraviolet, optical and infrared telescopes can discover HAS via black-body radiation.
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We propose a multi-messenger probe of QCD axion Dark Matter based on observations of black hole-neutron star binary inspirals. It is suggested that a dense Dark Matter spike may grow around intermediate mass black holes ($10^{3}-10^{5} mathrm{,M_{odot}}$). The presence of such a spike produces two unique effects: a distinct phase shift in the gravitational wave strain during the inspiral and an enhancement of the radio emission due to the resonant axion-photon conversion occurring in the neutron star magnetosphere throughout the inspiral and merger. Remarkably, the observation of the gravitational wave signal can be used to infer the Dark Matter density and, consequently, to predict the radio emission. We study the projected reach of the LISA interferometer and next-generation radio telescopes such as the Square Kilometre Array. Given a sufficiently nearby system, such observations will potentially allow for the detection of QCD axion Dark Matter in the mass range $10^{-7},mathrm{eV}$ to $10^{-5},mathrm{eV}$.
Advanced LIGO may be the first experiment to detect gravitational waves. Through superradiance of stellar black holes, it may also be the first experiment to discover the QCD axion with decay constant above the GUT scale. When an axions Compton wavelength is comparable to the size of a black hole, the axion binds to the black hole, forming a gravitational atom. Through the superradiance process, the number of axions occupying the bound levels grows exponentially, extracting energy and angular momentum from the black hole. Axions transitioning between levels of the gravitational atom and axions annihilating to gravitons can produce observable gravitational wave signals. The signals are long-lasting, monochromatic, and can be distinguished from ordinary astrophysical sources. We estimate up to O(1) transition events at aLIGO for an axion between 10^-11 and 10^-10 eV and up to 10^4 annihilation events for an axion between 10^-13 and 10^-11 eV. In the event of a null search, aLIGO can constrain the axion mass for a range of rapidly spinning black hole formation rates. Axion annihilations are also promising for much lighter masses at future lower-frequency gravitational wave observatories; the rates have large uncertainties, dominated by supermassive black hole spin distributions. Our projections for aLIGO are robust against perturbations from the black hole environment and account for our updated exclusion on the QCD axion of 6*10^-13 eV < ma < 2*10^-11 eV suggested by stellar black hole spin measurements.
The QCD axion is expected to form dense structures known as axion miniclusters if the Peccei-Quinn symmetry is broken after inflation. Miniclusters that have survived until today would interact with the population of neutron stars (NSs) in the Milky Way to produce transient radio signals from axion-photon conversion in the NS magnetosphere. Here, we quantify the rate, duration, sky location, and brightness of these interactions for two different minicluster internal density profiles. For both density profiles, we find that these interactions: will occur frequently ($mathcal{O}(1-100),mathrm{day}^{-1}$); last between a day and a few months; are spatially clustered towards the Galactic center; and can reach observable fluxes. Searching for these transient signatures, which are within the reach of current generation telescopes, therefore offers a promising pathway to discovering QCD axion dark matter.
The physics case for axions and axion-like particles is reviewed and an overview of ongoing and near-future laboratory searches is presented.
We study early and late time signatures of both QCD axion strings and hyperlight axion strings (axiverse strings). We focus on charge deposition onto axion strings from electromagnetic fields and subsequent novel neutralizing mechanisms due to bound state formation. While early universe signatures appear unlikely, there are a plethora of late time signatures. Axion strings passing through galaxies obtain a huge charge density, which is neutralized by a dense plasma of bound state Standard Model particles forming a one dimensional atom. The charged wave packets on the string, as well as the dense plasma outside, travel at nearly the speed of light along the string. These packets of high energy plasma collide with a center of mass energy of up to $10^{9}$ GeV. These collisions can have luminosities up to seven orders of magnitude larger than the solar luminosity, and last for thousands of years, making them visible at radio telescopes even when they occur cosmologically far away. The new observables are complementary to the CMB observables for hyperlight axion strings that have been recently proposed, and are sensitive to a similar motivated parameter range.
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