The Axion is a hypothetical particle, which has been originally proposed to solve the strong-CP problem of QCD. It was later realized that the axion is also a promising dark matter candidate. Axion-like fields can also be found in most versions of string theory and have implications for the foundations of physics, as we understand it today. Theory predicts that axions should have a (very) weak interaction of axions with the electromagnetic field. In particular, the Primakoff effect, where two photons interact with an axion, leads to photon-axion conversion whose probability (in the limit of small conversions) is ~g2B2R2/4 where g is the coupling constant, B the magnetic field, and R the coherence length of the magnetic field (e.g., the size of the system). The physical process is often termed photon-particle oscillations and is similar, in essence, to neutrino oscillations. Neither the coupling strength of axions to photons, nor the mass of the axion are predicted by theory. To date, such particles have not been detected.
Terrestrial experiments to search for axions usually exploit the Primakoff effect in which photons convert into axions (this also applies to gravitons). One type of approach is the “light shining through walls” design. In such experiments, a strong laser beam traverses a magnetized medium where some of the photons convert to axions. A wall blocks the photon flux but not the axion flux which propagates freely through such obsticales and could later reconvert to photons in the presence of a magnetic field and be detected behind the wall. Such experiments currently give only upper limit on the value of the coupling constant, g, within a certain mass range for the axion. Another type of experiments are helioscopes (e.g., CAST) that look at the sun which is presumably an axion source, and converts some axions to X-ray photons. Current best limits on g from such experiments are still several orders of magnitude above most theoretical model predictions with the major limitation originating from the relatively weak laboratory magnetic fields that can be produced and the small size of the experiments.
Compact astrophysical objects , in contrast to terrestrial apparatus, could have intense magnetic fields extending over large scales. A good example for such systems is that of magnetars whose size is of order 10km and possess a magnetic field of order 1e11 Tesla; i.e., about 10 orders of magnitude stronger than the strongest laboratory fields. Potentially, such intense fields and large size systems allow one to probe down to much lower values of the coupling constant. Nevertheless, to be able to detect axions or place reliable limits on their existence, one needs to know what signature to look for. This work, done at CITA and at the IAS (Princeton), models the spectral signatures arising from photon-axion oscillations in a variety of astrophysical environments. It turns out that under conditions that prevail in compact systems (such as pulsars, magnetars, and quasars), a prominent and broad spectral feature is expected to occur which could be detected using broad band photometry and low-to-medium resolution spectroscopy . Studying such features, one could probe down to coupling constants lower by 2-3 orders of magnitude than currently reached by other methods.
Spectral signatures of photon-axion oscillations were calculated in great detail and the phenomenology is rich with the shapes and energies of the predicted signals depending primarily on the stratification of the magnetic field and plasma density around the compact object. Generally, the predicted spectral feature looks like an absorption feature although the physics responsible to it is very different (e.g., no energy is dissipated in the oscillation process). The expected shapes differ dramatically from other known absorption features such as atomic lines and edges. In addition, depending on how the properties of compact objects (such as the magnetic field, and plasma density) depend on time, the shape and position of the spectral oscillation feature may vary with time in a particular way. The markedly different phenomenology of photon-particle oscillation features compared to atomic features allows for their secure detection in realistic astrophysical spectra .
Future prospects. Probing axion physics by compact object spectroscopy seems very promising. If our physical understanding of magnetars, pulsars, and quasars is qualitatively correct then using current photometric and spectroscopic data one may be able to probe previously unexplored regions of the axion parameter space. It is also possible to further optimize the observations to detect axions (by using e.g., multiple short exposure photometry of magnetars) pushing the g-limits on sub-meV axion detection by almost three orders of magnitude compared to other methods. Clearly, if the axion is detected then one may have solved two of the most fundamental physical problems of current times: the dark matter problem in cosmology and astrophysics, and the strong-CP problem of QCD.
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