http://arxiv.org/abs/1807.10773
Axion-like-particles (ALPs) produced in the core of a magnetar can convert to photons in the magnetosphere, giving rise to novel features in the X-ray spectrum. Since ALPs only mix with the parallel mode of the photon, the polarization of the soft and hard X-ray spectra is predicted to be different from the case where no ALP-to-photon conversion occurs. We quantify this prediction by investigating scenarios where X-ray photons are produced both by astrophysical processes and by ALP-photon conversion, in an uncorrelated fashion, and in different relative proportions, which we parametrize by the angle $\chi_0$. We then define a normalized astrophysics-subtracted Stokes parameter $R$ which only acquires non-zero values in the presence of ALP-to-photon conversion. We find, remarkably, that the parameter $R$ factorizes into a product of the ALP-to-photon conversion probability and $\cos(2\chi_0)$. We display $R$ as a function of the photon energy, finding significant deviation from zero in both the soft as well as the hard X-ray regimes, and go on to display the predicted polarization on ALP parameter space. A measurement of non-zero $R$ in future X-ray polarimeters, in conjunction with astrophysics modeling of the polarization due to the surface plasma, will be crucial to detect the presence of ALPs from magnetars.
J. Fortin and K. Sinha
Tue, 31 Jul 18
26/69
Comments: 1+19 pages, 3 figures
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