X-ray polarization signals from magnetars with axion-like-particles

Abstract

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 have an O-mode component, in addition to the mainly X-mode component given by most astrophysical models. The relative strength of the O-mode component depends on the intensity of ALPs produced in the core and the probability of conversion. We quantify our results by considering X-ray emission 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 χ 0 . We then define a normalized astrophysics-subtracted Stokes parameter R which only acquires non-zero values in the presence of ALP-photon conversion. We find, remarkably, that the parameter R factorizes into a product of the ALP-to-photon conversion probability and cos(2χ 0 ) and display R, as well as the usual Stokes parameter Q, as a function of the photon energy and relative fractions of ALP and photon intensities. For benchmark points currently allowed by the CAST experiment, the O-mode prediction can be tested in future X-ray polarimeters and used either to constrain ALPs or find evidence for them.July 2018

Astroparticle Physics with Compact Objects

“…Multiplying the theoretically expected axionic spectrum by the probability, it is now possible to obtain the spectrum of X-rays from the conversion and compare it to observations. In addition, polarization measurements could be used to constrain properties of axions: converted photons could have only single polarization E || , also known as O-mode (ordinary), while many models of NS atmospheres predict that NS emission, especially at lower energies <1 keV, is primarily polarized in perpendicular X-mode (extraordinary) [98] and an admixture of a differently polarized mode could be, in principle, detected [99].…”

“…Different classes of NSs were suggested to use for this type of searches. First, the magnetars [96,99]-NSs with extremely high magnetic fields exceeding 10 14 G and high central temperatures, up to ∼10 9 K, which boosts their axionic luminosities. There are obvious downsides as well: all of them are rather distant and that greatly decreases the flux observed at the Earth.…”

Astroparticle physics with compact objects

“…Multiplying theoretically expected axionic spectrum by the probability it is now possible to obtain spectrum of X-rays from the conversion and compare it to observations. Also polarization measurements could be used to constrain properties of axions: converted photons could have only single polarization E || , also known as O-mode (ordinary), while many models of NS atmospheres predict that NS emission, especially at lower energies < 1 keV, is primarily polarized in perpendicular X-mode (extraordinary) [98] and an admixture of differently polarized mode could be in principle detected [99].…”

“…Different classes of NSs was suggested to use for this type of searches. First, the magnetars [96,99] -NSs with extremely high magnetic fields exceeding 10 14 G and high central temperatures, up to ∼ 10 9 K which boosts their axionic luminosities. There are obvious downsides as well: all of them are rather distant and that greatly decreases the flux observed at the Earth.…”

Photon-axion mixing in thermal emission of isolated neutron stars

“…Their subsequent transition into photons in the magnetic field surrounding the star could lead to observable signatures in its hard X-ray spectrum [6,19,20]. Magnetars are natural targets for such investigation due to the extremely strong magnetic fields and high internal temperatures, and their study led to further astrophysical limits on axion parameters [21,22,23].…”