Abstract
Photons may convert to axion-like particles (ALPs) in external magnetic
fields. Under certain conditions, this effect should result in irregular
features in observed spectra of astrophysical sources. Lack of such
irregularities in particular spectra was used to constrain ALP parameters, with
two most popular sources being the radio galaxy NGC 1275 and the blazar PKS
2155-304. The effect and, consequently, the constraints, depend on the magnetic
fields through which the light from the source is propagated. Here, we revisit
ALP constraints from gamma-ray observations of NGC 1275 taking into account the
regular magnetic field of the X-ray cavity observed around this radio galaxy.
This field was not accounted for in previous studies, which assumed a model of
purely turbulent fields with coherence length much smaller than the cavity
size. For the purely regular field, ALP constraints are relaxed considerably,
compared to the purely turbulent one. Similar arguments hold also for PKS
2155-304. While the actual magnetic field around a source is an unknown sum of
the turbulent and ordered components, the difference in results gives an
estimate of the theoretical uncertainty of the study and calls for detailed
measurements of magnetic fields around sources used to constrain ALP properties
in this approach.
Description
On the impact of magnetic-field models in galaxy clusters on constraints on axion-like paricles from the lack of irregularities in high-energy spectra of astrophysical sources
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