Constraints on ultralight axions from compact binary systems
Tanmay Kumar Poddar, Subhendra Mohanty, Soumya Jana
Abstract
Ultralight particles (${m}_{a}\ensuremath{\sim}{10}^{\ensuremath{-}21}--{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$) with axionlike couplings to other particles can be candidates for fuzzy dark matter (FDM) if the axion decay constant ${f}_{a}\ensuremath{\sim}{10}^{17}\text{ }\text{ }\mathrm{GeV}$. If a compact star is immersed in such a low-mass axionic potential, it develops a long-range field outside the star. This axionic field is radiated away when the star is in a binary orbit. The orbital period of a compact binary decays mainly due to the gravitational wave radiation, which was confirmed first in the Hulse-Taylor binary pulsar. The orbital period can also decay by radiation of other light particles like axions and axionlike particles (ALPs). For axionic radiation to take place, the orbital frequency of the periodic motion of the binary system should be greater than the mass of the scalar particle which can be radiated. This implies that, for most of the observed binaries, particles with mass ${m}_{a}<{10}^{\ensuremath{-}19}\text{ }\text{ }\mathrm{eV}$ can be radiated, which includes FDM particles. In this paper, we consider four compact binary systems---PSR $\mathrm{J}0348+0432$, PSR $\mathrm{J}0737\ensuremath{-}3039$, PSR $\mathrm{J}1738+0333$, and PSR $\mathrm{B}1913+16$ (Hulse-Taylor binary)---and show that the observations of the decay in orbital period set a bound on the axion decay constant of ${f}_{a}\ensuremath{\lesssim}\mathcal{O}({10}^{11}\text{ }\text{ }\mathrm{GeV})$. This implies that fuzzy dark matter cannot couple to gluons.