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Binary pulsars as dark-matter probes

Abstract

During the motion of a binary pulsar around the Galactic center, the pulsar and its companion experience a wind of dark-matter particles that can affect the orbital motion through dynamical friction. We show that this effect produces a characteristic seasonal modulation of the orbit and causes a secular change of the orbital period whose magnitude can be well within the astonishing precision of various binary-pulsar observations. Our analysis is valid for binary systems with orbital period longer than a day. By comparing this effect with pulsar-timing measurements, it is possible to derive model-independent upper bounds on the dark-matter density at different distances DD from the Galactic center. For example, the precision timing of J1713+0747 imposes ρDM105GeV/cm3\rho_{\rm DM}\lesssim 10^5\,{\rm GeV/cm}^3 at D7kpcD\approx7\,{\rm kpc}. The detection of a binary pulsar at D10pcD\lesssim 10\,{\rm pc} could provide stringent constraints on dark-matter halo profiles and on growth models of the central black hole. The Square Kilometer Array can improve current bounds by 2 orders of magnitude, potentially constraining the local density of dark matter to unprecedented levels.Comment: 8+3 pages, 7 figures. To appear in Phys. Rev. D; v2: matches published versio

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