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Constraining axion mass through gamma-ray observations of pulsars.

Lloyd, Sheridan J. and Chadwick, Paula M. and Brown, Anthony M. (2019) 'Constraining axion mass through gamma-ray observations of pulsars.', Physical review D., 100 (6). 063005.


We analyze nine years of PASS 8 Fermi-LAT data in the 60–500 MeV range and determine flux upper limits (ULs) for 17 gamma-ray dark pulsars as a probe of axions produced by nucleon-nucleon Bremsstrahlung in the pulsar core. Using a previously published axion decay gamma-ray photon flux model for pulsars which relies on a high core temperature of 20 MeV, we improve the determination of the UL axion mass (ma), at 95% confidence level, to 9.6 × 10−3 eV, which is a factor of 8 improvement on previous results. We show that the axion emissivity (energy loss rate per volume) at realistic lower pulsar core temperatures of 4 MeV or less is reduced to such an extent that axion emissivity and the gamma-ray signal becomes negligible. We consider an alternative emission model based on energy loss rate per mass to allow ma to be constrained with Fermi-LAT observations. This model yields a plausible UL ma of 10−6 eV for pulsar core temperature less than 0.1 MeV, but knowledge of the extent of axion-to-photon conversion in the pulsar B field would be required to make a precise UL axion mass determination. The peak of the axion flux is likely to produce gamma rays in the less than 1 MeV energy range, and so future observations with medium-energy gamma-ray missions, such as AMEGO and e-ASTROGAM, will be vital to further constrain UL ma.

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Publisher statement:Reprinted with permission from the American Physical Society: Lloyd, Sheridan J., Chadwick, Paula M. & Brown, Anthony M. (2019). Constraining Axion Mass through Gamma-ray Observations of Pulsars. Physical Review D 100(6): 063005 © 2019 by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.
Date accepted:09 August 2019
Date deposited:14 August 2019
Date of first online publication:09 September 2019
Date first made open access:08 October 2019

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