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Register a new userTowards Robust Constraints on Axion Dark Matter using PSR J1745-2900
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Added by
Sankarshana Srinivasan Mr
Publication date
2021
fields
Physics
and research's language is
English
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We apply novel, recently developed plasma ray-tracing techniques to model the propagation of radio photons produced by axion dark matter in neutron star magnetospheres and combine this with both archival and new data for the galactic centre magnetar PSR J1745-2900. The emission direction to the observer and the magnetic orientation are not constrained for this object leading to parametric uncertainty. Our analysis reveals that ray-tracing greatly reduces the signal sensitivity to this uncertainty, contrary to previous calculations where there was no emission at all in some directions. Based on a Goldreich-Julian model for the magnetosphere and a Navarro-Frank-White model for axion density in the galactic centre, we obtain the most robust limits on the axion-photon coupling, to date. These are comparable to those from the CAST solar axion experiment in the mass range $sim 4.2-60,mu{rm eV}$. If the dark matter density is larger, as might predicted by a spike model, the limits could be much stronger. The dark matter density in the region of the galactic centre is now the biggest uncertainty in these calculations.
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Polarised radio emission from PSR J1745-2900 has already been used to investigate the strength of the magnetic field in the Galactic Centre, close to Sagittarius A*. Here we report how persistent radio emission from this magnetar, for over four years since its discovery, has revealed large changes in the observed Faraday rotation measure, by up to 3500 rad m$^{-2}$ (a five per cent fractional change). From simultaneous analysis of the dispersion measure, we determine that these fluctuations are dominated by variations in the projected magnetic field, rather than the integrated free electron density, along the changing line of sight to the rapidly moving magnetar. From a structure function analysis of rotation measure variations, and a recent epoch of rapid change of rotation measure, we determine a minimum scale of magnetic fluctuations of size ~ 2 au at the Galactic Centre distance, inferring PSR J1745-2900 is just ~ 0.1 pc behind an additional scattering screen.
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If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-$z$ galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and find that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range $z=6$-$10$, and the optical depth to reionization, $tau$, as measured from cosmic microwave background polarization. All probes we consider consistently exclude $m_alesssim 10^{-23}text{ eV}$ from contributing more than half of the DM, with our strongest constraint ruling this model out at more than $8sigma$ significance. In conservative models of reionization a dominant component of DM with $m_a=10^{-22}text{ eV}$ is in $3sigma$ tension with the measured value of $tau$, putting pressure on an axion solution to the cusp-core problem. Tension is reduced to $2sigma$ for the axion contributing only half of the DM. A future measurement of the UV-luminosity function in the range $z=10$-$13$ by JWST would provide further evidence for or against $m_a=10^{-22}text{ eV}$. Probing still higher masses of $m_a=10^{-21}text{ eV}$ will be possible using future measurements of the kinetic Sunyaev-Zeldovich effect by Advanced ACTPol to constrain the time and duration of reionization.
The dark photon, an new hypothetical light spin 1 field, constitutes a well-motivated dark matter candidate. It manifests as an oscillating electric field with a fixed direction, which can be observed in magnetometric records. In this letter, we use magnetometer data from the Voyager probes to look for the dark photon in the 10^-24 eV to 10^-19 eV mass range, corresponding to frequencies between 10^-9 Hz and 10^-4 Hz. We also discuss the sensitivity of possible future SQUID magnetometry experiments.
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