Abstract
An ultralight scalar field is a candidate for the dark matter. The ultralight scalar dark matter with mass around 10−23 eV induces oscillations of the pulse arrival time in the sensitive frequency range of the pulsar timing arrays. We search for the ultralight scalar dark matter using the North American Nanohertz Observatory for Gravitational Waves 11-year Data Set. We give the 95% confidence upper limit for the signal induced by the ultralight scalar dark matter. In comparison with the published Bayesian upper limits on the amplitude of the ultralight scalar dark matter obtained by Bayesian analysis using the Parkes Pulsar Timing Array 12-year data set (Porayko et al. 2018), we find three times stronger upper limit in the frequency range from 10−8.34 to 10−8.19 Hz which corresponds to the mass range from 9.45×10−24 to 1.34×10−23 eV. In terms of the energy density of the dark matter, we find that the energy density near the Earth is less than 7 GeV/cm3 in the range from 10−8.55 to 10−8.01 Hz (from 5.83×10−24 to 2.02×10−23 eV). The strongest upper limit on the energy density is given by 2 GeV/cm3 at a frequency 10−8.28 Hz (corresponding to a mass 1.09×10−23 eV). We find that the signal of the ultralight scalar dark matter is strongly covariant with the solar system ephemeris effect. Also, we reveal that the model of the solar system ephemeris effect prefers parameters which are contrary to the expectation that noise will be reduced on all pulsars.