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It is widely accepted that dark matter contributes about a quarter of the critical mass-energy density in our Universe. The nature of dark matter is currently unknown, with the mass of possible constituents spanning nearly one hundred orders of magnitude. The ultralight scalar field dark matter, consisting of extremely light bosons with $m sim 10^{-22}$ eV and often called fuzzy dark matter, provides intriguing solutions to some challenges at sub-Galactic scales for the standard cold dark matter model. As shown by Khmelnitsky and Rubakov, such a scalar field in the Galaxy would produce an oscillating gravitational potential with nanohertz frequencies, resulting in periodic variations in the times of arrival of radio pulses from pulsars. The Parkes Pulsar Timing Array (PPTA) has been monitoring 20 millisecond pulsars at two to three weeks intervals for more than a decade. In addition to the detection of nanohertz gravitational waves, PPTA offers the opportunity for direct searches for fuzzy dark matter in an astrophysically feasible range of masses. We analyze the latest PPTA data set which includes timing observations for 26 pulsars made between 2004 and 2016. We perform a search in this data set for evidence of ultralight dark matter in the Galaxy using Bayesian and Frequentist methods. No statistically significant detection has been made. We therefore place upper limits on the local dark matter density. Our limits, improving on previous searches by a factor of two to five, constrain the dark matter density of ultralight bosons with $m leq 10^{-23}$ eV to be below $6,text{GeV},text{cm}^{-3}$ with 95% confidence in the Earth neighborhood. Finally, we discuss the prospect of probing the astrophysically favored mass range $m gtrsim 10^{-22}$ eV with next-generation pulsar timing facilities.
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We describe 14 years of public data from the Parkes Pulsar Timing Array (PPTA), an ongoing project that is producing precise measurements of pulse times of arrival from 26 millisecond pulsars using the 64-m Parkes radio telescope with a cadence of approximately three weeks in three observing bands. A comprehensive description of the pulsar observing systems employed at the telescope since 2004 is provided, including the calibration methodology and an analysis of the stability of system components. We attempt to provide full accounting of the reduction from the raw measured Stokes parameters to pulse times of arrival to aid third parties in reproducing our results. This conversion is encapsulated in a processing pipeline designed to track provenance. Our data products include pulse times of arrival for each of the pulsars along with an initial set of pulsar parameters and noise models. The calibrated pulse profiles and timing template profiles are also available. These data represent almost 21,000 hrs of recorded data spanning over 14 years. After accounting for processes that induce time-correlated noise, 22 of the pulsars have weighted root-mean-square timing residuals of < 1 ${mu}$s in at least one radio band. The data should allow end users to quickly undertake their own gravitational-wave analyses (for example) without having to understand the intricacies of pulsar polarisation calibration or attain a mastery of radio-frequency interference mitigation as is required when analysing raw data files.
An ultralight scalar field is a candidate for the dark matter. The ultralight scalar dark matter with mass around $10^{-23},{rm eV}$ 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 $10^{-8.19},{ rm Hz}$ which corresponds to the mass range from $9.45times10^{-24}$ to $1.34times10^{-23},{rm eV}$. In terms of the energy density of the dark matter, we find that the energy density near the Earth is less than $7,{rm GeV/cm^3}$ in the range from $10^{-8.55}$ to $10^{-8.01},{ rm Hz}$ (from $5.83times10^{-24}$ to $2.02times10^{-23},{rm eV}$). The strongest upper limit on the the energy density is given by $2,{rm GeV/cm^3}$ at a frequency $10^{-8.28},{ rm Hz}$ (corresponding to a mass $1.09times10^{-23},{rm eV}$). We find that the signal of the ultralight scalar dark matter can be explained by 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.
A pulsar timing array (PTA), in which observations of a large sample of pulsars spread across the celestial sphere are combined, allows investigation of global phenomena such as a background of gravitational waves or instabilities in atomic timescales that produce correlated timing residuals in the pulsars of the array. The Parkes Pulsar Timing Array (PPTA) is an implementation of the PTA concept based on observations with the Parkes 64-m radio telescope. A sample of 20 millisecond pulsars is being observed at three radio-frequency bands, 50cm (~700 MHz), 20cm (~1400 MHz) and 10cm (~3100 MHz), with observations at intervals of 2 - 3 weeks. Regular observations commenced in early 2005. This paper describes the systems used for the PPTA observations and data processing, including calibration and timing analysis. The strategy behind the choice of pulsars, observing parameters and analysis methods is discussed. Results are presented for PPTA data in the three bands taken between 2005 March and 2011 March. For ten of the 20 pulsars, rms timing residuals are less than 1 microsec for the best band after fitting for pulse frequency and its first time derivative. Significant red timing noise is detected in about half of the sample. We discuss the implications of these results on future projects including the International Pulsar Timing Array (IPTA) and a PTA based on the Square Kilometre Array. We also present an extended PPTA data set that combines PPTA data with earlier Parkes timing data for these pulsars.
Light Axionic Dark Matter, motivated by string theory, is increasingly favored for the no-WIMP era. Galaxy formation is suppressed below a Jeans scale, of $simeq 10^8 M_odot$ by setting the axion mass to, $m_B sim 10^{-22}$eV, and the large dark cores of dwarf galaxies are explained as solitons on the de-Broglie scale. This is persuasive, but detection of the inherent scalar field oscillation at the Compton frequency, $omega_B= (2.5{rm , months})^{-1}(m_B/10^{-22}eV)$, would be definitive. By evolving the coupled Schrodinger-Poisson equation for a Bose-Einstein condensate, we predict the dark matter is fully modulated by de-Broglie interference, with a dense soliton core of size $simeq 150pc$, at the Galactic center. The oscillating field pressure induces General Relativistic time dilation in proportion to the local dark matter density and pulsars within this dense core have detectably large timing residuals, of $simeq 400nsec/(m_B/10^{-22}eV)$. This is encouraging as many new pulsars should be discovered near the Galactic center with planned radio surveys. More generally, over the whole Galaxy, differences in dark matter density between pairs of pulsars imprints a pairwise Galactocentric signature that can be distinguished from an isotropic gravitational wave background.
We estimate the merger timescale of spectroscopically-selected, subparsec supermassive black hole binary (SMBHB) candidates by comparing their expected contribution to the gravitational wave background (GWB) with the sensitivity of current pulsar timing array (PTA) experiments and in particular, with the latest upper limit placed by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). We find that the average timescale to coalescence of such SMBHBs is $langle t_{rm evol} rangle > 6times 10^4,$yr, assuming that their orbital evolution in the PTA frequency band is driven by emission of gravitational waves. If some fraction of SMBHBs do not reside in spectroscopically detected active galaxies, and their incidence in active and inactive galaxies is similar, then the merger timescale could be $sim 10$ times longer, $langle t_{rm evol} rangle > 6times 10^5,$yr. These limits are consistent with the range of timescales predicted by theoretical models and imply that all the SMBHB candidates in our spectroscopic sample could be binaries without violating the observational constraints on the GWB. This result illustrates the power of the multi-messenger approach, facilitated by the PTAs, in providing an independent statistical test of the nature of SMBHB candidates discovered in electromagnetic searches.
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