Constraining ultralight scalar dark matter couplings with the European Pulsar Timing Array second data release

Pulsar Timing Arrays (PTAs) offer an independent method for searching for ultralight dark matter (ULDM), whose wavelike nature induces periodic oscillations in the arrival times of radio pulses. In addition to this gravitational effect, the direct coupling between ULDM and ordinary matter results in pulsar spin fluctuations and reference clock shifts, leading to observable effects in PTAs. The second data release from the European PTA (EPTA) indicates that ULDM cannot account for all dark matter in the mass range ${m}_{\ensuremath{\phi}}\ensuremath{\in}[{10}^{\ensuremath{-}24.0}\ensuremath{-}{10}^{\ensuremath{-}23.3}]\text{ }\text{ }\mathrm{eV}$ based solely on gravitational effects. In this work, we derive constraints on the coupling coefficients by considering both gravitational and coupling effects. Our results demonstrate that for ULDM with lighter masses, i.e., approximately ${m}_{\ensuremath{\phi}}\ensuremath{\le}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}$, EPTA provides stronger constraints on these couplings than previous PTA experiments and establishes comparable or even tighter constraints than other precise experiments, such as the atomic clock measurements.