A New Avenue in Multimessenger Astronomy with Bursts of Ultralight Scalar Fields*

New ultralight scalar fields are ubiquitous in theories beyond the standard model. If these ultralight bosons exist, violent events that emit gravitational wave (GW) or electromagnetic (EM) signals may also emit coincident relativistic and high-density bursts of the scalar fields, which could be detected here on Earth as a transient event and correlated with the GW/EM signals. This would provide a new avenue in multimessenger astronomy. We study this scenario with ultralight scalar fields whose couplings to electrons, photons, and gluons lead to an observable variation of fundamental constants, to which quantum sensors are sensitive. Since the scalar fields are massive, there can be significant time delay between the scalar burst and GW/EM signal, due to either the emission spectrum, or backaction effects with ordinary matter. We show the regions of parameter space in which this new type of multimessenger astronomy is possible.