The Northern Cross Fast Radio Burst project

Context. The radio emission from magnetars is poorly understood and poorly characterized observationally, particularly for what concerns single pulses and sporadic events. Interest in this type of radio emission has been boosted by the detection of an extremely bright millisecond radio signal from the Galactic magnetar designated as SGR J1935+2154 in 2020, which occurred almost simultaneously with a typical magnetar short burst of X-rays. As of now, this event remains the Galactic radio pulse that is the most reminiscent of fast radio bursts, and it is the only one that has a sound association with a known progenitor. Aims. We aim to constrain the rate of impulsive radio events from magnetars by means of intensive monitoring using a high-sensitivity radio telescope. Methods. We performed a long-term campaign on seven Galactic magnetars (plus one candidate) using the Northern Cross transit radio telescope (in Medicina, Italy), searching for short timescales and dispersed radio pulses. Results. We obtained no detections in ∼560 hours of observation, setting an upper limit at a 95% confidence level of < 52 yr-1 on the rate of events with energy ≳1028 erg, which is consistent with limits in the literature. Furthermore, under some assumptions regarding the properties and energetic behavior of magnetars, we find that our upper limits point toward the fact that the entire population of observed fast radio bursts cannot be explained by radio bursts emitted by magnetars.