The Northern Cross Fast Radio Burst project

V. Search for transient radio emission from Galactic magnetars

¹ Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, Piazza della Vittoria 15, I-27100 Pavia, Italy
² Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38123 Povo, (TN), Italy
³ INAF–Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius, (CA), Italy
⁴ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via Corti 12, I-20133 Milano, Italy
⁵ INAF–Istituto di Radio Astronomia (IRA), Via Piero Gobetti 101, I-40129 Bologna, Italy
⁶ South African Radio Astronomy Observatory, Black River Park, 2 Fir Street, Observatory, Cape Town 7925, South Africa
⁷ Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
⁸ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
⁹ Dipartimento di Fisica e Astronomia, Università di Padova, Via Marzolo 8, I-35131 Padova, Italy
¹⁰ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
¹¹ INAF–Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monteporzio Catone, Italy
¹² ASI–Space Science Data Center, Via del Politecnico snc, I-00133 Roma, (RM), Italy
¹³ INAF–IAPS, Via del Fosso del Cavaliere 100, I-00133 Roma, (RM), Italy
¹⁴ INFN Tor Vergata, Via della Ricerca Scientifica, I-00133 Roma, (RM), Italy
¹⁵ Institute of Space Sciences and Astronomy (ISSA), University of Malta, Msida MSD 2080, Malta
¹⁶ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Roma, (RM), Italy

^⋆ Corresponding author: andrea.geminardi@iusspavia.it

Received: 5 March 2025
Accepted: 29 May 2025

Abstract

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.