Observing radio transients with Phased ALMA: Pulses from the Galactic Centre magnetar

Context. Radio transients, such as pulsars and fast radio bursts (FRBs), are primarily detected at centimetre (cm) radio wavelengths, where the highest luminosities are found. However, observations of sources in dense environments are heavily affected by propagation effects, such as scattering, which may hinder a detection. Millimetre (mm)-wave observations bypass this complication but require the largest radio telescopes to compensate for the lower flux densities. When used in phased mode, the ALMA radio telescope provides an equivalent dish size of ∼84 m, making it the most sensitive instrument at mm/sub-mm wavelengths. In combination with its high time resolution, it offers a unique opportunity to study radio transients in an unexplored frequency window.Aims. We studied the Galactic Centre (GC) magnetar, PSR J1745-2900, as a laboratory for magnetars in complex magneto-turbulent environments and for linking with FRBs. Through this pilot study, we showcase the potential of ALMA in its phased configuration to observe radio transients and to achieve, for some sources, the first ever detections outside the cm-wave range.Methods. We studied the GC magnetar using ALMA archival data of Sgr A* at Band 3, taken during the 2017 GMVA campaign. The data were searched in intensity, and the pulses were classified based on their circular and linear polarisation properties and arrival phase.Results. We detected eight highly polarised pulses from the GC magnetar with energies in the range of 10^{29 erg. We constructed its cumulative energy distribution and we fit a power law, assuming the event rate scales with the energy as R ∝ Eγ. The result is an exponent of γ = −2.4 ± 0.1, which is consistent with values reported for magnetars at cm-waves and repeating FRBs. With the γ-value and the system properties of the phased ALMA mode, we estimate that over 160 known pulsars could be detected by ALMA. For repeating FRBs, observing during their peak activity window could lead to detections of several bursts per hour.Conclusions. We expect that ALMA’s lower frequency bands with polarisation capabilities, will serve as a pioneer on mm-wave searches for pulsars and to study complex environments involving radio transients.}