Ultraviolet and Gravitational Wave Signatures of Magnetar Formation

We investigate the impact of gravitational wave (GW) emission by a newly born magnetar onto its electromagnetic (EM) counterparts in the Ultraviolet Transient Astronomy Satellite (ULTRASAT) band. In particular, we carry out a numerical calculation of magnetar-powered shock breakouts (SBOs) and supernova (SN) light curves. We find that an early SBO peak in the bolometric light curves provides a likely signature of a magnetar central engine, which can help constrain the magnetar’s spin period and magnetic dipole. Moreover, our results show that GW emission by the magnetar has only a minor effect on the SBO light curve. Such SBOs can be detected by ULTRASAT at a rate ∼1-2 yr⁻¹, provided that the magnetar wind is not efficiently thermalized during the early phases of the evolution. We additionally find that the subsequent SN light curves can be more luminous than SBOs in the ULTRASAT band and can thus be detected at a higher rate. Finally, we find a possible late-time signature of GW emission in the SN light curves, which may provide an additional tool for the detection of GW signals. Our results demonstrate that future ULTRASAT observations will provide crucial insights into the magnetar formation process and unique information for direct searches of long-transient signals with current and future generation GW detectors. In particular, we estimate a rate of multimessenger (UV+GW) detections of newly formed magnetars ≳1 every 2 yr with ULTRASAT and the Einstein Telescope.