Novae breves from magnetar giant flares: Potential probes of neutron star crusts

Abstract

Matter ejected from the magnetar crust during giant flares (GFs) may undergo $r$-process nucleosynthesis, producing short-lived optical transients termed "novae breves". Although intrinsically much fainter than kilonovae from compact binary mergers, novae breves may occur within or near the Galaxy, making them promising observational targets. We aim to investigate how the neutron star (NS) equation of state (EOS) and the mass of the central magnetar affect the ejecta properties following GFs and the resulting nova brevis emission. We employ a semi-analytical ejecta model combined with nuclear reaction network calculations to compute nucleosynthesis yields and multi-band light curves for different EOSs and magnetar masses, and assess their detectability with current and future facilities. We find that variations in the EOS and magnetar mass modify the ejecta mass and its density and velocity distributions, etc., leading to observable differences in nova brevis light curves. In particular, both the peak luminosity and the characteristic peak timescale are EOS-dependent. Assuming a fixed Galactic magnetar mass of 1.4 solar mass and taking the $u$ band as an example, we find that the minimum apparent AB magnitudes range from 7 mag (H4 EOS) to 8.5 mag (WFF EOS) with peak timescales of 100-1000 s. A more massive magnetar produces fainter emission with a shorter peak timescale. For a magnetar mass of 1.4 solar mass, novae breves associated with known magnetars may reach peak luminosities of 1e37-1e39 erg/s, enabling targeted searches, particularly following high-energy GF alerts. Moreover, a detection horizon of 10 Mpc or beyond is achievable with current and future facilities, allowing searches for novae breves from previously unknown magnetars in the Local Volume. Although challenging, detection of such rapidly evolving transients is feasible.