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

¹ Department of Astronomy, School of Physics, Peking University, Beijing 100871, China
² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
³ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 January 2026
Accepted: 2 April 2026

Abstract

Context. 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.

Aims. 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.

Methods. We employed a semi-analytical ejecta model combined with nuclear reaction network calculations to compute nucleosynthesis yields and multiband light curves for different EOSs and magnetar masses, and we assessed their detectability with current and future facilities.

Results. We find that variations in the EOS and magnetar mass modify the ejecta mass and its density and velocity distributions, among others, 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 M_⊙ 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 ≃10²–10³ s. A more massive magnetar produces fainter emission with a shorter peak timescale. For a magnetar mass of 1.4 M_⊙, novae breves associated with known magnetars may reach peak luminosities of ∼10³⁷–10³⁹ erg s⁻¹, enabling targeted searches, particularly following high-energy GF alerts. Larger ejecta masses yield higher peak luminosities. 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.

Conclusions. Although challenging, the detection of such rapidly evolving transients is feasible. Future searches for novae breves can help establish their observational existence and improve our understanding of the NS EOS and crustal properties.