Magnetar outburst models with cooling simulations

¹ Institute of Space Sciences (ICE-CSIC), Campus UAB, C/ de Can Magrans s/n, Cerdanyola del Vallès (Barcelona), 08193 Spain
² Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
³ Gran Sasso Science Institute (GSSI), Viale F. Crispi 7, L’Aquila, 67100 Italy
⁴ Departament de Física Aplicada, Universitat d’Alacant, Ap. Correus 99, E-03080 Alacant, Spain
⁵ Department of Physics and Astronomy, University of Padova, Via Marzolo 8, I-35131 Padova, Italy
⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey, RH5 6NT United Kingdom

^⋆ Corresponding author: degrandis@ice.csic.es

Received: 20 March 2025
Accepted: 25 July 2025

Abstract

Magnetar outbursts are among the most noteworthy manifestations of magnetism in neutron stars. They are episodes in which the X-ray luminosity of a strongly magnetised neutron star swiftly rises by several orders of magnitude to then decay over the course of several months. In this work, we present simulations of outbursts as a consequence of localised heat deposition in a magnetised neutron star crust, and the subsequent surface cooling. In particular, we employed a magnetothermal evolution code adapted to the study of short-term phenomena; that is, one including in its integration domain the outer layers of the star, where heat diffusion is faster. This choice entailed the development and use of heat blanketing envelope models that are thinner than those found in the literature as the surface boundary condition. We find that such envelopes can support a higher surface temperature than the thicker ones (albeit for less time), which can account for the typical luminosities observed in outbursts even when coming from small hotspots (few km in radius). We study several parameters related to the energetics and geometry of the heating region, concluding that the cooling of a crustal hotspot found in the outer part of the crust can account for the luminosity evolution observed in outbursts both in terms of peak luminosity and timescales. Finally, we discuss the key observables that must be studied in future observations to better constrain the nature of the underlying mechanism.