arXiv subjects

D. De Grandis

Publications and source records attributed to D. De Grandis.

3 recordsLinked to original sources

arXiv2025

A magnetar outburst with atypical evolution: the case of Swift J1555.2-5402

The magnetar Swift J1555.2-5402 was discovered in outburst on 2021 June 3 by the Burst Alert Telescope on board the Swift satellite. Early X-ray follow-up revealed a spin period P~3.86 s, a period derivative Pdot~3e-11 s/s, dozens of short bursts, and an unusually flux decline. We report here on the X-ray monitoring of Swift J1555.2-5402 over the first ~29 months of its outburst with Swift, NICER, NuSTAR, INTEGRAL and Insight-HXMT, as well as radio observations with Parkes soon after the outburst onset. The observed 0.3-10 keV flux remained at levels >~1e-11 erg/cm^2/s for nearly 500 days before dropping by a factor of ~10 from its June 2021 peak towards the end of the monitoring campaign. During this time span, the spectrum was dominated by a single blackbody, with temperature attaining approximately a constant value (~1.2 keV) while the inferred radius shrank from ~1.7 km to ~0.3 km (assuming a source distance of 10 kpc). The long-term spin-down rate (Pdot~3.6e-11 s/s) is only ~15 % higher than that measured in the first 30 days. No periodic or burst-like radio emission was detected, in line with what has been previously reported using different radio facilities. The persistently high temperature, shrinking hotspot, and a prolonged bright flux plateau followed by a fast dimming observed during the outburst evolution pose a challenge for the outburst mechanisms proposed so far.

astro-ph.HE↗

arXiv2025

Neutron star envelopes with machine learning: a single-hidden-layer neural network application

Thermal and magneto-thermal simulations are an important tool for advancing understanding of neutron stars, as they allow us to compare models of their internal structure and physical processes against observations constraining macroscopic properties such as the surface temperature. A major challenge in the simulations is in modelling of the outermost layers, known as the envelope, exhibiting a drop of many orders of magnitude in temperature and density in a geometrically thin shell. This is often addressed by constructing a separate envelope model in plane-parallel approximation that produces a relation between the temperature at the bottom of the envelope, $T_b$, and the surface temperature, $T_s$. Our aim is to construct a general framework for approximating the $T_b$-$T_s$ relation that is able to include the dependencies from the strength and orientation of the magnetic field. We used standard prescriptions to calculate a large number of magnetised envelope models to be used as a training sample and employed single-hidden-layer feedforward neural networks as approximators, providing the flexibility, high accuracy, and fast evaluation necessary in neutron star simulations. We explored the optimal network architecture and hyperparameter choices and used a special holdout set designed to avoid overfitting to the structure of the input data. We find that relatively simple neural networks are sufficient for the approximation of the $T_b$-$T_s$ relation with an accuracy $\sim 3\%$. The presented workflow can be used in a wide range of problems where simulations are used to construct approximating formulae.

astro-ph.HE↗

arXiv2025

Magnetar Outburst Models with Cooling Simulations

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 ondition. 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.

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