A magnetar outburst with atypical evolution: The case of Swift J1555.2–5402

¹ European Space Science (ESA), European Space Astronomy Center (ESAC) Camino Bajo del Castillo s/n E-28692 Villanueva de la Cañada Madrid, Spain
² Institute of Space Sciences (ICE, CSIC), Campus UAB Carrer de Can Magrans s/n E-08193 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC) E-08860 Castelldefels (Barcelona), Spain
⁴ INAF–Osservatorio Astronomico di Brera Via Bianchi 46 Merate (LC) I-23807, Italy
⁵ INAF–Osservatorio Astronomico di Roma via Frascati 33 I-00078 Monteporzio Catone, Italy
⁶ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano via A. Corti 12 I-20133 Milano, Italy
⁷ Università degli Studi di Milano Bicocca, Dipartimento di Fisica G. Occhialini Piazza della Scienza 3 I-20126 Milano, Italy
⁸ INAF–Osservatorio Astronomico di Cagliari Via della Scienza 5 I-09047 Selargius, Italy
⁹ Scuola Universitaria Superiore IUSS Pavia Piazza della Vittoria 15 I-27100 Pavia, Italy
¹⁰ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova via F. Marzolo 8 I-35131 Padova, Italy
¹¹ Mullard Space Science Laboratory, University College London Holmbury St. Mary Dorking Surrey RH5 6NT, UK
¹² Institut für Astronomie und Astrophysik, Universität Tübingen Sand 1 D-72076 Tübingen, Germany

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Received: 8 October 2025
Accepted: 8 December 2025

Abstract

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 Ṗ ≃ 3 × 10⁻¹¹ s s⁻¹, dozens of short bursts, and an unusual 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 ≳10⁻¹¹ erg cm⁻² s⁻¹ for nearly 500 days before dropping by a factor of ≃10 from its 2021 June peak towards the end of the monitoring campaign. During this time span, the spectrum was dominated by a single blackbody, whose temperature attained an approximately 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 (Ṗ ≃ 3.6 × 10⁻¹¹ s s⁻¹) 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 had been previously reported using different radio facilities. The persistently high temperature, shrinking hotspot, and prolonged bright flux plateau followed by a fast dimming observed during the outburst evolution pose a challenge for the outburst mechanisms proposed so far.