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 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.

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Journal
Astronomy and Astrophysics
Published
2025-12-17
DOI
https://doi.org/10.1051/0004-6361/202557603
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
0.00

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article

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

N. Rea, F. Coti Zelati, S. Mereghetti, Andrea Possenti et al.
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
article

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

N. Rea, F. Coti Zelati, S. Mereghetti, Andrea Possenti, M. Imbrogno, P. Esposito, A. Borghese, G. L. Israel, L. Ducci, Davide De Grandis, M. Burgay, M. Trudu, M. Pilia, Dominik P. Pacholski
article en

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.

Astronomy and AstrophysicsVol. 706
Institute of Space Sciences (ES), University of Padua (IT), Astronomical Observatory of Rome (IT), Istituto Universitario di Studi Superiori di Pavia (IT), European Space Agency (FR), Institut d'Estudis Espacials de Catalunya (ES), European Space Astronomy Centre (ES), Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano (IT), Osservatorio Astronomico di Cagliari (IT), University College London (GB), University of Milano-Bicocca (IT), University of Tübingen (DE), National Institute for Astrophysics (IT)
National Aeronautics and Space Administration, UK Space Agency, Australian Government, Commonwealth Scientific and Industrial Research Organisation, Goddard Space Flight Center
Openalex Percentile: Top 72%
Astrophysical Phenomena and Observations
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