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

Résumé fourni par la source

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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