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 á-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 ³10-11 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 (á-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.