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 P3.86 s, a period derivative Pdot3e-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 (Pdot3.6e-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.

💡 Research Summary

Swift J1555.2‑5402 was discovered on 2021 June 3 by the Swift/BAT instrument as a short X‑ray burst. Follow‑up observations with NICER measured a spin period of ≈3.86 s and a period derivative of ≈3 × 10⁻¹¹ s s⁻¹, confirming its magnetar nature. This paper presents a comprehensive multi‑instrument campaign spanning roughly 29 months, using Swift/XRT, NICER, NuSTAR, INTEGRAL, Insight‑HXMT for X‑ray monitoring and the Parkes Murriyang telescope for radio searches.

The 0.3–10 keV X‑ray flux remained relatively high (≥10⁻¹¹ erg cm⁻² s⁻¹) for about 500 days, then dropped by a factor of ten toward the end of the campaign. The decay can be described by two exponential components: an early e‑folding time of τ≈538 days and a later, steeper component with τ≈262 days. Simultaneously, the blackbody radius shrank from ≈1.7 km to ≈0.3 km (assuming a distance of 10 kpc) following an exponential trend with a similar time‑scale, while the temperature stayed remarkably constant at ≈1.2 keV throughout the entire monitoring period. This combination of a persistent high temperature and a rapidly contracting emitting area is atypical for magnetar outbursts, which usually show cooling temperatures as the hotspot expands or fades.

Broad‑band spectroscopy using the latest NuSTAR observation together with a simultaneous Swift/XRT spectrum (0.3–20 keV) required an absorbed blackbody plus a weak power‑law component (photon index Γ≈0.9) to achieve an acceptable fit. The power‑law contributes roughly 20 % of the 10–60 keV flux and shows a modest decline compared with earlier NuSTAR observations, indicating that the high‑energy tail is not the dominant energy reservoir.

Timing analysis confirms a long‑term spin‑down rate of ˙P≈3.6 × 10⁻¹¹ s s⁻¹, about 15 % higher than the value measured during the first 30 days of the outburst. This modest increase suggests a gradual evolution of the external magnetic torque, possibly linked to changes in the magnetospheric configuration. Phase‑resolved spectroscopy shows little variation of temperature or radius with rotational phase, reinforcing the picture that the main evolution is driven by the shrinking hotspot rather than by geometric effects.

Radio observations performed on 2021 June 4, 5, and 7 with the UWL receiver and two back‑ends (Medusa and PDFB4) yielded no detection of periodic or burst‑like emission. This non‑detection aligns with the general rarity of radio pulsations from magnetars, especially during outburst phases.

The authors argue that the observed phenomenology—constant high blackbody temperature, rapid hotspot contraction, an extended bright plateau followed by a sudden dimming—cannot be readily explained by the standard crust‑heating or magnetospheric‑twist models that have been successful for most magnetar outbursts. They propose that localized magnetic field re‑configuration within the crust, leading to an efficient but spatially confined heat transport, or radiative transfer through a thin, highly conductive layer, might be responsible for maintaining the temperature while the emitting area shrinks. The discrepancy between the flux decay time and the radius‑shrinkage time further supports a decoupling of thermal and geometric evolution.

In summary, Swift J1555.2‑5402 presents an atypical outburst evolution that challenges existing theoretical frameworks. The paper highlights the need for more sensitive, high‑cadence X‑ray monitoring, deeper radio searches, and advanced magneto‑thermal simulations to capture the physics of such unusual magnetar behavior.