A magnetar outburst with atypical evolution: the case of Swift J1555.2-5402

TL;DR

Swift J1555.2-5402’s first magnetar outburst displays an unusually long bright plateau with a nearly constant blackbody temperature and a shrinking hot spot, followed by a rapid late-time decline. Using ~29 months of X-ray data from Swift, NICER, NuSTAR, Insight-HXMT, INTEGRAL and deep Parkes radio searches, the study characterizes spectral and timing evolution, bursts, and absence of radio emission. The spectrum is well described by a stable $kT_{BB}\approx1.2$ keV blackbody with $R_{BB}$ contracting from ~$1.7$ km to ~$0.3$ km and a flux decaying exponentially with $\tau\approx262$ days; timing shows a variable spin-down ($P\approx3.86$ s, $\dot{P}$ ranging from $\sim3.6\times10^{-11}$ to $\sim2.1\times10^{-11}$ s s$^{-1}$) and significant timing noise. The results challenge both crustal cooling and twisted-magnetosphere models, suggesting either long-lived heating or multiple evolving twisted regions to explain the persistent brightness and torque evolution observed during this outburst.

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 Pdot~3e-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 (Pdot~3.6e-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.

Figures (6)

• Figure 1: Temporal evolution of the blackbody temperature and radius, and of the observed and unabsorbed flux (0.3--10 keV) of Swift J1555.2$-$5402 over a time span of about 900 days since the epoch of the first Swift/BAT trigger (on 2021 June 3 at 09:45:46 UT; 59368.40678 MJD). The green dashed lines indicate the fit with an exponential function for the radius and observed flux.
• Figure 2: Top: Values of the spin period measured in the single NICER, Swift, Insight-HXMT/LE and NuSTAR observations in 2021 as a function of time. The blue dashed line indicates the best-fitting model (for more details see Sect. \ref{['sec:timing']}). Post-fit residuals are shown at the bottom. Middle: Evolution of the background-subtracted pulsed fraction of Sw J1555 as a function of time. Bottom: Background-subtracted pulsed fraction as a function of energy for the simultaneous Swift (black circle) and NuSTAR (green diamond) observations. The upper limit is reported at 3$\sigma$ c.l..
• Figure 3: From top to bottom: background-subtracted pulse profile extracted from NICER data at the outburst peak in the 1.5--8 keV energy range, blackbody temperature and radius (assuming a distance of 10 kpc), 0.3--10 keV unabsorbed flux. All uncertainties are at 1$\sigma$ c.l. For display purposes, the pulse profile has been shifted arbitrarily in phase, and two cycles are shown.
• Figure 4: Comparison between the long-term temporal evolution of the normalised flux for the major magnetar outbursts occurred up to the end of 2016 with the outburst of Sw J1555 in red. Data taken from the Magnetar Outburst Online Catalog cotizelati18. For SGR J1745$-$2900, we included the latest observations presented by rea20. The outbursts highlighted in colour are those showing a substantial and fast decay of the flux in the final phase, as observed for Sw J1555.
• Figure 5: Comparison between the flux evolution of Sw J1555 and a model of hotspot cooling modelled with the methodology by 2025AA...701A.229D, showing the evolution of a $\sim10^{44}\,$erg injection in a 3 km radius hotspot over a time of $\sim400\,$days.