The multi-outburst activity of the magnetar in Westerlund I

Abstract

After two major outbursts in 2006 and 2011, on 2017 May 16 the magnetar CXOU J164710.2-455216, hosted within the massive star cluster Westerlund I, emitted a short (20 ms) burst, which marked the onset of a new active phase. We started a long-term monitoring campaign with Swift (45 observations), Chandra (5 observations) and NuSTAR (4 observations) from the activation until 2018 April. During the campaign, Swift BAT registered the occurrence of multiple bursts, accompanied by two other enhancements of the X-ray persistent flux. The long time span covered by our observations allowed us to study the spectral as well as the timing evolution of the source. After 11 months since the 2017 May outburst onset, the observed flux was about 15 times higher than its historical minimum level and a factor of 3 higher than the level reached after the 2006 outburst. This suggests that the crust has not fully relaxed to the quiescent level, or that the source quiescent level has changed following the multiple outburst activities in the past 10 years or so. This is another case of multiple outbursts from the same source on a yearly time scale, a somehow recently discovered behaviour in magnetars.

Accepted for publication in MNRAS, 14 pages, 7 figures

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• The first 7 months of the 2020 X-ray outburst of the magnetar SGR J1935+2154
• The long-term enhanced brightness of the magnetar 1E 1547.0-5408
• X-ray timing studies of the low-field magnetar CXOU J164710.2-455216
• eROSITA X-ray Analysis of the PeVatron Candidate Westerlund 1
• Magnetar Outburst Models with Cooling Simulations
• A month of monitoring the new magnetar Swift J1555.2-5402 during an X-ray outburst
• A magnetar outburst with atypical evolution: the case of Swift J1555.2-5402