CXOU J164710.2-455216: Swift/XRT flux enhancement following the detection of a burst

ATel #17312; Alice Borghese (ESA/ESAC), Matteo Imbrogno, Francesco Coti Zelati, Nanda Rea (ICE-CSIC, IEEC), Paolo Esposito (IUSS Pavia), Gian Luca Israel (INAF-OAR), on behalf of a larger collaboration
on 28 Jul 2025; 11:01 UT
Credential Certification: Alice Borghese (alice.borghese@gmail.com)

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On 2025 July 26 at 22:49:05 UT, Swift/BAT detected a new short burst from a direction consistent with the magnetar CXOU J164710.2-455216, hosted in the massive star cluster Westerlund 1 (GCN #41172, ATel #17311). This is the first reported flaring activity since the multi-outburst events accompanied by bursts reported in 2017 May, 2017 October and 2018 February (e.g., GCN #21095; GCN #22389; Atel #10877; Atel #11270; Borghese et al., MNRAS, 484, 2931, 2019).

An observation of our Swift monitoring program was performed with the XRT in PC mode on July 27, 2025, starting at 00:16:58 UT and ending at 23:46:53 UT, resulting in an on-source exposure time of about 4 ks. We modeled the spectrum with an absorbed blackbody, assuming a hydrogen column density of 2.5e22 cm^-2 (Borghese et al. 2019). The best fitting values are: blackbody temperature of 0.75(4) keV (errors are reported at 1 sigma confidence level) and radius of 0.52(6) km at a distance of 3.9 kpc (Kothes & Dougherty, A&A, 468, 993, 2007). The absorbed flux in the 0.3-10 keV energy band is 3.8(3)e-12 erg/cm^2/s. Moreover, we detected the spin pulsations at a period P = 10.61064(9) s, with a pulsed fraction (defined as the semi-amplitude of the best-fitting sinusoid divided by the source average count rate) PF = 47(8)%.

We compared these results with those obtained from the previous observation of the monitoring campaign, carried out on May 30, 2025 (ID: 00015870032). The 0.3-10 keV absorbed flux was 1.0(2)e-12 erg/cm^2/s. Therefore, after the detection of the burst, the persistent X-ray flux has increased by a factor ~3.5. We are currently monitoring the source to assess whether the burst with the corresponding flux enhancement has marked the beginning of a new outburst.