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CXOU J164710.2-455216/PSR J1647-4552: Swift XRT observations of this active magnetar

ATel #10877; George Younes (GWU), Chryssa Kouveliotou (GWU), Nicholas Gorgogne (GWU), Jamie Kennea (PSU), Brad Cenko (NASA/GSFC)
on 20 Oct 2017; 14:42 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Pulsar, Magnetar

Referred to by ATel #: 10878

We report on early Swift XRT observations of the magnetar CXOU J164710.2-455216/PSR J1647-4552 in the massive star cluster Westerlund 1 (Muno et al. 2006, ApJ 636L, 41M) taken on 2017 October 19 at 04:50:17.6 UT, 89.7 seconds after the first BAT trigger (GCN #22024). The total exposure of the observation is 7.5 ks. The source is detected with an average background-corrected count rate of 0.082 cts/s.

The source spectrum is well fit with either a combination of a blackbody (BB) and a power-law (PL) or 2 BBs. The former fit results in a BB temperature kT=0.53(4) keV and an emitting area of 1.3(4) km (assuming a distance of 3.9 kpc), and a hard PL index, Gamma=1.2(7). This PL index is much harder than the one derived in quiescence, Gamma=3.9. The 2 BBs fit results in a warm BB temperature kT= 0.52(5) keV and an emitting area of about 1.6(3) km, while the hot BB has a temperature of 2(1) keV and an emitting area of 0.12(9) km. The unabsorbed flux in the 2-10 keV range for both fits is about 6.1(5)e-12 erg/s/cm2. This flux level is more than 1 order of magnitude larger than the source quiescent flux in the same energy range, 0.25e-12 erg/s/cm2 (Olausen, S. A. & Kaspi, V. M. 2014, ApJS 212, 6O). The harder spectrum and the brighter flux indicate that magnetar CXOU J164710.2-455216 is currently experiencing an outburst. We note that this is the third outburst from the source since discovery (e.g., An, H. et al. 2013, ApJ 763, 82A; Rodrigez Castillo, G., et al. 2014, MNRAS, 441, 1305R).

We also searched for pulsations from the source in the XRT data using Z^2 (m=2). In the energy range 0.5-10 keV, we find a strong peak at a period of 10.61067(5), consistent with the pulse period from the source. The folded light curve shows a single pulse with a pulsed fraction of about 50% (max-min/max+min).

We encourage high energy and multi-wavelength monitoring of this source.