Swift XRT follow-up of the transient magnetar CXO J1647-4552

ATel #3653; G. L. Israel (INAF-OAR), P. Esposito (INAF-OAC), N. Rea (CSIC-IEEC) report on behalf of a larger collaboration
on 20 Sep 2011; 12:54 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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We report on the preliminary results of the first Swift-XRT observations of the transient magnetar CXO J1647-4552 after the BAT detection of four SGR-like bursts (Baumgartner et al. GCN 12359). We analyzed the observation performed on 2011, September 20 carried out in PC mode (8.6ks of effective exposure time). Data were corrected for pile-up when necessary.

Preliminary spectral study of this observation found the source, at the beginning of the pointing, at a persistent flux of about 7.8 x 10^-11 ergs/cm^2/s (in the 1-10keV energy band), >100 times higher than the flux recorded during the latest observation carried out by XMM in August 2009 (in the same energy band), and >250 times with respect to the pre-outburst level observed in September 2006 by XMM (2.7 x 10^-13 ergs/cm^2/s, Muno et al. 2007, MNRAS, 378, L44). The flux rapidly decreased to ~1.2 x 10^-11 ergs/cm^2/s towards the end of the pointing.

The spectrum is significantly harder than those of August 2009 and September 2006 (pre-outburst). The source spectra, during the first orbit and the remaining ones are fitted well by an absorbed (NH = (2.6+\-0.4) x 10^22 cm^-2) power-law component with the following parameters: Gamma = 1.5+\-0.7 and 2.2+\-0.3, respectively (reduced chi^2 of 1.1 for 64 degree of freedom; 90% c.l. are reported).

Timing analysis was performed after having removed bursts and corrected the photon arrival times to the barycenter of the Solar System. Pulsations at about 10.61s were clearly detected. A refined period measurement was obtained by means of a phase fitting technique, and corresponds to a best-fit value of P=10.6104(3)s (90% c.l.) consistent, within the uncertainty, to the August 2009 value. The pulse shape is almost sinusoidal with a background-subtracted pulsed fraction of 47+\-3%. The pulsed fraction and pulse shape are significantly different from those recorded at the beginning of the September 2006 outburst (pulsed fraction of about 10% and pulse shape characterized by three peaks; Israel et al. 2007, ApJ, 664, 448; Woods et al. 2011, ApJ, 726, 37), while are rather similar to those observed in August 2009 during the outburst decay (almost sinusoidal shape with a pulsed fraction of 43+\-1%). Note that the pulsed fraction in the quiescent state is about 92%+\-3% (Muno et al. 2007; Israel et al. 2007).