Spin-down Measurement of PSR J1745-2900: a New Magnetar

ATel #5046; Eric V. Gotthelf, Kaya Mori, Jules P. Halpern (Columbia University), Nicolas M. Barriere (UC Berkeley), Melania Nynka, Charles J. Hailey (Columbia University) , Fiona A. Harrison (Caltech), Jamie A. Kennea (Penn State), Victoria M. Kaspi (McGill University), Craig B. Markwardt (NASA/GSFC), John A. Tomsick (UC Berkeley), Shuo Zhang (Columbia University)
on 4 May 2013; 22:15 UT
Credential Certification: Eric Gotthelf (eric@astro.columbia.edu)

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Following the detection with the NuSTAR X-ray Observatory of a pulsed signal from a likely magnetar (ATel #5020) located 3" from Sgr A* (ATel #5032), we requested a Swift ToO observation to monitor its temporal and spectral evolution. Data were collected by the XRT in windowed timing (WT) mode with 1.8 ms resolution. We corrected the photon arrival times to the Solar System barycenter using the Chandra coordinates of Rea et al. (ATel #5032). No clock correction is available yet for absolute timing of the quicklook data at this time.

During the 15.5 ks observation a total of 4900 counts were collected from the source in a 0.4' radius aperture in the 0.3-10 keV XRT bandpass. A search around the expected 3.76 s pulsar period reveals a significant signal with a pulse shape and modulation consistent with the NuSTAR discovery observation, confirming our previous detection. Using the Z₃² test we obtain a period of 3.7635603(68) s at epoch MJD 56415.4186. The period uncertainty is determined using the Monte Carlo method described in Gotthelf, Vasisht, & Dotani 1999, ApJ, 522, L49.

Combining the Swift period measurement with a refined NuSTAR value, obtained using a total of 58.5 ks of data, we derive a preliminary Pdot = (2.5+/-1.1)e-11, implying a magnetic field B = 3e14 G, spin-down power Edot = 2e34 erg/s, and characteristic age P/2Pdot = 2.4 kyr, for the dipole model. The Edot is smaller than the concurrent X-ray luminosity by an order of magnitude, which rules out rotation power as the source of the X-ray emission. We note that the value of Pdot may be significantly larger than the long-term spin-down rate due to the possibility of recovery from a glitch, as is commonly observed in magnetars in outburst (see e.g. Kaspi et al 2003; Dib et al 2008).

Based on the X-ray timing properties, the pulsed radio emission (ATel #5035,#5040,#5043), the X-ray spectral measurements (ATel #5006,#5011,#5020), and the likely association with Swift flaring behavior (ATel #5006,#5009), we conclude that PSR/SGR J1745-2900 is a magnetar that has emerged from a quiescent state. This is similar to the behavior of the Transient Anomalous X-ray Pulsars, and further blurs any distinction among the different classes of magnetars.

We thank Neil Gehrels for approving this TOO request, and the Swift duty scientists and planners for carrying out the observations.