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The post-burst awakening of the anomalous x-ray pulsar in Westerlund

Israel, G. L. and Campana, S. and Dall’Osso, S. and Muno, M. P. and Cummings, J. and Perna, R. and Stella, L. (2007) The post-burst awakening of the anomalous x-ray pulsar in Westerlund. Astrophysical Journal, 664 (1). pp. 448-457. ISSN 0004-637X.

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On 2006 September 21, an intense (~10^39 erg s^-1) and short (20 ms) burst was detected by Swift BAT at a position consistent with that of the candidate anomalous X-ray pulsar (AXP) CXOU J164710.2-455216, discovered by Chandra in 2005. Swift follow-up observations began ~13 hr after the event and found the source at a 1–10 keV flux level of about 4.5 x 10^-11 erg cm^-2 s^-1, i.e., ~300 times brighter than measured 5 days earlier by XMM-Newton. We report the results obtained from Swift BAT observations of the burst and subsequent Swift XRT observations carried out during the first 4 months after the burst. These data are complemented with those from two XMM-Newton observations (carried out just before and after the BAT event) and four archival Chandra observations carried out between 2005 and 2007. We find a phase-coherent solution for the source pulsations after the burst. The evolution of the pulse phase comprises an exponential component decaying with timescale of 1.4 days, which we interpret as the recovery stage following a large glitch (Δv/v ~ 6 x 10^-5). We also detect a quadratic component corresponding to a spin-down rate of P ~ 9 x 10^-13 s s^-1, implying a magnetic field strength of 10^14 G. During the first Swift XRT observation taken 0.6 days after the burst, the spectrum showed a kT ~0.65 keV blackbody (R_(BB) ~ 1.5 km) plus a Γ ~ 2.3 power law accounting for about 60% of the 1–10 keV observed flux. Analysis of Chandra archival data, taken during 2005 when the source was in quiescence, reveal that the modulation in quiescence is 100% pulsed at energies above ~4 keV and consistent with the (unusually small-sized) blackbody component being occulted by the neutron star as it rotates. These findings demonstrate that CXOU J164710.2-455216 is indeed an AXP; we compare them with the properties of three other AXPs which displayed similar behavior in the past.

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Additional Information:© 2007 The American Astronomical Society. Received 2006 November 20; accepted 2007 March 15. This work is partially supported at OAR through Agenzia Spaziale Italiana (ASI), Ministero dell’Istruzione, Universitae Ricerca Scientifica e Tecnologica (MIUR-COFIN), and Istituto Nazionale di Astrofisica (INAF) grants. We acknowledge financial contribution from contract ASI-INAF I/023/05/0. We thank Neil Gehrels for approving the Swift ToO observation program and Norbert Schartel for approving the XMM-Newton postburst observation through the Director’s Discretionary Time program. We thank Patrizia Romano for her help in the quick look of the TDRSS data of the BAT event. We also thank Nanda Rea, Andrea Possenti, Marta Burgay, Diego Gotz, and Peter Woods for useful discussions.
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Agenzia Spaziale Italiana (ASI)I/023/05/0
Ministero dell’Istruzione, Universitae Ricerca Scientifica e Tecnologica (MIUR-COFIN)UNSPECIFIED
Istituto Nazionale di Astrofisica (INAF)UNSPECIFIED
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13436
Deposited By: Tony Diaz
Deposited On:24 Jun 2009 21:49
Last Modified:03 Oct 2019 00:38

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