The Post‐Burst Awakening of the Anomalous X‐Ray Pulsar in Westerlund 1

On September 21, 2006, an intense (~10^39 erg/s) and short (20 ms) burst wasdetected by Swift BAT at a position consistent with that of the candidateAnomalous X-ray Pulsar, CXOU J164710.2-455216, discovered by Chandra in 2005.Swift follow-up observations began about 13 hours after the event and found thesource at a 1-10keV flux level of about 4.5 x 10^-11 erg/s/cm^2, i.e. ~300times brighter than measured 5 days earlier by XMM. We report the resultsobtained from Swift BAT observations of the burst and subsequent Swift XRTobservations carried out during the first four months after the burst. Thesedata are complemented with those from two XMM observations (carried out justbefore and after the BAT event) and four archival Chandra observations carriedout between 2005 and 2007. We find a phase coherent solution for the sourcepulsations after the burst. The evolution of the pulse phase comprises anexponential component decaying with timescale of 1.4d which we interpret as therecovery stage following a large glitch (Delta nu / nu about 6 x 10^-5). Wealso detect a quadratic component corresponding to a spin-down rate of Pdot ~ 9x 10^-13, implying a magnetic field strength of 10^14 Gauss. During the first Swift XRT observation taken 0.6 days after the burst, thespectrum showed a kT = 0.65keV blackbody (R_BB = 1.5km) plus a Gamma=2.3power-law accounting for about 60% of the 1-10 keV observed flux. Analysis ofChandra archival data, taken during 2005 when the source was in quiescence,reveal that the modulation in quiescence is 100% pulsed at energies above 4 keVand consistent with the (unusually small-sized) blackbody component beingocculted by the neutron star as it rotates.Comment: Accepted for publication on ApJ Main Journal; 24 pages, 8 figure