XMM Observations of AXP CXO J167410.2-455216 Four Days Prior to Outburst

ATel #902; M. Muno (Caltech), B. Gaensler (Sydney), J. S. Clark (OpenU), S. Portegies Zwart (Amsterdam), D. Pooley (Berkeley), R. de Grijs (Sheffield), I. Stevens (Birmingham), and I. Negueruela (Alicante)
on 29 Sep 2006; 20:40 UT
Credential Certification: M. Muno (mmuno@srl.caltech.edu)

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The XMM Newton observatory observed the anomalous X-ray pulsar CXO J167410.2-455216 (ATEL #893, #894, #896, #901) on 2006 Sept 16 at 18:58 UT for 45 ks, only 4.3 days before a 20 msec burst observed with the Burst Alert Telescope on Swift (Krimm et al. GCN #5581) apparently initiated an outburst from the source. The pulsar was detected at a flux of 1.5e-13 erg cm^-2 s^-1 (0.5-8.0 keV), which was 50% below the discovery value in 2006 (Muno et al. 2006, ApJ, 636 L41), and 300 times fainter than was measured by Campana et al.(ATEL #893) 12.4 h after the burst. The spectrum could be described (reduced chi-squared <1) as a 0.54+/-0.02 keV blackbody with a radius of 0.16+/-0.06 km (for D=5 kpc). The inferred absorption column was equivalent to (1.4+/-0.1)e22 cm^-2 of H. This was lower than the reported post-outburst values, although the discrepancy was probably not physical, but a result of the fact that a power law component was not required in our model. The pulsar had a period of 10.61065(7) s on MJD 53994.786313(2) (pulse maximum), which was consistent with the discovery value, and the value reported by Gavriil et al. (ATEL #901). We confirm the spin-down rate was <9e-12 (3 sigma), which implies a B field of <3e14 G. The pulse was single-peaked with an rms amplitude in the fundamental of 63+/-2\%, and a harmonic with amplitude 20+/-3\%. There was no evidence for higher harmonics (<8\%). Gavriil et al. reported that the pulse profile had three peaks and a 20\% rms amplitude 6.2 days after the burst. Therefore, the outburst not only caused a large increase in the flux from the source, but also a dramatic change in the geometry of the X-ray emitting region.