Chandra Target of Opportunity Observations of CXO J164710.2-455216

ATel #901; F. P. Gavriil (NPP Fellow; GSFC), P. M. Woods (Dynetics), V. M. Kaspi (McGill U.)
on 28 Sep 2006; 18:40 UT
Credential Certification: Victoria Kaspi (vkaspi@physics.mcgill.ca)

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The Chandra X-ray Observatory observed the Anomalous X-ray Pulsar CXO J164710.2-455216, which is currently either in or recovering from an outburst (see ATELs #893, #894, #896), on Sept 27 for 15,100 ks starting at 7 UT. The source was observed in continuous clocking mode with the S3 chip on ACIS-S. We collected ~24,500 counts, with an average count rate of ~1.6 cps. There is no evidence for bursts in these data. Pulsations are clearly detected from the source. A preliminary analysis using a predictive orbital ephemeris finds a barycentric period of 10.61069(6) s (1 sigma uncertainty) at 54005.379 MJD (TDB). This is not significantly different from that measured by Muno et al. (2006) on the basis of Chandra data from May and June 2005. The upper limit on the implied long-term average period derivative is 9e-12, which suggests a surface dipolar magnetic field B < 3e14 G, under standard assumptions. However we note that an extremely large glitch or glitches (dP/P>3e-5) since the Muno et al measurement could be contaminating this limit. The CXO ToO pulsations are highly non-sinusoidal, showing 3 peaks of different amplitudes. This is in contrast to the sinusoidal pulsations detected by Muno et al. Similarly large pulse morphology changes have been seen following the 2002 outburst of AXP 1E 2259+586 (Woods et al ApJ 605, 378). The RMS pulsed fraction of CXO J1674710.2-455216 is ~20% (0.2-10 keV). A preliminary spectral analysis of the source yields the following spectral parameters for a power-law+blackbody model: N_H = 3.2(2)e22 cm^-2, kT=0.53(11) keV, Gamma=3.05(25), with reduced chi-squared of 1.1. 0.5-10 keV observed flux 1.8e-11 erg/s/cm^2. Comparing with the results reported in ATEL #896 for a Swift/XRT ToO observation made less than a day after the burst, the spectrum appears to be softening with time. Given the observed flux reported in ATEL #896, we derive an average power-law flux decay index over the 7 days since the reported X-ray burst of approximately -0.26. We note this is intriguingly close to the long-term flux decay power law index observed in the 2002 outburst of the AXP 1E 2259+586 (Woods et al. 2004 ApJ 605, 378). Chandra is scheduled to observe the source again on October 4 and October 15.