Discovery of a 70.5 ms Pulsar in AX J1838.0-0655 Associated with HESS J1837-069

ATel #1392; E. V. Gotthelf, J. P. Halpern, F. Camilo (Columbia U.), C. Markwardt, J. Swank (GSFC)
on 19 Feb 2008; 17:27 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)

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We have detected highly significant X-ray pulsations from the region of the TeV source HESS J1837-069 using the Rossi X-ray Timing Explorer (RXTE). The observatory was pointed at the steady X-ray source AX J1838.0-0655 (Bamba et al. 2003, ApJ, 589, 253) located within the HESS source, that has been conjectured to contain a pulsar wind nebula by Aharonian et al. 2006 (ApJ, 636, 777) and Malizia et al. 2005 (ApJ, 630, L157). This was recently confirmed using a Chandra observation that cleanly resolved AX J1838.0-0655 into a pulsar and its wind nebula (Halpern and Gotthelf 2008, ApJ, submitted). The Chandra unabsorbed 2-10 keV flux of the pulsar is 8.5E-12 ergs cm-2 s-1. We infer that AX J1838.0-0655 is the origin of the pulsations from the absence of any known comparably bright X-ray source in the RXTE field, and the evidence of a pulsar wind nebula. This result verifies that AX J1838.0-0655 is another example of a pulsar wind nebula connected with an extended TeV source.

RXTE targeted the Chandra location of AX J1838.0-0655 on 2008 February 17 19:53 UT to search for the expected pulsar signal. A total of 9.14 ks of good RXTE data were acquired with the Proportional Counter Array (PCA) in GoodXenon mode spanning three satellite orbits (14.3 ks) with an average of 2.0 detector units (PCUs) active. Standard filters were applied to the resulting data and the photon arrival times were corrected to the solar system barycenter using the target coordinates. An FFT of the 2-14 keV (top PCA layer only) light curve, binned into 5 ms steps, produced a power of S = 117 at a period of 70.498 ms. This corresponds to essentially nil false detection probability. The folded light curve around the FFT signal provided a refined period of 70.49821(3) ms corresponding to a square-wave like pulse profile with complex structure. The measured pulsed fraction of 4.8% suggests an intrinsic pulsar modulation of roughly ~40% after allowing for the estimated RXTE background. The signal is also present in summed ~100 s dwells on the source by the RXTE PCA from 2004 - 2007.

A prior pulsar search in the radio band using 6 ks of data acquired on 2007 September 14 with the ATNF Parkes telescope at a frequency of 3 GHz (to alleviate the effects of scattering due to the high column density) yields an upper limit on the flux density of S_3 < 0.075 mJy, assuming a pulsar duty cycle of 20%.

Further X-ray timing observations are under way to improve the period measurement and future observations are planned to determine the spin-down rate and energetics of the pulsar.