RXTE dips yield better orbital period determination for MAXI J1659-152

ATel #2926; T. M. Belloni (INAF-OAB, Italy), T. Muñoz-Darias (INAF-OAB, Italy), E. Kuulkers (ESA/ESAC, Spain)
on 12 Oct 2010; 14:24 UT
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Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

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We analyzed 32 RXTE/PCA observations of MAXI J1659-152 (Negoro et al. 2010, ATel #2873; Mangano et al. 2010, GCN #11296) from UT 2010 Sep 28 through to 2010 Oct 11. The total exposure is 72.5 ks. We produced light curves from PCU2 with 16-s bins over the full PCA range and over the 2-4.5 keV range. Since the average count rate of the source increases secularly because of the ongoing outburst, for each observation interval (corresponding to an RXTE orbit) we subtracted the mean count rate. Some dip-like structures with a typical length of ~2000 seconds are visible in some intervals, consistent with what is reported in by Kuulkers et al. (Atel #2912). We performed a phase dispersion minimization search over the range 0.25-12 hours and found a clear excess in the chi squared at a period of 2 hours and 24.85 minutes, corresponding to a deep minimum in the folded light curve (20 bins) centered on T0=55465.17784 MJD. We estimate an error on the period of less than a minute. This strongly confirms the period reported in ATel #2912. The period and time of the dips are consistent in both the total and soft X-ray light curves. Further X-ray observations should allow a more precise determination of the period. We note that other period search methods (e.g., Lomb-Scargle) do not give similar conclusive results, but we regard this to be due to the highly unevenly sampled nature of the data, introducing strong aliasing, as well as the high non-sinusoidal nature of the light curve.