Discovery of the Pulse Period of IGR J16493-4348 from RXTE PCA Observations

ATel #2766; R. H.D. Corbet, A. B. Pearlman, K. Pottschmidt (UMBC/NASA GSFC)
on 31 Jul 2010; 19:09 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

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The supergiant high-mass X-ray binary IGR J16493-4348 has recently been found to have an orbital period of 6.8 days (Corbet et al. 2010, ATel #2599; Cusumano et al. 2010, MNRAS, 406, L16). In order to search for pulsations from this source we performed RXTE PCA observations on 2010-07-09 from 19:59 to 22:29 with a total exposure time of approximately 8ks. The observations were centered on an orbital phase of 0.85 +/- 0.05, where phase 0 corresponds to the time of expected maximum flux (Corbet et al. 2010).

The power spectrum of the light curve shows modulation at a period of 1069 +/- 7 s. Although the PCA observations cover only about 7.5 complete pulse cycles, the formal statistical significance of the modulation is extremely high (false alarm probability < 10^-6). The modulation amplitude (semi-amplitude/mean) on this period, after allowing for the estimated background from the Galactic ridge, is approximately 20%.

RXTE PCA observations of IGR J16493-4348 were previously made by Markwardt et al. (2005, ATel #465) on 2005-04-14 (binary phase = 0.17 +/- 0.04) and 2004-04-15 (binary phase = 0.39 +/- 0.04) for 3.5 and 2.6 ks respectively. Although variability is present in the first of these observations that could be consistent with a ~1069 s modulation, this observation is too short to independently confirm the presence of pulsations. The second observation, as noted by Markwardt et al. (2005), shows no variability and these authors suggested that the source was in a quiescent state. We note that the source quiescence can now be attributed to an orbital minimum, probably an eclipse (Cusumano et al. 2010).

A 1069 s pulse period is consistent with the range of periods seen in other supergiant high-mass X-ray binary pulsars (e.g. Liu et al. 2006, A&A, 445, 1165).