A 6.8 Day Period in IGR J16493-4348 from Swift/BAT and RXTE/PCA Observations

ATel #2599; R. H.D. Corbet (UMBC/NASA GSFC), S. D. Barthelmy (NASA GSFC), W. H. Baumgartner (UMBC/NASA GSFC), H. A. Krimm (USRA/NASA GSFC), C. B. Markwardt (UMCP/NASA GSFC), G. K. Skinner (UMCP/NASA GSFC), J. Tueller (NASA GSFC)
on 5 May 2010; 05:13 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

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IGR J16493-4348 was discovered by Grebenev et al. (2005, ATel #457) and subsequent X-ray observations suggested that the source is an X-ray binary (e.g. Hill et al. 2008, MNRAS, 385, 423). Nespoli et al. (2010, arXiv:1004.4101v1) recently classified the optical counterpart as a B0.5 I supergiant.

We have analyzed light curves of IGR J16493-4348 obtained with: (i) the Swift BAT 58 month survey (Baumgartner et al. 2010, HEAD, 11, 1305) in an energy range of 15 - 100 keV, covering 2004-12-21 to 2009-09-30 (MJD 53,360 to 55,104). (ii) PCA Galactic plane scans in the energy range 2 - 10 keV between 2004-06-07 to 2010-04-11 (MJD 53,163 to 55,297). In the power spectra of both light curves highly statistically significant (false alarm probabilities < 1e-9) modulation is seen at a period near 6.8 days.

In order to characterize the modulation we fitted sine waves to the two light curves independently and derived:
Tmax (BAT) = MJD 54,301.39 +/- 0.14 + n x 6.7906 +/- 0.0020
Tmax (PCA) = MJD 54,301.55 +/- 0.14 + n x 6.7851 +/- 0.0016
where Tmax is the time of maximum flux.

The mean count rates are 9.5e-5 cts/s/detector element for the BAT, equivalent to approximately 2.5 mCrab, and 6.8 cts/s for the PCA. The flux modulation (semi-amplitude/mean) on the 6.8 day period is approximately 100% in both light curves.

We note that the cadence of PCA Galactic plane scans is twice per week. Although this has the potential to create artifacts in the power spectrum near a period of one week, the close agreement in both period and time of maximum flux between the BAT and the PCA shows that for IGR J16493-4348 the PCA is indeed detecting modulation from the source itself. A 6.8 day period is typical of the orbital period expected for a supergiant system and thus supports the supergiant classification of Nespoli et al. (2010).

In the power spectrum of the BAT light curve a lower amplitude peak, statistically significant at > 99.9%, is seen at a period of 20.07 +/- 0.02 days (Tmax = MJD 54,265.1 +/- 0.5). This differs significantly from three times the main peak which would have a value of 20.37 days. The power spectrum of the PCA light curve also shows a small peak with a consistent period (20.09 +/- 0.02 days) and epoch of maximum flux (Tmax = MJD 54,266.0 +/- 0.7). However, this peak in the PCA power spectrum has low statistical significance. If the 20.07 day modulation is astrophysical in origin it might be due to either a second (uncatalogued) X-ray binary, close on the sky to IGR J16493-4348, or to superorbital modulation of IGR J16493-4348 itself, similar to that seen in 2S 0114+650 (e.g. Farrell et al. 2008, MNRAS, 389, 608).