A 4.2 Day Period in the X-ray Pulsar IGR J16393-4643 from Swift/BAT and RXTE/PCA Observations

ATel #2570; R. H.D. Corbet (UMBC/NASA GSFC), H. A. Krimm (USRA/NASA GSFC), S. D. Barthelmy (NASA GSFC), W. H. Baumgartner (UMBC/NASA GSFC), C. B. Markwardt (UMCP/NASA GSFC), G. K. Skinner (UMCP/NASA GSFC), J. Tueller (NASA GSFC)
on 16 Apr 2010; 23:37 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

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The 910s X-ray pulsar IGR J16393-4643 was reported by Thompson et al. (2006, ApJ 649, 373) to have a 3.6875 +/-0.0006 day orbital period from a pulse timing analysis, although other solutions with orbital periods of 50.2 and 8.1 days could not be excluded. Thompson et al. proposed, on the basis of their orbital parameters, that IGR J16393-4643 is a supergiant wind-accretion powered HMXB. Nespoli et al. (2010, A&A in press, arXiv:0910.0990) instead suggest that this is a symbiotic X-ray binary with a 50 day period.

We have analyzed light curves of IGR J16393-4643 obtained with: (i) the Swift BAT (15 - 50 keV) obtained between 2005-02-12 and 2010-04-13 (MJD 53,413 - 55,299); and (ii) RXTE PCA Galactic bulge scans (2 - 10 keV) obtained between 2004-06-07 and 2010-04-11 (MJD 53,163 - 55,297). No modulation is seen in either light curve at any of the possible orbital periods reported by Thompson et al. (2006). Instead, highly significant modulation at a period near 4.24 days is seen in both light curves.

In order to characterize the modulation we fitted sine waves to the two light curves independently and derived:
Tmax (BAT) = MJD 54,352.50 (+/- 0.09) + n x 4.2368 (+/- 0.0007)
Tmax (PCA) = MJD 54,352.39 (+/- 0.11) + n x 4.2371 (+/- 0.0007)
where Tmax is the time of maximum flux.

The mean count rates are 0.0013 cts/cm2/s for the BAT (approximately 6 mCrab) and 22.9 +/- 0.4 cts/s for the RXTE PCA. For both light curves, the semi-amplitude of the modulation divided by the mean flux is approximately 25%.

The 4.24 day period is likely to be the orbital period of this system. This period is sufficiently close to the 3.7 day period of Thompson et al. (2006) that the source's location in the HMXB orbital period/pulse period diagram (Corbet 1986, MNRAS, 220, 1047) is essentially unchanged. IGR J16393-4643 is therefore still likely to be a supergiant HMXB and a symbiotic system can be excluded. Our period does not coincide with any of the periods suggested by Thompson et al. (2006). However, the pulse timing analysis performed by these authors was challenging due to the ambiguities in orbital parameters that came from the use of three widely separated observations, and an unknown pulse period change between the observations.

This telegram made use of Swift/BAT transient monitor results provided by the Swift/BAT team.

Scaled Map Transient Analysis for IGR J16393-4643