Swift/BAT and RXTE/ASM Discovery of the Orbital Period of IGR J16418-4532

ATel #779; R. Corbet (GSFC/USRA), L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), J. Tueller (GSFC), on behalf of the Swift-Survey team, and R. Remillard (MIT)
on 29 Mar 2006; 17:44 UT
Credential Certification: Robin Corbet (corbet@gsfc.nasa.gov)

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The Swift Burst Alert Telescope (BAT) light curve of IGR J16418-4532 (Tomsick et al. 2004 ATEL #224) obtained between 2004 Dec 21 and 2005 Sep 17 shows modulation at a period near 3.75 days. This period is confirmed and refined by observations made with the Rossi X-ray Timing Explorer All-Sky Monitor (ASM) between 1996 Jan 9 and 2006 March 16.

The mean BAT count rate of IGR J16418-4532 in the range 14 - 195 keV is 9.5 (+/- 0.9 statistical) E-4 counts/s/cm^2 which corresponds to approximately 3 mCrab. The mean count rate in the ASM in the range 1.5 - 12 keV is 0.39 +/- 0.01 (statistical) counts/s, equivalent to approximately 5 mCrab. However, IGR J16418-4532 is located only 0.7 degrees from the ~380 mCrab source GX 340+0. For the ASM this may lead to some contamination of the measured flux from IGR J16418-4532 by emission from GX 340+0.

The parameters of the modulation were quantified by fitting sine waves to the BAT and ASM light curves separately. For the BAT light curve the fit gives amplitude = 9.2 (+/- 1.2 ) E-4 counts/s/cm^2, period = 3.753 +/- 0.004 days, and epoch of maximum flux = MJD 53560.7 +/- 0.1. A sine wave fit to the ASM light curve gives amplitude = 0.09 +/- 0.02 counts/s, period = 3.7389 +/- 0.0004 days and epoch of maximum flux = MJD 53560.2 +/- 0.2. Although the period and epoch derived from the two data sets are formally statistically discrepant, the error bars may well be underestimates because of the presence of non-periodic modulation in the light curves and because the modulation is unlikely to be exactly sinusoidal. The BAT light curve when folded using the parameters derived from the ASM does indeed show modulation similar to that seen when the light curve is folded on the BAT-derived parameters.

We interpret this modulation as revealing the orbital period of a binary system. We note that the mean BAT flux at phase 0.5 is consistent with zero which may indicate the presence of a total eclipse. If there is a total eclipse, then the ~0.25 counts/s detected with the ASM at this phase implies that the background subtracted luminosity in the ASM energy band is about 1.5 mCrab. IGR J16418-4532 lies close to the Galactic plane and a number of sources found with INTEGRAL have been found to be high-mass X-ray binaries with supergiant primaries. The possible presence of an eclipse in the folded light curve is consistent with a supergiant classification.