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A 117-day Period in IGR J19294+1816

ATel #2008; R. H. D. Corbet (CRESST/GSFC/UMBC), H. A. Krimm (CRESST/GSFC/USRA)
on 6 Apr 2009; 19:53 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 2985, 3917, 4135, 5079, 5104

An analysis of the light curve of IGR J19294+1816 (Turler et al, ATel #1997) from the Swift/BAT hard X-ray transient monitor covering 2006 January 1 through 2009 April 2 reveals a strong signal with a period of 117 days. We associate this period with the orbital period of the binary system. The power spectrum of the light curve shows strong peaks near 117 days and its harmonic near 58 days. A folding analysis also reveals a modulation near 117 days. The smoothed light curve shows a non-sinusoidal variation with peaks at 117-day intervals. For the eleven 117-day cycles covered by the BAT data, eight predicted maxima correspond to peaks in the light curve where the daily average flux rose above 0.002 ± 0.0009 ct/s/cm2 (10 mCrab) in the 15-50 keV band. Two other high points were at roughly half the assumed orbital period and are possible evidence for an interaction between the neutron star and an equatorial disk around the mass-donating star. These high points at half the orbital period account for the previous suggestion by Krimm et al. (ATel #1999) that the orbital period of IGR J19294+1816 is 56 ± 4 days.

Using the 8 maxima in the light curve which coincide with the times predicted by a 117 day period we derive:
Tpeak (MJD) = 54,680.3 (+/- 0.6) + n x 117.2 (+/- 0.2)

Using the pulse period of the system (Strohmayer et al, ATel #2002) of 12.44 seconds, we can place this binary system on the plot of pulse period vs. orbital period (Corbet 1986, MNRAS, 220, 1047). IGR J19294+1816 falls within the region of parameter space expected for a Be system. Such an interpretation is also consistent with the transient nature of the source.

Swift BAT hard X-ray transient monitor light curves for IGR 19294+1816