Superorbital Modulation in the Wind-Accretion HMXBs IGR J16418-4532 and IGR J16479-4514

ATel #5126; Robin H. D. Corbet (UMBC/NASA GSFC), Hans A. Krimm (USRA/NASA GSFC)
on 12 Jun 2013; 18:18 UT
Credential Certification: Robin Corbet (corbet@umbc.edu)

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Following the detection of a superorbital period in the wind-accretion HMXB 4U 1909+07 (= X 1908+075) with Swift/BAT transient monitor data (ATel #5119), we have searched for superorbital modulation in similar systems. We again used light curves covering the energy range 15-50 keV and calculated power spectra for all sources for which transient monitor light curves exist. We find evidence for previously unreported superorbital modulation in the HMXB systems IGR J16418-4532 and IGR J16479-4514. For both sources the light curves cover time ranges of MJD 53416 to 56451 (2005-02-15 to 2013-06-08).

For IGR J16418-4532, in addition to modulation at the 3.74 day orbital period (e.g. ATel #779; Levine et al. 2011, ApJS, 196, 6), the power spectrum shows a peak near 14.7 days with a false alarm probability of < 1e-6. The light curve folded on this period shows an approximately sinusoidal modulation. From a sine wave fit to the light curve we obtain:

Tmax = MJD 55994.6 +/- 0.4 + n x 14.730 +/- 0.006 where Tmax is the time of maximum flux.

The full amplitude of the modulation, defined as (maximum - minimum)/ mean flux, from the sine fit is approximately 70%.

For IGR J16479-4514 the power spectrum of the BAT light curve shows, in addition to modulation at the 3.32 day orbital period (Jain et al. 2009, MNRAS, 397, L11) and harmonics of this, peaks near 11.9 days and its second harmonic. The second harmonic is stronger than the fundamental and from this we derive a superorbital period of 11.880 +/- 0.002 days. The light curve folded on this period shows a relatively sharp rise from minimum to maximum followed by a plateau. The time of minimum flux is approximately MJD 55993 +/- 1 with maximum flux occurring approximately 0.25 in phase after this. The full amplitude of the modulation is approximately 130%.

The ratios of the superorbital and orbital periods of IGR J16418-4532 and IGR J16479-4514 are 3.9 and 3.6 respectively. These are similar to the period ratios of the three other wind-accretion high-mass X-ray binaries for which superorbital periods have been reported (ATel #5119 and references therein). Although the total number of proposed superorbital periods in wind-accretion HMXBs is only 5, we note a monotonic relationship between superorbital and orbital periods in this limited dataset.