Superorbital Periodicity in the Wind-Accretion HMXB 4U 1909+07 (= X 1908+075)

ATel #5119; Robin H. D. Corbet (UMBC/NASA GSFC), Hans A. Krimm (USRA/NASA GSFC)
on 10 Jun 2013; 17:00 UT
Credential Certification: Robin Corbet (corbet@umbc.edu)

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Following the report of the detection of enhanced hard X-ray emission from 4U 1909+07 with INTEGRAL (ATel #5079) we have investigated the long-term light curve of this source using Swift BAT data from the transient monitor program covering the energy range 15 - 50 keV, and a time range of MJD 53416 to 56452 (2005-02-15 to 2013-06-09).

The power spectrum of this light curve shows, in addition to strong modulation on the 4.4 day orbital period, significant modulation at a superorbital period near 15.2 days (false alarm probability ~ 1e-5) and the second harmonic of this. Combining the detections at the fundamental and second harmonic, we determine a period of 15.180 +/- 0.003 days.

As expected from the presence of harmonics in the power spectrum, the light curve folded on the superorbital period shows a multi-peaked profile. The minimum is somewhat more clearly defined than the maximum. From an inspection of the folded light curve, the minimum occurs at approximately MJD 55999 +/- 1.5. The time of maximum flux occurs about 0.35 in phase after the minimum. The amplitude of the modulation, defined as (maximum - minimum)/ mean flux, is approximately 50%

The ratio of the superorbital and orbital periods of 4U 1909+07 is 3.4. This is similar to the two other wind-accretion high-mass X-ray binaries containing neutron stars for which a superorbital period has been found: 2S 0114+650, Porb = 11.6 days, Psuper = 30.7 days, ratio ~2.6 (e.g. Farrell et al. 2008, MNRAS 389, 608) and IGR J16493-4348, Porb = 6.79 days, Psuper = 20 days (ATel #2599, Pearlman et al. 2013, AAS 221, 142.38), ratio ~2.95. Thus, although the physical cause of the superorbital modulations has yet to be determined, a similar mechanism is suspected for all three systems.