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Confirmation of the superorbital modulation of the high mass X-ray binaries 4U 1909+07, IGR J16479-4514 and IGR J16418-4532 with INTEGRAL/IBIS

ATel #5131; S. P. Drave, A. J. Bird, M. E. Goossens (Univ. of Southampton, UK), L. Sidoli (INAF/IASF Milano, Italy), V. Sguera (INAF/IASF Bologna, Italy), M. Fiocchi and A. Bazzano (INAF/IASF Roma, Italy)
on 14 Jun 2013; 14:59 UT
Credential Certification: Sebastian Drave (sd805@soton.ac.uk)

Subjects: Binary, Variables

Following the recent announcement of the detection of superorbital periods in the wind-fed supergiant X-ray binaries 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 by Corbet and Krimm 2013 (Atels #5119 and #5126) we investigated archival INTEGRAL data to search for additional signatures of these periods. The 18-60 keV INTEGRAL/IBIS light curve used spanned the lifetime of the INTEGRAL mission (until approximately Oct. 2010). A Lomb-Scargle analysis of each light curve confirmed the presence of the superorbital modulation for all three sources named above, the specific details of which are given below.

A Lomb-Scargle analysis of the long term IBIS light curve of the SFXT IGR J16479-4514 showed a strong detection of both the second harmonic of the orbital period (3.32 days, Jain et al., 2009, MNRAS, 397, L1) and the fundamental superorbital period of 11.880 ± 0.002 days (Corbet and Krimm, ATel #5126). The superorbital modulation was detected with a confidence of greater than 99.999% at a period of 11.891 ± 0.002 days, which is consistent, to within 3 sigma, with the previous result. Folding the data set on this period, and using the previously defined ephemeris, resulted in a qualitatively similar profile to that reported in ATel #5126. The profile displayed a sharp rise in flux followed by a plateau at phase ~0.7 with a modulation full amplitude (as defined in ATel #5126) of approximately 160%. The minimum and maximum fluxes were separated by a phase interval of approximately 0.2.

In contrast to the Swift/BAT detection reported in ATel #5126, the second harmonic of the superorbital modulation of the candidate SFXT IGR J16418-4532 was detected most strongly in the IBIS data set. The period was detected with a confidence of greater than 99.999% at a period of 7.3421 ± 0.0004 days, corresponding to a fundamental period of 14.6842 ± 0.0008 days. While the IBIS and BAT detections are not formally consistent they are broadly similar, with possible reasons for the discrepancy being the underestimate of the formal uncertainties and the detection of different harmonics of the signal. Folding on the IBIS determination displayed a profile with an approximately sinusoidal envelope, superimposed with a multi-peaked structure and an amplitude of approximately 130%.

The superorbital modulation of the wind-fed HMXB 4U 1909+07 was also significantly detected in excess of the 99.999% confidence level in the IBIS light curve. The period of the modulation was determined as 15.104 ± 0.004 days, slightly outside of the formal uncertainties of the BAT determination of 15.180 ± 0.003 days (ATel #5119). Folding the IBIS light curve on our period determination displays a broadly sinusoidal variation with some additional structure. The flux maximum and minimum are separated by an orbital phase interval of ~ 0.4 and the modulation displays an amplitude of ~ 71%.

The INTEGRAL/IBIS detections reported above provide strong confirmations of the superorbital modulations of the HMXBs 4U 1909+07, IGR J16479-4514 and IGR J16418-4532 recently reported by Corbet and Krimm (ATels #5119 and #5126). Both IGR J16479-4514 and IGR J16418-4532 are eclipsing SFXTs with short orbits (3.32 days, Jain et al. 2009 and 3.74 days, Corbet et al. 2006 respectively) while 4U 1909+07 is a classical wind-fed SgXRB with a 4.40 day orbital period (Wen et al. 2000) that displays a smooth, sinusoidal orbital variation. Hence, while a mechanism for generating superorbital X-ray variations in wind-fed HMXBs is currently unknown, it cannot be solely an effect of system inclination. The investigation of INTEGRAL/IBIS light curves for the presence, or otherwise, of superorbital modulations in further SgXRBs is currently on going. We also encourage the monitoring of such systems in other wavebands, particularly the optical and IR, to aide in identifying the origins of these superorbital modulations.