A Study of the 20 day Superorbital Modulation in the High-mass X-Ray Binary IGR J16493-4348

Abstract

We report on Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels Swift Observatory (Swift) X-ray Telescope (XRT) and Swift Burst Alert Telescope (BAT) observations of IGR J16493-4348, a wind-fed Supergiant X-ray Binary (SGXB) showing significant superorbital variability. From a discrete Fourier transform of the BAT light curve, we refine its superorbital period to be 20.058 ± 0.007 days. The BAT dynamic power spectrum and a fractional root mean square analysis both show strong variations in the amplitude of the superorbital modulation, but no observed changes in the period were found. The superorbital modulation is significantly weaker between MJD 55,700 and MJD 56,300. The joint NuSTAR and XRT observations, which were performed near the minimum and maximum of one cycle of the 20 day superorbital modulation, show that the flux increases by more than a factor of two between superorbital minimum and maximum. We find no significant changes in the 3-50 keV pulse profiles between superorbital minimum and maximum, which suggests a similar accretion regime. Modeling the pulse-phase averaged spectra we find a possible Fe Kα emission line at 6.4 keV at superorbital maximum. The feature is not significant at superorbital minimum. While we do not observe any significant differences between the pulse-phase averaged spectral continua apart from the overall flux change, we find that the hardness ratio near the broad main peak of the pulse profile increases from superorbital minimum to maximum. This suggests the spectral shape hardens with increasing luminosity. We discuss different mechanisms that might drive the observed superorbital modulation.

Superorbital Modulation in the High-Mass X-ray Binary 4U 1538-52, and Possible Modulation in IGR J16393-4643

“…If 4U 1538−52 was fainter, it would not have been possible to detect the superorbital modulation as the su-perorbital peak in the power spectrum is much smaller than the orbital peak. The difficulty of detecting superorbital modulation is compounded when the modulation can either decrease in strength, as in IGR J16493-4348 (Coley et al 2019) or 2S 0114+650 (Hu et al 2017), or change modulation shape, as for 4U 1538−52. Thus the possibility remains that low-level superorbital modulation could be present in other similar systems where it has not yet been seen because they are either fainter and/or the superorbital modulation is less persistent.…”

“…However, in the most recent observations (after ∼MJD 57,650) there is a distinct change in the modulation profile which results in the decrease of the peak at the fundamental in the power spectrum and the presence of a peak near the second harmonic of the period. For comparison, in IGR J16493-4348 BAT observations also showed changes in the strength of the superorbital modulation (Coley et al 2019), but with the modulation preserving the phase and shape of the modulation before and after an interval where the modulation significantly weakened. Similarly, for 2S 0114+650 Hu et al (2017) reported that the superorbital period was stable, but that the modulation amplitude was highly variable.…”

“…Several mechanisms have been proposed for the origin of the superorbital modulation, see for example the discussion in Coley et al (2019), including a precessing accretion disk, a triple stellar system, precession of the donor star, corotating interaction regions (CIRs) in the stellar wind, and tidal oscillations in the primary. While a precessing disk and a triple system were both considered to be unlikely, the other proposed models could not be excluded.…”

“…, (b) Romano (2015), (c) Martínez-Núñez et al (2015), (d) Pearlman et al (2019), (e) Coley et al (2019). For parameters of 4U 1538−52, apart from the superorbital period, see Section 1.2.…”

Advances in Understanding High-Mass X-ray Binaries with INTEGRAL and Future Directions

“…In this case, the superorbital modulation is produced by a single-arm CIR with a rotational period of 10.3 days. Coley et al (2019) presented an attempt to observationally understand the nature of the superorbital modulation in IGR J16493−4348. They combined an analysis of long-term intensity changes traced via Swift BAT with a broad-band spectral analysis combining two quasi-simultaneous Swift XRT and NuSTAR observation at the maximum and minimum phase of the superorbital modulation within a single 20 days cycle.…”