Multi-mission Investigation of X-ray Superorbital Modulation in the Supergiant High Mass X-ray Binary 4U 1538-52

Abstract

Superorbital modulations has been detected in the supergiant High-Mass X-ray binary 4U 1538-52 using long-term monitoring with the Neil Gehrels Swift Observatory Burst Alert Telescope (BAT). The source also exhibits a long-term pulse period evolution as seen with Rossi X-ray Timing Explorer (RXTE), INTEGRAL, and Fermi Gamma-Ray Burst Monitor (GBM) that appears uncorrelated with changes in its X-ray flux. To investigate the mechanisms causing these superorbital modulations and its possible dependence on pulse period changes, we analyzed long-term monitoring with Swift-BAT and Monitor of All Sky X-ray Image Gas Slit Camera (MAXI-GSC) to construct dynamic power spectra and superorbital intensity profiles. In addition, we used pointed X-ray observations from Nuclear Spectroscopic Telescope Array (NuSTAR) and Neutron Star Interior Composition Explorer mission (NICER) to investigate the pulsation and spectral properties across different superorbital and orbital phase intervals. We find the presence of superorbital modulations in the MAXI-GSC 2-20 keV lightcurves, consistent with the periodicity observed with the Swift-BAT lightcurves. However, no significant changes are detected in the pulse profiles or spectral parameters across different superorbital, orbital, or pulse-change intervals. This lack of spectral or timing variations with orbital and superorbital phases suggests that the mechanisms driving the observed superorbital modulation and pulse period changes are likely associated with large-scale stellar wind structures, such as Co-Rotating Interaction regions, within the stellar wind of the supergiant companion.

Figures (9)

• Figure 1: Top: Spin-period of 4U 1538--52 from 15 years of Fermi-GBM observations (black points) along with pulse period measurements from RXTE (red points), and INTEGRAL (blue points) observations. Dashed lines indicate dates of pointed X-ray observations of the source: RXTE (red), INTEGRAL (blue), Suzaku (pink), NuSTAR (green) and NICER (purple). Epochs of pulse period changes are marked with arrows; Spin Epoch I (brown), Spin Epoch II (cyan), and Spin Epoch III (light green). Middle: The long-term lightcurves of Swift-BAT (15--50 keV) observations. Bottom: long-term lightcurves of MAXI-GSC (2--20 keV). The Swift-BAT and MAXI-GSC lightcurves have been re-binned with a time period of 14.9081 d (superorbital period)
• Figure 2: Power spectra of Swift-BAT data calculated using Lomb-Scargle periodograms. The fundamental frequency of the superorbital period (14.9081 d) is marked by the dotted blue line, and the second harmonic of the superorbital period (7.4541 d) is marked by the dotted red line. a: The power spectra of the time interval MJD 53415 till 57650. b: Power spectra of the time interval MJD 57650 till 58800. c: Power spectra of the time interval MJD 58800 till 60750. d: Power spectra of the entire observation period.
• Figure 3: Dynamic power spectra of Swift-BAT (a) and MAXI-GSC (b) data. The dates of pointed X-ray observations of 4U 1538--52 (see Table \ref{['Tab:observation_list']}) are marked by green (NuSTAR) and purple (NICER) dashed lines. The thin black lines represent the different epochs of pulse period changes as mentioned in Section \ref{['sec:Long-Term Observations']}. Top: The pulse period of 4U 1538--52 from Fermi-GBM (black) and INTEGRAL (green) observations are plotted here. Middle: Dynamic power spectra constructed using LSP. Right: Power spectrum of the lightcurve, normalized to the average power. The fundamental frequency of the superorbital period is marked by a black arrow and the second harmonic by a red arrow, with 99.9% and 99.99% significance levels indicated by the dashed blue and green lines respectively. Bottom: The height of the fundamental frequency of the superorbital modulations (black) and the second harmonic (red) relative to the mean power.
• Figure 4: a: Different segments of the Swift--BAT lightcurves as defined in Section \ref{['sec:dynamic power + intensity profiles']}, folded on the superorbital period of 14.9081 d. The fractional RMS ($f_{\rm{RMS}}$) was calculated for the superorbital intensity profiles. b: Segments of the MAXI-GSC lightcurves as defined in Section \ref{['sec:dynamic power + intensity profiles']}, folded in the same manner as Swift--BAT. c: Segments of Swift--BAT lightcurves, the data divided into epochs based on the torque reversals mentioned in Section \ref{['sec:intro']}. All lightcurves used MJD 56137.19, the epoch of maximum flux, as T$_0$.
• Figure 5: Left: The pulse profiles of the NICER observations of 4U 1538--52 arrayed to show the changes in pulse profiles as a function of orbital and superorbital phase. The folded pulse profiles are normalized by dividing by the average source intensity to create Normalized Intensity plotted on the y-axis. The observations are arrayed in order of increasing superorbital phase. Right: The pulse profiles of the NuSTAR observations of 4U 1538--52 in the different energy bands. The individual pulse profiles are given an arbitrary shift in the y-axis to enhance clarity.