Determination of parameters of Long-Term variability of the X-ray pulsar LMC X-4

Abstract

-We have investigated the temporal variability of the X-ray flux measured from the highmass Xray binary LMCX-4 on time scales from several tens of days to tens of years, i.e., exceeding considerably the orbital period (1.408 days). In particular, we have investigated the 30-day cycle of modulation of the X-ray emission from the source (superorbital or precessional variability) and refined the orbital period and its first derivative. We show that the precession period in the time interval 1991-2015 is near its equilibrium value P sup = 30.370 days, while the observed historical changes in the phase of this variability can be interpreted in terms of the "red noise" model. We have obtained an analytical law from which the precession phase can be determined to within 5% in the entire time interval under consideration. Using archival data from several astrophysical observatories, we have found 43 X-ray eclipses in LMC X-4 that, together with the nine eclipses mentioned previously in the literature, have allowed the parameters of the model describing the evolution of the orbital period to be determined. As a result, the rate of change in the orbital periodṖ orb /P orb = (1.21 ± 0.07) × 10 −6 yr −1 has been shown to be higher than has been expected previously.

Discovery of Pulsation Dropout and Turn-on during the High State of the Accreting X-Ray Pulsar LMC X-4

“…In Observation 02, we found the absolute magnitude of the period derivatives to be ≈ 3×10 −7 Hz s −1 and ≈ 3×10 −8 Hz s −1 in Observations 02 and 08, respectively. These frequency derivatives are much larger than the long term derivative of ∼ 1 × 10 −10 Hz s −1 found by Molkov et al (2015). Changes in pulse shape are also evident, from a singly pointy peak in the flare and afterglow intervals (intervals 2, 4, 6, and 7), to more complex shapes in intervals 1, 3, and 5.…”

NuSTAR observations of the X-ray pulsar LMC X-4: A constraint on the magnetic field and tomography of the system in the fluorescent iron line

“…A correction of the photon arrival times for the Solar system barycenter was made using the standard nustardas tools. A corresponding correction of the photon arrival times for the neutron star motion in the binary system was made out using the orbital parameters from Molkov et al (2015). Search for the pulse period was carried out using the epoch folding method (the efsearch tool of the xspec package).…”

“…The most accurate ephemeris for the orbital parameters, including the decay of the orbital period and the superorbital variability, were obtained in recent papers by and Molkov et al (2015). The maximum value of the persistent luminosity of the source in X-ray energy range is about L x ≃ (3 − 4) × 10 38 erg s −1 (La Barbera et al 2001;Tsygankov, Lutovinov 2005;Grebenev et al 2013), which is close to the Eddington limit of the luminosity of an accreting neutron star (note, that in a presence of a strong magnetic field this limit may be much higher, see Mushtukov et al 2015).…”

INTEGRAL/IBIS deep extragalactic survey: M81, LMC and 3C 273/Coma fields

“…This conclusion was supported by an independent detection of this source in the standard X-ray energy band by the INTEGRAL/JEM-X telescope, which allowed Grebenev et al (2013) to reconstruct the source spectrum in a broad energy band and demonstrate that it is typical for accreting X-ray pulsars. Based on the extended data set obtained with INTEGRAL and using the current ephemerides for LMC X-4 (Molkov et al 2015), we have repeated such an analysis and verified the result of Grebenev et al (2013). Summarizing the above, we can conclude that the hard X-ray emission detected by INTEGRAL from the vicinity of LMC X-4 is associated with the X-ray pulsar EXO 053109−6609.2 and that the hard X-ray sources IGR J05319−6601 and IGR J05305−6559 are actually the same source -the X-ray counterpart of EXO 053109−6609.2.…”