A Study of Superorbital Modulation in Wind-fed Supergiant X-ray Binaries

Joel Barry Coley
Assistant Professor
Howard University ~ Physics & Astronomy

Wed, February 12, 2020 - 4:00 PM
Karl Herzfeld Auditorium of Hannan Hall - Rm 108

X-ray binary star systems consisting of highly magnetized neutron stars and massive stellar companions are important astrophysical laboratories to study accretion physics in the presence of strong electromagnetic and gravitational potentials.  The light curves and spectra of neutron star X-ray binaries shows variability on a number of different timescales including their neutron star rotation periods, their binary orbits and in some cases on superorbital timescales.  Only six wind-fed Supergiant X-ray binaries have been found to show superorbital modulation.  In this talk, I present Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels Swift Observatory (Swift) X-ray Telescope (XRT) and Swift Burst Alert Telescope (BAT) observations of wind-fed Supergiant X-ray binaries that show significant superorbital variability.  Their superorbital periods show a possible correlation with their binary orbits, where they were found to be on the order of 3-4 times the length of their orbital period.  Dynamic power spectra and fractional root mean square analyses rms show that the amplitudes of modulation changes as a function of time, but no observed changes in the periods are found.  NuSTAR and XRT observations show that their spectral shape hardens with increasing luminosity, which indicates that the superorbital modulation is linked to the mass accretion rate.  I discuss different mechanisms that might drive the observed superorbital modulation.

Refreshments served at 3:45 PM

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