4U 2206+54: An Unusual High‐Mass X‐Ray Binary with a 9.6 Day Orbital Period But No Strong Pulsations

Abstract

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of the X-ray source 4U 2206+54, previously proposed to be a Be star system, show the X-ray flux to be modulated with a period of approximately 9.6 days. If the modulation is due to orbital variability, then this would be one of the shortest orbital periods known for a Be star X-ray source. However, the X-ray luminosity is relatively modest, whereas a high luminosity would be predicted if the system contains a neutron star accreting from the denser inner regions of a Be star envelope. Although a 392 s pulse period was previously reported from EXOSAT observations, a reexamination of the EXOSAT light curves does not show this or any other periodicity. An analysis of archival RXTE Proportional Counter Array observations also fails to show any X-ray pulsations. We consider possible models that may explain the properties of this source, including a neutron star with accretion halted at the magnetosphere and an accreting white dwarf.