Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

TL;DR

This paper reports the discovery of 5560-second X-ray pulsations in the high-mass X-ray binary 4U 2206+54, revealing new insights into its neutron star properties and orbital dynamics.

Contribution

The study presents the first detection of long (~1 hour) X-ray pulsations in 4U 2206+54, expanding understanding of its neutron star and orbital characteristics.

Findings

Detected 5560-s pulsations in 4U 2206+54

Observed a soft X-ray excess consistent with a hot spot

Estimated orbital eccentricity to be ~0.4

Abstract

The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 0.001-1000 s. A cyclotron line at ~30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Our new long and uninterrupted RXTE observations allow us to search for long (~1 hr) pulsations for the first time. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2-10 keV is 1.5 x 10^{35} erg s^{-1} with a ratio Fmax/Fmin ~ 5. This ratio implies an eccentricity of ~0.4, somewhat higher than previously suggested. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit.