On the orbital parameters of Be/X-ray binaries in the Small Magellanic Cloud

Abstract

The orbital motion of a neutron star about its optical companion presents a window through which we can study the orbital parameters of that binary system. This has been used extensively in the Milky Way to calculate these parameters for several high‐mass X‐ray binaries. Using several years of Rossi X‐ray Timing Explorer Proportional Counter Array data, we derive the orbital parameters of four Be/X‐ray binary systems in the Small Magellanic Cloud (SMC), increasing the number of systems with orbital solutions by a factor of 3. We find one new orbital period, confirm a second and discuss the parameters with comparison to the Galactic systems. Despite the low metallicity in the SMC, these binary systems sit amongst the Galactic distribution of orbital periods and eccentricities, suggesting that metallicity may not play an important role in the evolution of high‐mass X‐ray binary systems. A plot of orbital period against eccentricity shows that the supergiant, Be and low‐eccentricity OB transient systems occupy separate regions of the parameter space; akin to the separated regions on the Corbet diagram. Using a Spearman’s rank correlation test, we also find a possible correlation between the two parameters. The mass functions, inclinations and orbital semimajor axes are derived for the SMC systems based on the binary parameters and the spectral classification of the optical counterpart. As a by‐product of our work, we present a catalogue of the orbital parameters for every high‐mass X‐ray binary in the Galaxy and Magellanic Clouds for which they are known.

SXP 5.05 = IGR J00569-7226: using X-rays to explore the structure of a Be star's circumstellar disc

“…Substituting in the values from Section 2.3.4, we obtain a mass function of 10.56 M⊙. This is quite high relative to many other Be/X-ray binary systems, but not as large as most supergiant systems (see Townsend et al (2011) for a summary of known binary parameters and mass functions).…”

The Orbit of the Gamma-Ray Binary 1FGL J1018.6−5856

“…Fig. 5 shows the updated orbital period vs. eccentricity plot from Townsend et al (2011) for HMXBs with J1018 included from the derived orbital eccentricity in this work. J1018 lies around the transition zone between supergiant and Be X-ray binary systems, and follows the correlation of the two quantities which was noted by Casares et al (2012) for the GRBis with known orbital parameters.…”

Star formation history and X-ray binary populations: the case of the Large Magellanic Cloud

“…There are also 3 RLOF systems (Cen X-3, LMC X-4, and SMC X-1) shown in black squares. The data for the LMC are taken from the references listed in Table 1, while those for the SMC and the Milky Way from the compilations of Klus et al (2014) and Townsend et al (2011), respectively. We see that all but one of the LMC BeXRBs have orbital periods of less than ∼30 days.…”