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Register a new userOn the Spin Period Distribution in Be/X-ray Binaries
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There is a remarkable correlation between the spin periods of the accreting neutron stars in Be/X-ray binaries (BeXBs) and their orbital periods . Recently Knigge et al. (2011) showed that the distribution of the spin periods contains two distinct subpopulations peaked at $sim 10$ s and $sim 200$ s respectively, and suggested that they may be related to two types of supernovae for the formation of the neutron stars, i.e., core-collapse and electron-capture supernovae. Here we propose that the bimodal spin period distribution is likely to be ascribed to different accretion modes of the neutron stars in BeXBs. When the neutron star tends to capture material from the warped, outer part of the Be star disk and experiences giant outbursts, a radiatively-cooling dominated disk is formed around the neutron star, which spins up the neutron star, and is responsible for the short period subpopulation. In BeXBs that are dominated by normal outbursts or persistent, the accretion flow is advection-dominated or quasi-spherical. The spin-up process is accordingly inefficient, leading to longer periods of the neuron stars. The potential relation between the subpopulations and the supernova mechanisms is also discussed.
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Be/X-ray binary systems exhibit both periodic (Type I) X-ray outbursts and giant (Type II) outbursts, whose origin has remained elusive. We suggest that Type II X-ray outbursts occur when a highly misaligned decretion disk around the Be star becomes eccentric, allowing the compact object companion to capture a large amount of material at periastron. Using 3D smoothed particle hydrodynamics simulations we model the long term evolution of a representative Be/X-ray binary system. We find that periodic (Type I) X-ray outbursts occur when the neutron star is close to periastron for all disk inclinations. Type II outbursts occur for large misalignment angles and are associated with eccentricity growth that occurs on a timescale of about 10 orbital periods. Mass capture from the eccentric decretion disk results in an accretion disk around the neutron star whose estimated viscous time is long enough to explain the extended duration of Type II outbursts. Previous studies suggested that the outbursts are caused by a warped disk but our results suggest that this is not sufficient, the disk must be both highly misaligned and eccentric to initiate a Type II accretion event.
We present an optical and X-ray study of four Be/X-ray binaries located in the Small Magellanic Cloud (SMC). OGLE I-band data of up to 11 years of semi-continuous monitoring has been analysed for SMC X-2, SXP172 and SXP202B, providing both a measurement of the orbital period (Porb = 18.62, 68.90, and 229.9 days for the pulsars respectively) and a detailed optical orbital profile for each pulsar. For SXP172 this has allowed a direct comparison of the optical and X-ray emission seen through regular RXTE monitoring, revealing that the X-ray outbursts precede the optical by around 7 days. Recent X-ray studies by XMM-Newton have identified a new source in the vicinity of SXP15.3 raising doubt on the identification of the optical counterpart to this X-ray pulsar. Here we present a discussion of the observations that led to the proposal of the original counterpart and a detailed optical analysis of the counterpart to the new X-ray source, identifying a 21.7 d periodicity in the OGLE I-band data. The optical characteristics of this star are consistent with that of a SMC Be/X-ray binary. However, this star was rejected as the counterpart to SXP15.3 in previous studies due to the lack of H{alpha} emission.
The orbital motion of a neutron star about its optical companion presents a window through which to 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 RXTE PCA data, we derive the orbital parameters of four Be/X-ray binary systems in the SMC, increasing the number of systems with orbital solutions by a factor of three. 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 Spearmans 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.
We report on the long-term average spin period, rate of change of spin period and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the Small Magellanic Cloud. We also collect and calculate parameters of each system and use these data to determine that all systems contain a neutron star which is accreting via a disc, rather than a wind, and that if these neutron stars are near spin equilibrium, then over half of them, including all with spin periods over about 100 s, have magnetic fields over the quantum critical level of 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then their magnetic fields are inferred to be much lower, of the order of 10^6-10^10 G, comparable to the fields of neutron stars in low-mass X-ray binaries. Both results are unexpected and have implications for the rate of magnetic field decay and the isolated neutron star population.
Almost all Galactic black hole binaries with low mass donor stars are transient X-ray sources; we expect most of the X-ray transients observed in external galaxies to be black hole binaries also. Obtaining period estimates for extra-galactic transients is challenging, but the resulting period distribution is an important tool for modeling the evolution history of the host galaxy. We have obtained periods, or upper limits, for 12 transients in M31, using an updated relation between the optical and X-ray luminosities. We have monitored the central region of M31 with Chandra for the last ~12 years, and followed up promising transients with HST; 4sigma B magnitude limits for optical counterparts are ~26--29, depending on crowding. We obtain period estimates for each transient for both neutron star and black hole accretors. Periods range from <0.4 to 490+/-90 hours (<0.97 to <175 hrs if all are BH systems). These M31 transients appear to be somewhat skewed towards shorter periods than the Milky Way (MW) transients; indeed, comparing the M31 and MW transients with survival analysis techniques used to account for some data with only upper limits yield probabilities of ~0.02--0.08 that the two populations are drawn from the same distribution. We also checked for a correlation between orbital period and distance from the nucleus, finding a 12% probability of no correlation. Further observations of M31 transients will strengthen these results.
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