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Name:Atsuo Okazaki


Institution:Hokkai-Gakuen University


Country:Japan




Title:Modeling gamma-ray binaries with Be stars




Abstract:TeV gamma-ray binaries are a recently established, rare class of gamma-ray sources. Only four binaries show variable TeV emission. Given that all of these systems but one have Be stars, studying the interaction between the Be-star envelope and the compact companion is a key to understanding of physics of high energy emission from these systems.

In this paper, we report on the results from 3-D SPH simulations of two TeV binaries with Be stars, PSR B1259-63 and LS I +61 303. For PSR B1259-63, where a relativistic wind from a pulsar interacts with a disk and wind of a Be star, we study the effect of the Be disk density on the interaction structure. We find that, if the Be disk in this system has a typical base density (<= 10^{-10} g/cm^3), the pulsar wind strips off an outer part of the Be disk on the side of the pulsar, truncating the disk at a radius significantly smaller than the pulsar orbit. On the other hand, if the Be disk is denser (~10^{-9} g/cm^3), the outer part of the disk has the large enough inertia not to be quickly blown off, so that a pulsar wind bubble is created when the pulsar passes through/close to the disk.

For LS I +61 303, for which the nature of the compact object is not yet known, there are two competing models, the pulsar wind model and the microquasar model. Performing SPH simulations based on these models, we find that two models predict significantly different interaction structure. If the system has an accretor, a transient gas stream from the Be disk towards the compact object appears at each periastron passage and lasts only for about a few days. An accretion disk also forms, which stays persistently, despite the temporary mass transfer from the Be disk. On the other hand, if the system has a pulsar with a relativistic wind, the Be disk is truncated by the pulsar wind, so that neither the gas stream nor the accretion disk is formed. Such a difference should be detected by optical, high-dispersion spectroscopic observations.





Title:Be stars, Be disks & models in the context of explaining the BeXRB phenomenon




Abstract:I start this review by summarizing key observational features of Be stars and their circumstellar disks. Then, I describe a scenario that a Be disk is formed by viscous diffusion of matter ejected from the equatorial surface of the central star. I first apply this viscous decretion disk scenario to Be disks around isolated Be stars and show that the resulting disk structure and dynamics explain many of the key observational features. Then, I apply the model to Be disks in binaries, in particular emphasis on Be/X-ray binaries, where the effect of the companion has to be taken into account. After showing that Be disks in such binaries are likely truncated by tidal/resonant torques by the companion, I compare the results from such a truncated disk model with observed BeXRB phenomenon.








            

            
            

              

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