Non-axisymmetric Structure of Accretion Disks in Be/X-ray Binaries
Kimitake Hayasaki
The non-axisymmetric structure of accretion disks around the neutron star in Be/X-ray binaries is studied by analyzing the results from three dimensional (3D) Smoothed Particle Hydrodynamics (SPH) simulations. It is found that ram pressure due to the phase-dependent mass transfer from the Be-star disk excites a one-armed, trailing spiral structure in the accretion disk around the neutron star. The spiral wave has a transient nature; it is excited around the periastron, when the material is transferred from the Be disk, and is gradually damped afterwards. It is also found that the orbital phase-dependence of the mass-accretion rate is mainly caused by the inward propagation of the spiral wave excited in the accretion disk.
aipcheck.tex
%
% $Id: aipcheck.tex,v 1.7 2002/01/11 19:50:46 frank Exp $
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Testing for potential problems with this class
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newif\ifproblem
\newif\iftimesok

\makeatletter
\def\IfStandaloneCheck{\def\next{aipcheck}
  \edef\currjob{\jobname}
  \edef\next{\meaning\next}
  \edef\currjob{\meaning\currjob}
  \ifx\currjob\next
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\makeatother

\typeout{***********************************************}
\typeout{*}
\typeout{* Testing if all files required for the aipproc}
\typeout{* class are available ...}
\typeout{*}
\typeout{***********************************************}

\typeout{*}
\typeout{* Looking for LaTeX2e ... }
\ifx\documentclass\undefined
 \typeout{*}
 \typeout{* Sorry this is a fatal error:}
 \typeout{*}
 \typeout{* The aipproc class can only be used with LaTeX2e which is}
 \typeout{* the standard LaTeX since 1994!}
 \typeout{*}
 \typeout{* Please make sure that your version of LaTeX is up-to-date}
 \typeout{* before attempting to use this class.}
 \typeout{*}
 \expandafter\stop
\else
 \typeout{* ... ok }
\fi


\def\next#1/#2/#3\next{#1#2}
\typeout{*}
\typeout{* Testing that LaTeX2e is not too old ... }
\ifnum\expandafter\next\fmtversion\next<199612 \relax
 \typeout{* ... what a vintage! }
 \typeout{*}
 \typeout{* Sorry this is a fatal error:}
 \typeout{*}
 \typeout{* The aipproc class can only be used with a recent version}
 \typeout{* of LaTeX2e. Your version is dated \fmtversion\space --- but}
 \typeout{* at least the 1996/12/01 version is required!}
 \typeout{*}
 \typeout{* Please make sure that your version of LaTeX is up-to-date}
 \typeout{* before attempting to use this class.}
 \typeout{*}
 \expandafter\stop
\else
 \ifnum\expandafter\next\fmtversion\next<199806 \relax
   \typeout{* ... probably ok }
   \typeout{*}
   \typeout{* Your version of LaTeX2e is quite old --- the aipproc class}
   \typeout{* hasn't been tested with your release.}
   \typeout{*}
   \typeout{* We believe that it will probably work, but if you encounter}
   \typeout{* problems you will need upgrade your installation.}
   \typeout{*}
   \typein{* Type <return> to continue ...}
   \problemtrue
 \else
   \typeout{* ... ok }
 \fi
\fi


\typeout{*}
\typeout{* Looking for aipproc.cls ... }
\IfFileExists{aipproc.cls}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* Before you can use the aipproc class you have to unpack}
     \typeout{* it from the documented source.}
     \typeout{*}
     \typeout{* Run LaTeX on the file 'aipproc.ins', e.g.,}
     \typeout{*}
     \typeout{* \space\space latex aipproc.ins}
     \typeout{*}
     \typeout{* or whatever is necessary on your installation to process}
     \typeout{* a file with LaTeX. This should unpack a number of files for you:}
     \typeout{*}
     \typeout{* aipproc.cls \space and \space aip-*.clo}
     \typeout{*}
     \typeout{* After that retry processing this guide.}
     \typeout{*}
     \stop
}

\typeout{*}
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\IfFileExists{aipxfm.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aipxfm.sty file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for aip-8s.clo ... }
\IfFileExists{aip-8s.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-8s.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for aip-8d.clo ... }
\IfFileExists{aip-8d.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-8d.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}


\typeout{*}
\typeout{* Looking for aip-6s.clo ... }
\IfFileExists{aip-6s.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-6s.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}


\typeout{*}
\typeout{* Looking for aip-arlo.clo ... }
\IfFileExists{aip-arlo.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-arlo.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for fixltx2e.sty ... }
\IfFileExists{fixltx2e.sty}
    {
     \typeout{* ... ok }
    }
    {
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     \typeout{*}
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	 {
	  \typeout{* ... ok }
	 }
	 {
	  \typeout{* ... not found! }
	  \typeout{*}
	  \typeout{* Sorry this is a fatal error:}
	  \typeout{*}
	  \typeout{* Your LaTeX distribution contains neither fixltx2e.sty}
	  \typeout{* nor fix2col.sty.}
	  \typeout{*}
	  \typeout{* This means that it is either too old or incompletely}
	  \typeout{* installed.}
	  \typeout{*}
	  \typeout{* fixltx2e.sty is part of the standard LaTeX distribution}
	  \typeout{* since 1999; fix2col.sty is an earlier version of this}
	  \typeout{* package.}
	  \typeout{*}
	  \typeout{* Best solution is to get the latest LaTeX distribution.}
	  \typeout{* If this is impossible for you, download fix2col.sty.}
	  \typeout{* You can get this software from a CTAN host.}
          \typeout{* Refer to http://www.ctan.org and search for "fix2col".}
	  \typeout{*}
	  \typeout{* After you have updated your LaTeX distribution}
	  \typeout{* retry processing this guide.}
	  \stop
     }
}

\typeout{*}
\typeout{* Looking for fontenc.sty ... }
\IfFileExists{fontenc.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The fontenc package, which is part of standard LaTeX}
     \typeout{* (base distribution) has to be installed at the site to}
     \typeout{* run the aipproc class.}
     \typeout{*}
     \typeout{* The fact that it cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* improperly.}
     \typeout{*}
     \typeout{* Please make sure that your version of LaTeX is okay}
     \typeout{* before attempting to use this class. The LaTeX distribution}
     \typeout{* contains the file "ltxcheck.tex" which can be used to}
     \typeout{* test the basic functionality and integrity of your installation.}
     \typeout{*}
     \stop
    }

\typeout{*}
\typeout{* Looking for calc.sty ... }
\IfFileExists{calc.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The calc package, which is part of standard LaTeX}
     \typeout{* (tool distribution) has to be installed at the site}
     \typeout{* to run the aipproc class.}
     \typeout{*}
     \typeout{* The fact that it cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts.}
     \typeout{*}
     \typeout{* Please make sure that the tools distribution of LaTeX}
     \typeout{* is installed before attempting to use this class.}
     \typeout{*}
     \typeout{* (You might be able to get calc.sty separately for your}
     \typeout{* installation if you are unable to upgrade to a recent}
     \typeout{* distribution for some reason.)}
     \typeout{*}
     \stop
    }

\typeout{*}
\typeout{* Looking for varioref.sty ... }
\IfFileExists{varioref.sty}
    {
     \typeout{* ... ok }
     \gdef\variorefoptionifavailable{varioref,}
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    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The varioref package, which is part of standard LaTeX}
     \typeout{* (tool distribution) is not installed at this site.}
     \typeout{*}
     \typeout{* The fact that it cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts.}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you cannot make use of the options "varioref" or "nonvarioref".}
     \typeout{*}
     \typeout{* Please also note that the aipguide.tex documentation}
     \typeout{* normally uses the "varioref" option to show its}
     \typeout{* effects (which  will now fail).}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
     \gdef\variorefoptionifavailable{}
     \let\vpageref\pageref
     \let\vref\ref
    }

\typeout{*}
\typeout{* Looking for times.sty ... }
\IfFileExists{times.sty}
    {
     \begingroup
% load times and forget it immediately again
       \RequirePackage{times}
       \global\expandafter\let\csname ver@times.sty\endcsname\relax    
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         \timesoktrue
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         \endgroup
     \typeout{* ... obsolete! }
     \typeout{*}
     \typeout{* Serious problem detected:}
     \typeout{*}
     \typeout{* The times package, which is part of standard LaTeX}
     \typeout{* (psnfss distribution) is obsolete at this site.}
     \typeout{*}
     \typeout{* The fact that it contains incorrect code either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts with old files remaining!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but}
     \typeout{* you have to specify the option "cmfonts" which result in}
     \typeout{* documents which are not conforming to the AIP layout specification!}
     \typeout{*}
     \typeout{* You can also try using the class in the following way:}
     \typeout{*}
     \typeout{* \space\space \string\documentclass[cmfonts]{aipproc}}
     \typeout{* \space\space \string\usepackage{times}}
     \typeout{* \space\space ...}
     \typeout{*}
     \typeout{* With luck this will result in Times Roman output but chances}
     \typeout{* are that you will get a larger number of error messages in}
     \typeout{* which case you have to remove the \string\usepackage declaration.}
     \typeout{*}
     \typein{* Type <return> to continue ...}
          \problemtrue
          \gdef\psnfssproblemoption{cmfonts}
          \def\textdegree{$^\circ$}            % used below but now
                                               % not setup
       \fi
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Serious problem detected:}
     \typeout{*}
     \typeout{* The times package, which is part of standard LaTeX}
     \typeout{* (psnfss distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you have to specify the option "cmfonts" which result in}
     \typeout{* documents which are not conforming to the AIP layout specification!}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
     \gdef\psnfssproblemoption{cmfonts,}
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\iftimesok % don't bother testing other font options if times already
           % bad

\typeout{*}
\typeout{* Looking for t1ptm.fd or T1ptm.fd ... }
\IfFileExists{t1ptm.fd}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found, trying T1ptm.fd ... }
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          {
           \typeout{* ... ok }
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          {
           \typeout{* ... not found}
           \typeout{* Serious problem detected:}
           \typeout{*}
           \typeout{* The times package, which is part of standard LaTeX}
           \typeout{* (psnfss distribution) is available but the corresponding}
           \typeout{* .fd file (defining how to load Times Roman) is missing.}
           \typeout{*}
           \typeout{* The fact that this package is only partially installed}
           \typeout{* means that you LaTeX installation is unable to use Times}
           \typeout{* Roman fonts!}
           \typeout{*}
           \typeout{* You can use the aipproc class without this package but }
           \typeout{* you have to specify the option "cmfonts" which result in}
           \typeout{* documents which are not conforming to the AIP layout}
           \typeout{* specification!}
           \typeout{*}
           \typein{* Type <return> to continue ...}
           \problemtrue
           \timesokfalse
           \gdef\psnfssproblemoption{cmfonts,}
          }
    }

\fi


\newcommand\CheckFDFile[3]{%
  \typeout{*}
  \typeout{* Looking for #1#3.fd or #2#3.fd ... }
  \IfFileExists{#1#3.fd}
    {
     \typeout{* ... ok }
    }
    {
     \IfFileExists{#2#3.fd}
      {
       \typeout{* ... ok }
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      {\problemtrue
       \typeout{* ... not found! }
      }
    }
}

\iftimesok % don't bother testing other font options if Times already bad

%\CheckFDFile{ot1}{OT1}{ot1ztmcm}
%\CheckFDFile{oml}{OML}{omlztmcm}
%\CheckFDFile{oms}{OMS}{omsztmcm}
%\CheckFDFile{omx}{OMX}{omxztmcm}

\typeout{*}
\typeout{* Looking for mathptm.sty ... }
\IfFileExists{mathptm.sty}
    {
     \typeout{* ... ok }
     \CheckFDFile{ot1}{OT1}{ptmcm}
     \CheckFDFile{oml}{OML}{ptmcm}
     \CheckFDFile{oms}{OMS}{pzccm}
     \CheckFDFile{omx}{OMX}{psycm}
     \ifproblem
      \typeout{*}
      \typeout{* Problem detected:}
      \typeout{*}
      \typeout{* The mathptm package, which is part of standard LaTeX}
      \typeout{* (psnfss distribution) was found but some or all of its}
      \typeout{* support files describing which fonts to load are missing!}
      \typeout{*}
      \typeout{*}
      \typeout{* The fact that this package is only partially installed}
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      \typein{* Type <return> to continue ...}
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      \typeout{*}
      \typeout{* Looking for mathptmx.sty ... }
      \IfFileExists{mathptmx.sty}
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        \CheckFDFile{oml}{OML}{ztmcm}
        \CheckFDFile{oms}{OMS}{ztmcm}
        \CheckFDFile{omx}{OMX}{ztmcm}
        \ifproblem
	  \typeout{*}
	  \typeout{* Problem detected:}
	  \typeout{*}
	  \typeout{* The mathptmx package, which is part of standard LaTeX}
	  \typeout{* (psnfss distribution) was found but some or all of its}
	  \typeout{* support files describing which fonts to load are missing!}
	  \typeout{*}
	  \typeout{*}
	  \typeout{* The fact that this package is only partially installed}
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	  \typein{* Type <return> to continue ...}
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	  \gdef\psnfssproblemoption{mathptm,}
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       {
	\typeout{* ... not found! }
	\typeout{*}
	\typeout{* Problem detected:}
	\typeout{*}
	\typeout{* The mathptmx package, which is part of standard LaTeX}
	\typeout{* (psnfss distribution) can not be found.}
	\typeout{*}
	\typeout{* This is unfortunate but not a disaster as the older}
	\typeout{* version of the package "mathptm" (no x) seems to exist.}
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	\typeout{* You can use the aipproc class without this package but }
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	\typeout{* math formulas use the older version with upright greek letters.}
	\typeout{*}
	\typein{* Type <return> to continue ...}
	\problemtrue
	\gdef\psnfssproblemoption{mathptm,}
       }
      \fi
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The mathptm package, which is part of standard LaTeX}
     \typeout{* (psnfss distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you have to specify the option "nomathfonts" so that}
     \typeout{* math formulas will be typeset using Computer Modern.}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
     \gdef\psnfssproblemoption{nomathfonts,}
    }

\typeout{*}
\typeout{* Looking for mathtime.sty ... }
\IfFileExists{mathtime.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* The mathime package can not be found.}
     \typeout{*}
     \typeout{* This is not a serious problem because this package is}
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     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you cannot use the "mathtime" option of the class.}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
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\else
\fi % iftimesok

\typeout{*}
\typeout{* Looking for graphicx.sty ... }
\IfFileExists{graphicx.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The graphics package, which is part of standard LaTeX}
     \typeout{* (graphics distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you cannot use commands like \protect\includegraphics
                or \protect\resizebox}
     \typeout{* in this case.}
     \typeout{*}
     \typeout{* Please note that you will get a further error message below}
     \typeout{* about: "graphicx.sty not found" because the class will try}
     \typeout{* to load this package! Type return in response to that error.}
     \typeout{*}
     \typeout{* As a result the illustrations in aipguide will look strange.}
     \typeout{*}
     \typein{* Type <return> to continue ...}

     \gdef\resizebox##1##2{}
     \gdef\includegraphics{\textbf{graphics package missing:}}
     \problemtrue
    }

\typeout{*}
\typeout{* Looking for textcomp.sty ... }
\IfFileExists{textcomp.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The textcomp package, which is part of standard LaTeX}
     \typeout{* (base distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you will always get the error: "textcomp.sty not found"}
     \typeout{* because the class will try to load this package!}
     \typeout{* Type return in response to that error.}
     \typeout{*}
     \typein{* Type <return> to continue ...}

     \def\textdegree{$^\circ$}         % used below but now
                                       % not set up
     \problemtrue
    }


\typeout{*}
\typeout{* Looking for url.sty ... }
\IfFileExists{url.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The url package, which should be part of a good LaTeX}
     \typeout{* distribution, can not be found.}
     \typeout{*}
     \typeout{* Without this package you will not be able to use the \string\url}
     \typeout{* command. Try to download this package from a CTAN  host.}
     \typeout{* Refer to http://www.ctan.org and search for "url".}
     \typeout{*}
     \typein{* Type <return> to continue ...}

     \problemtrue
    }


\makeatletter

\typeout{*}
\typeout{* Looking for natbib.sty ... }
\IfFileExists{natbib.sty}
    {
     \IfStandaloneCheck
       {\begingroup
        \let\@listi\relax
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hayasaki.tex
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\documentclass[
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\begin{document}

\title{
Non-axisymmetric Structure of Accretion Disks in Be/X-ray Binaries}

\classification{97.10.Gz, 97.80.Jp, 97.60.Jd, 97.10.Fy}
\keywords      {Accretion and accretion disks, X-ray binaries, Neutron stars, Circumstellar shells, clouds, and expanding envelopes; circumstellar masers}

\author{Kimitake Hayasaki}
{
  address={Department of Applied Physics, Graduate School of Engineering, Hokkaido University, 
           Kitaku N13W8, Sapporo 060-8628, Japan}
%  ,altaddress={Devision of Physics, Graduate School of Science, Hokkaido
%  University, Kitaku, Sapporo 060-0810, Japan},
  ,altaddress={Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn Victoria 3122, Australia},
,email={hayasaki@topology.coe.hokudai.ac.jp}
}
\author{Atsuo T. Okazaki}{
  address={Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan}
}

\begin{abstract}
The non-axisymmetric structure of accretion disks around the neutron star
in Be/X-ray binaries is studied by analyzing the results 
from three dimensional (3D) Smoothed Particle Hydrodynamics (SPH) simulations.
It is found that ram pressure due to the phase-dependent
mass transfer from the Be-star disk excites
a one-armed, trailing spiral structure
in the accretion disk around the neutron star.
The spiral wave has a transient nature;
it is excited around the periastron,
when the material is transferred from the Be disk,
and is gradually damped afterwards.
It is also found that
the orbital phase-dependence of the mass-accretion rate
is mainly caused by the inward propagation of
the spiral wave excited in the accretion disk.
\end{abstract}

\maketitle

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%% MAINMATTER
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\section{INTRODUCTION}

The majority of high-mass X-ray binaries
have been identified as Be/X-ray binaries.
These systems generally consist of a neutron star and
a Be star with a cool ($\sim 10^{4}K$) equatorial disk, which is
geometrically thin and nearly Keplerian.
Be/X-ray binaries are distributed over a wide range of orbital
periods 
$(10\,{\rm d} \le P_{\rm{orb}} \le 300\,\rm{d})$ and
eccentricities ($e \le 0.9$).

Most Be/X-ray binaries show only transient activity in 
X-ray emission and are termed Be/X-ray transients. Be/X-ray transients
show periodic (Type I) outbursts, which are separated by the orbital
period and have lumiosity $L_{\rm{X}}=10^{36-37}\rm{erg\,s}^{-1}$, and giant (Type II)
outbursts of 
$L_{\rm{X}}\ge10^{37}\rm{erg\,s}^{-1}$ with no
orbital modulation.
These outbursts have features that strongly suggest the presence of
an accretion disk around the neutron star.

Recently, \citet{kh} studied the
accretion flow around the neutron star in a Be/X-ray binary with a
short period ($P_{\rm orb}=24.3\,{\rm d}$) and a moderate eccentricity
($e=0.34$), using a 3D SPH code 
and the imported data from \citep{oka2002}.
They found that a time-dependent accretion disk is formed around the
neutron star. 
They also discussed the evolution of the
azimuthally-averaged structure of the disk, 
in which a one-armed spiral structure is seen.
It is important to note that Be/X-ray
binaries are systems with double circumstellar disks (the Be
decretion disk and the neutron star accretion disk), which
interact mainly via mass transfer, and that this gives a new
point of view to understand the interactions in Be/X-ray
binaries (see Fig.~\ref{fig:bex}). 

In this paper, we show that 
ram pressure due to the material transferred from the
Be disk around periastron temporarily excites the one-armed spiral wave
in the accretion disk around the neutron star in Be/X-ray binaries.

%%%%%%%%%%%%%%%%%%
%=== figure1 =====
%%%%%%%%%%%%%%%%%%
\begin{figure}
\centerline{\includegraphics[height=5cm]{hayasaki1.ps}}
\caption
{A new schmatic diagram in a typical Be/X-ray binary.
The system has double circumstellar disks 
(the Be disk around the Be star 
and the accretion disk around the neutron star) which interact mainly via the mass transfer.
}
\label{fig:bex}
\end{figure}

\section{one-armed spiral wave excited by a ram pressure}

Our simulations were performed by using the same 3D SPH code as in
\citep{kh}, which was based on a version originally developed by Benz
(\citep{benz}; \citep*{benz1}) and later by \citep*{bate}.
In order to study the effect of the ram pressure on the
accretion disk, we compare results from model~1 in \citep{kh} (hereafter,
model~A) with those from a simulation (hereafter, model~B) in which
the mass transfer from the Be disk is artificially stopped for one
orbital period. Except for this difference, the two simulations have the
same model parameters: The orbital period $P_{\rm orb}$ is
24.3\,d, the eccentricity $e$ is 0.34, and the Be disk is coplanar
with the orbital plane. The inner radius of the simulation region
$r_{\rm{in}}$ is $3.0\times10^{-3}a$, where $a$ is the semi-major axis
of the binary. The polytropic equation of state with the exponent
$\Gamma=1.2$ is adopted. The Shakura-Sunyaev viscosity parameter
$\alpha_{\rm{SS}}=0.1$ throughout the disk.

Fig.~\ref{fig:contours} gives a sequence of snapshots of the accretion
disk around the neutron star for $7\le{t}\le8$ in model~A,
where the unit of time is the orbital period $P_{\rm orb}$.
The left panels show the contour maps of the surface density,
whereas the non-axisymmetric components of the surface density
and the velocity field are shown in the right panels.
Annotated in each left panel are the time in units of $P_{\rm orb}$
and the mode strength $S_{1}$, a measure of the amplitude of
the one-armed spiral wave, which is defined by using the azimuthal Fourier decomposition
of the surface density distribution \citep[see][Sec~2.2 for detail]{kh1}.
It is noted from the figure
that the one-armed, trailing spiral is excited at periastron
and is gradually damped towards the next periastron.
The disk changes its topology from circular to eccentric 
with the development of the spiral wave, and then
from eccentric to circular with the decay of the wave 
during one orbital period.

For comparison, we present the results for model~B, in which
the mass transfer is artificially turned off for $7 \le t \le 8$.
Fig.~\ref{fig:contours2} shows
the surface density (the left panel)
and the non-axisymmetric components of the surface density
and the velocity field (the
right panel) at the time corresponding to the middle panel of
Fig.~\ref{fig:contours}. 
It should be noted that
disk deformation due to the one-armed mode is not seen in model~B.
The disk is more circular and has a larger radius in model~B than in
model~A.
This strongly suggests that
the excitation of the one-armed spiral structure in the accretion disk
is induced by the ram pressure from the material transferred from the
Be disk at periastron.

%%%%%%%%%%%%%%%%%%
%=== figure2 =====
%%%%%%%%%%%%%%%%%%
\begin{figure}
\resizebox{\hsize}{!}{
  \includegraphics*{hayasaki2}\\
  \includegraphics*{hayasaki3}\\
  \includegraphics*{hayasaki4}}
  \caption{Snapshots of the accretion disk for model~A.
   The left panels show the surface density in a range of
   three orders of magnitude in the logarithmic scale,
   while the right panels show the non-axisymmetric components of
   the surface density (gray-scale plot)
   and the velocity field (arrows) in the linear scale.
   In the right panels, the region in gray (white) denotes the region
   with positive (negative) density enhancement.
   The periastron is in the $x$-direction and
   the disk rotates counterclockwise.
   Annotated in each left panel are the time
   in units of $P_{\rm orb}$ and the mode strength $S_{1}$.}
 \label{fig:contours}
\end{figure}

%%%%%%%%%%%%%%%%%%
%=== figure3 =====
%%%%%%%%%%%%%%%%%%

\begin{figure}
\centerline{\includegraphics[height=2.65cm]{hayasaki5.ps}}
	\caption{Same as Fig. 2, but for model~B.}
 \label{fig:contours2}
\end{figure}


\subsection{Phase dependence of the mass-accretion rate}
\label{sec:accrate}

After the accretion disk has developed ($t \ge 5$), 
the mass-accretion rate has double peaks per orbit,
a relatively-narrow, low peak at periastron and a broad, high peak afterwards
[see Fig.~15(a) of \citep{kh}].
While the first low peak at periastron could be artificial, being related to
the presence of the inner simulation boundary,
the origin of the second high peak was not clear.
Below we show that the one-armed spiral wave is responsible for the second peak
in the mass-accretion rate.

Fig.~\ref{fig:accrate} shows
the time dependence of the mass-accretion rate for $7\le{t}\le8$.
The thick line denotes the mass-accretion rate in
model~A, in which the mass transfer from the Be disk 
is taken into account, whereas the thin line denotes that in model~B. 
The difference between the accretion rate profiles for these two models is striking.
The accretion rate in model~B monotonically decreases over one orbital period,
whereas that of model~A shows a broad peak centred at $t\sim7.32-7.35$,
which corresponds to the second peak found in \citep{kh}.
The phase lag of this peak behind the peak of the mass-transfer rate, which occurs at periastron, 
results from the inward propagation of the wave 
from the disk outer radius to the inner simulation boundary.

%%%%%%%%%%%%%%%%%%
%=== figure4 =====
%%%%%%%%%%%%%%%%%%
\begin{figure}
\centerline{\includegraphics[height=10cm]{hayasaki6.ps}}
	\caption{Time dependence of the mass-accretion rate for $7\le{t}\le8$.
	The thick and thin lines are for model~A and model~B, respectively.
	The right axis shows the X-ray luminocity corresponding
	to the mass-accretion rate.}
\label{fig:accrate}
\end{figure}

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\begin{theacknowledgments}
This work has been supported by Grant-in-Aid for the 21st Century 
COE Scientific Research Programme on "Topological Science and Technology"
from the Ministry of 
Education, Culture, Sport, Science and Technology of Japan (MECSST) and
in part by Nukazawa Science Fundation.
\end{theacknowledgments}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%% of the above.
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\begin{thebibliography}{9}

\bibitem[Hayasaki and Okazaki(2004)]{kh}
K.~Hayasaki, and A T.~Okazaki, \emph{MNRAS},\textbf{350}, 971, (2004). 

\bibitem[Okazaki et al.(2002)]{oka2002}
A~T.~Okazaki,  M~R.~Bate,  G.I.~Ogilvie  \& J.E. Pringle
, \emph{MNRAS}, \textbf{337}, 967, (2002).

\bibitem[Hayasaki and Okazaki(2004)]{kh1}
K.~Hayasaki, and A T.~Okazaki, submitted to \emph{MNRAS}, (2004). 

\bibitem[Benz(1990)]{benz}
W.~Benz, ``Smoothed Particle Hydrodynamics - a Review'' in
\emph{The Numerical Modelling of Nonlinear Steller Pulsations Problems and Prospects}, edited by
  J.~R. Buchler, Kluwer, Dordrecht, 1990, pp. 269.

\bibitem[Benz et al.(1990)]{benz1}
W.~Benz, L R.~Bowers, A G W.~Cameron \& H W.~Press \emph{ApJ}, \textbf{348},
  647, (1990).

\bibitem[Bate, Bonnell and Price(1995)]{bate}
M~R.~Bate, I~A.~Bonnell \& N~M.~Price \emph{MNRAS}, \textbf{285},
  33, (1995).

\end{thebibliography}

\end{document}
\endinput

astro-ph0412203.extracted_bib
\begin{thebibliography}{9}

\bibitem[Hayasaki and Okazaki(2004)]{kh}
K.~Hayasaki, and A T.~Okazaki, \emph{MNRAS},\textbf{350}, 971, (2004). 

\bibitem[Okazaki et al.(2002)]{oka2002}
A~T.~Okazaki,  M~R.~Bate,  G.I.~Ogilvie  \& J.E. Pringle
, \emph{MNRAS}, \textbf{337}, 967, (2002).

\bibitem[Hayasaki and Okazaki(2004)]{kh1}
K.~Hayasaki, and A T.~Okazaki, submitted to \emph{MNRAS}, (2004). 

\bibitem[Benz(1990)]{benz}
W.~Benz, ``Smoothed Particle Hydrodynamics - a Review'' in
\emph{The Numerical Modelling of Nonlinear Steller Pulsations Problems and Prospects}, edited by
  J.~R. Buchler, Kluwer, Dordrecht, 1990, pp. 269.

\bibitem[Benz et al.(1990)]{benz1}
W.~Benz, L R.~Bowers, A G W.~Cameron \& H W.~Press \emph{ApJ}, \textbf{348},
  647, (1990).

\bibitem[Bate, Bonnell and Price(1995)]{bate}
M~R.~Bate, I~A.~Bonnell \& N~M.~Price \emph{MNRAS}, \textbf{285},
  33, (1995).

\end{thebibliography}