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higher x - ray emissivity arcs , termed ripples , are observed in the intracluster medium ( icm ) of the perseus cluster ( fabian et al .
2003 , 2006 ; sanders & fabian 2007 ) , as well as in a2052 ( blanton et al . 2007 ) .
the ripples are thought to be sound waves that are excited by the inflation of bubbles near the cluster s center .
some of the ripples are observed on the outer edge of bubbles ; they are compressed icm that will turn into shock waves , and later to sound waves .
it is possible that some of the other ripples are compressed by weak jets ( soker & pizzolato 2005 ) , rather than being sound waves detached from their parent bubble . beside being interesting by their own merit
, the sound waves can heat the icm and partially offset the radiative cooling of the icm ( e.g. , brggen et al . 2005 ; fabian et al . 2005 ; fujita & suzuki 2005 ; graham et al . 2008 ) .    to obtain repeated sound waves previous numerical simulations assumed repeating bubble inflation episodes ( e.g. , ruszkowski et al .
2004a , b ; sijacki & springel 2006a , b ) .
this had to be done because these simulations used _ artificial bubbles _ ,
i.e. , numerically injected spherical bubbles at off - center locations , as is commonly done ( e.g. , brggen 2003 ; brggen & kaiser 2001 ; gardini 2007 ; jones & de young 2005 ; pavlovski et al . 2008 ; reynolds et al . 2005 ; robinson et al . 2004 ; ruszkowski et al .
2004a , 2004b , 2007 , 2008 ; scannapieco & brggen 2008 ) .
this of course is not the way bubbles are formed in clusters .
bubble are formed by jets [ a different approach of injecting magnetic energy instead of a jet ( nakamura et al .
2008 ; xu et al .
2008 ) requires further study ] .
when large , almost spherical bubbles ( termed _ fat bubbles _ ) are inflated by jets , some phenomena appear , that are not revealed when artificial bubbles are used ( sternberg & soker 2008b ) . among these
are : ( 1 ) more stable bubbles , and ( 2 ) bubble - icm boundary that is stochastically vibrates due to the vortices inside the bubble and the back flowing icm .    in previous works we found that in order for the jets to inflate fat bubbles they should have a large opening angle , or they should rapidly precess ( soker 2004 , 2006 ; sternberg et al . 2007
; sternberg & soker 2008a ) .
in addition , jets that are relatively slow , @xmath2 , and with a mass loss rate of the order of @xmath3 ( for both jets ) , are more likely to inflate fat bubbles .
the same results can be seen in the simulations of omma et al .
( 2004 ) , alouani bibi et al .
( 2007 ) and binney et al .
( 2007 ) , who use similar parameters to ours , but instead of conical jets with wide opening angles they use cylindrical jets with large initial radius .
this is also shown by the simulation of heinz et al .
( 2006 ) , who manages to inflate a bubble ( although not a bubble attached to the center ) by launching a jet with a velocity of @xmath4 and a mass loss rate of @xmath5 in one jet . in this paper
we continue our study of jet - inflated bubbles , and show that such bubbles can excite several consecutive sound waves without the need to invoke periodic jet launching episodes .
we do not deal with the propagation of sound waves and their properties .
these seem to require more sophisticated treatment ( graham et al .
we limit ourself to show that a proper treatment of bubble inflation can better explain the excitation of sound waves in the icm .
the simulations were performed using the _ virginia hydrodynamics - i _ code ( vh-1 ; blondin et al . 1990 ; stevens et al . 1992 ) , as described in sternberg & soker ( 2008b ) . in this paper
we mention only the important features of the code .
the unperturbed icm temperature is set to @xmath6 .
gravity is included , but radiative cooling is not included .
we study a three - dimensional axisymmetric flow with a 2d grid ( referred to as 2.5d ) .
we simulate half of the meridional plane using the two - dimensional version of the code in spherical coordinates .
the symmetry axis of all plots shown in this paper is along the x ( horizontal ) axis .
we run two cases . in the wide jets case ,
the two opposite jets were injected at a radius of @xmath7 , with constant mass flux of @xmath8 per one jet , and a constant radial velocity of @xmath9 , inside a half opening angle of @xmath10 .
the total power of the two jets is @xmath11 .
the jets were active for a period of @xmath12 , from @xmath13 until @xmath14 .
for more detail on the numerical code and the properties of the bubbles see sternberg & soker ( 2008b ) .    in the case of the precessing jets
the two opposite jets were injected at a radius of @xmath7 with @xmath8 per one jet , and a constant radial velocity of @xmath9 ( the same parameters of the wide jets ) , but within a half opening angle of @xmath15 . in our axisymmetric code
the jets are precessing very rapidly around the symmetry axis ( i.e. , in 3d we actually inject a torus ) .
we vary the angle between the symmetry axis and the jets axis @xmath16 in a random way .
namely , the jet axis has the same probability to take any direction within an angle of @xmath17 .
this is done by changing @xmath16 periodically and taking @xmath18 to be constant .
the precession period , i.e. , the time the jet returns to the same angle @xmath16 , is @xmath19 .
the jet s interaction with the icm is similar to that of a wide jet with a half opening angle of @xmath20 .
the jets were active for a duration of @xmath21 between @xmath22 and @xmath14 .
gravity was included as was in the wide jets case .
in fig . [ fig : jeti ] we show the density and velocity map of the wide - jet - inflated bubble at @xmath23myr , and at @xmath14 when the jet is shut off .
there are some flow structures which characterize the jet - inflated bubble that are not present when artificial bubbles are used ( sternberg & soker 2008b ) .
most relevant for us are the following : ( 1 ) the bubble and the dense shell around it have radial momentum .
in particular , the shell s front is relatively dense and has an outward velocity .
this will form the ripple observed at the front edge of the bubbles in perseus .
( 2 ) as the bubbles grow their shapes change .
this relative motion of the bubble - icm boundary excites sound waves .
( 3 ) there is a circular flow , a vortex , around the lowest density region in the bubble .
this vortex and the back flow of the icm along the boundary of the bubble change the shape of the bubble , a processes leading to excitation of sound waves .
we also note the back flow of the icm around the bubble , and that at early times the bubble expands supersonically , e.g. , an average mach number of @xmath24 between @xmath23myr and @xmath14 , and the shell is a weak shock . at later times the bubble moves subsonically .
[ cols="^ " , ]
we followed the response of the icm to the inflation of bubbles by jets .
the inflation of a bubble by a jet results in vortices inside the bubble and a backflow of the icm around the bubble ( sternberg et al . 2007 ; sternberg & soker 2008a , b ) .
both processes cause the bubble - icm to change shape with time and to become corrugated . in the case of a precessing jet ( section [ sec : precessing ] ) , the changes in the jet axis cause a more prominent change in the bubble - icm shape .
this motion of the bubble - icm boundary sends shocks and sound waves into the icm .
this effect can not be reproduced by artificial bubbles , i.e. , by numerically inserting spherical bubbles at off - center locations .
our main result is that one episode of a bubble pair inflation can excite several sound waves along each radial direction .
these form high - density arcs , the ripples .
the front ripple is actually a weak shock .
there is no need to introduce periodic ( or semi - periodic ) episodes of bubble inflation .
nonetheless , multiple episodes will make more ripples , and make the ripple structure more complicated .
this is the case in perseus , where the main central bubble pair was formed by jets that changed their direction and intensity ( dunn et al .
2006 ) .
we also observe a dense arc at the bubble s front that is apparent part of the time .
the dense arc is the excitation of a new sound wave at the compression phase .
part of the time the region at the bubble s front is at the rarefaction phase , the wave trough , and no dense arc is observed there .
our results can be incorporated into a broader scope . in a previous paper ( sternberg & soker 2008b ) we found
that to follow the evolution of bubbles in the icm one must inflate them by jets , rather than introduce them artificially .
the evolution of bubbles and their influence on the icm , e.g. , sound waves , is crucial for the deposition of heat from the central active galactic nuclei to the icm .
our works show that in order to understand the heating of the icm one must properly inflate bubbles .
we thank john blondin for his immense help with the numerical code .
this research was supported by the asher fund for space research at the technion , and by the israeli science foundation ( grant no .
89/08 ) .
blanton , e. l. , douglass , e. m. , sarazin , c. l. , clarke , t. e. , & mcnamara , b. r. 2007 , in heating versus cooling in galaxies and clusters of galaxies , eso astrophysics symposia ( springer - verlag , berlin heidelberg ) , 109
we show that repeated sound waves in the intracluster medium ( icm ) can be excited by a single inflation episode of an opposite bubble pair . to reproduce this behavior in numerical simulations 
 the bubbles should be inflated by jets , rather than being injected artificially . 
 the multiple sound waves are excited by the motion of the bubble - icm boundary that is caused by vortices inside the inflated bubbles and the backflow ( ` cocoon ' ) of the icm around the bubble . 
 these sound waves form a structure that can account for the ripples observed in the perseus cooling flow cluster . 
 we inflate the bubbles using slow massive jets , with either a wide opening angle or that are precessing . 
 the jets are slow in the sense that they are highly sub - relativistic , @xmath0 , and they are massive in the sense that the pair of bubbles carry back to the icm a large fraction of the cooling mass , i.e. , @xmath1 . 
 we use a two - dimensional axisymmetric ( referred to as 2.5d ) hydrodynamical numerical code ( vh-1 ) .    _ subject headings : _ ( galaxies : ) cooling flows galaxies : clusters : general galaxies : jets