You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
two typical types of sources , z and atoll sources , in neutron star ( ns ) low mass x - ray binaries ( lmxbs ) describe different tracks in ccds on different timescales and display distinct @xmath1 relations of lower khz qpos ( * ? ? ?
* hasinger & van der klis ( 1989 ) ) , ( * ? ? ?
* wang et al . ( 2012 ) ) .
however , the @xmath1 relations of lower khz qpos of sco x-1 displays a mildly similar trend to that of atoll sources ( * ? ? ?
* wang et al . ( 2012 ) ) .
moreover , recent studies show that the characteristics of both z and atoll types present in two sources ( * ? ? ?
* ding et al . ( 2011 ) ) ding et al .
2010 for a review ) .
in addition , a compilation of rxte data for three transient atoll sources presents new branch which connects to the top of the c - shaped ( atoll ) path and forms a horizontal track  turning c - shape into a `` z '' when they go down to very low luminosity ( * ? ? ? * gierli@xmath2ski & done ( 2002 ) ) .
these phenomena enlighten the investigation for the links between z and atoll sources .
many evidences indicate that there is a connection between z and atoll sources . we suppose an evolutionary scenario that the lmxbs containing a ns may evolve from a cyg - like z phase at the beginning and transform into the sco- like z phase .
eventually , they enter into atoll phase , going through a phase characterized by hybrid atoll / z phase like the sources gx 13 + 1 ( * ? ? ?
* hasinger & van der klis ( 1989 ) ) , ( * ? ? ?
* schnerr et al . ( 2003 ) ) , ( * ? ? ?
* homan et al . (
the variation of accretion rate dominates the whole process . at the beginning ,
the matter with high falling velocity releases substantial gravitational energy and forms a radiation dominant inner disk ( * ? ? ?
* frank et al . ( 2002 ) ) .
the accreting material piles up around the ns at high accretion rate ( near critical eddington accretion rate , even supercritical rate ) , describing a z - shape track in ccds . during this process
, the disk may puff up and become geometrically thick due to some instabilities ( * ? ? ?
* paczysky & wiita ( 1980 ) ) , resulting in two effects .
firstly , more and more material pile up near the polar cap and bury the magnetic field on ns sur- face ( * ? ? ?
* zhang & kojima ( 2006 ) ) , which leads to the decrease of magnetic pressure .
furthermore , the viscosity and friction between the accreted matter slow down the falling velocity , resulting in the decrease of ram pressure and low accretion rate .
this scenario is consistent with the atoll phase .
then the nb and fb turn into the is and banana branch from the sco - like phase , respectively . besides , the low accretion rate results in a long timescale for the trace of atoll patterns in ccds , as well as a wide range of luminosity .
because of the inhomogeneities in the inner accretion disk ( * ? ? ?
* romanova , kulkarni & lovelace ( 2007 ) ) , the friction is different from here to there .
consequently , the accretion rate may become high at some position , which generates a horizon pattern connect- ing to the top of the c - shape for atoll sources , consistent with the scenario presenting in ( * ? ? ?
* gierli@xmath2ski & done ( 2002 ) ) .
we suggest that the z type and atoll type sources are two phases during different evolutionary phase of ns lmxbs , instead of two distinct sources .
it is known that the z and atoll sources are two typical types of neutron - star sources in low mass x - ray binaries ( lmxbs ) , which present very different q-@xmath0 relations of lower khz qpos . 
 we propose that the z and atoll sources are two different phases in the evolutionary track of neutron star in lmxbs , instead of two types of distinct sources .