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cataclysmic variables ( cvs ) are close binary systems consisting of a mass - accreting white dwarf ( wd ) and a mass - donating late - type main - sequence companion star . intermediate polars ( ips ) , an important subclass of cvs , are considered to involve wds with magnetic field of @xmath4 g. in an ip , gas from the companion forms an accretion disk down to a radius @xmath5 where gravity is counter - balanced by the magnetic pressure .
then the gas is captured by the wd s magnetosphere , and accretes onto the wd surface to form accretion columns due to strong magnetic field . in the accretion column ,
the gas is heated up to @xmath6 k by a standing shock , and lands onto the wd surface emitting thermal x - rays .
if @xmath5 is far enough from the wd surface , the temperature @xmath7 just below the shock is proportional to gravitational potential of the wd ( aizu 1973 ) : @xmath8 where @xmath9 is mean molecular weight and @xmath10 is the proton mass .
therefore the wd can be estimated by combining the measured @xmath7 with the standard mass v.s .
radius ( @xmath11-@xmath12 ) relation ( nauenberg 1972 ) .
an x - ray spectrum from ips is a superposition of optically - thin thermal emissions of various temperatures , from @xmath7 downwards . to determine @xmath7
, it is hence important to accurately measure both the hard x - ray continuum , and the ratio of fe xxv and xxvi lines at @xmath13 kev .
this is because the hard x - ray continuum is sensitive to the hottest components ( with temperature @xmath14 ) , whereas the latter tells us contributions from cooler components arising closer to the wd surface .
this method has been established with x - ray satellites such as _ ginga _
( e.g. ishida 1991 ) , _ asca _
( e.g. fujimoto & ishida 1997 , ezuka & ishida 1997 ) , _ rxte _
( e.g. suleimanov et al . 2005 ) , _ integral _ ( e.g. falanga et al . 2005 ) , _ swift _ ( e.g. brunschweiger et al .
2009 ) , and _ suzaku _
( e.g. yuasa et al .
2010 , hayashi et al .
2011 ) .
gk persei , a cataclysmic variable system , is interestingly categorized as both a dwarf nova and an ip . after a classical nova explosion in 1901 ( williams 1901
; hale 1901 ) , it repeats , every 23 years , dwarf nova outbursts each lasting for 2 months ( e.g. imon 2001 ) . during outbursts , the optical and x - ray luminosities increase by a factor of 1020 .
the distance was measured to be @xmath15 pc ( harrison & bornak 2013 ) .
reinsch ( 1994 ) and morales - rueda et al .
( 2002 ) optically determined the wd mass as @xmath16 and @xmath17 , respectively .
an outburst from gk persei started in march 2015 , and continued for 2 months ( wilber et al .
2015 ) . during this outburst , a too ( target of opportunity ) observation of _
nustar _ was triggered ( zemko et al . 2016 ) .
suleimanov et al .
( 2016 ) analyzed the too data and constrained the wd mass as @xmath18 .
_ suzaku _ was serendipitously pointed at the onset of this outburst ( yuasa et al .
2016 ) . with _ nustar _ , we observed gk persei again , after the object returned to its quiescence .
the present paper describes a combined analysis of the outburst and quiescence data from _
the x - ray satellite _ nustar _ ( harrison et al .
2013 ) , launched on 2012 june 13 , has a 379 kev sensibility .
it is thus suited for our purpose , because we can study both the hard x - ray continuum and the fe - k lines .
as described in  1 , gk persei was observed twice with _ nustar _
, once at the outburst peak and the other after the outburst terminated .
details of the observations are shown in table 1 .
we extracted the data from the two focal plane modules ( fpma and fpmb ) with nupipeline and nuproducts included in nustardas version 1.5.1 .
the source region was chosen to be a circle of @xmath19 radius for the outburst data , and @xmath20 for the quiescence data .
the remaining region was used for background .
we used nustar caldb version 20150904 .
the x - ray spectra were analyzed with xspec version 12.9.0 .
( arnaud 1996 ) .
aizu , k. 1973 , prog .
phys . , 50 ,
344 arnaud , k. a. 1996 , astronomical data analysis software and systems v , 101 , 17 brunschweiger , j. , greiner , j. , ajello , m. , osborne , j. 2009 , a&a , 496 , 121 done , c. , & osborne , j. p. 1997 ,
mnras , 288 , 649 ezuka , h. , & ishida , m. 1999 , apjs , 120 , 277 falanga , m. , bonnet - bidaud , j. m. , suleimanov , v. 2005 , a&a , 444 , 561 fujimoto , r. , & ishida , m. 1997 , apj , 474 , 774 ghosh , p. , & lamb , f. k. 1979 , apj , 234 , 296 hale , g. e. 1901 , apj , 13 , 173 harrison , f.a . , craig , w.w . , christensen , f.e .
, et al . 2013 , apj , a103 harrison , t. e. , bornak , j. , mcarthur , b. e. , benedict , g. f. 2013 apj , 767 , 11 hayashi , t. , ishida , m. , terada , y. , bamba , a. , shionome , t. 2011 , pasj , 63 , 739 ishida , m. 1991 , phd thesis , univ . of tokyo magdziarz , p. , & zdziarski , a. a. 1995 , mnras , 273 , 837 morales - rueda , l. , still , m. d. , roche , p. , wood , j. h. , lockley , j. j. 2002 , mnras , 329 , 597 nauenberg , m. 1972 , apj , 175 , 417 reinsch k. 1994 , a&a , 281 , 108 imon , v. 2002 , a&a , 382 , 910 suleimanov , v. , revnivtsev , m. , ritter , h. 2005 , a&a , 435 , 191 suleimanov , v. , doroshenko , v. , ducci , l. , zhukov , g. v. , werner , k. 2016 , a&a , 591 , 12 wilber , a. , neric , m. , starreld , s. , wagner , r. m. , woodward , c. e. 2015 , the astronomer s telegram , 7217 , 1 williams , a. s. 1901 ,
mnras , 61 , 337 yuasa , t. , nakazawa , k. , makishima , k. , saitou , k. , ishida , m. , ebisawa , k. , mori , h. , yamada , s. 2010 , a&a , 520 , a25 yuasa , t. , hayashi , t. , ishida , m. 2016 , mnras , 459 , 779 zemko , p. , orio , m. , luna , g. , mukai , k. 2016 , arxiv:1603.03286 [ astro-ph.sr ]
we report on _ nustar _ observations of the intermediate polar gk persei which also behaves as a dwarf nova . 
 it exhibited a dwarf nova outburst in 2015 march - april . 
 the object was observed in 379 kev x - rays with _ 
 nustar _ 
 , once at the outburst peak , and again in 2015 september during quiescence . 
 the 5 - 50 kev flux during the outburst was 26 times higher than that during the quiescence . with a multi - temperature emission model and a reflection model , we derived the post - shock temperature as @xmath0 kev in the outburst , and @xmath1 kev in the quiescence . 
 this temperature difference is considered to reflect changes in the radius at which the accreting matter , forming an accretion disk , is captured by the magnetosphere of the white dwarf ( wd ) . assuming that this radius scales as the power of -2/7 of the mass accretion rate , and utilizing the two temperature measurements , as well as the standard mass - radius relation of wds , we determined the wd mass in gk persei as @xmath2 solar masses . 
 the magnetic field is estimated as @xmath3 g.