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in this paper we present preliminary results of a systematic study on the x - ray spectral variability properties of the pg quasar sample @xcite with xmm - newton @xcite .
the main goals of this projects are :    * the identification of the physical driver underlying spectral variability in active galactic nuclei ( agn ) , by determining the typical timescales on which it occurs in x - ray unobscured agn * the study of the correlation between emission in the soft ( @xmath1  kev ) and hard ( @xmath4  kev ) energy bands .
the results of this study can shed some light on the physical origin of the x - ray emission . while the hard x - ray emission is probably due to comptonization of hard disk soft photons in a compact corona within a few schwarzschild radii from the supermassive black hole @xcite , the nature of a prominent and almost ubiquitous `` soft excess '' is still matter of debate . thermal emission from the disk @xcite , bremsstrahlung emission from the ionized skin of the accretion disk ( nayakshin , kazanas & kallman 2000 ) , comptonization by cool , thick gas @xcite , or an extremely relativistic warm outflow @xcite are among the possible explanations .
it is of paramount importance that x - ray variability studies are performed with the same instrument , as the cross - calibration among detectors flown on different mission is not yet accurate enough to guarantee against spurious results .
xmm - newton is ideally suitable for such a study , thanks to its unprecedented effective area in the whole 0.115  kev energy band , which allows to expand the pioneering studies conducted by rxte ( markovitz et al .
2003 ; mchardy et al .
2004 ) to a larger sample of comparatively weaker agn .
our parent sample is the whole set of xmm - newton observations of pg quasars available in the public archive as of august 2004 .
they cover 42 objects , with largely inhomogeneous exposure times .
their spectral properties are presented by porquet et al .
( 2004 ) and jimnez - bailn et al .
( 2004 ) . in this paper
we will concentrate on the spectral variability pattern in objects dominated by comptonization in the whole xmm - newton energy bandpass ( 0.110  kev ) .
consequently , we will consider in the following only the 20 pg quasars , whose xmm - newton epic spectra are well fit with a double power - law model by jimnez - bailn et al .
( 2004 ) .      in fig .
[ fig1 ] we show the 310  kev versus 0.11  kev hardness ratio ( hr )    against the 0.110  kev count rate as measured on light curves with a time binning @xmath6  s. in only 2 objects ( @xmath710% of the cases ) one observes a correlation more significant then the 99% confidence level according to a k - s test .
moreover , these correlations have two opposite signs : positive in pg0947 + 396 , and negative in pg1202 + 281 .
we conclude that on such short timescales variations of the x - ray emission produced by the two comptonization media are largely independent .
they are probably dominated by local fluctuations in the accretion flow or in the disk illumination , which do not affect the disk - corona system as a whole .
there are only two pg quasars , for which xmm - newton observations at multiple epochs are available : pg1407 + 265 and pg1440 + 356 .
the former was observed twice , in january and december 2001 .
the latter was observed by xmm - newton 4 times in the framework of a specific program to determine the timescale of the spectral variability pattern .
the first observation was performed on december 2001 , the remaining three in january 2003 , at the relative distance of three days each . in order to spectrally characterize their variability properties
, we fit the data with a combination of two comptonizing continua , trying different geometries for the seed photons influx and the comptonizing media .
the following results refer to a physical scenario where the two spherical reservoirs of comptonizing electrons ( a two - phase corona ) share the same soft photons input flux ( a region at the peak of the accretion disk thermal distribution , and smaller than the disk gradient temperature scale ) .
we show in fig .
[ fig2 ]    the iso-@xmath9 contour plot for the temperature ( @xmath10 ) versus plasma optical depth ( @xmath11 ) for the comptonizing medium producing the bulk of the x - ray emission in the soft x - ray band . in both cases
the best - fit parameters corresponding to observations separated by the longer intervals ( @xmath71  year ) are different at a confidence level larger then 99% for two interesting parameters .
intriguingly enough , in pg1407 + 265 a change in the optical depth of the comptonized medium is favored , whereas in pg1440 + 356 a feedback mechanism between these quantities may force the contours to follow a source - specific `` track '' in the kt vs. @xmath11 plane . given the well known degeneracy of the above parameters in comptonization models @xcite , one should refrain from attributing a too literal meaning to the best - fit parameters values .
however , regardless of their true values , one can firmly conclude that in these two cases the mechanism responsible for the spectral variability operates on time scales longer than about 1  week and shorter than about 1  year .
although this is still a rather loose constraint ( which could be improved in the future if xmm - newton will pursue similar monitoring programs sampling intermediate timescales ) , it already rules out dynamical timescales playing an important role .
viscous instabilities in the accretion flow are a more likely candidate for the ultimate physical driver of the long - term variability .
we are undertaking a systematic study of the x - ray variability pattern of the pg quasars using xmm - newton observations . in this paper
we focus our attention on the variability pattern in `` naked '' quasars , whose emission across the whole xmm - newton band is due to comptonization .
the main results can be summarized as follows :    * on short ( @xmath12hours ) timescales there is no correlation between the variability in the soft and the hard x - ray regime * variability of the comptonizing plasma physical parameters is observed when observations separated by more than 1 week ( and less than 1 year ) are compared
this paper presents preliminary results of a systematic study on the x - ray spectral variability of pg quasars with xmm - newton . 
 we concentrate on those objects , whose x - ray spectra are well fit in the framework of the `` double comptonization '' model . on short ( @xmath0  s ) timescales , variability between the soft ( @xmath1  kev ) and the hard ( @xmath2  kev ) bands is uncorrelated . in the two objects , for which observations at different epochs are available 
 , spectral variability in the soft x - ray regime is unveiled . 
 its timescale is constrained in the range between @xmath31 week and @xmath31 year .