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blazars are well - known jet - dominated extragalactic objects characterized by the emission of strongly variable and polarized non - thermal radiation across the entire electromagnetic spectrum , from radio to high energy @xmath4-rays .
the strong emission of blazars at all wavelengths makes them the dominant type of extragalactic sources in the radio , @xmath5-wave , and @xmath4-ray bands where the accretion and other thermal emission processes do not produce significant amounts of radiation .    for these reasons
blazars are hard to find at optical and x - ray frequencies , while dominating the @xmath5-wave and @xmath4-ray high galactic latitude sky . the advent of the fermi @xcite and , more recently , of the planck satellite @xcite , capable of probing deeply these two last observing windows , combined with the versatility of the swift observatory @xcite , and the observations by a number of ground based observatories , is giving us the unprecedented opportunity to collect multi - frequency data for very large samples of blazars in order to assemble simultaneous or quasi - simultaneous broad - band spectra .
in order to explore the blazars parameters space from different viewpoints we have adopted different criteria to select the list of blazars to be observed simultaneously by planck , swift and fermi .
the first three samples of blazars are flux limited in the high energy part of the electromagnetic spectrum @xcite . _
the soft x - ray flux limited sample _ , including 43 sources , was defined starting from the rosat all - sky survey bright source catalog ( 1rxs @xcite ) , selecting all the blazars with count rate larger than @xmath6 in the @xmath7 energy band , and radio flux density @xmath8 . _ the hard x - ray flux limited sample _ , including 34 sources , was defined starting from the @xmath9-bat 54  month source catalog @xcite , selecting all blazars with x - ray flux @xmath10 in the @xmath1 energy band and radio flux density @xmath11 . _ the @xmath4-ray flux limited sample _ , including 50 sources ,
was created starting from the @xmath12-lat bright source list @xcite , selecting all the high galactic ( @xmath1310@xmath14 ) blazars detected with high significance ( @xmath15 ) , with a flux cut @xmath16 and radio flux density @xmath17 .
the last sample @xcite , including 104 sources , is _ the radio flux density limited sample _
@xcite : it includes all northern and equatorial radio - loud agn with declination @xmath18 that have a measured average radio flux density @xmath19 .
@xmath12-lat data taken from august  4 ,  2008 to november  4 ,  2010 have been analyzed selecting for each source in the previously defined samples @xcite @xcite , only photons above @xmath20 , belonging to the diffuse class @xcite which have a low probability of background contamination , within a @xmath21 region of interest ( roi ) centered around the source itself .
the data were analyzed with a binned maximum likelihood technique @xcite using the analysis software ( _ gtlike _ ) developed by the lat team . a model accounting for the diffuse emission as well as for the nearby @xmath4-ray sources is included in the fit .
for the evaluation of the @xmath4-ray seds , the whole energy range from @xmath20 to @xmath22 has been divided into 2 equal logarithmically spaced bins per decade . in each energy bin the standard gtlike binned analysis has been applied assuming a power law spectrum for all the point sources in the model , with photon index fixed to 2 .
the flux of the source in all selected energy bins has been evaluated , requiring in each bin a test statistics ( ts ) greater than 10 and the ratio between the flux and its error greater than 0.5 .
if these conditions were not satisfied , an upper limit ( ul ) has been evaluated in that energy bin . for each source in the four samples , three different integration periods have been considered :    * simultaneous observations ( data accumulated during the period of planck observation of the source ) ; * quasi - simultaneous observations ( data integrated over a period of two months centered on the planck observing period of the source ) ; * 27 month fermi integration ( data integrated over a period of 27 months from august 4 , 2008 to november 4 , 2010 ) .
the plot of radio to @xmath4-ray flux distributions in the @xmath23 , widely known as a spectral energy distribution ( sed ) , is a powerful method of studying the physics of blazars .
the seds of pks  1124 - 186 ( from the soft x - ray sample ) , pks  b1830 - 210 ( from the hard x - ray sample ) , pks  1502 + 106 ( from the @xmath4-ray sample ) and pks  1510 - 089 ( from the radio sample ) have been reported respectively in fig .  [
fig : sed1 ] , fig .
[ fig : sed2 ] , fig .
[ fig : sed3 ] , fig .
[ fig : sed4 ] . in all plots ,
red filled points ( or ul ) show simultaneous multi - frequency data , green points ( or ul ) show @xmath4-ray data integrated over a period of 2 months centered on the planck observing period , or ground - based data taken quasi - simultaneously and blue points ( or ul ) show @xmath12-lat data integrated over 27 months ; literature or archival data are shown in light gray .
we have collected planck , swift , fermi and ground based simultaneous multi - frequency data for a great number of blazars included in four statistically well defined samples .
the acquisition of this unprecedented multifrequency multi - satellite data set was used to build well sampled simultaneous seds .
the seds of these sources clearly show the typical two - bump signature usually attributed to synchrotron and inverse compton emission .
the comparison between our simultaneous data with literature archival measurements shows that seds built with non - simultaneous data suffer from uncertainties in the @xmath5-wave region that are relatively modest and generally limited to about a factor 2 while the high energy part of the spectrum is much more affected with uncertainties due to flux variations of up to a factor of ten or more .
the @xmath12 lat collaboration acknowledges support from a number of agencies and institutes for both development and the operation of the lat as well as scientific data analysis .
these include nasa and doe in the united states , cea / irfu and in2p3/cnrs in france , asi and infn in italy , mext , kek , and jaxa in japan , and the k.  a.  wallenberg foundation , the swedish research council and the national space board in sweden . additional support from inaf in italy and cnes in france for science analysis during the operations phase is also gratefully acknowledged .
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blazars are jet - dominated extragalactic objects characterized by the emission of strongly variable non - thermal radiation across the entire electromagnetic spectrum . 
 therefore , the study of blazars ( and in general of radio loud agn ) through the use of multi - frequency simultaneous data is essential in order to understand the physical processes that take place in these objects . with planck , fermi and swift 
 simultaneously on orbit , complemented with other space and ground - based observatories , it is possible to assemble high - quality multi - frequency simultaneous broad - band spectra of large and statistically well - defined samples of radio - loud agn . in particular , four samples of sources have been selected . 
 the first three samples are flux limited in the high energy part of the electromagnetic spectrum : the soft x - ray ( @xmath0 ) sample includes 43 sources from the rosat all sky survey bright source catalog , the hard x - ray ( @xmath1 ) sample includes 34 sources from the swift - bat 54 months source catalog and the gamma - ray sample includes 50 sources from the fermi - lat 3 months bright agn source list . 
 the fourth sample is radio flux limited , including 104 bright northern and equatorial radio - loud agn ( most of which have been monitored at metsahovi radio observatory for many years ) with average radio flux density at @xmath2 greater than @xmath3 . 
 we present the methods applied and the results of the analysis performed using fermi - lat data for all sources in the four different samples of agn .