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[ [ the - diagnostic - plot ] ] * the diagnostic plot * + + + + + + + + + + + + + + + + + + + + +    despite their cosmological relevance , less than two dozens of confirmed heavily obscured , compton  thick ( nh@xmath010@xmath1 @xmath2 ) agn have been found so far .
we propose a new diagnostic plot to select compton - thick agn candidates in the local universe .
the plot is based on the combination of the f(210 kev)/(@xmath3 f@xmath4 ) flux ratio ( where ir is either 24 @xmath5 m ( spitzer ) or 25 @xmath5 m ( iras ) ) with the xmm - newton color ( hardness ratio , hr ) .
while the f(2 - 10 kev)/(@xmath3 f@xmath4 ) flux ratio allows us to separate heavily obscured agn candidates and starburst galaxies from less obscured ( nh@xmath65@xmath710@xmath8 @xmath2 ) agn ( @xcite ) , the x - ray color is able to separate starburst ( hr@xmath6 - 0.1 ) from obscured agn ( hr@xmath0 - 0.1 ) .
the region of compton - thick agn candidates is marked in figure 1 ( left panel ) .      to test the proposed diagnostic plot in selecting compton - thick agn ,
we have cross - correlated the iras point source catalog ( psc ) v2.1 with the bright end ( f(4.5 - 12 kev)@xmath010@xmath9 erg @xmath2 s@xmath10 ) of the incremental version of the 2xmm catalogue ( @xcite ) .
we find that 47 iras sources populate the region of the compton - thick agn candidates , 46 of which are extra - galactic sources ( hereafter the iras-2xmm sample , fig .
1 , right panel ) . for all but one source ,
the spectroscopic classification is already available in the literature . up to now , we have performed a preliminary x  ray analysis for 40 out of the 46 sources .
twenty  four of them are already known as compton 
thick agn in the literature ( 10 have also high energy data that confirm their nature ) . among the remaining 16 sources , there are 9 obscured but compton - thin sources and 7 newly discovered compton - thick candidates ( i.e. with 210 kev properties consistent with a typical compton  thick emission ) .
for one of them a preliminary analysis of swift - bat ( @xcite ) and suzaku ( proprietary data ) data confirms our classification as a compton 
thick agn ( severgnini et al . in prep . ) . in conclusion , of the 40 sources
already analyzed so far we find that all of them are obscured and at least @xmath1175% of them are compton 
thick agn .
we thus demonstrate that the x - ray / ir flux ratio , along with the x  ray color , can be used to efficiently select local ( z<0.1 ) , heavily obscured and compton - thick agn .
f@xmath12 ) vs. hr4 ( defined using the two following bands : 2 - 4.5 kev and 4.5 - 12 kev ) for a large sample of agn already studied in the literature . filled triangles in the upper quadrants are compton - thin agn ( nh@xmath1310@xmath1 @xmath2 ) taken from the hbs sample ( @xcite ) and the xmds survey ( @xcite ) .
the only triangle in the lower 
right quadrant is a compton - thick candidate according to @xcite .
stars are a sample of local starburst galaxies ( @xcite ) and open squares are local `` confirmed '' compton - thick agn from @xcite .
_ right panel : _
f(2 - 12 kev)/(@xmath14f@xmath15 ) vs. hr4 diagnostic plot for the sources resulting from the iras-2xmm cross - correlation .
the sources belonging to the iras-2xmm sample populate the lower 
right quadrant .
open squares are the 24 compton 
thick already known in the literature , filled squares are the 7 newly discovered compton  thick candidates and the filled triangles are the 9 compton - thin agn .
no x  ray analysis has been performed yet for small filled circles.,title="fig : " ]   f@xmath12 ) vs. hr4 ( defined using the two following bands : 2 - 4.5 kev and 4.5 - 12 kev ) for a large sample of agn already studied in the literature .
filled triangles in the upper quadrants are compton - thin agn ( nh@xmath1310@xmath1 @xmath2 ) taken from the hbs sample ( @xcite ) and the xmds survey ( @xcite ) .
the only triangle in the lower 
right quadrant is a compton - thick candidate according to @xcite .
stars are a sample of local starburst galaxies ( @xcite ) and open squares are local `` confirmed '' compton - thick agn from @xcite .
_ right panel : _
f(2 - 12 kev)/(@xmath14f@xmath15 ) vs. hr4 diagnostic plot for the sources resulting from the iras-2xmm cross - correlation .
the sources belonging to the iras-2xmm sample populate the lower 
right quadrant .
open squares are the 24 compton 
thick already known in the literature , filled squares are the 7 newly discovered compton  thick candidates and the filled triangles are the 9 compton - thin agn .
no x  ray analysis has been performed yet for small filled circles.,title="fig : " ]
we present here a new powerful diagnostic plot to select heavily obscured agn in the local universe by combining infrared ( spitzer , iras ) and x - ray ( xmm ) information . on the basis of this plot , we selected a sample of x  ray obscured sources in the 2xmm catalogue and found seven newly discovered compton - thick agn candidates .     
 address = inaf - osservatorio astronomico di brera , altaddress = e - mail : paola.severgnini@brera.inaf.it     address = inaf - osservatorio astronomico di brera     address = inaf - osservatorio astronomico di brera     address = inaf - osservatorio astronomico di brera     address = dip . 
 di astronomia , univ . 
 degli studi di bologna     address = inaf - iasf palermo     address = inaf - iasf palermo