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direct imaging of stars still remains exceptional , and approximate geometric information on the coronae of stars must be obtained from indirect methods such as image reconstruction from rotational modulation or eclipse mapping .
the latter is a powerful means to image the corona of a star when applied to algol - type systems which are semi - detached binaries composed of a massive early - type star orbiting an evolved late - type star .
the latter fills its roche lobe , hence a gas stream flows from the l1 lagrange point ( @xcite ) . the coronal emission originates exclusively from the late - type companion ; therefore , the simultaneous x - ray and radio eclipses at phase 0.5 can constrain the location of the coronal emitting regions and provide powerful information on the co - spatiality of the emission regions , with important implications for the structure of the outer atmospheres .    in algol - type systems ,
the formation of an accretion disk around the primary depends on the location of the binary in the @xmath1-@xmath2 diagram , where @xmath1 is the radius of the primary in units of the binary separation , and @xmath2 is the mass ratio of the secondary to the primary ( @xcite ) . in brief , short - period algols , such as our target ,
tend to populate a region of the @xmath1-@xmath2 diagram where the gas stream directly impacts on the primary while eventually forming a transient accretion disk . although the presence of circumstellar material has been indirectly inferred from h@xmath3 difference profiles ( @xcite ) ,
its detection is still elusive in the x - rays .
however , cool material can absorb x - rays if located along the line of sight , thus producing an extra column density detectable in x - rays .
we present results of a multiwavelength coordinated campaign on in x - rays with _ xmm - newton _   and in radio with the vla .
rz cas is a nearby ( @xmath4  pc ) algol - type binary , consisting of an a3  v primary and a k3  iv secondary with similar radii ( @xmath5 , @xmath6 , @xmath7 ; @xcite ) , orbiting with synchronous rotation ( @xmath8  d ; @xcite ) .
their masses are , however , different , with @xmath9 and @xmath10 .
the secondary fills its roche lobe , and a flow of material falls directly onto the primary ( @xcite ; @xcite ) .
the orbital inclination angle ( @xmath11 ) leads to almost complete eclipses .
@xcite found that rz cas displays variable x - ray luminosity , with a spread of an order of magnitude ( @xmath12  erg  s@xmath13 ) .
they also found that a two - temperature optically thin plasma model was required to fit their _ asca _ and _ rosat _ pspc data , along with abundances about 0.2 times the solar photospheric values .
their _ rosat _ observations showed significant intensity variations , apparently unrelated to the eclipse , but probably originating from either rotational modulation , flare activity , or inhomogeneous distributions of coronal structures on the secondary .
a core - halo structure was proposed from radio data by @xcite , whereas @xcite observed a radio modulation close to the primary eclipse seemingly correlating with x - rays .
_ xmm - newton _   observed rz cas for 110  ks from august 22 ( 5h23m54s ut ) to august 23 , 2003 ( 13h41m23s ut ) , whereas the vla observed on august 22 , 2003 from 4h41m20s ut to 19h38m30s ut .
the vla was in a - array configuration , and we observed alternatively in the c ( 6.1 cm ) and x ( 3.55 cm ) bands , with 0137 + 331 ( 3c 48 ) as flux calibrator and 0228 + 673 as phase calibrator .
figure  [ fig : campaign ] shows a diagram of the coordinated observations .
the x - ray data were reduced with the _ xmm - newton _   sas software ( version 6 ) using standard procedures .
the radio data were reduced with aips ( december 31 , 2003 version ) using standard techniques . the phases before 9h ut and after 16h ut were significantly degraded due to strong cumuloform clouds .
no reliable solutions could be found during these periods ( see shaded regions in fig .  [
fig : vlaxmm ] ) .
we used different techniques to obtain the radio light curves in both bands and to check their consistency .
figure  [ fig : vlaxmm ] shows the x - ray pn light curve for events from 0.25 to 8  kev ( panel a ) and a light curve of a hardness ratio ( panel b ) defined here as the ratio between the count rates in the hard band ( @xmath14  kev ) and the soft band ( @xmath15  kev ) .
the x - ray eclipse is almost absent , with a local x - ray minimum about an hour _ after _ the optical eclipse of the k3 subgiant .
the light curve modulation suggests , instead , the presence of extended x - ray sources which are not significantly eclipsed and are rotationally modulated ( e.g. , in particular in the phase range @xmath16 ) .
it could also reflect the evolution of active regions in rz cas .
a flare was observed at @xmath17 , i.e. , at the time of the eclipse of the primary a3  v star .
however , the synchronous rotation of the stars does not suggest that @xmath17 should be a special orbital phase for flares .
we found also that rz cas significantly became significantly and steadily hotter in the second half of the observation , as shown in the hardness ratio light curve .
no obvious flare can explain such hardening , therefore we hypothesize that hot , active regions rose in this time span , probably due to some reconfiguration of the magnetic topology in rz cas .    the radio light curves in x and c bands are also shown in figure  [ fig : vlaxmm ] in panels ( c ) and ( d ) , respectively .
a similar flux level was observed in both frequencies .
panel ( e ) of fig .
[ fig : vlaxmm ] shows a light curve of the spectral index @xmath3 ( assuming a power law for the radio spectrum , i.e. , @xmath18 ) .
the shaded time ranges in the radio light curves correspond to time spans when an accurate calibration could not be obtained .
the index @xmath3 appears to be shallow ( @xmath19 ) between 4.9  ghz and 8.4  ghz .
this is consistent with the core - halo model proposed by @xcite for rz cas .
the radio light curves show a steady flux increase , an a possible shallow eclipse is observed about 1.5  hours _ before _ the optical secondary eclipse .
the spectral index @xmath3 possibly flattens during the radio eclipse .
the marked difference in i ) eclipse timing , and ii ) in flux modulation in the x - rays and radio strongly suggests that the emitting sources are not co - spatial , at least during this coordinated campaign .
this does not support the findings of @xcite .
in addition , the shallow eclipses preclude compact sources in both wavelength regimes .
furthermore , the absence of eclipse at phase @xmath17 indicates that the primary a - type star does not emit significant x - rays . finally ,
the lack of significant eclipses at both primary and secondary eclipses precludes intrabinary ( or accretion spot ) x - ray and radio emissions in rz cas .
instead , the light curves imply rotational modulation of extended sources and active regions rise in the second half of the x - ray observation .
-4 mm    figure  [ fig : rgs ] shows the average _ xmm - newton _   rgs spectrum of rz cas .
strong emission lines of o  viii , ne  x , ne  ix , and several fe l - shell lines can be observed on top of an underlying continuum .
the high ne  x / ne  ix and o  viii / o  vii ratios indicate a dominant hot plasma in rz cas s corona .
the faint o  vii triplet suggests a high intercombination - to - forbidden line ratio , albeit with a large error bar .
although such a ratio could be indicative of a high - density ( @xmath20  @xmath21 ) plasma , it is in fact artificial and due to the strong uv field from the early - type primary ( see @xcite ) .
the poor signal - to - noise ratio in the triplet prevented us from obtaining a phase - dependent line ratio , which would have confirmed the influence of the uv field ( minimal close to @xmath22 , maximal close to @xmath17 ) .
we performed a multi-@xmath23 fit of the rz cas data using both mos spectra in the @xmath24     range and both rgs spectra above @xmath25   .
we also discarded several wavelength ranges of the rgs data to avoid bias from poorly known l - shell lines of low - z ions ( see @xcite ) .
a 4-@xmath23 model fits the data adequately ( @xmath26 for 1202 d.o.f . ) , with @xmath27  mk , @xmath28  mk , @xmath29  mk , @xmath30  mk , @xmath31 @xmath21 , @xmath32  @xmath21 , @xmath33 @xmath21 , @xmath34  @xmath21 .
coronal abundances ( relative to the solar photospheric abundances ; @xcite ) are @xmath35 , @xmath36 , @xmath37 , @xmath38 , @xmath39 , @xmath40 , @xmath41 , @xmath42 , @xmath43 , @xmath44 . all quoted uncertainties are 90 % confidence ranges ( @xmath45 ) .
the high n / c abundance ratio is a strong signature of cno cycle processed material .
this has been observed in algol ( @xcite ) and in the rapidly rotating giant star yy men ( @xcite ) .
we also performed time - dependent spectroscopy using the epic mos spectra only .
we observed no significant variation in abundances .
however , consistently with the hardness ratio light curve ( fig .
[ fig : vlaxmm ] ) , we obtained slightly hotter plasma in the second half of the observation , in particular in the last high - flux time segment .
we also observed no variation in the spectral slope at low energies , implying that photoelectric absorption ( e.g. , due to the accretion disk near @xmath17 ) remained constant and was consistent with interstellar absorption .
based on observations obtained with _ xmm - newton _ , an esa science mission with instruments and contributions directly funded by esa member states and nasa .
the nrao is a facility of the nsf operated under cooperative agreement by associated universities , inc .
m.  a. and j.  r.  d. acknowledge support from nasa grant nng04ga42 g , and m.  g. from the swiss nsf ( grant 20 - 66875.01 ) .
_ xmm - newton _   and the vla simultaneously observed the eclipsing algol - type binary in august 2003 . 
 the secondary eclipse ( k3 iv companion behind the a3 v primary ) was placed at the center of the 15-hour radio campaign , while the x - ray satellite monitored a full 1.2-day orbital period . 
 we present results of the x - ray and radio campaigns . 
 the x - ray light curve shows significant modulation probably related to rotational modulation and active region evolution , and even small flares . 
 however , the x - ray eclipse is not deep , implying that the coronal x - ray emitting material is spatially extended . 
 the reflection grating spectrometer ( rgs ) spectrum shows a variety of bright emission lines from fe , ne , o , n. a strong [ c / n ] depletion probably reflects the surface composition of the secondary which fills its roche lobe and loses material onto the primary . 
 the o  vii he - like triplet reflects a low forbidden - to - intercombination ratio ; while it generally suggests high electron densities , the ratio is here modified by photoexcitation by the strong uv flux of the primary a3 v star . 
 the radio light curve shows no similarity to the x - ray light curve . 
 the eclipse timings are different , and the radio flux increased while the x - ray flux decreased . 
 the radio spectral slope is shallow ( @xmath0 ) .