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Paper text:
a number of x  ray luminous galaxy clusters show large  scale synchrotron radio emission associated to the intracluster medium .
these diffuse sources are know as radio halos , when they are located at the centre of the hosting cluster and show low or negligible polarization , and radio relics , when found in the cluster outskirts and highly polarized .
both halos and relics have low surface brightness , large linear size ( from @xmath10.5 mpc to more than 1 mpc ) and steep integrated spectra , i.e. @xmath2 ( s @xmath3 @xmath4 ) .
radio halos and relics represent the most striking evidence for @xmath5 g magnetic fields on cluster scale and relativistic electrons diffused within the whole cluster volume ( for a recent review see giovannini & feretti 2002 ) .
the existence of this class of radio sources is believed to be connected to cluster mergers , since thus far they have only been found in clusters with significant signs of a current or recent merging event . in particular
the leading hypothesis for the origin of the observed radio emission from these objects is a re  acceleration process , probably via turbolence powered by cluster mergers , of a population of relatively low energy ( @xmath6 ) electrons , initially injected in the intracluster medium ( two - phase model , brunetti et al .
the cluster a3562 is the easternmost component of the a3558 complex , a chain of three clusters and two sc groups which form a single physically connected structure , whose complex dynamical stage of merging is supported by a large amount of observational data in all bands ( see for instance ettori et al . 1997 , ettori et al . 2000 ) .
the existence of a radio halo at the a3562 centre was confirmed by deep radio observations at 1.4 ghz carried out with vla ( venturi et al .
2003 ) .    in the framework of the re  acceleration model ,
the halo in a3562 seems to play a very special role , since its largest linear size ( lls @xmath1 600 kpc ) and 1.4 ghz radio power ( p@xmath7 = 2.14@xmath8 w hz@xmath9 ) are among the lowest values found for this class of radio sources .
the halo nicely fits the correlations existing between the halo radio power and the cluster x  ray luminosity and temperature ( bacchi et al . 2003 ) , extending them to lower values for all quantities involved .    according to the two - phase model , the halo in a3562
could be interpreted either as the result of a low efficiency re - acceleration process or as a young source at the beginning of the re - acceleration phase .
it has been established that the head  tail radio galaxy j13333141 , located within the halo emission , has deposited in the intracluster medium a number of electrons high enough to feed the halo if it has been active for a considerable fraction of its crossing time ( t@xmath10 yrs ) .
this result seems to suggest that the halo at the centre of a3562 is young as compared to the other radio halos known to date ( venturi et al .
however only low frequencies observations allow to discriminate between the two possibilities .
with the purpose of studing the morphology of the halo in a3562 and determine the shape of its synchrotron spectrum at low frequencies , we carried out observations at 235 mhz , 327 mhz and 610 mhz with the giant metrewave radio telescope ( gmrt , pune , india ) .
thanks to the u  v coverage of this instrument , a range of resolutions at all frequencies ( from few arcsec to tens of arcsec ) is allowed .
this ensures an accurate determination of the total radio flux density of the halo , after a careful subtraction of the individual point sources embedded in the halo emission .
an extended radio emission , located south  west of the halo , surrounds the radio source j13323146 ( figure 1 ) , identified with the brightest galaxy in the sc 1329313 group ( giacintucci et al .
the nuclear component , detected at 1.4 ghz , disappears at low frequency .
this seems to rule out any connection between the low brightness extended emission and an active nucleus , suggesting that its origin is most likely due to cluster merger re  acceleration .
the preliminary results of our low frequency data suggest that the spectrum of the radio halo is steep up to @xmath1 610 mhz , with @xmath11 , and it flattens beyond 610 mhz , with @xmath12 .
this seems to be in good agreement with a very recent re  acceleration phase , supporting the idea that the radio halo in a3562 is young and that its existence is strongly connected to the ongoing major merger in the a3558 complex ( venturi et al . in prep ) .
we present the preliminary results of 235 mhz , 327 mhz and 610 mhz observations of the galaxy cluster a3562 in the core of the shapley concentration . 
 the purpose of these observations , carried out with the giant metrewave radio telescope ( gmrt , pune , india ) , was to study the radio halo located at the centre of a3562 and determine the shape of its radio spectrum at low frequencies , in order to understand the origin of this source . in the framework of the re  acceleration model 
 , the preliminary analysis of the halo spectrum suggests that we are observing a young source ( few @xmath0 yrs ) at the beginning of the re  acceleration phase .