Collaborators: Murgia M. (IRA-INAF-CA), Feretti L. (IRA-INAF- BO), Govoni F. (OAC-INAF-CA), Giovannini G. (University of Bologna), Ferrari C. (Observatoire de la Còte Azur) Schindler S. (Innsbruck University), Vacca V. (University of Cagliari). Spectral index in radio halos Emanuela Orrù

diffuse radio emission found in merging clusters Not associated with any cluster galaxy Associated with the ICM Radio Halos 500 kpc synchrotron emission extended about 1 Mpc presence of relativistic electrons and magnetic fields on large scales. thermal and non-thermal morphologies are similar. Reviews: Giovannini & Feretti 2002; Ferrari et al. 2008

Origin of diffuse radio sources: Difficult to explain the presence of relativistic particles spread over Mpc distances Origin of the electrons?  loss < 10 8 yr <  diff B  G implies  >>10 3 with v diff  100 km s -1

Observable radio frequencies: RADIAL STEEPENING OF THE SPECTRUM COMPLEX SPATIAL SPECTRAL INDEX DISTRIBUTION SIGNIFICANT VARIATIONS OF  Primary electrons models: CRe injected by AGN, stellar winds etc. Re-accelerated via shock ( Ensslin et al ) or turbulence ( Brunetti et al & 2004, Cassano & Brunetti 2005 ).

Observable radio frequencies: RADIAL TREND CONSTANT SMALL SPECTRAL INDEX VARIATIONS  Secondary electrons models: Relativistic protons (CRp) have negligible losses they can diffuse at large distances. Relativistic electrons (CRe) are produced by hadronic interaction between CRp and ICM gas protons via pions decay. CRe are continuosly injected ( Blasi & Colafrancesco 1999, Dolag & Ensslin 2000, Pfrommer et al & 2004 & 2008 ).  N(  )  =2  =0.5  =3  =1

I(r) r 327 MHz 1400 MHz Secondary models B(r) r …osservable behaviour… I(r) r 1400 MHz 327 MHz Primary models

Spectral index maps 327 MHz -1.4 GHz A2744 A2219 A665 A2163 Orru’ et al. 2007Feretti et al COMA Giovannini et al A3562 Giacintucci et al. 2005

GLOBAL Spectral index profiles: 1.4 GHz & 327 MHz  Brightness fitted with an exp. Law.  r e-folding radius  Global azimuthally averaged spectral index is constant   ~1 within the errors up to 1 Mpc See Orrù et al. 2007

LOCAL Spectral index profiles: 1.4 GHz & 327 MHz α dev ≈ Err_α≈

Simulated LOFAR frequencies: Murgia et al 2004, FARADAY, Vacca et al In prep. 30 & 120 MHz Simulated halo, model fit of halo A665 B0=1.5 uG, Power spectrum Kolmogorov fluctuations from 1 up to 300 kpc. Spectral index  =3 Equipartition between magnetic field and particles 30 MHz 25”X25” Rms  2 mJy/beam 120 MHz 6”X6”rms~ 0.07mJy/b eam 120 MHz 25”X25” rms~0.29 mJy/beam Halo in A665 as seen by LOFAR full-array with 1 hour of integration and 4 MHz of bandwidth (from LOFAR home page). The first contour is 3 sigma.

Spectral index map Error_Spectral index map Simulated SPECTRAL LOFAR frequencies:

SUMMARY Spectral index in radio halos with LOFAR  Important for the models  studied local and global  LOFAR will improve our knowledge in a new range of frequencies for α<1.5  LOFAR will discover and give unprecedent info about α>1.5