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

Spectral index in radio Spectral index in radio haloshalos

Emanuela OrrùEmanuela Orrù

diffuse radio emission found in merging clusters

Not associated with any cluster galaxy

Associated with the ICM

Radio HalosRadio 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:Origin of diffuse radio sources:

Difficult to explain the presence of relativistic particles spread over Mpc distances

Origin of the electrons?Origin of the electrons?

lossloss < < 10 1088 yr yr < < diffdiff BG implies >>103 with vdiff 100 km s-1

Observable expectation @ radio frequencies:

RADIAL STEEPENING OF THE SPECTRUMRADIAL STEEPENING OF THE SPECTRUM

COMPLEX SPATIAL SPECTRAL INDEX COMPLEX SPATIAL SPECTRAL INDEX DISTRIBUTIONDISTRIBUTION

SIGNIFICANT VARIATIONS OF SIGNIFICANT VARIATIONS OF

Primary electrons models:Primary electrons models:CRe injected by AGN, stellar winds etc.Re-accelerated via shock (Ensslin et al. 1998) or turbulence (Brunetti et al. 2001 & 2004, Cassano & Brunetti 2005).

Observable expectation @ radio frequencies:

RADIAL TREND CONSTANT RADIAL TREND CONSTANT

SMALL SPECTRAL INDEX VARIATIONS SMALL SPECTRAL INDEX VARIATIONS

Secondary electrons models: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. 2003 & 2004 & 2008).

N() =2 =0.5

=3 =1

I(r)

r

327 MHz327 MHz

1400 MHz1400 MHz

Secondary modelsSecondary models

B(r)

r

……osservable behaviour…osservable behaviour…

I(r)

r

1400 MHz1400 MHz

327 MHz327 MHz

Bbreak2

Primary modelsPrimary models

Spectral index Spectral index mapsmaps

327 MHz -1.4 GHz327 MHz -1.4 GHz

)(S

A2744

A2219

A665

A2163

Orru’ et al. 2007 Feretti et al. 2004

+COMA Giovannini et al.

1993 A3562 Giacintucci et al. 2005

)(SGLOBAL Spectral index profiles: 1.4 GHz & 327 MHzGLOBAL 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 MHzLOCAL Spectral index profiles: 1.4 GHz & 327 MHz

αdev ≈0.2-0.3

Err_α≈0.2-0.3

Simulated halo @ Simulated halo @ LOFAR frequencies:LOFAR frequencies:

Murgia et al 2004, FARADAY,

Vacca et al. 2008 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” Rms2

mJy/beam

120 MHz

6”X6”rms~0.07mJy/

beam

120 MHz25”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 mapSpectral index map Error_Spectral index mapError_Spectral index map

Simulated SPECTRAL INDEX @ Simulated SPECTRAL INDEX @ LOFAR frequencies:LOFAR frequencies:

SUMMARYSUMMARY

Spectral index in radio halos with Spectral index in radio halos with LOFARLOFAR

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