Emanuela Orrù Spectral index in radio halos 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), VaccaV. (University of Cagliari).
diffuse radio emission found in merging clusters • Not associated with any cluster galaxy • Associated with the ICM 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. Radio Halos Reviews: Giovannini & Feretti 2002; Ferrari et al. 2008
Origin of diffuse radio sources: loss < 108 yr < diff with vdiff 100 km s-1 BG implies >>103 Difficult to explain the presence of relativistic particles spread over Mpc distances Origin of the electrons?
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 STEEPENING OF THE SPECTRUM COMPLEX SPATIAL SPECTRAL INDEX DISTRIBUTION SIGNIFICANT VARIATIONS OF
=0.5 =2 N() =3 =1 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). Observable expectation @ radio frequencies: RADIAL TREND CONSTANT SMALL SPECTRAL INDEX VARIATIONS
Primary models Secondary models I(r) B(r) 327 MHz 1400 MHz r r I(r) 327 MHz 1400 MHz r …osservable behaviour…
A665 A2744 A2219 Spectral index maps 327 MHz -1.4 GHz A2163 Orru’ et al. 2007 Feretti et al. 2004 + A3562 Giacintucci et al. 2005 COMA Giovannini et al. 1993
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 GLOBAL Spectral index profiles: 1.4 GHz & 327 MHz See Orrù et al. 2007
LOCAL Spectral index profiles: 1.4 GHz & 327 MHz αdev ≈0.2-0.3 Err_α≈0.2-0.3
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 MHz 25”X25” rms~0.29 mJy/beam Murgia et al 2004, FARADAY, Vacca et al. 2008 In prep. Simulated halo @ LOFAR frequencies: 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 INDEX @ 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