Galactic radio loops. Philipp Mertsch with Subir Sarkar. The Radio Synchrotron Background Workshop, University of Richmond 21 July 2017

Transcription

1 Galactic radio loops Philipp Mertsch with Subir Sarkar The Radio Synchrotron Background Workshop, University of Richmond 21 July 2017

2 Foregrounds in B-modes Adam et al., arxiv: (Planck (Planck Int. results 2015 XXX) results X)

3 Haslam et al., A&AS 47 (1982) 1 Haslam 408 MHz

4 The galactic radio background is predominantly synchrotron of CR electrons on the galactic magnetic fields wher e and and we look at its line-of-sight projections:

5 Ingredients CR electrons: sources: SNRs! pulsars? PWNe? large-scale distribution?! conceptual: stochasticity of sources? propagation: diffusive! convective? reacceleration? energy losses? Galactic magnetic fields: large-scale, ordered component anisotropic random (also called striated) component small-scale, turbulent component

6 Local e - spectrum and synchrotron Strong et al., A&A 534 (2011) A54 require break in IS electron spectrum, e.g. at source: fix local (turbulent) magnetic field: constrain certain propagation models, e.g. disfavour reacceleration

7 Haslam vs GALPROP averaging over large parts of the sky assumes factorisation in longitude and latitude leads to loss of sensitivity for structures on intermediate scales

8 Difference: Haslam - GALPROP

9 Angular power spectrum 1. radio sky: 1. spherical harmonics: 2. angular power spectrum: advantages: information ordered by scale statistically meaningful quantities natural for some applications, e.g. CMB foreground subtraction

10 Haslam APS even/odd structure reflects symmetries of sky map 2. smoother for higher multipoles 3. power-law with (Kolmogorov turbulence)

11 Smooth component only Mertsch & Sarkar, JCAP 06 (2013) 041 synchrotron: smooth emissivity (GALPROP) free-free: WMAP MEM-template unsubtracted sources: shot noise

12 Smooth component only Mertsch & Sarkar, JCAP 06 (2013) 041 synchrotron: smooth emissivity (GALPROP) free-free: WMAP MEM-template unsubtracted sources: shot noise

13 Angular Powerspectrum log(c l / C l O12) Deficit in RM Beck et al., JCAP 05 (2016) 056 Observation O12 L max =30pc L max =100pc L max =300pc L max =1kpc L max =3kpc Spherical harmonics l

14 Turbulence cascade plasma perturbations described by MHD modes, e.g. Alfvén waves two-point correlation function: Fourier transform power spectrum: observed in space plasma and simulations r for with (Kolmogorov turbulence)

15 Scaling the synchrotron emissivity GALPROP assumes a smooth distribution of RMS values for the turbulent field: can rescale GALPROP s volume emissivity compute small-scale turbulent field scaling factor exploit scaling of synchrotron emission to find: with

16 Turbulence in projection consider two-point correlations on sphere power-law in wavenumber reflected by power-law in angle (or multipole ) Chepurnov, Astron. Astrophys. Transact. 17 (1999) 281

17 including turbulent component Mertsch & Sarkar, JCAP 06 (2013) 041 synchrotron: smooth emissivity and power-law turbulence in wavenumber free-free: reflected by power-law in WMAP angle MEM-template or multipole : unsubtracted sources: shot noise Chepurnov, Astron. Astrophys. Transact. 17 (1999) 281; also: Cho & Lazarian, ApJ 575 (2002) 63; Regis, Astropart. Phys. 35 (2011) 170

18 Page et al., ApJS 170 (2007) 335 Radio loops probably shells of old SNRs can only observe 4 radio loops directly in radio sky total Galactic population of up to O(1000) can contribute on all scales

19 Radio loops

20 Haslam et al., A&AS 47 (1982) 1 Haslam 408 MHz

21 Haslam - GALPROP

22 Unsharp masked Haslam Vidal et al., MNRAS 452 (2015) 656

23 Bennett al., ApJS 208 (2013) 20 WMAP9 polarisation

24 WMAP9 23 GHz polarisation Page et al., ApJS 170 (2007) 335

25 Planck 30 GHz polarisation Planck collaboration

26 Modelling individual shells Mertsch & Sarkar, JCAP 06 (2013) 041 assumption: flux from one shell factorises into angular part and frequency part: frequency part : magnetic field gets compressed in SNR shell electrons get betatron accelerated emissivity increased with respect to ISM angular part : assume constant emissivity in thin shell:

27 Modelling individual shells Mertsch & Sarkar, JCAP 06 (2013) 041 assumption: flux from one shell factorises into angular part and frequency part: frequency part : magnetic field gets compressed in SNR shell electrons get betatron accelerated emissivity increased with respect to ISM angular part : assume constant emissivity in thin shell: add up contribution from all shells

28 including ensemble of shells Mertsch & Sarkar, JCAP 06 (2013) 041 O(1000) shells of old SNRs present in Galaxy we know 4 local shells (Loop I-IV) but others are modeled in MC approach they contribute exactly in the right multipole

29 Best fit of local shells and Mertsch & Sarkar, JCAP 06 (2013) 041 ensemble O(1000) shells of old SNRs present in Galaxy we know 4 local shells (Loop I-IV) but others are modeled in MC approach they contribute exactly in the right multipole

30

31 Anomalies in ILC9 (l 20) Liu, Mertsch & Sarkar, ApJL 789 (2014) 29

32 Mean temperature WMAP 9yr ILC map smoothing to l lmax = 20 4 wide band around Loop I (Berkhuijsen et al., 1971) compute <T> p-values from 10 4 simulations with WMAP 9yr best-fit APS p-value: 0.01 (ILC9 and SMICA)

33 Clustering analysis 20 wide band around Loop distance modulus map: G j = arccos(n j. n ctr )-radius Pearson s correlation coefficient: p-values from 10 4 simulations with WMAP 9yr best-fit power spectrum p-value: (ILC9) (SMICA)

34 What do we know about spatially correlates with Loop I unlikely synchrotron (checked with our synchrotron model) frequency dependence: 1. ILC method efficiently suppresses power laws in frequency: over most of the sky: anomaly? in the Loop I region synch free-free thermal dust 2. pixelwise correlation between WMAP W- and V-bands with ICL9: Liu, PM & Sarkar, ApJL 789 (2014) 29

35 Magnetic dipole radiation close to black body, i.e. what is assumed to uniquely define CMB! Draine & Lazarian, ApJ 508 (1998) 157, ibid., ApJ 512 (1999) 740 Draine & Hensley, ApJ 765 (2013) 169

36 Magnetised dust in SMC thermal dust typical spinning dust low- foreground magnetic dust very flat spectra, ; dependence on compound, grain size, shape Draine & Hensley, ApJ 757 (2012) 103

37 Summary 1. Lack of angular power in the Galactic radio background for l= Small-scale turbulence cannot explain it 3. A population of O(1000) shells from old supernova remnants provides the angular power needed 4. Excess in CMB map. Magnetised dust?