Measurement of the intergalactic magnetic fields with gamma-ray telescopes

1. Measurement of the intergalactic magnetic fields with gamma-ray telescopes Dmitri Semikoz APC , Paris

2. Overview What we know about EBL and IGMF? How to measure weak magnetic fields in the voids of large scale structure • Extended emission around point sources • Time delay of signal after big flaires • Change in the spectrum of blazars and low bounds of IGMF and EBL measurements Conclusions

3. Introduction: connection to Extragalactic Background Light

5. Gamma-ray horizon A.Domingues et al, arXiv:1305.2162

6. Measurements with high tau A.Domingues et al, arXiv:1305.2162

7. EBL problem and new physics or protons or? A.Neronov, D.S., A.Taylor and Ye.Vovk, arXiv:1207.1962 M.Meyer, D.Horns, M.Raue, 1211.6405

8. PKS 1424+240 A.Furnis et al, arXiv:1304.4859

11. PKS 1424+240 W.Essey and A.Kusenko, arXiv:1310.3440

12. Measurement of weak IGMF in the voids of LSS

13. A.Neronov, D.S., PRD 2009, arXiv:0910.1920 • Magnetic fields might be generated via "battery" effects during phase transitions in the Early Universe. • In principle, the initial magnetic field energy density might provide non-negligible contribution to the overall energy density of the Universe. • Magnetic field correlation length could not exceed the size of cosmological horizon; strength of magnetic field averaged over large distance scales could not exceed the "causality" limit • Damping processes remove small-scale magnetic fields in the course of cosmological evolution.

14. IGMF measurement with gamma-ray telescopes • Plaga '95 • Neronov & D.S. '07, '09 • Murase et al 08 • ------------------------------ • Neronov & Vovk '10 • Tavecchio et al. '10 • Dolag et al. '10 • .... • γ-rays with energies above ~0.1 TeV are absorbed by the pair production on the way from the source to the Earth. • e+e- pairs re-emit γ-rays via inverse Compton scattering of CMB photons. • Inverse Compton γ-rays could be detected at lower energies.

15. Cascade component • Fraction of electron energy in secondary photons in direction of observer • Fraction of voids on the way of primary photon • Ratio of point source flux at Eg and Eg0

16. Dependence of the measurement on the IGMF correlation length • γ • If the correlation length of EGMF is large, deflection angle is • B e- e- • γ • If the correlation length of EGMF is small, (λB<<De) deflection angle is • B

17. IGMF from galactic winds? Galactic winds expanding into the intergalactic medium form "bubbles" around galaxies, similar to the stellar wind bubbles blown by massive stars in the interstellar medium. Bubbles are able to expand up to ~100 kpc distances around small galaxies (up to 1010 MSun) and up to ~1 Mpc distances in the case of Milky Way like galaxies. Bubbles are blown as long as star formation or AGN activity in the galaxy is strong enough. They might contract after the end of the star formation activity. Volume filling factor of these galactic wind blown bubbles is uncertain. State-of-art simulations are not able to model the bubble evolution "from the first principles". • Pinsonneault et al. '10

18. Extended emission around point sources

19. Imaging of cascade: 3-d cascade needed • 3-d cascade in turbulent EGMF • A.Neronov, D.S., M.Kachelriess, S.Ostapchenko and A.Elyev , 2009

20. Imaging of cascade: jet opening angle • θjet==0 • θjet==3o • θjet==6o • θjet==9o • Imaging: cascade component forms an extended emission around initially point source. • – detectability depends on the telesope PSF and on the scale of angular deflections of e+e- pairs • Neronov et al. ’09

21. Imaging of cascade: EGMF • B=10-17G • B=10-16G • B=10-15G • B=10-14G • Imaging: cascade component forms an extended emission around initially point source. • – detectability depends on the telesope PSF and on the scale of angular deflections of e+e- pairs (i.e. on the strength of EGMF)

22. Time delay of secondary emission • T=0 • T=106yr • T=3×106yr • T=107yr • Timing: cascade component emission is delayed, compared to the direct signal from the point source • – detectability depends on the telesope sensitivity and on the scale of time delay • Plaga '95 • Murase et al. '08

23. Time delay • T=0 • T=106yr • T=3×106yr • T=107yr • Timing: cascade component emission is delayed, compared to the direct signal from the point source • – detectability depends on the telesope sensitivity and on the scale of time delay(i.e. on the strength of EGMF)

24. Time delay of signal after big flaires

25. Search for the time-delayed cascade emission • Blazars which are most interesting for the search of the cascade emission have hard spectra: they are bright in the VHE (0.1-10 TeV) energy band and should preferentially be not-so-bright in the HE (0.1-10 GeV) band, to make the cascade emission visible on top of the direct primary source emission. Good examples are the brighest VHE blazars, like Mrk 421 and Mrk 501. • Mrk 501 has produced a bright flare during Fermi monitoring period! • Neronov, D.S., Taylor '12

26. Search for the time-delayed cascade emission • B~10-18 – 10-16 G • The flare occurred during the multiwavelength campaing, including HE and VHE observations. • Fermi data indicate that the flare lasted 30-50 days, but the VHE observations cover only the first three days of the flare. • Fermi data indicate a peculiar hardening of the spectrum above ~10 GeV during the flare. One possibility for the explanation of the hard component is the cascade emission suppressed at low energies by too-large time delay. • Neronov, DS, Taylor '12

27. e.-m.cascade signatures in the spectrum of blazars

28. Cascade emission signal from non-flaring blazars • Taylor, Vovk, Neronov '11 • So far, (published) HE-VHE monitoring campaigns did not succeed to catch exceptionally bright flares of VHE blazars simultaneously in Fermi and ground-based telescopes, which would be most suitable for the search of the time-delayed cascade emission. • .... waiting for the Big one • Meanwhile, non-observation of the cascade signal in sources in which it would be detectable in the B=0 case, imposes lower bounds on the strength of intergalactic magnetic field at the level of ................

31. The hardest VHE blazar 1ES 0229+200 • Blazar 1ES 0229+200 is considered to be the best candidate for the search of the cascade emission because it has very hard VHE spectrum extending into the ~10 TeV energy band, where γ-ray emission is strongly attenuated by the pair production effect. • Most of the primary γ-ray beam power is removed and transferred to the cascade emission which should appear in the GeV energy band. • The source is extremely weak in the Fermi energy band. It is detected only in the 3-year long exposure. • The source is stable in the VHE band: no variability is found between observations made over ~5 yr time span. • Vovk, Taylor, Neronov, and DS 1112.2534

32. EGMF and EBL from 1ES 0229+200

33. EGMF and EBL from 1ES 0229+200

34. Lower bound on IGMF • Flux, nW/m2 sr • λ, μm • Meyer, Mazin, Raue, Horns '12 • Vovk, Taylor, D.S., Neronov '12 • Strength of the suppression of the VHE gamma-ray flux via the pair production depends on the density/spectrum of the EBL • . Uncertainty of our knowledge of EBL density was a factor of ~2 up to recent studies. • Now it is 20-30 % This uncertainty introduces an order-of-magnitude (factor 2) uncertainty in the lower bound on the intergalactic magnetic field.

35. EGMF limits from 5 blazars • Finke et al, 1510.02485

36. Conclusions Gamma-ray observations provide for the first time an evidence for non-zero magnetic field in the intergalactic medium outside of LSS (if plasma heating is not relevant). This bound depends on EBL density and it is about B>10-17 G for modern n_EBL models It is not clear at the moment if the intergalactic magnetic fields have primordial origin or they were spread recently by galactic winds. High-quality coordinated multi-wavelength observations of blazar flares in the GeV-TeV band might provide measurements of IGMF if it is in the range 10-17 – 10-16 G. Deep exposures with Fermi and Cherenkov telescopes might provide measurements of IGMF via detection of extended emission, if IGMF is in the range 10-16 – 10-12 G.

37. Conclusions We can study possibility of IGMF detection by CTA with very hard spectrum Fermi sources like 1ES 0229+200, if spectrum measured well both at high and low energies.