T. Vachaspati (ASU), W. Chen, F. Ferrer (WUS) or Search for Parity-odd signatures in the gamma ray sky Hiroyuki TASHIRO Nagoya University 2016 Jan. IBS CTPU Based on arXiv: , , ,

Introduction Cosmological magnetic fields and Helicity Magnetic helicity and cascade photons from TeV blazars Application to diffuse Gamma ray sky Summary Outline

JAXA/NASA Question: Magnetic fields on cosmological scales exist? Magnetic fields on different scales ESA Ferreti et al. 2012

Cosmological magnetic fields Cosmic relics of physics in the early universe Cosmological magnetic field generation During inflation (Turner & Widrow 1988, Ratra 1992) In cosmological phase transitions (Harrison 1970, Vachaspati 1991) In the epoch of recombination (Ichiki et al. 2006) In the epoch of reionization (Langer et al. 2005) The scale of the magnetic fields from pc to the horizon scale

Constraint on cosmological magnetic fields From BBN (Grasso & Rubinstein 1996) From CMB (Subramanian & Barrow1998, Planck collaboration 2015) at 1 Mpc From large scale structures (HT et al. 2010, 2011 Kahniashvili 2010) at 1 Mpc From gamma ray observations of blazars Neronov, Volk (2010)

CMB temperature anisotropy Large scales Stress-energy tensor of magnetic fields small scales Lorentz force in cosmic plasma Shaw & Lewis 2009

Constraint on cosmological magnetic fields From BBN (Grasso & Rubinstein 1996) From CMB (Subramanian & Barrow1998, Planck collaboration 2015) at 1 Mpc From large scale structures (HT et al. 2010, 2011 Kahniashvili 2010) at 1 Mpc From gamma ray observations of blazars Neronov, Volk (2010)

TeV gamma ray photons produce cascade GeV photons Observer High energy photon blazar TeV photons EBL (extragalactic background light)CMB photons GeV photons TeV photon produces an electron-positron pair by scattering with EBL Mean free path of a TeV photons : scatter CMB photons to produce cascade GeV gamma rays and Typical length scale of energy loss:

Neronov and Volk (2010) Primary TeV spectrum HESS observation Cascade photons Fermi upper bound

Cosmological magnetic fields in IGM Observer blazar

Magnetic fields deviate electron and positron trajectories Cascade Gamma rays are extended Observer TeV photons GeV photons blazar Neronov, Volk (2010)

Observer TeV photons GeV photons blazar observed cascade gamma ray flux from a point source is suppressed The cascade emission appears as extended emission around the initial point source The size of extend emission is larger than the resolution of the observation, Non-detection of Fermi Neronov & Volk (2010)

Constraint on cosmological magnetic fields From BBN (Grasso & Rubinstein 1996) From CMB (Subramanian & Barrow1998, Planck collaboration 2015) at 1 Mpc From large scale structures (HT et al. 2010, 2011 Kahniashvili 2010) at 1 Mpc From gamma ray observations of blazars Neronov, Volk (2010)

: vector potential Right-handedLeft-handed Magnetic Helicity describing how much a magnetic structure (field line) is twisted (spiral)

Why helicity? Basic quantity for cosmological magnetic fields (stochastic Gaussian divergence-free fields) Mode decomposition of magnetic fields divergence-free eigenfunctions of the Laplacian operator

Power spectrum of a stochastic Gaussian and isotropic fields The ensemble average of the magnetic field energy density The ensemble average of magnetic field helicity

Magnetic helicity in cosmology Inverse cascade : magnetic field energy transfer from small scales to large scales Helicity evolution : resistivity Magnetic helicity is a conservation quantity In the early Universe

Helical magnetic fields Non-helical magnetic fields Magnetic field amplification occurs on large scales, in order to conserve the total helicity Banerjee & Jedamzik 2004 Inverse cascade

Durrer & Neronov 2013 Magnetic field scale 1 pc for QCD → 100 kpc 5x10^-4 pc for EW → 3 kpc Magnetic field evolution in cosmology

Helicity : remnant of the physics in the early universe? Baryogenesis via “sphaleron” predicts left-handed helicity Vachaspati (2001) Chiral-magnetic field and chiral-vorticity effects produce the helicity of magnetic feilds Vilenkin (1979, 1980) Axion inflation model with produces helical mangetic fileds Anber & Sorbo (2004)

Constraint on cosmological magnetic helicity Parity odd cross-correlation of CMB anisotropy for maximal helical magnetic fields on 1 Mpc C. Caprini, R. Durrer and T. Kahniashvili (2004) T. Kahniashvili and B. Ratra (2005) K. E. Kunze (2012)

Observer Can we get the constraint on the helicity of IGM magnetic fields from blazar observation? blazar

In the linear order of the magnetic field strength (the bending angle is small) Cascade photon from blazar

Two-point correlation function of magnetic fields Helicity Arriving direction of cascade gamma rays from a blazar

12 Gamma ray correlator Different energy combinations can probe the correlation on different length scales

Non helical magnetic fields Helical magnetic fields on the observation plane can probe a spiral in the sky

Gamma ray photon distribution on the sky for the case 4 Long and Vachaspati (2015)

Fermi Large Area Telescope measure the diffuse gamma rays NASA/DOE/Fermi LAT Collaboration

Cascade photons from unseen blazars

Most of diffusion gamma rays are not cascade photons Do they contaminate our values ? the distribution of non-cascade photons is a random uniform distribution the contributions of them cancel each other No left- or right handed feature Hypothesis:

Fermi LAT pass 7 “clean” data Photon data = Cascade photons (signal) + non-cascade photons (noise) MC with homogeneous distribution

Fermi LAT pass 7 data Magenta : error bars from MC simulations with homogeneous photon distribution Signal Red : 2-sigma deviation from zero Statistic probability : 1 % (8 convective 2-sigma signal)

Estimation of Magnetic field strength (1) Signal appears at R=13 deg.

Estimation of Magnetic field strength (2) Signal amplitude on (10, 40, 50) GeV

Fermi LAT pass 7 data Magenta : error bars from MC simulations with homogeneous photon distribution Signal Red : 2-sigma deviation from zero Statistic probability : 1 % (8 convective 2-sigma signal)

Estimation of Magnetic field strength (2) Signal amplitude On (10, 40, 50) GeV

Magnetic field strength evaluated from the bending angle and from the signal amplitude Summary Parity odd correltor can probe helical magnetic fields Fermi LAT data set gives non-zero signals