13th Patras Workshop on Axions, WIMPs and WISPs, Patras 2017, Thessaloniki, Greece, 15 May 2017 - 19 May 2017; Hamburg : Verlag Deutsches Elektronen-Synchrotron, DESY-PROC, (2018). doi:10.3204/DESY-PROC-2017-0

Mirizzi, A.

Montanino, D.

Vazza, F.

Viel, M.

English

Archivio Istituzionale della Ricerca- Università del Salento

Conversion of TeV photons in realistic extragalac-tic magnetic fieldDaniele Montanino1, Franco Vazza2,3, Alessandro Mirizzi4 Matteo Viel5,6,71Università del Salento, and INFN, sezione di Lecce, Italy2INAF, Istituto di Radio Astronomia di Bologna, Italy3Universität Hamburg, Germany4Università di Bari and INFN, sezione di Bari, Italy5International School for Advanced Studies, Trieste, Italy6INAF, Osservatorio Astronomico di Trieste, Italy7INFN, sezione di Trieste, ItalyDOI: http://dx.doi.org/10.3204/DESY-PROC-2017-02/montanino danieleVery High Energy (TeV) photons can evade absorption by conversion into axion-like (ALPs)particles in the extragalactic magnetic field (EMF). We show that using realistic simula-tions of the EMF we obtain an even stronger conversion than those obtained with thesimplified cell model. This effect would be probed with the upcoming Cherenkov Tele-scope Array detector.1 Photon-ALP conversion in Extragalactic Magnetic FieldAxion-like particles (ALPs) are ultralight pseudo-scalar bosons a with a two-photon vertex aγγ,predicted by several extensions of the Standard Model. In the presence of an external magneticfield, the aγγ coupling leads to the phenomenon of photon-ALP mixing [1]. This effect allowsfor the possibility of direct searches of ALPs in laboratory experiments.Due to the aγγ coupling, ultra-light ALPs can also play an important role in astrophysicalobservations. In particular, an intriguing hint for ALPs has been recently suggested by VeryHigh-Energy (VHE) γ-ray experiments. In this respect, recent observations of cosmologicallydistant γ-ray sources by ground-based γ-ray Imaging Atmospheric Cherenkov Telescopes haverevealed a surprising degree of transparency of the Universe to VHE photons [2, 3], where onewould have expected a significant absorption of VHE photons by pair-production processes(γVHE + γEBL → e+e−) on the extragalactic background light (EBL).However, the range of the parameters where ALPs would impact the cosmic transparencyis constrained from other observations as shown in Fig. 1. In particular, for ALPs with massesma ≤ 10−9 eV, the strongest bound on gaγ is derived from the absence of γ-rays from SN 1987A(see [4] for a review).Anyway, the distribution of the extragalactic magnetic fields has been poorly characterizedin previous works on ALP conversions. In particular, a cell-like structure (hereafter the cellmodel) has been adopted with many domains of equal size (l ∼ 1 Mpc) in which the magneticPatras 2017 1168 PATRAS 2017Figure 1: Limits on ALP parameter space in the plane (ma, gaγ). The parameter space whereALPs could explain the low γ-ray opacity is shown in light blue. The horizontal grey bandsrepresent the sensitivity of future ALPS-II and IAXO experiments. The two small squaresrepresent cases of low-mass (LM, ma = 10−10 eV and gaγ = 4× 10−12 GeV−1) and high-mass(HM, ma = 2 × 10−8 eV and gaγ = 10−11 GeV−1) ALPs where conversions in realistic extra-galactic magnetic fields would affect the TeV photon transparency. Contour lines correspondingto Tγ = 10−6 for the cell and realistic models are also shown. See the text for more details.field has (constant) random values and directions [5]. Only recently it has been pointed outthat in more realistic situations, the magnetic field direction would vary continuously alongthe propagation path, and this would lead to sizable differences in the ALP conversions withrespect to the cell model.We have studied for the first time the photon conversions into ALPs using recent magneto-hydrodynamical cosmological simulations [6, 7]. In this more realistic case the magnetic fieldscan locally fluctuate in filaments of matter up to two orders of magnitude larger that what foundin the cell model and photon-ALP conversions are enhanced compared to previous estimates.Indeed, significant conversions are found both in the low-mass region below the SN 1987Abound both in the high-mass region on the right of the recent Fermi-LAT bound. These tworanges are indicated with small squares in Fig. 1.In Fig. 1 dashed lines represent the contour lines corresponding to the photon transferfunction Tγ = 10−6 for the cell model and simulated model of the extragalactic B-field for asource at z = 0.3 and at energy E = 20 TeV. Each contour corresponds to the 95th percentileof the distribution of Tγ . In other words, there is a 5% probability that Tγ is larger than theindicated value. From these curves is evident the enhancement of the area probed with therealistic model at a fixed value of Tγ as we will see in the following section.2 Patras 2017CONVERSION OF TEV PHOTONS IN REALISTIC EXTRAGALACTIC MAGNETIC FIELDPATRAS 2017 169Figure 2: Photon transfer function Tγ(E) for a source at redshift z = 0.3 for LM and HM cases.Upper panels refer to realistic models of extragalactic magnetic field, while lower panels are forthe cell model. See the text for details.We have solved the propagation equation for the photon-ALP ensemble, both with a randomcell model and with the realistic magnetic field. In the first case denoting bk a random unitvector inside each cell, during their path with a total length L along the line of sight, thebeam crosses n = L/lc domains, where lc is the size of each domain: The set {Bk}1≤k≤n ={B0 · bk}1≤k≤n represents a given random realization of the beam propagation. We have useda fixed (comoving) value of B0 = 1.9 nG which corresponds to the r.m.s. of the strength of therealistic magnetic field on all configurations and a fixed size of l = 1.4 Mpc per cell, which is thetypical coherence length of magnetic fields in the realistic model, based on spectral analysis.2 ResultsIn Fig. 2 we present the photon transfer function Tγ(E) in function of the photon energy E fora source at redshift z = 0.3. In the left panels we consider, the parameters corresponding to LMparameters while in the right panels we the HM parameters marked in Fig. 1. Upper panels referto realistic simulations for magnetic field, while lower ones are for the cell model. The blacksolid curve represents the Tγ expected in the presence of only absorption onto EBL. The solidgrey curve represents the median Tγ in the presence of ALPs conversions. The orange curvecorresponds to conversions for a particular realization of the extragalactic magnetic field. Theshaded band is the envelope of the results on all the possible realizations of the extragalacticmagnetic field at 68 % (dark blue), 90 % (blue) and 99 % (light blue) C.L., respectively bysimulating 103 different realizations of the extragalactic magnetic field in the cell case, orPatras 2017 3DANIELE MONTANINO ET AL.170 PATRAS 2017extracting an equivalent number of 1-dimensional beams of cells randomly extracted from theoutputs of the cosmological simulation at increasing redshift. According to conventional physics,it turns out that the Tγ gets dramatically suppressed at high energies (E > 2 TeV). As expected,including ALP conversions with cell magnetic fields, the enhancement of Tγ with respect to thestandard case is modest since it is suppressed by the small coupling (left panel) or the highALP mass (right panel). However, when we consider ALP conversions in realistic magneticfields the enhancement of Tγ is striking.ALP conversions in such models would produce a considerable hardening of the spectrumat high enough energies, thereby making it possible to detect VHE photons in a range where noobservable signal would be expected according to conventional physics or to conversions withcell magnetic fields. An example of a particular realization is shown by the orange curve. Inthis specific case we see that the observable photon flux at high energies can be significantlylarger than the average one. On this specific line of sight the enhancement of Tγ with respectto the standard case would reach 3 order of magnitudes.Depending on the particular magnetic realization crossed by the photons, it is also possibleto observe a suppression of the photon flux stronger than in the presence of conventional physics.Nevertheless, from Fig. 2 one infers that the cases in which Tγ is enhanced at high energiesare much more probable. From these results it is evident that using realistic models of theextragalactic magnetic fields has a strong impact on the mechanism of photon-ALP conversionsto reduce the cosmic opacity.3 ConclusionsWe have studied the conversions of VHE photons into ALPs proposed as a mechanism to reducethe absorption onto EBL, using for the first time realistic models of extragalatic magnetic fields,obtained from magneto-hydrodynamical cosmological simulations. We find an enhancement ofthe magnetic field with respect to what predicted in the naive cell model, due to the factthat simulated magnetic fields display larger fluctuations, correlated with density fluctuationsof the cosmic web. This effect would give a significant boost to photon-ALP conversions.Notably, using the cell model the parameter space for photon-ALP conversions at VHE energieswas strongly constrained by SN 1987A and Fermi-LAT data. However, using realistic modelsof the magnetic field we have found significant conversions also in regions of the parameterspace consistent with previous bounds. This mechanism can produce a significant hardeningof the VHE photon spectrum from faraway sources and we expect such signature to emerge atenergies E ≥ 1 TeV. Therefore, this scenario is testable with the present generation of ImagingAtmospheric Cherenkov Telescope, covering energies in the range from ∼ 50 GeV to ∼ 50 TeV.References[1] G. Raffelt and L. Stodolsky, Phys. Rev. D 37, 1237 (1988).[2] F. Aharonian et al. [H.E.S.S. Collaboration], Nature 440, 1018 (2006).[3] D. Horns and M. Meyer, JCAP 1202, 033 (2012).[4] M. Meyer, D. Horns and M. Raue, Phys. Rev. D 87, no. 3, 035027 (2013).[5] A. De Angelis, O. Mansutti and M. Roncadelli, Phys. Rev. D 76, 121301 (2007).[6] F. Vazza, M. Brggen, C. Gheller and P. Wang, Mon. Not. Roy. Astron. Soc. 445, no.4, 3706 (2014).[7] D. Montanino, F. Vazza, A. Mirizzi and M. Viel, Phys. Rev. Lett. 119, no. 10, 101101 (2017).4 Patras 2017CONVERSION OF TEV PHOTONS IN REALISTIC EXTRAGALACTIC MAGNETIC FIELDPATRAS 2017 171

Conversion of TeV photons in realistic extragalactic magnetic field

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Conversion of TeV photons in realistic extragalactic magnetic field

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