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Register a new userA possible blazar spectral irregularity case caused by photon--axionlike-particle oscillations
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Jianeng Zhou
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We report detection of a line-like feature in the $gamma$-ray spectrum of the blazar B0516$-$621, for which the data obtained with the Large Area Telescope onboard {it Fermi Gamma-Ray Space Telescope (Fermi)} are analyzed. The feature is at $sim$7,GeV and different analyses are conducted to check its real presence. We determine that it has a significance of 2.5--3.0$sigma$, and cautiously note the presence of possible systematics in the data which could reduce the significance. This putative feature is too narrow to be explained with radiation processes generally considered for jet emission of blazars. Instead, it could be a signal due to the oscillations between photons and axion-like particles (ALPs) in the sources jet. We investigate this possibility by fitting the spectrum with the photon-ALP oscillation model, and find that the parameter space of ALP mass $m_aleq 10^{-8}$,eV and the coupling constant (between photons and ALPs) $g_{agamma}$=1.16--1.48$times 10^{-10}$,GeV$^{-1}$ can provide a fit to the line-like feature, while the magnetic field at the emission site of $gamma$-rays is fixed at 0.7,G. The ranges for $m_a$ and $g_{agamma}$ are in tension with those previously obtained from several experiments or methods, but on the other hand in line with some of the others. This spectral-feature case and its possible indication for ALP existence could be checked from similar studies of other blazar systems and also suggest a direction of effort for building future high-energy facilities that would have high sensitivities and spectral resolutions for searching for similar features.
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Axionlike-particles (ALPs) are one promising type of dark matter candidate particle that may generate detectable effects on $gamma$-ray spectra other than the canonical weakly interacting massive particles. In this work we search for such oscillation effects in the spectra of supernova remnants caused by the photon-ALP conversion, using the Fermi Large Area Telescope data. Three bright supernova remnants, IC443, W44, and W51C, are analyzed. The inclusion of photon-ALP oscillations yields an improved fit to the $gamma$-ray spectrum of IC443, which gives a statistical significance of $4.2sigma$ in favor of such spectral oscillation. However, the best-fit parameters of ALPs ($m_{a}=6.6,{rm neV}$, $g_{agamma}=13.4 times 10^{-11},{rm GeV}^{-1}$) are in tension with the upper bound ($g_{agamma}< 6.6 times 10^{-11},{rm GeV}^{-1}$) set by the CAST experiment. It is difficult to explain the results using the systematic uncertainties of the flux measurements. We speculate that the irregularity displayed in the spectrum of IC443 may be due to the superposition of the emission from different parts of the remnant.
Many theories beyond the Standard Model of particle physics predict the existence of axionlike particles (ALPs) that mix with photons in the presence of a magnetic field. One prominent indirect method of searching for ALPs is to look for irregularities in blazar gamma-ray spectra caused by ALP-photon mixing in astrophysical magnetic fields. This requires the modelling of magnetic fields between Earth and the blazar. So far, only very simple models for the magnetic field in the blazar jet have been used. Here we investigate the effects of more complicated jet magnetic field configurations on these spectral irregularities, by imposing a magnetic field structure model onto the jet model proposed by Potter & Cotter. We simulate gamma-ray spectra of Mrk 501 with ALPs and fit them to no-ALP spectra, scanning the ALP and B-field configuration parameter space and show that the jet can be an important mixing region, able to probe new ALP parameter space around $m_asim$ 1-1000 neV and $g_{agamma}gtrsim$ $5times10^{-12}$ GeV$^{-1}$. However, reasonable (i.e. consistent with observation) changes of the magnetic field structure can have a large effect on the mixing. For jets in highly magnetized clusters, mixing in the cluster can overpower mixing in the jet. This means that the current constraints using mixing in the Perseus cluster are still valid.
The conversion between photons and axionlike particles (ALPs) in the Milky Way magnetic field could result in the detectable oscillation phenomena in $gamma$-ray spectra of Galactic sources. In this work, the GeV (Fermi-LAT) and TeV (MAGIC/VERITAS/H.E.S.S.) data of three bright supernova remnants (SNRs, ie. IC443, W51C and W49B) have been adopted together to search such the oscillation effect. Different from our previous analysis of the sole Fermi-LAT data of IC443, we do not find any reliable signal for the photon-ALP oscillation in the joint broadband spectrum of each SNR. The reason for the inconsistence is that in this work we use the latest revision (P8R3) of Fermi-LAT data, updated diffuse emission templates and the new version of the source catalog (4FGL), which lead to some modification of the GeV spectrum of IC443. Then we set constraints on ALP parameters based on the combined analysis of all the three sources. Though these constraints are somewhat weaker than limits from the CAST experiment and globular clusters, they are supportive of and complementary to these other results.
G. Galanti and M .Roncadelli recently made public some comments on the article by D. Wouters and P. Brun about irregularities induced by photon mixing to axion-like particles in astrophysical media [Phys. Rev. D86, 043005 (2012)]. They claim in particular to have found some mistakes in the article. This note is a response to their comments, we refute their arguments and show that the results presented in the article are correct. It turns out most of the misunderstandings come from the definition of the beam initial state, some clarifications about which are given here.
The jets of blazars are renowned for their multi-wavelength flares and rapid extreme variability; however, there are still some important unanswered questions about the physical processes responsible for these spectral and temporal changes in emission properties. In this paper, we develop a time-dependent particle evolution model for the time-varying emission spectrum of blazars. In the model, we introduce time-dependent electric and magnetic fields, which consistently include the variability of relevant physical quantities in the transport equation. The evolution on the electron distribution is numerically solved from a generalized transport equation that contains the terms describing the electrostatic, first-order and second-order emph{Fermi} acceleration, escape of particles due to both advection and spatial diffusion, as well as energy losses due to the synchrotron emission and inverse-Compton scattering of both synchrotron and external ambient photon fields. We find that the light curve profiles of blazars are consistent with the particle spectral evolution resulting from time-dependent electric and magnetic fields, rather than the effects of the acceleration or the cooling processes. The proposed model is able to simultaneously account for the variability of both the energy spectrum and the light curve profile of the BL Lac object Mrk 421 with reasonable assumptions about the physical parameters. The results strongly indicate that the magnetic field evolution in the dissipated region of a blazar jet can account for the variabilities.
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