Constraints on axionlike particles with H.E.S.S. from the irregularity of the PKS $2155-304$ energy spectrum

Axionlike particles (ALPs) are hypothetical light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ALPs can modify the energy spectrum of the gamma rays for a sufficiently large coupling between ALPs and photons. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H.E.S.S. observations of the distant BL Lac object PKS $2155-304$ ($z=0.116$) are used to derive upper limits at the 95% C.L. on the strength of the ALP coupling to photons, $g_{\gamma a}<2.1\times {10}^{-11}\mathrm{Ge}{\mathrm{V}}^{-1}$ for an ALP mass between 15 and 60 neV. The results depend on assumptions on the magnetic field around the source, which are chosen conservatively. The derived constraints apply to both light pseudoscalar and scalar bosons that couple to the electromagnetic field.

Figure 1

Survival probability for gamma rays mixing with ALPs in a galaxy cluster magnetic field (see text for details). Top panel: Raw function. Bottom panel: The same function convolved with the energy resolution and bias of H.E.S.S. The instrumental response functions at 50 GeV are extrapolated to lower energies, not reachable with H.E.S.S.

Figure 2

Time-averaged energy spectrum of PKS $2155-304$ for the data set used in the analysis. Top panel: The blue line is the best fit of a log-parabola modulated by absorption on the EBL to the data. Bottom panel: Relative residuals of the fit normalized to the errors.

Figure 3

Schematic view of the procedure used to quantify spectral fluctuations.

Figure 4

Predicted probability density functions of the irregularity estimator for two ALP parameter sets. The vertical band corresponds to the rms of the fluctuations of the measurement when varying the binning. The dashed line indicates the value used to set the limits.

Figure 5

Evolution of the one-tailed 95% lower bound of the PDF constructed for the estimator normalized to its measured value as a function of the system parameters. Each panel shows variation of one parameter around the value $g_{\gamma a}\times B={10}^{-10}{\mathrm{GeV}}^{-1}\times 1\mu \mathrm{G}$, $m=30\mathrm{neV}$ and conversion in the galaxy cluster with a turbulence power spectrum slope $\alpha =5/3$ on scales between 1 and 10 kpc. For the middle panel and the right panel, the value used in the analysis is represented by a marker. From left to right, variations of the coupling constant, of the maximal turbulence scale compared to the total system radius of 370 kpc (number of corresponding domains) and of the turbulence power spectrum slope are displayed.

Figure 6

Constraints on the ALP parameters expressed in reduced variables independent of the magnetic field strength (see text for details) for both the IGMF (left panel) and the galaxy cluster magnetic field (right panel).

Figure 7

H.E.S.S. exclusion limits on the ALP parameters $g_{\gamma a}$ and $m$. The dashed region on the left is obtained considering photon-ALP mixing in the IGMF with an optimistic scenario with a 1 nG field strength. The dashed region on the right is obtained considering photon-ALP mixing in the galaxy cluster of PKS $2155-304$.