Revisiting the analysis of axion-like particles with the Fermi-LAT gamma-ray observation of NGC1275

Abstract

In this work, we re-analyze the Fermi-LAT observation of NGC 1275 to search for axion-like particle (ALP) effects and constrain ALP parameters. Instead of fitting the observed spectrum with ALP models, we adopt a new method for the analysis of this source which calculates the irregularity of the spectrum. With the newly used method, we find no spectral oscillation for the NGC 1275 and rule out couplings g aγ > 3 × 10 −12 GeV −1 around ALP mass of m a ∼ 1 neV at 95% confidence level, which is more stringent than the previous results. We also show that the constraints can be further improved by combining the observation of PKS 2155-304. We suggest that with more sources taken into account, we could obtain a much wider exclusion region.

Axion-Like Particle Searches with IACTs

“…where d is the number of degrees of freedom, k runs over the N bins, and φ w/oALP ( θ) is a global fit without ALPs with spectral parameters θ. This method relies on the accuracy of the assumed shape of the spectrum, and is, therefore, subject to possible bias, but can be used in the case when the global fit represents a good estimate on the spectrum [71]. Expanding on this, Abramowski et al [36] searched for irregularities avoiding a global fit and using only a spectral shape over three adjacent points in the energy spectrum (a triplet i):…”

“…Excluded couplings are g aγγ > 3 × 10 −12 GeV −1 for masses of the ALPs m a ∼ 1 neV at a 95 % confidence level. The results of this search show the possibility of further improving the constraints by combining NGC 1275 observations with observations of another source PKS 2155-304 [71].…”

“…In [83] the highest energy spectra of AGN studied by Fermi-LAT and IACTs are compared, showing that the inclusion of proton-ALPs oscillation effects improves the agreement of the standard AGN model with the data. Recently, the analysis of Ajello et al [41] was revisited by Cheng et al [71] using a different analysis method, calculating the irregularity of the spectrum of NGC 1275. Aiming to measure the irregularity of the spectrum, an estimator needs to be chosen.…”

“…Looking back to the article by H.E.S.S [36], one could decide to use the estimator from Equation (14). A possible problem arises in the case of a large number of energy bins (∼100) (as in [71]), since the ALPs signatures might become wider than the bin size, making this kind of estimator insensitive to such alternations. Using the energy windows, instead of the spectral points triplets, and following the assumption of a power-law model in those energy windows, Cheng et al [71] proposed an alternative version of the estimator,…”

“…where i and j represent the energy window and bin, respectively, while φ pl is the flux assumed by the power-law spectral fit in each energy window, and φ and σ are the measured values of the flux and uncertainty, respectively [71]. Each of the simulated ALPs models were fitted assuming a baseline log parabola.…”