Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars

We present constraints on the nature of axions and axionlike particles (ALPs) by analyzing gamma-ray data from neutron stars using the Fermi Large Area Telescope. In addition to axions solving the strong $CP$ problem of particle physics, axions and ALPs are also possible dark matter candidates. We investigate axions and ALPs produced by nucleon-nucleon bremsstrahlung within neutron stars. We derive a phenomenological model for the gamma-ray spectrum arising from subsequent axion decays. By analyzing five years of gamma-ray data (between 60 and 200 MeV) for a sample of four nearby neutron stars, we do not find evidence for an axion or ALP signal; thus we obtain a combined 95% confidence level upper limit on the axion mass of $7.9\times {10}^{-2}\mathrm{eV}$, which corresponds to a lower limit for the Peccei-Quinn scale $f_a$ of $7.6\times {10}^7\mathrm{GeV}$. Our constraints are more stringent than previous results probing the same physical process, and are competitive with results probing axions and ALPs by different mechanisms.

Figure 1

A Feynman diagram for the nucleon-nucleon bremsstrahlung process $NN\to NNa$, according to the OPE assumption. $N_1$ and $N_2$ are incoming nucleons, and $N_3$ and $N_4$ are outgoing nucleons. $a$ represents the axion. $\pi$ represents a pion. In the case $nn\to nna$, we consider ${\pi }^0$. We also represent the decay process $a\to \gamma \gamma$ in this diagram.

Figure 2

The function ${\omega }^4{S}_{\sigma }(\omega )$, which shows the energy dependence of the emissivity. The spin structure function $S_{\sigma }(\omega )$ is computed according to Eq. (3), assuming nonrelativistic nucleons and nucleon energy $E=p^2/2M_N$. We plot values of $S_{\sigma }(\omega ;\mu ,T)$ for different values of $\mu$ and $T$. In the data analysis, we use $T=20\mathrm{MeV}$, $\mu /T=10$, which corresponds to the solid green curve.

Figure 3

The spectral energy distribution of gamma rays from axion decays, for the neutron star J0108-1431, derived according to Eq. (12).

Figure 4

Smoothed residual maps computed according to (counts model)/model, in celestial coordinates for the sources (a) J0108-1431, (b) $\mathrm{J}0953+0755$, (c) J0630-2834, (d) $\mathrm{J}1136+1551$. The energy range is 60–200 MeV. The residual maps are plotted in a $28°\times 28°$ square, with the neutron star at the center, with a pixel size of 0.5°. The images are smoothed with a Gaussian of $\mathrm{sigma}=5°$. 2FGL sources are denoted with green crosses; the putative neutron star source is denoted with a red cross in the center.

Figure 5

Spectral residuals over the ROI, as a function of energy, according to (counts model)/model, for the sources (a) J0108-1431, (b) $\mathrm{J}0953+0755$, (c) J0630-2834, (d) $\mathrm{J}1136+1551$. The error bars along the $x$ axis denote the range of energies considered at each point, and the error bars along the $y$ axis are statistical uncertainties.

Figure 6

Exclusion ranges for $m_a$ and $f_a$ derived here (red), compared with those of other experiments (blue) [50, 51, 52] and theoretically motivated inclusion regions (green) [53, 54, 55]; values within the shaded boxes (blue and red) are excluded, with the lower bound corresponding to the 95% C.L. upper limit.

Figure 7

Exclusion plot for the $(m_a,f_a)$ parameter space for ALPs at 95% C.L. NS describes the region derived from this work, SN 1987A describes the region derived from Fermi LAT analysis of SN 1987A [16]. The axion line (black) shows the parameters allowed by Peccei-Quinn axions.

Figure 8

A histogram of limits on $m_a$ from the joint likelihood analysis of the blank fields considered. The red dashed line corresponds to the upper limit of 0.079 eV obtained from the analysis of the four selected neutron stars.