Pulsar interpretation for the AMS-02 result

The AMS-02 Collaboration has just published a high-precision measurement of the cosmic positron fraction $e^{+}/(e^{-}+e^{+})$, which rises with energy from $\sim 5\mathrm{GeV}$ to $\sim 350\mathrm{GeV}$. The result indicates the existence of primary electron/positron sources to account for the positron excess. In this work, we investigate the possibility that the nearby mature pulsars with ages of $O({10}^5)\mathrm{yr}$ are the primary positron sources. By fitting the data we find that the positrons from a single nearby pulsar, such as Geminga or Monogem, with the spectral index $\alpha \sim 2$ can interpret the AMS-02 result. We also investigate the possibility that high-energy positrons are generated by multiple known pulsars in the ATNF catalogue. Such a scenario can also fit the AMS-02 data well. Future precise measurements of fine structures in the positron spectrum would be a support to the pulsar scenario.

Figure 1

The positron fraction (left) and electron flux (right) for the exotic $e^{\pm }$ from Geminga [$\mathrm{J}0633+1746$] (upper) and Monogem [$\mathrm{B}0656+14$] (lower). Also shown are the positron fraction data from AMS01 [47], $\mathrm{HEAT}94+95$ [48], HEAT00 [49], PAMELA [2] and AMS02 [1], and electron flux data from PAMELA [32], ATIC [50], HESS [51, 52] and Fermi-LAT [53].

Figure 2

$1\sigma$ and $2\sigma$ confidence regions in the plane of $\alpha$ vs $E_{\mathrm{tot}}$ for Geminga (solid contours) and Monogem (dashed contours), respectively. Horizontal lines denote the maximum $E_{\mathrm{out}}$ derived by Eq. (3) with ${\tau }_0={10}^4\mathrm{yr}$ and ${\eta }_{e^{\pm }}=1$. Also shown are points for best fits.

Figure 3

$1\sigma$ (solid contour) and $2\sigma$ (dashed contour) confidence regions in the pulsar parameter space to fit the positron fraction and electron spectrum for $\alpha =2$ and $E_{\mathrm{cut}}=1\mathrm{TeV}$. The regions are projected onto the $E_{\mathrm{tot}}$ vs $d$ (upper left), $E_{\mathrm{tot}}$ vs $T$ (upper right), and $T$ vs $d$ (lower) planes. The circles and bars denote the 177 pulsars in the ATNF catalogue within $d<3\mathrm{kpc}$ and $5\times {10}^4<T(\mathrm{yr})<{10}^7$. $E_{\mathrm{tot}}$ for each pulsar is estimated in the range of $5\%\le {\eta }_{e^{\pm }}\le 50\%$. Seven nearby pulsars with $d<0.5\mathrm{kpc}$ and $T<{10}^6\mathrm{yr}$ are marked by colors.

Figure 4

The contributions to the positron fraction (left) and electron flux (right) of all the 177 pulsars in the ATNF catalogue with $d<3\mathrm{kpc}$ and $5\times {10}^4<T(\mathrm{yr})<{10}^7$. The parameters for each pulsar are randomly assigned in the following ranges: $700\le E_{\mathrm{cut}}(\mathrm{GeV})\le 3000$, $1.5\le \alpha \le 2.3$, and $5\%\le {\eta }_{e^{\pm }}\le 30\%$. For a particular combination of the parameters, the resulting spectrum including the contributions of all pulsars is represented by a grey line, while a golden line shows only the exotic contributions from the pulsars. A representative choice is shown by black lines (the solid line for the total result, and the dot-dashed line for the exotic contribution).

Figure 5

Illustration of the positron fraction from both the diffuse pulsars and the seven nearby mature pulsars. The parameters of the seven pulsars are chosen randomly.

Figure 6

The positron fraction (left) and electron flux (right) for the background together with an exotic component from multiple pulsars or DM annihilation in ${\tau }^{+}{\tau }^{-}$ channel. The “mock” DAMPE data are assumed to be contributed by multiple pulsars and are generated using the pulsar set denoted by black dot-dashed line in Fig. 4. The DM contribution corresponds to the best fit of an MCMC parameter scan.