Cosmic rays, neutrinos, and GeV-TeV gamma rays from starburst galaxy NGC 4945

The detection of high-energy astrophysical neutrinos and ultra-high-energy cosmic rays (UHECRs) provides a new way to explore sources of cosmic rays. One of the highest energy neutrino events detected by IceCube, tagged as IC35, is close to the UHECR anisotropy region detected by Pierre Auger Observatory. The nearby starburst (SB) galaxy, NGC 4945, is close to this anisotropic region and inside the mean angular error of the IC35 event. Considering the hypernovae contribution located in the SB region of NGC 4945, which can accelerate protons up to $\sim {10}^{17}\mathrm{eV}$ and inject them into the interstellar medium, we investigate the origin of this event around this starburst galaxy. We show that the interaction of these protons with the SB region’s gas density could explain Fermi-LAT gamma-ray and radio observations if the magnetic field’s strength in the SB region is the order of $\sim \mathrm{mG}$. Our estimated PeV neutrino events, in ten years, for this source is approximately 0.01 ($4\times {10}^{-4}$) if a proton spectral index of 2.4 (2.7) is considered, which would demonstrate that IC35 is not produced in the central region of this SB galaxy. Additionally, we consider the superwind region of NGC 4945 and show that protons can hardly be accelerated in it up to UHECRs.

Figure 1

This figure shows the attractive region of the skymap, including the galaxy NGC 4945 (green point) and a region of 27° centered at the biggest red point (red circle) corresponding to Auger hot spot [8]. The neutrino IC35 corresponds to the blue point and its median angular error (blue circle). UHECRs are shown as red points.

Figure 2

Total optical depth due to pair production in the SB region’s radiation field: the IR radiation, disk radiation, and the external radiation field due to the extragalactic background.

Figure 3

CR proton timescales: losses via ionization and inelastic $pp$ interaction are taken. The escape is due to advection and diffusion mechanism.

Figure 4

CR electron timescales: $e^{\pm }$ energy losses via ionization, Bremsstrahlung, inverse Compton, and synchrotron process in the magnetic field of value $B=100\mu \mathrm{G}$ and $B=1\mathrm{mG}$, while escape is only due to advection (diffusion is neglected).

Figure 5

Spectral energy distribution for radio emission. The total flux in the central SB for spectral index 2.4 (red solid line) and 2.7 (blue solid line) is produced by primary (dotted line) and secondary (dashed line) electrons. We observed that if the spectral index is steeper, we demand a stronger magnetic field to explain the data. Data are taken from [74, 80].

Figure 6

Spectral energy distribution for $\gamma$ rays. The total flux in the central SB region for spectral index 2.4 (red solid line) and 2.7 (blue solid line) is shown. The contribution from $pp$ model (dashed line), Bremsstrahlung (dotted-dashed-dashed line), external IC for secondary (dotted-dotted-dashed line), and primary (dotted line) are shown. The contribution from $\gamma \gamma$ absorption has no significant contribution and is not plotted.

Figure 7

Neutrino flux comparison among the resulting by our model (solid-lines), the 90% C.L. upper limit for muon-neutrino at 1 TeV set by [22] (black arrow), and the associated flux to the IC35 event considering 10 years of observations and assuming spectral index of 2.4 (red dashed-dotted line) and 2.6 (blue dashed-dotted line).