Supernova Origin of Cosmic Rays from a $\gamma$-Ray Signal in the Constellation III Region of the Large Magellanic Cloud

Cosmic rays could be produced via shock acceleration powered by supernovae. The supernova hypothesis implies that each supernova injects, on average, some $10^{50}\mathrm{erg}$ in cosmic rays, while the shock acceleration model predicts a power law cosmic ray spectrum with the slope close to 2. Verification of these predictions requires measurement of the spectrum and power of cosmic ray injection from supernova population(s). Here, we obtain such measurements based on $\gamma$-ray observation of the Constellation III region of the Large Magellanic Cloud. We show that $\gamma$-ray emission from this young star formation region originates from cosmic rays injected by approximately two thousand supernovae, rather than by a massive star wind powered by a superbubble predating supernova activity. Cosmic ray injection power is found to be $(1.1_{-0.2}^{+0.5})\times {10}^{50}\mathrm{erg}/\mathrm{supernova}$ (for the estimated interstellar medium density $0.3{\mathrm{cm}}^{-3}$). The spectrum is a power law with slope $2.09_{-0.07}^{+0.06}$. This agrees with the model of particle acceleration at supernova shocks and provides a direct proof of the supernova origin of cosmic rays.

Figure 1

History of the SFR of the CON III region from Ref. [18]. Grey shaded band shows the supernova rate time evolution derived from the SFR. Band width indicates the uncertainty of the supernova rate estimate.

Figure 2

LAT count map of LMC in the energy band $E>10\mathrm{GeV}$ smoothed with 0.3 degree Gaussian. Small green circles show positions of known supernova remnants [24]. White dashed circle of radius 1° outlines the extent of the CON III region. White arclike curves show positions of CON III star forming complexes from Ref. [18]. Green dashed circle shows “$E2$” source [17].

Figure 3

$\gamma$-ray spectrum of the CON III region (black data points). Blue semitransparent data points show rescaled spectrum of Cygnus cocoon [13]. Grey solid line shows the best fit power law spectrum. Dashed curve shows the best fit pion decay spectrum.