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Register a new userA serendipitous discovery of GeV gamma-ray emission from supernova 2004dj in a survey of nearby star-forming galaxies with Fermi-LAT
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The interaction between a supernova ejecta and the circum-stellar medium drives a strong shock wave which accelerates particles (i.e., electrons and protons). The radio and X-ray emission observed after the supernova explosion constitutes the evidence of the electron acceleration. The accelerated protons are expected to produce GeV-TeV gamma-ray emission via $pp$ collisions, but the flux is usually low since only a small fraction of the supernova kinetic energy is converted into the shock energy at the very early time. The low gamma-ray flux of the nearest supernova explosion, SN 1987A, agrees with this picture. Here we report a serendipitous discovery of a fading GeV gamma-ray source in spatial coincidence with the second nearest supernova--SN 2004dj from our gamma-ray survey of nearby star-forming galaxies with Fermi-LAT. The total gamma-ray energy released by SN 2004dj is about $6times10^{47}{rm erg}$. We interpret this gamma-ray emission arising from the supernova ejecta interacting with a surrounding high-density shell, which decelerates the ejecta and converts ~1% of the ejectas kinetic energy to relativistic protons. In addition, our gamma-ray survey of nearby star-forming galaxies discovers GeV emissions from two star-forming galaxies, i.e., Arp 299 and M33, for the first time.
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We report on the first detection of GeV high-energy gamma-ray emission from a young supernova remnant with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope. These observations reveal a source with no discernible spatial extension detected at a significance level of 12.2$sigma$ above 500 MeV at a location that is consistent with the position of the remnant of the supernova explosion that occurred around 1680 in the Cassiopeia constellation - Cassiopeia A. The gamma-ray flux and spectral shape of the source are consistent with a scenario in which the gamma-ray emission originates from relativistic particles accelerated in the shell of this remnant. The total content of cosmic rays (electrons and protons) accelerated in Cas A can be estimated as $W_{mathrm{CR}} approx (1-4) times 10^{49}$ erg thanks to the well-known density in the remnant assuming that the observed gamma-ray originates in the SNR shell(s). The magnetic field in the radio-emitting plasma can be robustly constrained as B $gt 0.1$ mG, providing new evidence of the magnetic field amplification at the forward shock and the strong field in the shocked ejecta.
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We report the detection of GeV $gamma$-ray emission from supernova remnant HESS J1731-347 using 9 years of {it Fermi} Large Area Telescope data. We find a slightly extended GeV source in the direction of HESS J1731-347. The spectrum above 1 GeV can be fitted by a power-law with an index of $Gamma = 1.77pm0.14$, and the GeV spectrum connects smoothly with the TeV spectrum of HESS J1731-347. Either a hadronic-leptonic or a pure leptonic model can fit the multi-wavelength spectral energy distribution of the source. However, the hard GeV $gamma$-ray spectrum is more naturally produced in a leptonic (inverse Compton scattering) scenario, under the framework of diffusive shock acceleration. We also searched for the GeV $gamma$-ray emission from the nearby TeV source HESS J1729-345. No significant GeV $gamma$-ray emission is found, and upper limits are derived.
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