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تسجيل مستخدم جديدFermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way
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It is widely accepted that cosmic rays (CRs) up to at least PeV energies are Galactic in origin. Accelerated particles are injected into the interstellar medium where they propagate to the farthest reaches of the Milky Way, including a surrounding halo. The composition of CRs coming to the solar system can be measured directly and has been used to infer the details of CR propagation that are extrapolated to the whole Galaxy. In contrast, indirect methods, such as observations of gamma-ray emission from CR interactions with interstellar gas, have been employed to directly probe the CR densities in distant locations throughout the Galactic plane. In this article we use 73 months of data from the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV to search for gamma-ray emission produced by CR interactions in several high- and intermediate-velocity clouds located at up to ~ 7 kpc above the Galactic plane. We achieve the first detection of intermediate-velocity clouds in gamma rays and set upper limits on the emission from the remaining targets, thereby tracing the distribution of CR nuclei in the halo for the first time. We find that the gamma-ray emissivity per H atom decreases with increasing distance from the plane at 97.5% confidence level. This corroborates the notion that CRs at the relevant energies originate in the Galactic disk. The emissivity of the upper intermediate-velocity Arch hints at a 50% decline of CR densities within 2 kpc from the plane. We compare our results to predictions of CR propagation models.
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Cosmic rays up to at least PeV energies are usually described in the framework of an elementary scenario that involves acceleration by objects that are located in the disk of the Milky Way, such as supernova remnants or massive star-forming regions,
and then diffusive propagation throughout the Galaxy. Details of the propagation process are so far inferred mainly from the composition of cosmic rays measured near the Earth and then extrapolated to the whole Galaxy. The details of the propagation in the Galactic halo and the escape into the intergalactic medium remain uncertain. The densities of cosmic rays in specific locations can be traced via the gamma rays they produce in inelastic collisions with clouds of interstellar gas. Therefore, we analyze 73 months of Fermi-LAT data from 300 MeV to 10 GeV in the direction of several high- and intermediate-velocity clouds that are located in the halo of the Milky Way. These clouds are supposed to be free of internal sources of cosmic rays and hence any gamma-ray emission from them samples the large-scale distribution of Galactic cosmic rays. We evaluate for the first time the gamma-ray emissivity per hydrogen atom up to ~7 kpc above the Galactic disk. The emissivity is found to decrease with distance from the disk, which provides direct evidence that cosmic rays at the relevant energies originate therein. Furthermore, the emissivity of one of the targets, the upper intermediate-velocity Arch, hints at a 50% decline of the cosmic-ray intensity within 2 kpc from the disk.
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0 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities above 250 MeV are (5.9 +/- 0.1(stat) (+0.9/-1.0)(sys)), (10.2 +/- 0.4(stat) (+1.2/-1.7)(sys)), and (9.1 +/- 0.3(stat) (+1.5/-0.6)(sys)) x10^(-27) photons s^(-1) sr^(-1) H-atom^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ~20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, Xco = N(H2)/Wco, is found to be (0.96 +/- 0.06(stat) (+0.15/-0.12)(sys)), (0.99 +/- 0.08(stat) (+0.18/-0.10)(sys)), and (0.63 +/- 0.02(stat) (+0.09/-0.07)(sys)) x10^20 H2-molecule cm^(-2) (K km s^(-1))^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of Xco in the vicinity of the solar system. From the obtained values of Xco, the masses of molecular gas traced by Wco in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ~5x10^3, ~10^3, and ~3.3x10^4 Msolar, respectively. A comparable amount of gas not traced well by standard HI and CO surveys is found in the regions investigated.
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