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Register a new userA discovery of a low-energy excess in cosmic-ray iron: an evidence of the past supernova activity in the Local Bubble
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Since its launch, the Alpha Magnetic Spectrometer - 02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, $bar{p}$, $e^{pm}$, and nuclei, $_1$H-$_8$O, $_{10}$Ne, $_{12}$Mg, $_{14}$Si, which resulted in a number of breakthroughs. One of the latest long awaited surprises is the spectrum of $_{26}$Fe just published by AMS-02. Because of the large fragmentation cross section and large ionization energy losses, most of CR iron at low energies is local, and may harbor some features associated with relatively recent supernova (SN) activity in the solar neighborhood. Our analysis of new iron spectrum together with Voyager 1 and ACE-CRIS data reveals an unexpected bump in the iron spectrum and in the Fe/He, Fe/O, and Fe/Si ratios at 1-2 GV, while a similar feature in the spectra of He, O, Si, and in their ratios is absent, hinting at a local source of low-energy CRs. The found excess fits well with recent discoveries of radioactive $^{60}$Fe deposits in terrestrial and lunar samples, and in CRs. We provide an updated local interstellar spectrum (LIS) of iron in the energy range from 1 MeV nucleon$^{-1}$ to $sim$10 TeV nucleon$^{-1}$. Our calculations employ the GalProp-HelMod framework that is proved to be a reliable tool in deriving the LIS of CR $bar{p}$, $e^{-}$, and nuclei $Zle28$.
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Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, $bar{p}$, $e^{pm}$, and nuclei (H-O, Ne, Mg, Si, Fe), which resulted in a number of breakthroughs. The most recent AMS-02 result is the measurement of the spectrum of CR fluorine up to $sim$2 TV. Given its very low solar system abundance, fluorine in CRs is thought to be mostly secondary, produced in fragmentations of heavier species, predominantly Ne, Mg, and Si. Similar to the best-measured secondary-to-primary boron to carbon nuclei ratio that is widely used to study the origin and propagation of CR species, the precise fluorine data would allow the origin of Si-group nuclei to be studied independently. Meanwhile, the secondary origin of CR fluorine has never been tested in a wide energy range due to the lack of accurate CR data. In this paper, we use the first ever precise measurements of the fluorine spectrum by AMS-02 together with ACE-CRIS and Voyager 1 data to actually test this paradigm. Our detailed modeling shows an excess below 10 GV in the fluorine spectrum that is most likely due to the primary fluorine component. We also provide an updated local interstellar spectrum (LIS) of fluorine in the rigidity range from few MV to $sim$2 TV. Our calculations employ the self-consistent GalProp-HelMod framework that has proved to be a reliable tool in deriving the LIS of CR $bar{p}$, $e^{-}$, and nuclei $Zle28$.
Cosmic ray electrons and positrons are tracers of particle propagation in the interstellar medium (ISM). A recent measurement performed using H.E.S.S. extends the all-electron (electron+positron) spectrum up to 20TeV, probing very local sources and transport due to the $sim$10~kyr cooling time of these particles. An additional key local measurement was the recent estimation of the ISM diffusion coefficient around Geminga performed using HAWC. The inferred diffusion coefficient is much lower than typically assumed values. It has been argued that if this diffusion coefficient is representative of the local ISM, pulsars would not be able to account for the all-electron spectrum measured at the Earth. Here we show that a low diffusion coefficient in the local ISM is compatible with a pulsar wind nebula origin of the highest energy electrons, if a so far undiscovered pulsar with spin-down power $sim 10^{33-34}$ erg/s exists within 30 to 80~pc of the Earth. The existence of such a pulsar is broadly consistent with the known population and may be detected in near future survey observations.
We report the analysis of the Fermi-Large Area Telescope data from six nearby giant molecular clouds (MCs) belonging to the Gould Belt and the Aquila Rift regions. The high statistical {gamma}-ray spectra above 3 GeV well described by power laws make it possible to derive precise estimates of the cosmic-ray (CR) distribution in the MCs. The comparison of {gamma}-ray spectra of Taurus, Orion A, and Orion B clouds with the model expected from Alpha Magnetic Spectrometer (AMS-02) CR measurements confirms these clouds as passive clouds, immersed in an AMS-02-like CR spectrum. A similar comparison of Aquila Rift, Rho Oph, and Cepheus spectra yields significant deviation in both spectral indices and absolute fluxes, which can imply an additional acceleration of CRs throughout the entire clouds. Besides, the theoretical modeling of the excess {gamma}-ray spectrum of these clouds, assuming {pi}0-decay interaction of CRs in the cloud, gives a considerable amount of an enhanced CR energy density and it shows a significant deviation in spectral shapes compared to the average AMS-02 CR spectrum between 30 GeV and 10 TeV. We suggest that this variation in the CR spectrum of Cepheus could be accounted for by an efficient acceleration in the shocks of winds of OB associations, while in Rho Oph, similar acceleration can be provided by multiple T-Tauri stars populated in the whole cloud. In the case of Aquila Rift, the excess in absolute CR flux could be related to an additional acceleration of CRs by supernova remnants or propagation effects in the cloud.
The ultrahigh-energy cosmic-ray anisotropies discovered by the Pierre Auger Observatory give the potential to finally address both the particles origins and properties of the nearby extragalactic magnetic field (EGMF). We examine the implications of the excess of ~ 10^20 eV events around the nearby radio galaxy Centaurus A. We find that, if Cen A is the source of these cosmic rays, the angular distribution of events constrains the EGMF strength within several Mpc of the Milky Way to > 20 nG for an assumed primary proton composition. Our conclusions suggest that either the observed excess is a statistical anomaly or the local EGMF is stronger then conventionally thought. We discuss the implications of this field, including UHECR scattering from more distant sources, time delays from transient sources, and the possibility of using magnetic lensing signatures to attain tighter constraints.
We report Suzaku results for soft X-ray emission to the south of the Galactic center (GC). The emission (hereafter GC South) has an angular size of ~42 x 16 centered at (l, b) ~ (0.0, -1.4), and is located in the largely extended Galactic ridge X-ray emission (GRXE). The X-ray spectrum of GC South exhibits emission lines from highly ionized atoms. Although the X-ray spectrum of the GRXE can be well fitted with a plasma in collisional ionization equilibrium (CIE), that of GC South cannot be fitted with a plasma in CIE, leaving hump-like residuals at ~2.5 and 3.5 keV, which are attributable to the radiative recombination continua of the K-shells of Si and S, respectively. In fact, GC South spectrum is well fitted with a recombination-dominant plasma model; the electron temperature is 0.46 keV while atoms are highly ionized (kT = 1.6 keV) in the initial epoch, and the plasma is now in a recombining phase at a relaxation scale (plasma density x elapsed time) of 5.3 x 10^11 s cm^-3. The absorption column density of GC South is consistent with that toward the GC region. Thus GC South is likely to be located in the GC region (~8 kpc distance). The size of the plasma, the mean density, and the thermal energy are estimated to be 97 pc x 37 pc, 0.16 cm^-3, and 1.6 x 10^51 erg, respectively. We discuss possible origins of the recombination-dominant plasma as a relic of past activity in the GC region.
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