LOCAL H i EMISSIVITY MEASURED WITHFERMI-LAT AND IMPLICATIONS FOR COSMIC-RAY SPECTRA

Abstract

Cosmic-ray (CR) electrons and nuclei interact with the Galactic interstellar gas and produce high-energy γ rays. The γ-ray emission rate per hydrogen atom, called emissivity, provides a unique indirect probe of the CR flux. We present the measurement and the interpretation of the emissivity in the solar neighborhood for γ-ray energy from 50 MeV to 50 GeV. We analyzed a subset of 4 years of observations from the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope (Fermi) restricted to absolute latitudes 10 • < |b| < 70 • . From a fit to the LAT data including atomic, molecular and ionized hydrogen column density templates as well as a dust optical depth map we derived the emissivities, the molecular hydrogen to CO conversion factor X CO = (0.902 ± 0.007) × 10 20 cm −2 (K km s −1 ) −1 and the dust-to-gas ratio X DU ST = (41.4 ± 0.3) × 10 20 cm −2 mag −1 . Moreover we detected for the first time γ-ray emission from ionized hydrogen. We compared the extracted emissivities to those calculated from γ-ray production cross-sections and to CR spectra measured in the heliosphere. We observed that the experimental emissivities are reproduced only if the solar modulation is accounted for. This provides a direct detection of solar modulation observed previously through the anticorrelation between CR fluxes and solar activity. Finally we fitted a parametrized spectral form to the heliospheric CR observations as well as to the Fermi-LAT emissivity and obtained compatible local interstellar spectra for proton and Helium kinetic energy per nucleon between between 1 and 100 GeV and for electron-positrons between 0.1 and 100 GeV.

Quantifying the Interstellar Medium and Cosmic Rays in the MBM 53, 54, and 55 Molecular Clouds and the Pegasus Loop Using Fermi-Lat Gamma-Ray Observations

“…To gauge the uncertainty in the emissivity model [mainly due to the uncertainty of the elemental composition of CRs and the cross sections other than proton-proton (p-p) collisions], we also plotted the model curve for ǫ M = 1.45 (the lowest value referred to in Mori 2009) which gives 15%-20% lower emissivity. We also plotted the emissivity spectrum of the local H I clouds (in different regions of the sky) measured by Abdo et al (2009b) and Casandjian (2015) for comparison, in the analysis for which different LAT event selections and response functions were employed (the so-called Pass 6 and Pass 7 by Abdo et al (2009b) and Casandjian (2015), respectively). Most recent studies of high-latitude regions by Fermi-LAT, e.g., local H I emissivities in Tibaldo et al (2015) and H I emissivity of the Chamaeleon complex in Planck Collaboration (2015) find H I emissivity spectra similar to that of Casandjian (2015).…”

“…It can be understood, at least partially, due to the assumption of the H I optical thickness. Abdo et al (2009b) and Casandjian (2015) assumed a uniform T s of 125 K and 140 K, respectively, and Tibaldo et al (2015) and Planck Collaboration (2015) assumed the optically thin case. If uniform T s of such values (greater than or equal to 125 K) is applied to our region, we will have smaller N (H tot ) on average than that we obtained in Figure 9, and larger H I emissivity.…”

“…Thanks to its wide field of view, the Fermi-LAT is an ideal telescope to study Galactic diffuse γ rays. Past studies of Galactic diffuse emission by Fermi-LAT can be found in, e.g., Ackermann et al (2012a), andCasandjian (2015).…”

Cosmic rays, gas, and dust in local clouds

“…Thus at the precision level of the current LAT observations, we find no evidence of CR exclusion or CR concentration in the clouds, up to the 12 CO-bright molecular regions. The Fermi-LAT analyses indicate that the CR population permeating the various phases of the different clouds has the same energy distribution as the average in the local ISM (Casandjian 2015), so γ rays should reliably trace most of the 65 interstellar gas. …”

“…Those assumptions still need to be tested in a variety of clouds and across complex phase changes, especially on consideration that dust grains and their radiation properties vary from the diffuse ISM to molecular clouds (Stepnik et al 2003;Flagey et al 2009;Martin et al 2012;Roy et al 2013;Ysard et al 2013;Planck Collaboration 2014a,b;20 Planck and Fermi Collaborations 2015). One can also test the uniformity of the Local γ-ray emissivity spectrum of the gas (Casandjian 2015) in well-resolved nearby clouds to verify the smooth penetration of CRs with energies above ∼1 GeV (Skilling & Strong 1976;Cesarsky & Volk 1978;Morlino & Gabici 2015).…”