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

Abstract

A study of the interstellar medium (ISM) and cosmic rays (CRs) using Fermi Large Area Telescope (LAT) data, in a region encompassing the nearby molecular clouds MBM 53, 54, and 55 and a farinfrared loop-like structure in Pegasus, is reported. By comparing Planck dust thermal emission model with Fermi-LAT γ-ray data, it was found that neither the dust radiance (R) nor the dust opacity at 353 GHz (τ 353 ) were proportional to the total gas column density N (H tot ) primarily because N (H tot )/R and N (H tot )/τ 353 depend on the dust temperature (T d ). The N (H tot ) distribution was evaluated using γ-ray data by assuming the regions of high T d to be dominated by optically thin atomic hydrogen (H I) and by employing an empirical linear relation of N (H tot )/R to T d . It was determined that the mass of the gas not traced by the 21-cm or 2.6-mm surveys is ∼25% of the mass of H I in the optically thin case and is larger than the mass of the molecular gas traced by carbon monoxide by a factor of up to 5. The measured γ-ray emissivity spectrum is consistent with a model based on CR spectra measured at the Earth and the nuclear enhancement factor of ≤1.5. It is, however, lower than local H I emissivities reported by previous Fermi-LAT studies employing different analysis methods and assumptions on ISM properties by 15%-20% in energies below a few GeV, even if we take account of the statistical and systematic uncertainties. The origin of the discrepancy is also discussed.

Probing local cosmic rays using Fermi-LAT observations of a mid-latitude region in the third Galactic quadrant

“…We use the bracketing A eff method to propagate the uncertainty to the spectral parameters. 10 To investigate the largest influence, we replace the A eff with the upper and lower bound of the uncertainty for the sources with spectral indexes freed except the isotropic background and the merged template for weak sources. It results in a 3%−5% change of the emissivity.…”

Study of the Cosmic Rays and Interstellar Medium in Local H i Clouds Using Fermi-LAT Gamma-Ray Observations

“…As we saw in Section 2.1 (Figure 1), the correlation between W H i and D em depends on T d , and the temperature dependence is significantly different in the cases of R or τ 353 . Although the model with N H proportional to R is preferred to the one proportional to τ 353 (and the one proportional to W H i ) in terms of ln L by the γ-ray data analysis as described in Section 4.1, the true N H distribution could be appreciably different from either of them; see Section 2.1 for the prerequisites for D em to be proportional to N H and also Mizuno et al (2016), who reported the apparent T d dependence of N H /R and N H /τ 353 in the MBM 53, 54, and 55 clouds and the Pegasus loop.…”

Study of the Interstellar Medium and Cosmic Rays in the MBM 53-55 Clouds and the Pegasus Loop

“…The shaded band shows the systematic uncertainty of our H I emissivity spectrum. (shown in [15], reproduced courtesy of the AAS)…”

“…In the following sections, we describe γ-ray modeling, data analysis, and discussion of the ISM and CR properties in Section 2, 3, and 4, respectively. Readers are also referred to [15] for more details. The Large Area Telescope (LAT) on board the Fermi satellite is a pair-tracking γ-ray telescope, detecting photons from ∼20 MeV to more than 300 GeV [16,17].…”