The cosmic-ray content of the Orion-Eridanus superbubble

Abstract

Aims. The nearby Orion-Eridanus superbubble, which was blown by multiple supernovae several million years ago, has likely produced cosmic rays. Its turbulent medium is still energised by massive stellar winds and it can impact cosmic-ray transport locally. The γ radiation produced in interactions between cosmic rays and interstellar gas can be used to compare the cosmic-ray spectrum in the superbubble and in other regions near the Sun. It can reveal spectral changes induced in GeV to TeV cosmic rays by the past and present stellar activity in the superbubble. Methods. We used ten years of data from the Fermi Large Area Telescope (LAT) in the 0.25-63 GeV energy range to study the closer (Eridanus) end of the superbubble at low Galactic latitudes. We modelled the spatial and spectral distributions of the γ rays produced in the different gas phases (atomic, molecular, dark, and ionised) of the clouds found in this direction. The model included other non-gaseous components to match the data. Results. We found that the γ-ray emissivity spectrum of the gas along the outer rim and in a shell inside the superbubble is consistent with the average spectrum measured in the solar neighbourhood. It is also consistent with the cosmic-ray spectrum directly measured in the Solar System. This homogeneity calls for a detailed assessment of the recent supernova rate and current census of massive stellar winds in the superbubble in order to estimate the epoch and rate of cosmic-ray production and to constrain the transport conditions that can lead to such homogeneity and little re-acceleration. We also found significant evidence that a diffuse atomic cloud lying outside the superbubble, at a height of 200-250 pc below the Galactic plane, is pervaded by a 34% lower cosmic-ray flux, but with the same particle energy distribution as the local one. Super-GeV cosmic rays should freely cross such a light and diffuse cirrus cloud without significant loss or spectral distorsion. We tentatively propose that the cosmic-ray loss relates to the orientation of the magnetic field lines threading the cirrus, which point towards the halo according to the dust polarisation data from Planck. Finally, we gathered the present emissivity measurements with previous estimates obtained around the Sun to show how the local cosmic-ray flux decreases with Galactic height and to compare this trend with model predictions.

Cosmic-ray variations in the solar neighbourhood

“…2, 3], have severely overlooked interstellar complications (cloud confusion along sightlines, non uniform conversions of CO line intensities to H 2 column densities, masses of optically-thick H and CO-dark H 2 , severe non-linearities in the use of dust optical depths to trace the total gas, etc). The CR homogeneity near the Sun was, however, significanty challenged by the study of the light Eridu H cirrus, at a height of 200-250 pc below the Galactic plane, which presents a 34% lower γ-ray emissivity, but similar spectrum, than the local ISM average [4]. This emissivity is at variance with the dispersion seen among other nearby clouds and with the CR flux decline expected towards the halo (see Figure 13 of [4]).…”

“…The CR homogeneity near the Sun was, however, significanty challenged by the study of the light Eridu H cirrus, at a height of 200-250 pc below the Galactic plane, which presents a 34% lower γ-ray emissivity, but similar spectrum, than the local ISM average [4]. This emissivity is at variance with the dispersion seen among other nearby clouds and with the CR flux decline expected towards the halo (see Figure 13 of [4]). The emissivity of the Eridu cirrus was compared to the average [15] found in the atomic gas at latitudes 10 • ≤ |b| ≤ 70 • which span distances to typically 1 kpc.…”

“…See Figure 2 for the other electron references. The emissivities are compared with the average found in the atomic gas within ∼1 kpc from the Sun (left plot, shaded green area for the satistical uncertainty, from [15]) and in the nearby clouds of Eridu, Taurus, and Chamaeleon (right plot, with statistical uncertainties as error bars and the impact of systematic uncertainties in the H mass as shaded areas, from [4,22,23]).…”

“…The emissivity measurement relies on the spatial correlation between the γ rays and the gas column densities in the H -bright phase. The Eridu cirrus is located 200-300 pc away from the Sun and at a height of 200-250 pc below the Galactic plane [4]. It shares with the Sun a common location in a long, rather hollow valley of the local ISM that is surrounded by denser, star forming, molecular clouds.…”

Investigating gamma-ray emission of the Cygnus cocoon with 12 years of Fermi-LAT data

“…A study of the Cygnus X region has already unveiled a cocoon of freshly accelerated [2] and more recently the Cygnus cocoon has been observed using HAWC data above 1TeV [3]. Other studies show the existence of potential -ray signals toward some MSFRs ( [4], [5], [6], [7], [8], [9]), but not others ( [10], [11]). The nature of the Cygnus cocoon, as well as of the gamma-ray emission from other MSFRs, and the particle acceleration and propagation mechanisms at play in MSFRs are still poorly understood.…”