The cosmic-ray content of the Orion-Eridanus superbubble

T Joubaud,J M Casandjian,R Schlickeiser,T Tolksdorf,I A Grenier

doi:10.1051/0004-6361/201937205

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. Theγradiation 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 theFermiLarge 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 theγrays 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 theγ-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 fromPlanck. 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.

Highlights

Superbubbles form around starburst regions under the combined and sustained influence of the ionising radiation and energetic winds of massive stars plus a series of supernovae (SNe)
H I optical depth correction In the range of NH I column densities studied here, Nguyen et al (2019) showed that a simple isothermal correction of the H I emission spectra, with a uniform spin temperature (TS) across the cloud, provides better than 10% estimates of the more precise NH I values inferred from the combination of emission and absorption H I spectra
In order to account for the unknown level of H I opacity in the different NH I maps, we have repeated our dust and γ-ray analyses for ten spin temperatures (100, 125, 150, 200, 300, 400, 500, 600, 700, 800 K) and for the optically thin case which yields the minimum amount of gas

Summary

Introduction

Superbubbles form around starburst regions under the combined and sustained influence of the ionising radiation and energetic winds of massive stars plus a series of supernovae (SNe). It asymptotically tends toward a power law for p2 f (p) ∝ p−γ, with a γ index ranging between two and three depending on the particle injection momentum: γ = 3 for the acceleration of suprathermal particles and γ = 2 for the reacceleration of relativistic CR. Such superbubbles can transfer 10% or more of their kinetic power to CR (Bykov 2001)

Results

Discussion

Conclusion