Aims. The Orion-Eridanus superbubble has been blown by supernovae and supersonic winds of the massive stars in the Orion OB associations. It is the nearest site at which stellar feedback on the interstellar medium that surrounds young massive clusters can be studied. The formation history and current structure of the superbubble are still poorly understood, however. It has been pointed out that the picture of a single expanding object should be replaced by a combination of nested shells that are superimposed along the line of sight. We have investigated the composite structure of the Eridanus side of the superbubble in the light of a new decomposition of the atomic and molecular gas. Methods. We used H i 21 cm and CO (J=1−0) emission lines to separate coherent gas shells in space and velocity, and we studied their relation to the warm ionised gas probed in H α emission, the hot plasma emitting X-rays, and the magnetic fields traced by dust polarised emission. We also constrained the relative distances to the clouds using dust reddening maps and X-ray absorption. We applied the Davis-Chandrasekhar-Fermi method to the dust polarisation data to estimate the plane-of-sky components of the magnetic field in several clouds and along the outer rim of the superbubble. Results. Our gas decomposition has revealed several shells inside the superbubble that span distances from about 150 pc to 250 pc. One of these shells forms a nearly complete ring filled with hot plasma. Other shells likely correspond to the layers of swept-up gas that is compressed behind the expanding outer shock wave. We used the gas and magnetic field data downstream of the shock to derive the shock expansion velocity, which is close to ∼20 km s −1 . Taking the X-ray absorption by the gas into account, we find that the hot plasma inside the superbubble is over-pressured compared to plasma in the Local Bubble. The plasma comprises a mix of hotter and cooler gas along the lines of sight, with temperatures of (3-9) and (0.3 − 1.2) × 10 6 K, respectively. The magnetic field along the western and southern rims and in the approaching wall of the superbubble appears to be shaped and compressed by the ongoing expansion. We find plane-of-sky magnetic field strengths from 3 to 15 µG along the rim.

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.

A solution of the transport equation for cosmic rays in turbulent magnetic fields in a spherically symmetric geometry is presented. The results are applied to particle propagation in superbubbles. In the fully analytical calculation, various energy-loss processes are considered. From the distribution function of the cosmic-ray particles, the distribution for pions from continuous losses is computed. Folding with the appropriate cross section yields the gamma-ray distribution. It is shown that in the case of efficient acceleration the volume-integrated π 0-decay gamma-ray emission from the superbubble exhibits the characteristic hard differential number density spectrum at large gamma-ray energies, E γ  ≫ 70 MeV. This prediction is useful for identifying efficient superbubble cosmic-ray accelerators from gamma-ray observations.

We describe an update to the Herschel-SPIRE Fourier-Transform Spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, η ff . This significant correction affects all FTS extended-source calibrated spectra in sparse or mapping mode, regardless of the spectral resolution. Line fluxes and continuum levels are underestimated by factors of 1.3-2 in the Spectrometer LongWavelength band (SLW, 447-1018 GHz; 671-294 µm) and 1.4-1.5 in the Spectrometer ShortWavelength band (SSW, 944-1568 GHz; 318-191 µm). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since Feb 2017. Studies based on extended-source calibrated spectra produced prior to this pipeline version should be critically reconsidered using the current products available in the Herschel Science Archive. Once the extended-source calibrated spectra are corrected for η ff , the synthetic photometry and the broadband intensities from SPIRE photometer maps agree within 2-4% -similar levels to the comparison of point-source calibrated spectra and photometry from point-source calibrated maps. The two calibration schemes for the FTS are now selfconsistent: the conversion between the corrected extended-source and point-source calibrated spectra can be achieved with the beam solid angle and a gain correction that accounts for the diffraction loss.

The Orion-Eridanus superbubble, formed from the winds and the explosions of Orion's massive stars, could be a cosmic-ray acceleration site. Inside the superbubble, the large level of magnetohydrodynamics turbulence and the core-collapse supernovae have created a turbulent medium which effect on cosmic rays can be probed comparing their flux and spectrum in the superbubble to the average in nearby interstellar clouds. To study cosmic rays in the superbubble, we first rely on Fermi Large Area Telescope data. Eight years of data and gamma rays above 250 MeV have been used. We are particularly interested in gamma rays resulting from the decay of neutral pions produced by the interaction of cosmic rays with interstellar gas. Hence, knowing both gas distribution and gamma-ray emission allows to obtain the cosmic-ray flux. We thus developed a model of the interstellar emission using multiwavelength tracers for the gas column densities in the different phases (atomic, molecular, ionized) of the superbubble in which atomic and molecular phases are further divided into several complexes based on coherence in spatial distribution and velocity. The model also includes other ancillary components such as inverse-Compton emission and point sources. Preliminary results show that the emissivity spectrum of the main HI cloud is consistent with the average spectrum measured in nearby clouds located outside the superbubble, but within the Gould Belt. This uniformity calls for a detailed assessment of the recent supernova rate and the energetics of massive stellar winds in the superbubble in order to estimate the diffusion properties of the young cosmic rays and to evaluate the need, or not, to advect them away in the Gould Belt or to the halo via the local Galactic wind.

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The Orion-Eridanus superbubble, formed by the supernovae and supersonic winds of massive stars in the Orion OB associations, has likely fostered cosmic-ray production. The large level of magnetohydrodynamic turbulence in the bubble can also locally alter the diffusion of cosmic rays in space and in momentum. We have studied the cosmic-ray spectrum inside the superbubble by measuring the γ-ray emissivity of gas clouds located in the bubble and along its rim. We have compared these spectra with the average found in other clouds in the solar neighbourhood. To this aim, we have used ten years of data from the Fermi Large Area Telescope (LAT) between 250 MeV and 63 GeV. We have modelled the γ radiation from hadronic interactions using multiwavelength tracers for the hydrogen column densities in the different gas phases, and using the gas distribution in space and velocity to separate the superbubble medium from its foregrounds and backgrounds. The model includes other ancillary components such as inverse-Compton emission, point sources, and solar and lunar emissions. Despite the stellar activity in the superbubble, we find the gas emissivity spectrum to be consistent with the average spectrum measured in the local interstellar medium (ISM). Yet, a small cirrus cloud lying just outside the superbubble exhibits a significantly lower cosmic-ray flux. We will discuss this change in the light of the magnetic-field geometry inferred from the Planck data on dust polarisation.