Gamma-ray haloes can exist around galaxies due to the interaction of escaping galactic cosmic rays with the surrounding gas. We have searched for such a halo around the nearby giant spiral Andromeda galaxy M31 using almost 7 years of Fermi LAT data at energies above 300 MeV. The presence of a diffuse gamma-ray halo with total photon flux 2.6 ± 0.6 × 10 −9 cm −2 s −1 , corresponding to a luminosity (0.3-100 GeV) of (3.2±0.6)×1038 erg s −1 (for a distance of 780 kpc) was found at a 5.3σ confidence level. The halo form does not correspond to the extended baryonic HI disc of M31, as would be expected in hadronic production of gamma photons from cosmic ray interaction, nor it is spherically symmetric, as could be in the case of dark matter annihilation. The best-fit halo template corresponds to two 6-7.5 kpc bubbles symmetrically located perpendicular to the M31 galactic disc, similar to the 'Fermi bubbles' found around the Milky Way centre, which suggests the past activity of the central supermassive black hole or a star-formation burst in M31.

Abstract. We have searched for an extended high-energy gamma-ray emission around the nearby giant spiral Andromeda galaxy M31 using almost 7 years of Fermi LAT data at energies above 300 MeV. The presence of a diffuse gamma-ray halo with total photon flux 2.6±0.6×10−9 cm −2 s −1 , corresponding to a luminosity (0.3-100 GeV) of (3.2±0.6)×10 38 erg s −1 (for a distance of 780 kpc) was found at a 5.3σ confidence level. The best-fit halo template corresponds to two 6-7.5 kpc bubbles symmetrically located perpendicular to the M31 galactic disc, similar to the 'Fermi bubbles' found around the Milky Way centre.

We have detected new components in stationary emission lines of SS 433; these are the superbroad components that are low-contrast substrates with a width of 2000-2500 km s −1 in He I λ4922 and Hβ and 4000-5000 km s −1 in He II λ4686. Based on 44 spectra taken during four years of observations from 2003 to 2007, we have found that these components in the He II and He I lines are eclipsed by the donor star; their behavior with precessional and orbital phases is regular and similar to the behavior of the optical brightness of SS 433. The same component in Hβ shows neither eclipses nor precessional variability. We conclude that the superbroad components in the helium and hydrogen lines are different in origin. Electron scattering is shown to reproduce well the superbroad component of Hβ at a gas temperature of 20-35 kK and an optical depth for Thomson scattering τ ≈ 0.25-0.35. The superbroad components of the helium lines are probably formed in the wind from the supercritical accretion disk. We have computed a wind model based on the concept of Shakura-Sunyaev supercritical disk accretion. The main patterns of the He II line profiles are well reproduced in this model: not only the appearance of the superbroad component but also the evolution of the central two-component part of the profile of this line during its eclipse by the donor star can be explained.

Theories of galaxy formation predict the existence of extended gas halo around spiral galaxies. If there are 10-100 nG magnetic fields at several ten kpc distances from the galaxies, extended galactic cosmic ray (CR) haloes could also exist. Galactic CRs can interact with the tenuous hot halo gas to produce observable gamma-rays. We have performed search for a gamma-ray halo around the M31 galaxy -the closest large spiral galaxy. Our analysis of almost 7 years of the Fermi LAT data revealed the presence of a spatially extended diffuse emission excess around M31. The data can be fitted using the simplest morphology of a uniformly bright circle. The best fit gave a 4.7σ significance for a 0.9 • (12 kpc) halo with a photon flux of ∼ (3.2 ± 1.0) × 10 −9 cm −2 s −1 and a luminosity of (4.0±1.5)×10 38 erg s −1 in the energy range 0.3-100 GeV. Our results also imply a low level of the flux from the disc of the M31 galaxy (3.3±1.0)×10 −10 cm −2 s −1 . The corresponding gamma-ray luminosity, 5×10 37 erg s −1 is several times smaller than the luminosity of the Milky Way. This difference could be explained by a lower star formation rate in M31: there are less CRs and the level of the ISM turbulence is lower, which in turn leads to a shorter time of CR containment.

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