High-energy emission from galactic sources
At the opposite of the radio domain in the electromagnetic spectrum, high-energy photons constitute another highly valuable source of information for investigating the physics of astronomical sources. In this group, stellar sources constitute targets of prime importance.
Relevant sources of high-energy emission are notably hot, massive stars (OB-stars and their evolved counterparts referred to as Wolf-Rayet stars). They produce strong stellar winds accelerated up to velocities of a few km/s, through which they undergo a significant mass loss. These winds are not smooth and give rise to shocks between different parts of the outflow that are not traveling at the same speed. These shocks heat a part of the wind plasma up to temperatures of a few million K, leading to a significant X-ray emission that can be measured by in-orbit X-ray satellites such as XMM-Newton or Chandra, for instance. In addition, when massive stars are part of a binary, or higher multiplicity system, their powerful winds are likely to collide. These wind collision produce additional shocks, at higher speeds, heating the shocked plasma up to temperatures of tens of million K, leading to an addition contribution to the soft X-ray emission (below 10 keV) from these massive systems. Such systems are frequently referred to as colliding-wind binaries (CWBs).
In CWBs, the X-ray emission coming from the wind-wind interaction region may be variable, depending on the eccentricity of the orbit. On top of that, soft X-rays are likely to be absorbed by the stellar wind material, leading to an additional orbital modulation depending on the orientation of the system as seen by the observer. Their investigation requires thus to collect time series of their X-ray emission to monitor their behaviour as a function of the orbital phase.
Projected orbit of WR140 along with X-ray spectra obtained with XMM-Newton at various orbital phases. Figure taken from De Becker et al. 2011, BSRSL, 80, 653.
On top of the thermal X-ray emission from the hot plasma heated by shocks, non-thermal X-ray emission can be produced in CWBs provided they accelerate electrons to relativistic velocities. In this case, the emission process is inverse Compton scattering, where UV and visible photons are up-scattered to high energies thanks to an energy transfer from relativistic electrons. This has been measured in a very few systems, such as the exceptional WR+WR binary Apep. The information on the non-thermal emission from these systems at high energies is of prime importance for the study of galactic particle accelerators.
Soft and hard X-ray spectrum of Apep obtained with NuStar showing a significant hard X-ray component attributable to inverse Compton scattering. Figure taken from del Palacio et al. 2023, A&A, 672, A109.
When higher energies (gamma-rays) are considered, galactic sources fit clearly in the category of particle accelerators, deserving complementary studies in the radio domain.
Beside massive stars, activities in this group also include the investigation of so-called cataclysmic variables, i.e. binary systems made of a white dwarf and another still evolving regular star. The X-ray emission from such systems is driven by the accretion of stellar material onto the compact companion’s surface. Finally, the investigation of some fields around massive stars leads to the detection and study of other point sources, such as pre-main sequence stars displaying a flaring behaviour indicative of X-ray emission associated to their magnetic activity.
In the general framework of the multi-wavelength investigation of galactic sources, high energies constitute an important source of information that is complementary with the radio domain and additional studies led in the visible and infrared domains by members of this group.
