Constraining the most energetic cosmic rays in the Fermi Bubbles with LEM

After more than 10 years since their discovery, the nature of the Fermi Bubbles remains unclear. Extending above and below the Galactic plane, the bubbles are nearly symmetrically aligned with the Galactic Center and shine bright in the GeV gamma-ray band. The gamma-rays are believed to be the inverse Compton emission produced by the population of cosmic ray electrons that also generates the synchrotron “microwave haze” observed by WMAP and Planck. In some models, the most energetic of those cosmic rays may generate detectable synchrotron emission in the soft X-rays, highly dependent on the cosmic ray power spectrum at energies above those directly probed by Fermi and on the magnetic fields in the bubbles. The Line Emission Mapper (LEM), with its ability to resolve the bright emission lines from the Milky Way’s 10⁶-10⁷ K plasmas that dominate the soft X-ray sky, may be able to detect the X-ray continuum synchrotron emission from those cosmic rays. LEM will cover the entire Fermi Bubbles as part of its shallow all-sky survey. LEM constraints – or positive detection – will provide essential insights into the energetics of the electron population and the magnetic field strength within the bubbles.