Inverse Compton origin of the hard X-ray and soft gamma-ray emission from the Galactic ridge

A recent redetermination of the nonthermal component of the hard X-ray to soft gamma-ray emission from the Galactic ridge, using the SPI instrument on the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), is shown to be well reproduced as inverse Compton emission from the interstellar medium. Both cosmic-ray primary electrons and secondary electrons and positrons contribute to the emission. The prediction uses the GALPROP model and includes a new calculation of the interstellar radiation field. This may solve a long- standing mystery of the origin of this emission, and potentially opens a new window on Galactic cosmic rays.