Detection of large-scale synchrotron radiation from the molecular envelope of the Sgr B cloud complex at the Galactic centre

We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival Green Bank Telescope (GBT), Murchison Widefield Array, and Very Large Array (VLA) images at 333, 88, and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud associated with the halo of the Sgr B complex on a scale of similar to 60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic ray electrons. Correlation of the Fe I 6.4 keV K alpha line and synchrotron emission provides compelling evidence that the low energy cosmic ray electrons are responsible for producing the K alpha line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has a mean spectral index alpha similar to -1 +/- 1, which gives the magnetic field strength similar to 100 mu G for cloud densities n(H) = 10(4)-10(5) cm(-3), and estimated cosmic ray ionization rates between 10(-13) and 10(-14) s(-1). Furthermore, the energy spectrum of primary cosmic ray electrons is constrained to be E-3 +/- 1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and gamma-ray emission detected from this cloud. These measurements have important implications on the role that high cosmic ray electron fluxes at the Galactic centre play in production of radio synchrotron emission, the Fe I K alpha line emission at 6.4 keV, and similar to GeV gamma-ray emission throughout the Central Molecular Zone.