2019MNRAS.485.2447M

We report quasi-simultaneous Giant Metrewave Radio Telescope observations of seven extragalactic radio sources at 150, 325, 610, and 1400 MHz, in an attempt to accurately define their radio continuum spectra, particularly at frequencies below the observed spectral turnover. We had previously identified these sources as candidates for a sharply inverted integrated radio spectrum whose slope is close to, or even exceeds α_c = +2.5, the theoretical limit due to synchrotron self-absorption (SSA) in a source of incoherent synchrotron radiation arising from relativistic particles with the canonical (i.e. power law) energy distribution. We find that four of the seven candidates have an inverted radio spectrum with a slope close to or exceeding +2.0, while the critical spectral slope α_c is exceeded in at least one case. These sources, together with another one or two reported in very recent literature, may well be the archetypes of an extremely rare class, from the standpoint of violation of the SSA limit in compact extragalactic radio sources. However, the alternative possibility that free-free absorption is responsible for their ultra-sharp spectral turnover cannot yet be discounted.