2014MNRAS.443.2824G

We present the first results of a systematic search for the rare extragalactic radio sources showing an inverted (integrated) spectrum, with spectral index α ≥ +2.0, a previously unexplored spectral domain. The search is expected to yield strong candidates for α ≥ +2.5, for which the standard synchrotron self-absorption (characterized by a single power-law energy distribution of relativistic electron population) would not be a plausible explanation, even in an ideal case of a perfectly homogeneous source of incoherent synchrotron radiation. Such sharply inverted spectra, if found, would require alternative explanations, e.g. free-free absorption, or non-standard energy distribution of relativistic electrons which differs from a power law (e.g. Maxwellian). The search was carried out by comparing two sensitive low-frequency radio surveys made with sub-arcminute resolution, namely, the Westerbork In the Southern Hemisphere (WISH) survey at 352 MHz and TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains 7056 WISH sources classified as `single' and brighter than 100 mJy at 352 MHz. We focus here on the seven of these sources for which we find α > +2.0. Two of these are undetected at 150 MHz and are particularly good candidates for α > +2.5. Five of the seven sources exhibit a `Gigahertz-Peaked-Spectrum'.