Extragalactic radio sources with sharply inverted spectrum at metre wavelengths

摘要

We present the first results of a systematic search for the rareextragalactic radio sources showing an inverted (integrated) spectrum, withspectral index $\alpha \ge +2.0$, a previously unexplored spectral domain. Thesearch is expected to yield strong candidates for $\alpha \ge +2.5$, for whichthe standard synchrotron self-absorption (characterized by a single power-lawenergy distribution of relativistic electron population) would not be aplausible explanation, even in an ideal case of a perfectly homogeneous sourceof incoherent synchrotron radiation. Such sharply inverted spectra, if found,would require alternative explanations, e.g., free-free absorption, ornon-standard energy distribution of relativistic electrons which differs from apower-law (e.g., Maxwellian). The search was carried out by comparing two sensitive low-frequency radiosurveys made with sub-arcminute resolution, namely, the WISH survey at 352 MHzand TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains7056 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 $\alpha > +2.0$.Two of these are undetected at 150 MHz and are particularly good candidates for$\alpha > +2.5$. Five of the seven sources exhibit a`Gigahertz-Peaked-Spectrum' (GPS).