Extragalactic radio sources with sharply inverted spectrum at metre wavelengths

TL;DR

This study systematically searches for rare extragalactic radio sources with sharply inverted spectra at low frequencies, aiming to identify candidates that challenge standard synchrotron self-absorption models and suggest alternative physical mechanisms.

Contribution

First systematic search for extragalactic radio sources with spectral index ≥ +2.0 at metre wavelengths, identifying candidates with extremely inverted spectra that imply non-standard absorption or electron energy distributions.

Findings

Seven sources with spectral index > +2.0 found in survey overlap region.

Two sources are strong candidates for spectral index > +2.5.

Most sources exhibit Gigahertz-Peaked-Spectrum characteristics.

Abstract

We present the first results of a systematic search for the rare extragalactic radio sources showing an inverted (integrated) spectrum, with spectral index $\alpha \ge +2.0$, a previously unexplored spectral domain. The search is expected to yield strong candidates for $\alpha \ge +2.5$, 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 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 $\alpha > +2.0$. 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).