GMRT observations of a first sample of Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) candidates in the Northern sky

TL;DR

This study extends the search for Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) in the northern sky, confirming two candidates with ultra-inverted spectra and providing detailed spectral and structural analysis.

Contribution

First GMRT observations of northern EISERS candidates, confirming two with spectra steeper than the theoretical SSA limit, and characterizing their radio structures.

Findings

Two EISERS confirmed with spectral slope > +2.5

Three sources have spectra near the critical slope

Nine sources exhibit GPS-type radio spectra

Abstract

We present an extension of our search for Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) to the northern celestial hemisphere. With an inverted radio spectrum of slope $\alpha$ > +2.5, these rare sources would either require a non-standard particle acceleration mechanism (in the framework of synchrotron self-absorption hypothesis), or a severe free-free absorption which attenuates practically all of their synchrotron radiation at metre wavelengths. By applying a sequence of selection filters, a list of 15 EISERS candidates is extracted out by comparing two large-sky radio surveys, WENSS (325 MHz) and TGSS-ADR1 (150 MHz), which overlap across 1.03$\pi$ steradian of the sky. Here we report quasi-simultaneous GMRT observations of these 15 EISERS candidates at 150 MHz and 325 MHz, in an attempt to accurately define their spectra below the turnover frequency. Out of the 15 candidates observed, two are confirmed as EISERS, since the slope of the inverted spectrum between these two frequencies is found to be significantly larger than the critical value $\alpha_c$ = +2.5: the theoretical limit for the standard case of synchrotron self-absorption (SSA). For another 3 sources, the spectral slope is close to, or just above the critical value $\alpha_c$. Nine of the sources have GPS type radio spectra. The parsec-scale radio structural information available for the sample is also summarised.