Towards building a first northern-sky sample of 'Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS)'

TL;DR

This paper extends the search for extremely inverted spectrum extragalactic radio sources (EISERS) to the northern hemisphere, identifying 15 candidates with unique spectral properties that challenge standard models of particle acceleration.

Contribution

It introduces a new northern-sky sample of EISERS candidates using WENSS and ADR-TGSS surveys, expanding the known population and providing detailed spectral and structural data.

Findings

Identified 15 EISERS candidates with inverted spectra > +2.5

Most candidates are associated with known quasars

Eight sources exhibit GPS-type spectra

Abstract

We present here an extension of our search for EISERS (Extremely Inverted Spectrum Extragalactic Radio Sources) to the northern 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. A list of 15 EISERS candidates is presented here. It was assembled by applying a sequence of selection filters, starting with the two available large-sky radio surveys, namely the WENSS (325 MHz) and the ADR-TGSS (150 MHz). These surveys offer the twin advantages of being fairly deep (typical rms $<$ 10 mJy/beam) and having a sub-arcminute resolution. Their zone of overlap spreads over 1.3$\pi$ steradian in the northern hemisphere. Radio spectra are presented for the entire sample of 15 EISERS candidates, of which 8 spectra are of GPS type. Eleven members of the sample are associated with previously known quasars. Information on the parsec-scale radio structure, available for several of these sources, is also summarized.