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

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 α > +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 1.03π 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 α 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 α c . Nine of the sources have GPS type radio spectra. The parsec-scale radio structural information available for the sample is also summarised.A few years ago, we combined the TIFR GMRT SKY SURVEY (TGSS/DR5) at 150 MHz with the 352 MHz WISH survey, to search for extragalactic radio sources whose

Quasars That Have Transitioned from Radio-quiet to Radio-loud on Decadal Timescales Revealed by VLASS and FIRST

“…Our sources have optically thick spectral index constraints that are consistent with SSA, though improved spectral coverage at low frequencies (<1 GHz) will ultimately be needed (Callingham et al 2015;Collier et al 2018;Keim et al 2019;Mhaskey et al 2019aMhaskey et al , 2019b. The identification of FFA associated with any of the newly radio-loud AGNs in our sample would be of particular interest in the context of galaxy evolution since simulations have shown that it may arise from jet−ISM interactions (Bicknell et al 1997(Bicknell et al , 2018.…”

“…In a future study, we will incorporate new data from forthcoming VLA and Giant Metre-wave Radio Telescope (GMRT) observations spanning a broader frequency range (the full contiguous frequency range of the VLA from 1 to 50 GHz and deep measurements below 1 GHz using the VLA and GMRT). This will enable a more rigorous radio SED modeling analysis that will allow us to test whether SSA or FFA is responsible for the observed spectral curvature (Mhaskey et al 2019a(Mhaskey et al , 2019b.…”

Compact steep-spectrum and peaked-spectrum radio sources

“…FFA has been suggested to occur in a disk (Marr et al, 2001), a torus (Peck et al, 1999;Kameno et al, 2000Kameno et al, , 2001Kameno et al, , 2003, and in ambient clouds shocked by the expanding radio source, producing a power-law distribution of electron density in the ionized ambient clouds (Bicknell et al, 1997(Bicknell et al, , 2018. There is supporting evidence from individual sources whose spectra are better fit by FFA rather than by SSA (e.g., Bicknell et al, 1997;Peck et al, 1999;Kameno et al, 2000Kameno et al, , 2001Kameno et al, , 2003Marr et al, 2001Marr et al, , 2014Callingham et al, 2015;Mhaskey et al, 2019a) and see Krishna et al (2014), Callingham et al (2017) and Mhaskey et al (2019b) for additional candidates. The FFA is generally consistent with an ionized medium with a temperature T∼ 10 4 K, path length ∼ 1 − 100 pc, and an electron density n e ∼ 10 3 − 10 4 cm −3 .…”

The peaked spectrum and compact steep spectrum sources: Issues and opportunities

“…There are some PS sources for which free‐free absorption (FFA) is a better fit to the radio spectrum turnover than synchrotron self‐absorption (Bicknell et al 1997; Callingham et al 2015; Kameno et al 2000, 2001, 2003; Marr et al 2001, 2014; Mhaskey et al 2019; Peck et al 1999; Tingay et al 2015). The required dense clouds of ionized thermal electrons may be produced by shock ionization of dense gas clouds by the expanding radio source (Bicknell et al 1997) or by clouds engulfed by the cocoon (Begelman 1999).…”