Abstract. It is important to quantify the underestimation of rms photometric errors returned by the commonly used APPHOT algorithm in the IRAF software, in the context of differential photometry of pointlike AGN, because of the crucial role it plays in evaluating their variability properties. Published values of the underestimation factor, η, using several different telescopes, lie in the range 1.3 -1.75. The present study aims to revisit this question by employing an exceptionally large data set of 262 differential light curves (DLCs) derived from 262 pairs of non-varying stars monitored under our ARIES AGN monitoring program for characterizing the intra-night optical variability (INOV) of prominent AGN classes. The bulk of these data were taken with the 1-m Sampurnanad Telescope (ST). We find η = 1.54±0.05 which is close to our recently reported value of η = 1.5. Moreover, this consistency holds at least up to a brightness mismatch of 1.5 mag between the paired stars. From this we infer that a magnitude difference of at least up to 1.5 mag between a point-like AGN and comparison star(s) monitored simultaneously is within the same CCD chip acceptable, as it should not lead to spurious claims of INOV.

We report quasi-simultaneous GMRT observations of seven extragalactic radio sources at 150, 325, 610 and 1400 MHz, in an attempt to accurately define their radio continuum spectra, particularly at frequencies below the observed spectral turnover. We had previously identified these sources as candidates for a sharply inverted integrated radio spectrum whose slope is close to, or even exceeds α c = +2.5, the theoretical limit due to synchrotron self-absorption (SSA) in a source of incoherent synchrotron radiation arising from relativistic particles with the canonical (i.e., power-law) energy distribution. We find that four out of the seven candidates have an inverted radio spectrum with a slope close to or exceeding +2.0, while the critical spectral slope α c is exceeded in at least one case. These sources, together with another one or two reported in very recent literature, may well be the archetypes of an extremely rare class, from the standpoint of violation of the SSA limit in compact extragalactic radio sources. However, the alternative possibility that free-free absorption is responsible for their ultra-sharp spectral turnover cannot yet be discounted.

No abstract

We discuss the nature of the multicomponent radio continuum and H i emission associated with the nearby galaxy group comprised of two dominant ellipticals, NGC 5898 and NGC 5903, and a dwarf lenticular ESO 514−G003. Striking new details of radio emission are unveiled from the second Data Release of the ongoing TIFR GMRT Sky Survey (TGSS) which provides images with a resolution of ∼24 × 18 arcsec2 and a typical rms noise of 5 mJy at 150 MHz. Previous radio observations of this compact triplet of galaxies include images at higher frequencies of the radio continuum as well as H i emission, the latter showing huge H i trails originating from the vicinity of NGC 5903 where H i is in a kinematically disturbed state. The TGSS 150‐MHz image has revealed a large asymmetric radio halo around NGC 5903 and also established that the dwarf S0 galaxy ESO 514−G003 is the host to a previously known bright double radio source. The radio emission from NGC 5903 is found to have a very steep radio spectrum (α∼−1.5) and to envelope a network of radio continuum filaments bearing a spatial relationship to the H i trails. Another noteworthy aspect of this triplet of early‐type galaxies highlighted by the present study is that both its radio‐loud members, namely NGC 5903 and ESO 514−G003, are also the only galaxies that are seen to be connected to an H i filament. This correlation is consistent with the premise that cold gas accretion is of prime importance for triggering powerful jet activity in the nuclei of early‐type galaxies.

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

We report the detection of HI 21-cm absorption in a member of the rare and recently discovered class of compact radio sources: extremely inverted spectrum extragalactic radio sources (EISERS). The EISERS conceivably form a special subclass of the inverted spectrum radio galaxies since the spectral index of the optically thick part of the spectrum for these sources crosses the synchrotron self-absorption limit of α = +2.5 (S(ν) ∝ να). We searched for HI absorption in two EISERS using the recently upgraded Giant Metrewave Radio Telescope (uGMRT) and detected an absorption feature in one of them. The strong associated HI absorption detected against the source J1209−2032 (z = 0.4040) implies an optical depth of 0.178 ± 0.02, corresponding to an HI column density of 34.8 ± 2.9 × 1020 cm−2, for an assumed HI spin temperature of 100 K and covering factor of 1. This is among the highest known optical depth and HI column densities found for compact radio sources of peaked spectrum type and supports the free-free absorption model for the steeply inverted radio spectrum of this source. For the other source, J1549+5038 (z = 2.171), no HI absorption was detected in our observations.

We point out that the remarkable linearity of the ultra-steep radio spectra of high redshift radio galaxies reflects a previously reported general trend for powerful radio galaxies, according to which the spectral curvature is lesser for sources having steeper spectra (measured near rest-frame 1 GHz). We argue based on existing theoretical and observational evidence that it is premature to conclude that the particle acceleration mechanism in sources having straight, ultra-steep radio spectra gives rise to an ultra-steep injection spectrum of the radiating electrons. In empirical support to this we show that the estimated injection spectral indices, available for a representative sample of 35 compact steep spectrum (CSS) radio sources are not correlated with their rest-frame (intrinsic) rotation measures, which are known to be typically large, indicating a dense environment, as is also the case for high-z radio galaxies.

We report our ongoing search for extremely inverted spectrum compact radio galaxies, for which the defining feature in the radio spectrum is not the spectral peak, but instead the slope of the spectrum (𝛼) in the high-opacity (i.e., lower frequency) part of the radio spectrum. Specifically, our focus is on the spectral regime with a spectral index, 𝛼 thick > +2.5. The motivation for our study is, first, extragalactic sources with such extreme spectral index are extremely rare, because of the unavailability of the right combination of sensitivity and resolution over a range of low frequencies. The second reason is more physically motivated since α = +2.5 is the maximum slope theoretically possible for a standard radio source emitting synchrotron radiation. Therefore such sources could be the test-bed for some already proposed alternative scenarios for synchrotron self-absorption (SSA), like the free-free absorption (FFA) highlighting the importance of jet-ISM interaction in the radio galaxy evolution.