On the photometric error calibration for the `differential light curves' of point-like Active Galactic Nuclei

It is important to quantify the underestimation of rms photometric errors returned by the commonly used \emph APPHOT algorithm in the \emph IRAF software, in the context of differential photometry of point-like AGN, because of the crucial role it plays in evaluating their variability properties. Published values of the underestimation factor, $\eta$, 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 $\eta$ = 1.54$\pm$0.05 which is close to our recently reported value of $\eta$ = 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.Comment: 20 pages, 3 figures and 1 table; accepted for publication in JAp

uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

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)'. EISERS conceivably form a special sub-class 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 $\alpha=+2.5$ (S($\nu$) $\propto$ $\nu^{\alpha}$). We have 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$=0.4040) implies an optical depth of 0.178$\pm$0.02 corresponding to an HI column density of 34.8$\pm$2.9 $\times$10$^{20}$ 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 GPC/CSS 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.Comment: Accepted for publication in A&

GMRT observations of extragalactic radio sources with steeply inverted spectra

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 $\alpha_c$ = +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 $\alpha_c$ 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.Comment: Accepted for publication in MNRAS. Comments are welcom

Extragalactic radio sources with sharply inverted spectrum at metre wavelengths

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).Comment: 8 pages, 2 figures, accepted for publication in MNRA

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

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.Comment: Accepted for publication in MNRAS. Comments are welcom