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 stro
ng 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).
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 p
lays 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.
We discuss the nature of the multi-component radio continuum and HI emission associated with the nearby galaxy group comprised of two dominant ellipticals, NGC 5898 and NGC 5903, and a dwarf lenticular ESO514-G003. Striking new details of radio emiss
ion are unveiled from the 2nd Data Release of the ongoing TIFR.GMRT.SKY.SURVEY (TGSS) which provides images with a resolution of ~ 24 arcsec x 18 arcsec 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 HI emission, the latter showing huge HI trails originating from the vicinity of NGC 5903 where HI 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 SO galaxy ESO514-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 ({alpha} ~ -1.5) and to envelope a network of radio continuum filaments bearing a spatial relationship to the HI 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 ESO514-G003, are also the only galaxies that are seen to be connected to an HI 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 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 havi
ng 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.
