M87 is a giant elliptical galaxy in the Virgo cluster of galaxies. The radio source has a core which coincides with the nucleus of the galaxy and a jet of emission which is detected from radio to X-ray bands. A supermassive black hole is assumed to be at the centre of M87 which sends out relativistic particles in the form jets along its axis of rotation. Relativistic particles accelerated in a magnetic field, give out synchrotron radiation. The centre is surrounded by an accretion disc, which is the closest that we can probe into a black hole. High resolution observations are needed to examine the nature of the radio emission closest to the centre of M87. An array of millimetre-band telescopes across the globe were used as an interferometer, called the Event Horizon Telescope, (EHT) to probe the nuclear region. The angular resolution of this interferometer array is 25 microarc sec, at a wavelength of 1.3mm and the data was carefully calibrated and imaged. The resulting image shows an asymmetric ring which is consistent with the predictions of strong gravitational lensing of synchrotron emission from hot plasma near the event horizon. In this paper, we review the results of the observations of the radio galaxy, M87, using the Event Horizon Telescope

A non-negligible fraction of quasars are red at optical wavelengths, indicating (in the vast majority of cases) that the accretion disc is obscured by a column of dust which extinguishes the shorter-wavelength blue emission. In this paper we summarise recent work by our group, where we find fundamental differences in the radio properties of SDSS optically selected red quasars. We also present new analyses, using a consistent colour-selected quasar parent sample matched to four radio surveys (FIRST, VLA Stripe 82, VLA COSMOS 3 GHz and LoTSS DR1) across a frequency range 150 MHz-3 GHz and four orders of magnitude in radio flux. We show this enhancement is driven by systems with small-scale radio emission (∼kpc) and peaks around the radio-quiet threshold (defined as the ratio of 1.4 GHz luminosity to 6μm luminosity) across the four radio samples. Exploring the potential mechanisms behind this enhancement, we rule out star-formation and propose either small-scale jets or dusty winds interacting with the interstellar medium; this will be tested in detail using new multi-band uGMRT data. Overall our results cannot be explained with a simple viewing angle hypothesis, and so may point towards red quasars representing a key phase in the evolution of galaxies.

In 2011, by 1.3 mm wavelength VLBI radio wave observations of the SgrA*, Fish, V. L. et al showed that the emissions tightly related to the formation of a black hole shadow have a remarkably large time-varying feature within a region of less than 50 μas. The present paper suggests that the origin of the time variation in the observed emission is due to effects of the orbital motion of the existing super-massive black hole binary orbiting at SgrA* with a period of 2150±2.5 s. This suggestion is based on observations of decameter radio wave pulses from SgrA*. We show a good correlation between the time variation in the coherent flux density of the VLBI results and the time variation model of estimated emission intensities based on the periodic motion of the super-massive black hole binary by applying parameters deduced from the decameter radio wave pulse observation model (DRWP-Model). With further confirmation by Fourier analyses of the potential periodicity of the VLBI data that show the same periods of DRWP Model, we conclude that the time variation detected by the 1.3 mm wavelength radio wave VLBI is evidence of an existing super-massive black hole at Sgr A*.

The search for periodic signals from blazars has become an actively pursued field of research in recent years. This is because periodic signals bring us information about the processes occurring near the innermost regions of blazars, which are mostly inaccessible to our direct view. Such signals provide insights into some of the extreme conditions that take place in the vicinity of supermassive black holes that lead to the launch of the relativistic jets. In addition, studies of characteristic timescales in blazar light curves shed light on some of the challenging issues in blazar physics that includes disk-jet connection, strong gravity near fast rotating supermassive black holes and release of gravitational waves from binary supermassive black hole systems. However, a number of issues associated with the search for quasi-periodic oscillations (QPOs) in blazars e.g., red-noise dominance, modest significance of the detection, periodic modulation lasting for only a couple of cycles and their transient nature, make it difficult to estimate the true significance of the detection. Consequently, it also becomes difficult to make meaningful inferences about the nature of the on-going processes. In this proceedings, results of study focused on searching for QPOs in a number of blazar multi-frequency light curves are summarized. The time series analyses of long term observations of the blazars revealed the presence of year-timescale QPOs in the sources including OJ 287 (optical), Mrk 501 (gamma-ray), J1043+2408 (radio) and PKS 0219 -164 (radio). As likely explanations, we discuss a number of scenarios including binary supermassive black hole systems, Lense–Thirring precession, and jet precession.

Search for periodic signals from blazars has become widely discussed topic in recent years. This is because periodic signals bear imprints of the processes occurring near the innermost regions of blazars which are mostly inaccessible to our direct view. Such signals provide insights into various aspect of blazar studies including disk-jet connection, magnetic field configuration and, more importantly, strong gravity near the supermassive black holes and release of gravitational waves from the binary supermassvie black hole systems. In this work, we report detection of a periodic signal in the radio light curves of the blazar J1043+2408 spanning ~10.5 years. We performed multiple methods of time series analysis, namely, epoch folding, Lomb-Scargle periodogram, and discrete auto-correlation function. All three methods consistently reveal a repeating signal with a periodicity of ~563 days. To robustly account for the red-noise processes usually dominant in the blazar variability and other possible artifacts, a large number of Monte Carlo simulations were performed. This allowed us to estimate a high significance (99.9% local and 99.4% global) against possible spurious detection. As possible explanations, we discuss a number of scenarios including binary supermassive black hole system, Lense-Thirring precession, and jet swing and precession.