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We present a review of the topics of X‐ray stellar tidal disruption events (TDEs) and changing‐look active galactic nuclei (AGN). Stars approaching a supermassive black hole (SMBH) can be tidally disrupted and accreted. TDEs were first discovered in the X‐ray regime and appear as luminous, giant‐amplitude flares from inactive galaxies. The early X‐ray observations with ROSAT also established the extreme X‐ray spectral softness of these events with temperatures of order 50–100 eV that continues to be seen in the majority of more recently identified events. While the majority of X‐ray TDEs has been identified from inactive galaxies and some showed the highest amplitudes of variability recorded from galaxy cores (amplitudes exceeding factors of 1000–6000), a small fraction of AGN has been found to be highly variable as well. In AGN, this so‐called changing‐look phenomenon often comes with a strong change in the optical broad emission lines, leading to Seyfert‐type changes between class 1 and class 2. These two forms of activity represent the extremes of variability among active and quiescent galaxies, and have opened up a new window to understanding accretion physics under extreme conditions. Finally, we introduce the term "frozen‐look AGN" to describe systems that show constant line emission despite dramatic changes in the observed ionizing continuum. These systems are best explained by strong changes of absorption along our line‐of‐sight. [ABSTRACT FROM AUTHOR]

We are carrying out the densest and longest multiyear, multiwavelength monitoring project of OJ 287 ever done. The project MOMO (Multiwavelength Observations and Modeling of OJ 287) covers wavelengths from the radio to the high‐energy regime. A few selected observations are simultaneous with those of the Event Horizon Telescope (EHT). MOMO aims at understanding disk‐jet physics and at testing predictions of the binary black hole scenario of OJ 287. Here, we present a discussion of extreme outburst and minima states in context, and then focus on the recent flux and spectral evolution between 2021 and May 2022, including an ongoing bright radio flare. Further, we show that there is no evidence for precursor flare activity in our optical–UV–X‐ray light curves that would be associated with any secondary supermassive black hole (SMBH) disk impact and that was predicted to start as thermal flare on December 23, 2021. [ABSTRACT FROM AUTHOR]

This paper is devoted to the memory of Dr Victor Afanasiev and his immense legacy. The report highlights the capabilities of two new instruments tested at the 1‐m Zeiss‐1000 telescope of SAO RAS: The Stokes Polarimeter (StoP) and the MAGIC focal reducer. Optimized for the study of active galactic nuclei, methodically, these instruments are suitable for a wide range of small telescope tasks. The fields of view of StoP and MAGIC are 6′ and 13′ for direct images, respectively. The StoP device allows one to conduct photometric observations and polarimetric ones with a double Wollaston prism; the spectral mode was added to MAGIC. For a star‐like target up to 14 mag in medium‐band filters with a seeing of 1″ for 20 min of total exposure, the photometry accuracy is better than 0.01 mag and the polarization accuracy is better than 0.6%. The available spectral range obtained with the volume phase holographic grating in MAGIC is 4,000–7,200 AA with a dispersion of 2A/px. StoP and MAGIC received the first light in 2020 and are used in test mode at the Zeiss‐1000. The report discusses the first results obtained by the authors with new instruments, as well as further prospects. [ABSTRACT FROM AUTHOR]

Most bulge‐dominated galaxies host black holes (BHs) with masses that tightly correlate with the masses of their bulges. This may indicate that the BHs may regulate galaxy growth, or vice versa, or that they may grow in lock‐step. The quest to understand how, when, and where those BHs formed motivates much of extragalactic astronomy. Here we focus on a population of galaxies with active BHs in their nuclei (active galactic nuclei [AGN]), that are fully or partially hidden by dust and gas: The emission from the broad line region is either completely or partially obscured with a visual extinction of one or above. This limit, though not yet precise, appears to be the point at which the populations of AGN may evolve differently. We highlight the importance of finding and studying those dusty AGN at redshifts between one and three, the epoch when the universe may have gone through its most dramatic changes. We emphasize the need for future large multiplexed spectroscopic instruments that can perform dedicated surveys in the optical and near‐infrared to pin down the demographics of such objects, and study their reddening properties, star‐formation histories, and excitation conditions. These key studies will shed light on the role of BHs in galaxy evolution during the epoch of peak growth activity. [ABSTRACT FROM AUTHOR]

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, αthick>+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. [ABSTRACT FROM AUTHOR]

In this paper, we investigate radio properties of active galaxies taken from the reference [Véron‐Cetty & Véron (2010)] catalog. The galaxies are limited to magnitudes in the range of 12m − 19m. We have cross‐correlated the list with radio catalogs and selected those galaxies that have data on six or more radio fluxes at different wavelengths. As a result, we have 198 galaxies that satisfy these conditions. Using SDSS DR15, we have obtained 96 spectroscopic identifications of the 198 objects. After the classification, 85% of the 96 objects have changed their types. Available data on the classification of these objects and our classification showed that 56.7% of them are Seyfert galaxies. For all the objects, we have built radio spectra and estimated radio spectral indices. As a result, we obtain α‾=−0.6089±0.056(α‾Seyfert=−0.6013±0.027, α‾LINER=−0.5955±0.025, α‾HII=−0.6672±0.039, α‾Comp.=−0.7128±0.043). We discuss the radio properties of active galaxies based on their radio spectral indices. [ABSTRACT FROM AUTHOR]

Observations of the X-ray sky after the next decade will most likely be dominated by Athena (Advanced Telescope for High ENergy Astrophysics), the second large mission of ESA's Cosmic Vision 2015-2035 programme. Athena has been conceived to address the "Hot and Energetic Universe" science theme, which focuses on the assembly and evolution of hot baryons in cosmic structures as well as the physics and energetic output generated by accreting super-massive black holes, along with the relationship between the two processes, dubbed cosmic feedback. Thanks to its transformational capabilities, Athena will enable bringing all of today's XMM-Newton's hot science topics to a new stage. In this paper, we summarize the Athena science case, science requirements, as well as the expected mission performance, mission concept, and its status. [ABSTRACT FROM AUTHOR]