In the early 1960s, astronomers were puzzled by quasars — sources of intense radio emission that seemed to be stars, but had unintelligible optical spectra. In 1963, Maarten Schmidt solved the puzzle by recognizing the Balmer lines of hydrogen, strongly redshifted, in the spectrum of the quasar 3C 273. Schmidt reached the "most direct and least objectionable" conclusion, that 3C 273 was no star, but the enormously bright nucleus of a distant galaxy.

3C 273: A Star-like Object with Large Red-Shift

Since the observational detection of black holes is inherently difficult, it is important to begin with a clear idea of the general possibilities for such detection as they seem practical from observations of existing type. FIGURE 1 shows a schematic view of the paths leading to this objective. The primary attribute by which we hope to recognize a black hole is its gravitational mass, which is discernible through its effect on macroscopic bodies (orbital motions) or microscopic bodies (compressional heating with emission of x-rays). The combination of large mass, small radius, and small luminosity constitutes the unique signature of a massive black hole, and there do exist invisible components to numerous binaries that may fulfill these conditions. Since other small objects (white dwarfs and neutrons stars) cannot have masses exceeding about 1.5 Mo, large mass means M 2 2 Mo. The notions of small radius and small luminosity can be taken in general to mean that these quantities are small compared with the values expected for any conceivable stable astronomical body that could be present. Those binaries for which fairly serious black hole arguments have been made to date are e Aur,\"34,10 p Lyrae,3,\" Cygnus X1 = HDE 226868,',e and BM Ori.\" Space does not permit discussion of the merits of these cases, although counterarguments have been made regarding ,8 Lyrae',* and might well be made against Cyg X-1 and BM Ori. It has been predicted for most of a decade, if not longer, that black holes in binary systems might be x-ray sources. Indeed, Cygnus X-1 may be one such case. However, x-rays are not the only evidence that might uncover collapsed stars, and it is fortunate that this is so because of the intrinsic difficulty of unambiguous identification of a black hole. That is, we would like, if at all possible, to have several independent demonstrations that a black hole is present. A possible path is offered by spectroscopic and photometric observations of certain unusual binaries that have long histories of observational peculiarities and, in addition, have massive invisible secondary components. It would be quite convincing if we could travel both paths of FIGURE 1 for some particular binary system. For example, E Aur might turn on as an x-ray source, or Cyg X-1 might, on detailed examination, show properties similar to those of BM Ori. An appreciation for the left path of FIGURE 1 (optical observations) may be gained through consideration of the eclipsing binary E Aurigae. The most striking peculiarity of e Aur is that the eclipse is apparently total (flat bottom)-yet the spectrum of the eclipsed component remains visible at all times. This, of course, proves that the eclipse really is not total, so we must find another explanation. It can easily be shown' that the eclipse also is not annular and that the disk models by Huang,' by Kopal,5 and by Cameron' are not compatible with the observations.'\" Useful observations of E Aur have been made since 1848, and models for the eclipse mechanism have been offered over the past 40 years. To explain

Black holes in binary systems

High-contrast adaptive optics imaging is a powerful technique to probe the architectures of planetary systems from the outside-in and survey the atmospheres of self-luminous giant planets. Direct imaging has rapidly matured over the past decade and especially the last few years with the advent of high-order adaptive optics systems, dedicated planet-finding instruments with specialized coronagraphs, and innovative observing and post-processing strategies to suppress speckle noise. This review summarizes recent progress in high-contrast imaging with particular emphasis on observational results, discoveries near and below the deuterium-burning limit, and a practical overview of large-scale surveys and dedicated instruments. I conclude with a statistical meta-analysis of deep imaging surveys in the literature. Based on observations of 384 unique and single young ($\approx$5--300~Myr) stars spanning stellar masses between 0.1--3.0~\Msun, the overall occurrence rate of 5--13~\Mjup \ companions at orbital distances of 30--300~AU is 0.6$^{+0.7}_{-0.5}$\% assuming hot-start evolutionary models. The most massive giant planets regularly accessible to direct imaging are about as rare as hot Jupiters are around Sun-like stars. Dividing this sample into individual stellar mass bins does not reveal any statistically-significant trend in planet frequency with host mass: giant planets are found around 2.8$^{+3.7}_{-2.3}$\% of BA stars, $<$4.1\% of FGK stars, and $<$3.9\% of M dwarfs. Looking forward, extreme adaptive optics systems and the next generation of ground- and space-based telescopes with smaller inner working angles and deeper detection limits will increase the pace of discovery to ultimately map the demographics, composition, evolution, and origin of planets spanning a broad range of masses and ages.

https://iopscience.iop.org/article/10.1088/1538-3873/128/968/102001/pdf

Imaging Extrasolar Giant Planets

The universe is so much bigger than the bit we can see in our observable bubble   and we know nothing about what lies out there in the cosmos. The challenge is so  much bigger than is shown on this chart and it is evident that we really only  understand a tiny fraction of the whole cosmos.  Theories ​for Dark Energy have been around for a long time but we still have no  satisfactory explanation. None of the theories are adequate, some are clearly wrong    – an error of 10^120 is a really big miss!  Scott Dodelson of Carnegie Mellon University, one of the lead scientists behind the  Dark Energy Survey’s new map of cosmic structure says:

Dark Energy

The requirement of intermediate line component in the recently observed spectra of several AGNs points to possibility of the existence of a physically separate region between broad line region (BLR) and narrow line region (NLR). In this paper we explore the emission from intermediate line region (ILR) by using the photoionization simulations of the gas clouds distributed radially from the AGN center. The gas clouds span distances typical for BLR, ILR and NLR, and the apperance of dust at the sublimation radius is fully taken into accout in our model. Single cloud structure is calculated under the assumption of the constant pressure. We show that the slope of the power law cloud density radial profile does not affect the existence of ILR in major types of AGN. We found that the low ionization iron line, Fe~II, appears to be highly sensitive for the presence of dust and therefore becomes potential tracer of dust content in line emitting regions. We show that the use of disk-like cloud density profile computed at the upper part of the accretion disc atmosphere reproduces the observed properties of the line emissivities. In particular, the distance of H${\beta}$ line inferred from our model agrees with that obtained from the reverberation mapping studies in Sy1 galaxy NGC 5548.

Intermediate line emission in AGN: the effect of gas density prescription

The origin of the broad line region in active galactic nuclei

Weak emission line quasars are a rare and puzzling group of objects. In this paper we present one more object of this class found in the Sloan Digital Sky Survey (SDSS). The quasar SDSS J094533.99+100950.1, lying at z = 1.66, has practically no C IV emission line, a red continuum very similar to the second steepest of the quasar composite spectra of Richards et al., is not strongly affected by absorption and the Mg II line, although relatively weak, is strong enough to measure the black hole mass. The Eddington ratio in this object is about 0.45, and the line properties are not consistent with the trends expected at high accretion rates. We propose that the most probable explanation of the line properties in this object, and perhaps in all weak emission line quasars, is that the quasar activity has just started. A disk wind is freshly launched so the low ionization lines which form close to the disk surface are already observed but the wind has not yet reached the regions where high ionization lines or narrow line components are formed. The relatively high occurrence of such a phenomenon may additionally indicate that the quasar active phase consists of several sub-phases, each starting with a fresh build-up of the Broad Line Region.

SDSS J094533.99+100950.1 - the remarkable weak emission line quasar

The nature of dark energy, driving the accelerated expansion of the Universe, is one of the most important issues in modern astrophysics. In order to understand this phenomenon, we need precise astrophysical probes of the universal expansion spanning wide redshift ranges. Quasars have recently emerged as such a probe, thanks to their high intrinsic luminosities and, most importantly, our ability to measure their luminosity distances independently of redshifts. Here we report our ongoing work on observational reverberation mapping using the time delay of the Mg II line, performed with the South African Large Telescope (SALT).

K. Hryniewicz

Papers

Intermediate line emission in AGN: the effect of gas density prescription

The origin of the broad line region in active galactic nuclei

SDSS J094533.99+100950.1 - the remarkable weak emission line quasar

Tracing dark energy with quasars