The appearance of active galactic nuclei (AGN) depends so strongly on orientation that our current classification schemes are dominated by random pointing directions instead of more interesting physical properties. Light from the centers of many AGN is obscured by optically thick circumnuclear matter and in radio-loud AGN, bipolar jets emanating from the nucleus emit light that is relativistically beamed along the jet axes. Understanding the origin and magnitude of radiation anisotropies in AGN allows us to unify different classes of AGN; that is, to identify each single, underlying AGN type that gives rise to different classes through different orientations. This review describes the unification of radio-loud AGN, which include radio galaxies, quasars, and blazars. We describe the classification and properties of AGN and summarize the evidence for anisotropic emission. We outline the two most plausible unified schemes for radio-loud AGN, one linking quasars and luminous radio galaxies and another linking BL~Lac objects and less luminous radio galaxies. Using the formalism appropriate to samples biased by relativistic beaming, we show the population statistics for two schemes are in accordance with available data. We analyze the possible connections between low- and high-luminosity radio-loud AGN. We review potential difficulties with unification and conclude that none currently constitutes a serious problem. We discuss likely complications to unified schemes that are suggested by realistic physical considerations; these will be important to consider when more comprehensive data for larger complete samples become available. We conclude with a list of the ten questions we believe are the most pressing in this field.Comment: 88 pages, latex file, uses aaspp.sty macro (available via ftp from ftp://aas.org/pubs/aastex/). Accompanying 22 figures and 3 tables available at http://itovf2.roma2.infn.it/padovani/review.html. (Abstract is abridged.) The only change is that the revised version indicates this paper is an invited review for PASP, in press, September 1995 issu

This Special Issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multi--band imaging data obtained with the {\it Hubble Space Telescope} ({\it HST}) and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 square arcminutes in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the {\it Chandra} X-ray Observatory (\cxo) and ground-based facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory (ESO) and from the Kitt Peak National Observatory (KPNO). Deep observations with the {\it Space Infrared Telescope Facility} (SIRTF) are scheduled. Reduced data from all facilities are being released worldwide within three to six months of acquisition. Together, this data set provides two deep reference fields for studies of distant normal and active galaxies, supernovae, and faint stars in our own galaxy. This paper serves to outline the survey strategy and describe the specific data that have been used in the accompanying letters, summarizing the reduction procedures and sensitivity limits.Comment: 19 pages, 4 figures. Accepted for publication in the Astrophysical Journal, Letters. See manuscript for the full authorship list. The data and the GOODS-group papers can be found at http://www.stsci.edu/science/goods

Neutrino emission from a flaring blazar: Neutrinos interact only very weakly with matter, but giant detectors have succeeded in detecting small numbers of astrophysical neutrinos. Aside from a diffuse background, only two individual sources have been identified: the Sun and a nearby supernova in 1987. A multiteam collaboration detected a high-energy neutrino event whose arrival direction was consistent with a known blazar—a type of quasar with a relativistic jet oriented directly along our line of sight. The blazar, TXS 0506+056, was found to be undergoing a gamma-ray flare, prompting an extensive multiwavelength campaign. Motivated by this discovery, the IceCube collaboration examined lower-energy neutrinos detected over the previous several years, finding an excess emission at the location of the blazar. Thus, blazars are a source of astrophysical neutrinos. Science , this issue p. 147 , p. eaat1378

We explain the properties of X-ray selected BL Lacertae objects, under the assumption that they constitute the small minority of the BL Lac population with energy cutoff located in the UV/X-ray band, as suggested by their multifrequency spectra. In particular, we derive their X-ray luminosity function, log N-log S, and radio flux distribution starting from radio selected BL Lacs using a method that does not require any free parameters. Our hypothesis is at variance with the most common explanation for the existence of two classes of BL Lacs, namely that X-ray selected objects are viewed significantly off the beaming axis while radio-selected ones have their jets more aligned with the line of sight. In this case, in fact, X-ray selected sources should be intrinsically more numerous than objects selected in the radio band at a fixed value of some isotropic luminosity such as that of the diffuse radio emission. While also accounting for most of the properties of BL Lacs detected in current X-ray surveys, this latter scenario would predict their radio emission to be dominated by the extended (unbeamed) component, in contrast with observations. We also address the question of the negative evolution apparently exhibited by X-ray selected BL Lacs, showing that neither recognition problems nor a break in the X-ray spectrum are likely to be responsible for it. Our hypothesis explains in a simple way the bimodal distribution of BL Lacs in the alpha_ro - alpha_ ox diagram. It also makes specific predictions for the X-ray, radio, and optical properties of BL Lacs which will be tested by future deep X-ray surveys.Comment: 49 pages, PostScript paper and figure files. To appear in The Astrophysical Journal. Send all Comments, questions, requests, to padovani@roma2.infn.i