P. Padovani

Bio: P. Padovani is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 182 citations.

Report on the ESO Workshop ''Active Galactic Nuclei: what's in a name?''

Abstract: Active galactic nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different “flavours” in the literature that now comprise a complex and confusing AGN “zoo”. It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their “big picture” through observations in each electromagnetic band from radio to $$\gamma $$γ-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude, we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.

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

Abstract: 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.

Black holes in binary systems

Abstract: 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

Identifying Luminous AGN in Deep Surveys: Revised IRAC Selection Criteria

Abstract: Spitzer IRAC selection is a powerful tool for identifying luminous AGN. For deep IRAC data, however, the AGN selection wedges currently in use are heavily contaminated by star-forming galaxies, especially at high redshift. Using the large samples of luminous AGN and high-redshift star-forming galaxies in COSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys. The new IRAC criteria are designed to be both highly complete and reliable, and incorporate the best aspects of the current AGN selection wedges and of infrared power-law selection while excluding high redshift star-forming galaxies selected via the BzK, DRG, LBG, and SMG criteria. At QSO-luminosities of log L(2-10 keV) (ergs/s) > 44, the new IRAC criteria recover 75% of the hard X-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN candidates have X-ray counterparts, a fraction that rises to 52% in regions with Chandra exposures of 50-160 ks. X-ray stacking of the individually X-ray non-detected AGN candidates leads to a hard X-ray signal indicative of heavily obscured to mildly Compton-thick obscuration (log N_H (cm^-2) = 23.5 +/- 0.4). While IRAC selection recovers a substantial fraction of luminous unobscured and obscured AGN, it is incomplete to low-luminosity and host-dominated AGN.

Colors of 2625 Quasars at 0 < z < 5 Measured in the Sloan Digital Sky Survey Photometric System

Abstract: We present an empirical investigation of the colors of quasars in the Sloan Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625 quasars with SDSS photometry: 1759 quasars found during SDSS spectroscopic commissioning and SDSS follow-up observations on other telescopes, 50 matches to FIRST quasars, 573 matches to quasars from the NASA Extragalactic Database, and 243 quasars from two or more of these sources. The quasars are distributed in a 25 wide stripe centered on the celestial equator covering ~529 deg2. Positions (accurate to 02) and SDSS magnitudes are given for the 898 quasars known prior to SDSS spectroscopic commissioning. New SDSS quasars, which range in brightness from i* = 15.39 to the photometric magnitude limit of the survey, represent an increase of over 200% in the number of known quasars in this area of the sky. The ensemble average of the observed colors of quasars in the SDSS passbands are well represented by a power-law continuum with αν = -0.5 (fν ∝ να) and are close to those predicted by previous simulations. However, the contributions of the small blue (or λ3000) bump and other strong emission lines have a significant effect upon the colors. The color-redshift relation exhibits considerable structure, which may be of use in determining photometric redshifts for quasars from their colors alone. The range of colors at a given redshift can generally be accounted for by a range in the optical spectral index with a distribution αν = -0.5 ± 0.65 (95% confidence), but there is a red tail in the distribution. This tail may be a sign of internal reddening, especially since fainter objects at a given redshift tend to exhibit redder colors than the average. Finally, we show that there is a continuum of properties between quasars and Seyfert galaxies, and we test the validity of the traditional dividing line (MB = -23) between the two classes of active galactic nuclei.

Dissecting the region around IceCube-170922A: the blazar TXS 0506+056 as the first cosmic neutrino source

Abstract: We present the dissection in space, time, and energy of the region around the IceCube-170922A neutrino alert. This study is motivated by: (1) the first association between a neutrino alert and a blazar in a flaring state, TXS 0506+056; (2) the evidence of a neutrino flaring activity during 2014 - 2015 from the same direction; (3) the lack of an accompanying simultaneous $\gamma$-ray enhancement from the same counterpart; (4) the contrasting flaring activity of a neighbouring bright $\gamma$-ray source, the blazar PKS 0502+049, during 2014 - 2015. Our study makes use of multi-wavelength archival data accessed through Open Universe tools and includes a new analysis of Fermi-LAT data. We find that PKS 0502+049 contaminates the $\gamma$-ray emission region at low energies but TXS 0506+056 dominates the sky above a few GeV. TXS 0506+056, which is a very strong (top percent) radio and $\gamma$-ray source, is in a high $\gamma$-ray state during the neutrino alert but in a low though hard $\gamma$-ray state in coincidence with the neutrino flare. Both states can be reconciled with the energy associated with the neutrino emission and, in particular during the low/hard state, there is evidence that TXS 0506+056 has undergone a hadronic flare with very important implications for blazar modelling. All multi-messenger diagnostics reported here support a single coherent picture in which TXS 0506+056, a very high energy $\gamma$-ray blazar, is the only counterpart of all the neutrino emissions in the region and therefore the most plausible first non-stellar neutrino and, hence, cosmic ray source.