117979829
A crucial hypothesis for Inflation
As is well known there are many different inflationary models that can explain the accelerated expansion occurred in the early Universe. It is possible that there exists a fundamental property of this period that could provide the corresponding field theory. Our hypothesis is that the Ricci scalar should assume a positive constant value during Inflation. Considering a single scalar field that drives the inflationary phase, we obtain uniquely the scalar potential and the coupling term between the scalar field and spacetime geometry. For this potential, that is well known in the inflationary scenario, the theoretical prediction of the spectral index ns is in perfect agreement with the experimental data. Interestingly in two dimensions, our model is equivalent to Liouville gravity.
120919640
Gravitational Behaviour of an Effective Topological Field Theory
Effective topological field theories describe the properties of Dirac fermions in the low-energy regime. In this work, we introduce a new emergent gravity model by considering Dirac fermions invariant under local de Sitter transformations in four-dimensional open manifolds. In the context of Cartan geometry, fermions couple to spacetime through a $Spin(5)$ Cartan connection that can be decomposed in spin connection and tetrads. In presence of a gravitational instanton background, we show that the corresponding effective topological field theory becomes equivalent to a dynamical gravitational theory with positive cosmological constant and Barbero-Immirzi parameter. At classical level and in absence of matter, this theory is compatible with general relativity.
34016084
Abelian Chern-Simons-Maxwell theory from a tight-binding model of spinless fermions.
Abelian Chern-Simons-Maxwell theory can emerge from the bosonization of the (2+1)-dimensional Thirring model that describes interacting Dirac fermions. Here we show how the Thirring model manifests itself in the low energy limit of a two-dimensional tight-binding model of spinless fermions. To establish that, we employ a modification of Haldane's model, where the "doubling" of fermions is rectified by adiabatic elimination. Subsequently, fermionic interactions are introduced that lead to the analytically tractable Thirring model. By local density measurements of the lattice fermions we can establish that for specific values of the couplings the model exhibits the confining (2+1)-dimensional QED phase or a topological ordered phase that corresponds to the Chern-Simons theory. The implementation of the model as well as the measurement protocol are accessible with the current technology of cold atoms in optical lattices.
140606740
From (under)ground to space: The birth of Space Science in Italy and the growth of astrophysics in its National Institutions
The first attempt by Italian scientists to carry on Astrophysical observations from Space can be traced back to the early 60ʼs just after the launch of the first satellites. Dominated by Physicists for decades, slowly space astrophysics has become one of the major fields of modern Astronomy. From the first experiments flown on balloons and rockets to the construction of todayʼs sophisticated satellites a historical scenario will be presented in the framework of Italian Science and, more generally, of European and World growth of the field. The first experiments in Universities, the setting of the CNR (National Research Council) labs, later transformed in institutes, to the recent merging of all Astronomical Observatories, will be discussed with mention to historical episodes. Main results and achievements will be outlined. How politics has affected the development of this branch of astronomical research will also be briefly considered within the limits assigned to the present talk. Some of the information contained in this paper comes from original documents, some from interviewing people who played a major role in the events, some from the literature but most of it is from personal reminiscences.
3198083
The low luminosity AGN in the LINER galaxy M 81: BeppoSAX discovery of highly ionized gas
The LINER nucleus of the nearby spiral galaxy M 81 was pointed by BeppoSAX, which caught it at the highest (2- 10) keV flux level observed so far. The LECS, MECS and PDS data, extending over (0.1-100) keV, are used to investi- gate the physical similarities and differences between LINERs and AGNs. The continuum is well fitted by a power law of pho- ton index 1:84, modified by little absorption due to cold material; this extends to (0.1-100) keV the validity of a similar ASCA result. Superimposed on the continuum BeppoSAX de- tects a 6.7 keV emission line (confirming another ASCA result) and an absorption edge at 8:6 keV. Both spectral features are consistent with being produced by iron at the same high ion- ization level, and probably also with the same column density. So, we suggest that they originate from transmission through highly ionized thin material. Concerning the origin of the con- tinuum emission, we do not observe signs of reflection from the optically thick material of an accretion disk, as usually found in Seyfert 1's (a 6.4 keV emission line and a broad bump peaking at 10-20 keV). The low bolometric luminosity of the nucleus of M 81 is consistent with being produced by advection dominated accretion; in this case the X-ray emission should be dominated by Comptonization, rather than by bremsstrahlung, in order to reproduce the steep spectrum observed over the (0.1-100) keV band.
16333116
Asca and rosat x-ray spectra of high-redshift radio-loud quasars
Results are presented on the X-ray properties of 9 high-redshift (1.2 < z < 3.4) radio-loud quasars (RLQs) observed by the Advanced Satellite for Cosmology and Astrophysics (ASCA; 10 observations) and ROSAT (11 observations, for a subset of six quasars). New ASCA observations of S5 0014+81 (z = 3.38) and S5 0836+71 (z = 2.17) and ROSAT observations of PKS 2126-158 for which results were never presented elsewhere are included. A simple model consisting of a power law plus cold, uniform absorption gives acceptable fits to the spectra of all sources. The ASCA spectra of the six brightest objects show evidence for absorption in excess of the Galactic value at a 99% confidence level. Comparison with the ROSAT data suggests that absorption has significantly varied (ΔNH ~ 8 × 1020 cm-2) in the case of S5 0836+71, on a timescale of approximately 0.8 yr in the quasar frame. For the remaining five sources for which ROSAT spectra were available, the two instruments gave consistent results, and the data were combined yielding unprecedented spectral coverage (typically about 0.4-40 keV in the quasar frame) for high-z quasars. This allows us to put severe limits on several different descriptions of the continuum (e.g., broken power law, bremsstrahlung, reflection component). No Fe Kα emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe Kα at the quasar redshift is marginally detected in S5 0014+81. Possible origins for the observed low energy absorption are discussed. In particular, contributions from the molecular clouds and dust present in our Galaxy (usually disregarded) are carefully considered. In light of the new results for S5 0836+71 and S5 0014+81, absorption intrinsic to the quasars is considered and discussed. The average slope obtained from the eight ASCA spectra in the observed ~0.5-10 keV energy band is Γ0.5-10 keV 1.61 ± 0.04, with a dispersion σ0.5-10 keV 0.10 ± 0.03. The average photon index in the observed 2-10 keV band, where the effect of absorption is negligible, is Γ2-10 keV 1.53 ± 0.05, with a dispersion σ2-10 keV 0.12. Furthermore, the implications of the present results on the calculations of the contribution of quasars to the cosmic X-ray and γ-ray backgrounds are briefly discussed.
33709843
X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfert galaxies using XMM–Newton are reported. The sample was optically selected, well defined, complete in B  magnitude, and distance limited: it consists of the nearest ($D \la22$ Mpc) 27 Seyfert galaxies (9 of type 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315) sample. This is one of the largest atlases of hard X-ray spectra of low-luminosity active galaxies ever assembled. All nuclear sources except two Seyfert 2s are detected between 2 and 10 keV, half for the first time ever, and average spectra are obtained for all of them. Nuclear luminosities reach values down to 10 38  erg s -1 . The shape of the distribution of X-ray parameters is affected by the presence of Compton-thick objects ($\ga$30% among type 2s). The latter have been identified either directly from their intense FeK line and flat X-ray spectra, or indirectly with flux diagnostic diagrams which use isotropic indicators. After taking into account these highly absorbed sources, we find that (i) the intrinsic X-ray spectral properties (i.e., spectral shapes and luminosities above 2 keV) are consistent between type 1 and type 2 Seyferts, as expected from “unified models”; (ii) Seyfert galaxies as a whole are distributed fairly continuously over the entire range of N H , between 10 20 and 10 25  cm -2 ; and (iii) while Seyfert 1s tend to have lower N H and Seyfert 2s tend to have the highest, we find 30% and 10% exceptions, respectively. Overall the sample is of sufficient quality to well represent the average intrinsic X-ray spectral properties of nearby active galactic nuclei, including a proper estimate of the distribution of their absorbing columns. Finally, we conclude that, with the exception of a few cases, the present study agrees with predictions of unified models of Seyfert galaxies, and extends their validity down to very low luminosities.
119368661
The Ultra Luminous Infrared Galaxy Mrk 231: new clues from BeppoSAX and XMM-Newton
We present BeppoSAX and XMM-Newton observations of Mrk 231. These observations and in particular the BeppoSAX PDS data allowed us to unveil, for the first time, the highly absorbed (NH∼2×1024 cm−2) AGN component. We find that: a) the AGN powering Mrk 231 has an intrinsic 2-10 keV luminosity of 1+1−0.5×1044 erg s−1; b) the strong starburst activity contributes significantly in the 0.1-10 keV energy range. We propose that the starburst activity strongly contributes to the far infrared luminosity of Mrk 231; this is also suggested by the multiwavelength properties of Mrk 231.