2,564 research outputs found
X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation
We have incorporated the description of the X-ray properties of Active
Galactic Nuclei (AGNs) into a semi-analytic model of galaxy formation, adopting
physically motivated scaling laws for accretion triggered by galaxy encounters.
Our model reproduces the level of the cosmic X-ray background at 30 keV; we
predict that the largest contribution (around 2/3) comes from sources with
intermediate X-ray luminosity 10^{43.5}< L_X/erg/s <10^{44.5}, with 50 % of the
total specific intensity produced at z<2. The predicted number density of
luminous X-ray AGNs (L_X>10^{44.5} erg/s in the 2-10 keV band) peaks at z
around 2 with a decline of around 3 dex to z=0; for the low luminosity sources
(10^{43}<L_X/erg/s <10^{44}) it has a broaderand less pronounced maximum around
z 1.5. The comparison with the data shows a generally good agreement. The model
predictions slightly exceed the observed number of low-luminosity AGNs at z
around 1.5, with the discrepancy progressively extending to
intermediate-luminosity objects at higher redshifts; we discuss possible
origins for the mismatch. Finally, we predict the source counts and the flux
distribution at different redshifts in the hard (20-100 keV) X-ray band for the
sources contributing to the X- ray background.Comment: 27 pages, accepted for publication in Ap
Towards reason: political disputes, public attention and the use of expert knowledge in policymaking
This article examines expert knowledge utilization in public policy processes. We study how much expert knowledge is employed and the extent to which decision-makers deliberate on the information provided by the experts, under various conditions of political disputes and public attention. We suggest four hypotheses. It is proposed that expert knowledge will be used more, but that there will be less deliberation in situations of political disputes. It is also suggested that expert knowledge will be consulted more and the decision-makers will take a more deliberative approach when there is a lot of attention from citizens. Our empirical findings, based on original data from local politics in Sweden, are in line with the hypotheses. The findings highlight the importance of both studying the extent of expert knowledge use and the way expertise is utilized. Another important insight is that politics seem to matter in relation to the role expert knowledge plays in public policymaking.Expert knowledge; Public policy; Political disputes; Public attention; Deliberation; Local government Sweden
Bell-type inequalities for cold heteronuclear molecules
We introduce Bell-type inequalities allowing for non-locality and
entanglement tests with two cold heteronuclear molecules. The proposed
inequalities are based on correlations between each molecule spatial
orientation, an observable which can be experimentally measured with present
day technology. Orientation measurements are performed on each subsystem at
diferent times. These times play the role of the polarizer angles in Bell tests
realized with photons. We discuss the experimental implementations of the
proposed tests, which could also be adapted to other high dimensional quantum
angular momenta systems.Comment: 4 page
On the anomalous X-ray afterglows of GRB 970508 and GRB 970828
Recently, BeppoSAX and ASCA have reported an unusual resurgence of soft X-ray
emission during the afterglows of GRB 970508 and GRB 970828, together with
marginal evidence for the existence of Fe-lines in both objects. We consider
the implications of the existence of a torus of iron-rich material surrounding
the sites of gamma ray bursts as would be expected in the SupraNova model; in
particular, we show that the fireball will quickly hit this torus, and bring it
to a temperature ~3x10^7 K. Bremsstrahlung emission from the heated up torus
will cause a resurgence of the soft X-ray emission with all expected
characteristics (flux level, duration and spectral hardening with time)
identical to those observed during the reburst. Also, thermal emission from the
torus will account for the observed iron line flux. These events are also
observable, for instance by new missions such as SWIFT, when beaming away from
our line sight makes us miss the main burst, as Fast (soft) X-ray Transients,
with durations ~10^3 s, and fluences ~10^-7-10^-4 erg cm^-2. This model
provides evidence in favor of the SupraNova model for Gamma Ray Bursts.Comment: To appear in MN Pink pages, MN-LateX, no figure
Most Supermassive Black Holes must be Rapidly Rotating
We use the integrated spectrum of the X-ray background and quasars Spectral
Energy Distribution to derive the contribution of quasars to the energy output
of the Universe. We find a lower limit for the energy from accretion onto black
holes of 6%, of the total luminosity of the Universe and probably more, with
15% quite possible. Comparing these values with the masses of black holes in
the center of nearby galaxies we show that the accretion process must be on
average very efficient: at least 15% of the accreted mass must be transformed
into radiated energy. This further implies that most supermassive black holes
are rapidly rotating.Comment: 8 Pages, Astrophysical Journal Letters, in pres
X-ray reprocessing in Seyfert Galaxies: simultaneous XMM-Newton/BeppoSAX observations
We selected a sample of eight bright unobscured (at least at the iron line
energy) Seyfert Galaxies observed simultaneously by XMM-Newton and BeppoSAX,
taking advantage of the complementary characteristics of the two missions. The
main results of our analysis can be summarized as follows: narrow neutral iron
lines are confirmed to be an ubiquitous component in Seyfert spectra; none of
the analyzed sources shows unambiguously a broad relativistic iron line; all
the sources of our sample (with a single exception) show the presence of a
Compton reflection component; emission lines from ionized iron are observed in
some sources; peculiar weak features around 5-6 keV (possibly arising from
rotating spots on the accretion disk) are detected in two sources. The scenario
emerging from these results strongly requires some corrections for the
classical model of reprocessing from the accretion disk. As for materials
farther away from the Black Hole, our results represent a positive test for the
Unification Model, suggesting the presence of the torus in (almost) all
sources, even if unobscured.Comment: Accepted for publication in A&
Broad band spectral properties of Seyfert 1 galaxies observed with BeppoSAX
We will present some results on the broad--band observations of BeppoSAX of
the bright Seyfert galaxies NGC 4151 and NGC 5548.Comment: 5 pages, 4 figures, Proc. of the 32 COSPAR Ass., Session E1.1 "Broad
Band X-Ray Spectra of Cosmic Sources, ed.s K. Makishima, L. Piro, T.
Takahashi, Advances in Space Research, in pres
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