1,601 research outputs found
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&
Nonthermal hard X-ray excess in the Coma cluster: resolving the discrepancy between the results of different PDS data analyses
The detection of a nonthermal excess in the Coma cluster spectrum by two
BeppoSAX observations analyzed with the XAS package (Fusco-Femiano et al.) has
been disavowed by an analysis (Rossetti & Molendi) performed with a different
software package (SAXDAS) for the extraction of the spectrum. To resolve this
discrepancy we reanalyze the PDS data considering the same software used by
Rossetti & Molendi. A correct selection of the data and the exclusion of
contaminating sources in the background determination show that also the SAXDAS
analysis reports a nonthermal excess with respect to the thermal emission at
about the same confidence level of that obtained with the XAS package
(~4.8sigma). Besides, we report the lack of the systematic errors investigated
by Rossetti & Molendi and Nevalainen et al. taking into account the whole
sample of the PDS observations off the Galactic plane, as already shown in our
data analysis of Abell 2256 (Fusco-Femiano, Landi & Orlandini). All this
eliminates any ambiguity and confirms the presence of a hard tail in the
spectrum of the Coma cluster.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter
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
Sequential double cross-validation for assessment of added predictive ability in high-dimensional omic applications
Enriching existing predictive models with new biomolecular markers is an important task in the new multi-omic era. Clinical studies increasingly include new sets of omic measurements which may prove their added value in terms of predictive performance. We introduce a two-step approach for the assessment of the added predictive ability of omic predictors, based on sequential double cross-validation and regularized regression models. We propose several performance indices to summarize the two-stage prediction procedure and a permutation test to formally assess the added predictive value of a second omic set of predictors over a primary omic source. The performance of the test is investigated through simulations. We illustrate the new method through the systematic assessment and comparison of the performance of transcriptomics and metabolomics sources in the prediction of body mass index (BMI) using longitudinal data from the Dietary, Lifestyle, and Genetic determinants of Obesity and Metabolic syndrome (DILGOM) study, a population-based cohort from Finland
- …