1,256 research outputs found
Unveiling GRB hard X-ray afterglow emission with Simbol-X
Despite the enormous progress occurred in the last 10 years, the Gamma-Ray
Bursts (GRB) phenomenon is still far to be fully understood. One of the most
important open issues that have still to be settled is the afterglow emission
above 10 keV, which is almost completely unexplored. This is due to the lack of
sensitive enough detectors operating in this energy band. The only detection,
by the BeppoSAX/PDS instrument (15-200 keV), of hard X-ray emission from a GRB
(the very bright GRB 990123), combined with optical and radio observations,
seriously challenged the standard scenario in which the dominant mechanism is
synchrotron radiation produced in the shock of a ultra-relativistic fireball
with the ISM, showing the need of a substantial revision of present models. In
this respect, thanks to its unprecedented sensitivity in the 10-80 keV energy
band, Simbol-X, through follow-up observations of bright GRBs detected and
localized by GRB dedicated experiments that will fly in the >2010 time frame,
will provide an important breakthrough in the GRB field.Comment: 4 pages, 2 figures. Paper presented at "Simbol-X: the hard X-ray
universe in focus", held in Bologna, Italy, on 14-16 May 2007. To be
published in Memorie della Societa' Astronomica Italian
Studying the WHIM Content of the Galaxy Large-Scale Structures along the Line of Sight to H 2356-309
We make use of a 500ks Chandra HRC-S/LETG spectrum of the blazar H2356-309,
combined with a lower S/N spectrum of the same target, to search for the
presence of warm-hot absorbing gas associated with two Large-Scale Structures
(LSSs) crossed by this sightline at z=0.062 (the Pisces-Cetus Supercluster,
PCS) and at z=0.128 ("Farther Sculptor Wall", FSW). No statistically
significant (>=3sigma) individual absorption is detected from any of the strong
He- or H-like transitions of C, O and Ne at the redshifts of the structures.
However we are still able to constrain the physical and geometrical parameters
of the associated putative absorbing gas, by performing joint spectral fit of
marginal detections and upper limits of the strongest expected lines with our
self-consistent hybrid ionization WHIM spectral model. At the redshift of the
PCS we identify a warm phase with logT=5.35_-0.13^+0.07 K and log N_H
=19.1+/-0.2 cm^-2 possibly coexisting with a hotter and less significant phase
with logT=6.9^+0.1_-0.8 K and log N_H=20.1^+0.3_-1.7 cm^-2 (1sigma errors). For
the FSW we estimate logT=6.6_-0.2^+0.1 K and log N_H=19.8_-0.8^+0.4 cm^-2. Our
constraints allow us to estimate the cumulative number density per unit
redshifts of OVII WHIM absorbers. We also estimate the cosmological mass
density obtaining Omega_b(WHIM)=(0.021^+0.031_-0.018) (Z/Z_sun)^-1, consistent
with the mass density of the intergalactic 'missing baryons' for high
metallicities.Comment: 29 pages, 8 figures, 4 tables. Accepted for publication in Ap
A New Method for Obtaining Binary Pulsar Distances and its Implications for Tests of General Relativity
We demonstrate how measuring orbital period derivatives can lead to more
accurate distance estimates and transverse velocities for some nearby binary
pulsars. In many cases this method will estimate distances more accurately than
is possible by annual parallax, as the relative error decreases as t^-5/2.
Unfortunately, distance uncertainties limit the degree to which nearby
relativistic binary pulsars can be used for testing the general relativistic
prediction of orbital period decay to a few percent. Nevertheless, the measured
orbital period derivative of PSR B1534+12 agrees within the observational
uncertainties with that predicted by general relativity if the proper-motion
contribution is accounted for.Comment: 4 pages, latex, uuencoded compressed postscript + source, no figures,
uses aaspptwo.sty and dec.sty, accepted for publication in ApJL, omitted
reference now include
On the nature of the z=0 X-ray absorbers: I. Clues from an external group
Absorption lines of OVII at redshift zero are observed in high quality
Chandra spectra of extragalactic sightlines. The location of the absorber
producing these lines, whether from the corona of the Galaxy or from the Local
Group or even larger scale structure, has been a matter of debate. Here we
study another poor group like our Local Group to understand the distribution of
column density from galaxy to group scales. We show that we cannot yet rule out
the group origin of z=0 systems. We further argue that the debate over Galactic
vs. extragalactic origin of z=0 systems is premature as they likely contain
both components and predict that future higher resolution observations will
resolve the z=0 systems into multiple components.Comment: Submitted to ApJ
The Lack of BLR in Low Accretion Rate AGN as Evidence of their Origin in the Accretion Disk
In this paper we present evidence suggesting that the absence or presence of
Hidden Broad Line Regions (HBLRs) in Seyfert 2 galaxies is regulated by the
rate at which matter accretes onto a central supermassive black hole, in units
of Eddington rate. We use the intrinsic (i.e. unabsorbed) X-ray luminosities of
these sources and their black hole masses (estimated by using the well known
relationship between nuclear mass and bulge luminosity in galaxies) to derive
the nuclear accretion rate in units of Eddington. We find that virtually all
HBLR sources have accretion rate larger than a threshold value of
(in Eddington units), while non-HBLR sources
lie at \dot{m} \ls \dot{m}_{thres}.
These data nicely fit predictions from a model proposed by Nicastro (2000),
in which the Broad Line Regions (BLRs) are formed by accretion disk
instabilities occurring in proximity of the critical radius at which the disk
changes from gas pressure dominated to radiation pressure dominated. This
radius diminishes with decreasing accretion rates; for low enough accretion
rates (and therefore luminosities), the critical radius becomes smaller than
the innermost stable orbit, and BLRs cannot form.Comment: 11 pages, 1 figure. Accepted for publication in the ApJ
Scan without evidence of dopaminergic deficit: A 10-year retrospective study
123I-ioflupane SPECT is a powerful method to assess nigrostriatal dopamine system integrity. Several independent studies have shown that 1â15% of patients with suspected degenerative parkinsonism, mainly PD, have scans without evidence of dopaminergic deficit (SWEDD). It has been proposed that most SWEDD patients either present with a non-degenerative condition mimicking PD, such as atypical tremor or dystonia, or demonstrate an abnormal scan when repeated later. We here hypothesized that scan interpretation methods may also play a crucial yet underestimated role in this issue.Methods We previously established age-dependent reference values of striatal uptake by analyzing scans from a cohort of patients with non-degenerative conditions. We then studied a large population with well-established degenerative parkinsonism (N = 410, 80% with PD), using identical imaging protocol, to evaluate the prevalence of patients with normal scans based on routine visual assessment. Each scan was eventually reassessed using the same automated method as for controls and a detailed 3D analysis.Results Ten potential SWEDD cases (2.4%) were identified. However, both reassessment methods independently showed that these scans were all outside reference limits and/or visually abnormal when reexamined carefully, except for one case (0.2%) with corticobasal syndrome.Conclusion SPECT misinterpretation emerges as an important contributor to the SWEDD population, suggesting that suspected SWEDD cases should prompt not only a serious diagnosis challenge but, equally important, a detailed scan reassessment. True SWEDD cases seem extremely rare in degenerative parkinsonism. We propose that the very concept of SWEDD is more confusing than helpful and should be definitely abandoned
The X-ray absorber of PKS2126-158
BeppoSAX observed the z=3.27 quasar PKS2126-158 on 1999 May 24-28 when its
2-10keV and 0.1-2.5keV fluxes were 1.1E-11 and 4.4E-12 cgs respectively, a
factor of 2 higher than in all previous ROSAT and ASCA observations and 40%
higher than in two more recent Chandra and XMM-Newton observations. The
shortest detected rest frame variability timescale is of a few months,
comparable to the causal timescale associated to an emission region of ~10
Schwarzschild radii around a few 1E10 solar masses black hole. The source is
detected with a signal to noise ratio S/N>=3 up to ~50 keV, 215 keV rest frame.
The BeppoSAX observations confirm the presence of low energy absorption along
the line of sight, independent on the continuum model adopted, at high
confidence level. Despite the limited spectral resolution of the BeppoSAX LECS
and MECS it is possible to put constraints on different absorption and
continuum models, but not to unambiguously determine the redshift of the
absorber. If the absorber is not significantly ionized the BeppoSAX data do
prefer an absorber at z<=2.7. Strong and complex metal line systems along the
line of sight to PKS2126-158 have been found at z=0.6631 and at 2.64<z<2.82.
They could well be associated to the X-ray absorption. Conversely, an ionized
(``warm'') absorber at the quasar redshift provides a good fit only if the iron
abundance is smaller than ~0.3 solar, while that of the other elements is fixed
to the solar value.Comment: A&A in pres
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