128 research outputs found
Quantum-spacetime scenarios and soft spectral lags of the remarkable GRB130427A
We process the Fermi LAT data on GRB130427A using the Fermi Science Tools,
and we summarize some of the key facts that render this observation truly
remarkable, especially concerning the quality of information on high-energy
emission by GRBs. We then exploit this richness for a search of spectral lags,
of the type that has been recently of interest for its relevance in
quantum-spacetime research. We do find some evidence of systematic soft
spectral lags: when confining the analysis to photons of energies greater than
5 GeV there is an early hard development of minibursts within this long burst.
The effect turns out to be well characterized quantitatively by a linear
dependence, within such a miniburst, of the detection time on energy. With the
guidance of our findings for GRB130427A we can then recognize that some support
for these features is noticeable also in earlier Fermi-LAT GRBs, particularly
for the presence of hard minibursts whose onset is marked by the highest-energy
photon observed for the GRB. A comparison of these features for GRBs at
different redshifts provides some encouragement for a redshift dependence of
the effects of the type expected for a quantum-spacetime interpretation, but
other aspects of the analysis appear to invite the interpretation as intrinsic
properties of GRBs
Demography of high redshift AGN
High redshift AGN holds the key to understand the early structure formation
and to probe the Universe during its infancy. We review the latest searches for
high-z AGN in the deepest X-ray field so far, the Chandra Deep Field South
(CDFS) 4 Msecond exposure. We do not confirm the positive detection of a signal
in the stacked Chandra images at the position of z~6 galaxies recently reported
by Treister and collaborators. We present z>3 X-ray sources number counts in
the 0.5-2 keV band obtained joining CDFS faint detections with Chandra-COSMOS
and XMM-COSMOS detections. We use these number counts to make predictions for
surveys with three mission concepts: Athena, WFXT and a Super-Chandra.Comment: Advances in Astronomy Special Issue "Seeking for the Leading Actor on
the Cosmic Stage: Galaxies versus Supermassive Black Holes
Time delays between Fermi LAT and GBM light curves of GRBs
Most Gamma-Ray Bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope
exhibit a delay of up to about 10 seconds between the trigger time of the hard
X-ray signal as measured by the Fermi GBM and the onset of the MeV-GeV
counterpart detected by the LAT. This delay may hint at important physics,
whether it is due to the intrinsic variability of the inner engine or it is
related to quantum dispersion effects in the velocity of light propagation from
the sources to the observer. It is critical to have a proper assessment of how
these time delays affect the overall properties of the light curves. We
cross-correlated the 5 brightest GRBs of the 1st Fermi LAT Catalog by means of
the continuous correlation function (CCF) and of the Discrete Correlation
Function (DCF). A maximum in the DCF suggests the presence of a time lag
between the curves, whose value and uncertainty are estimated through a
Gaussian fitting of the DCF profile and light curve simulation via a Monte
Carlo approach. The cross-correlation of the observed LAT and GBM light curves
yields time lags that are mostly similar to those reported in the literature,
but they are formally consistent with zero. The cross-correlation of the
simulated light curves yields smaller errors on the time lags and more than one
time lag for GRBs 090902B and 090926A; for all 5 GRBs, the time lags are
significantly different from zero and consistent with those reported in the
literature, when only the secondary maxima are considered for those two GRBs.
The DCF method evidences the presence of time lags between the LAT and GBM
light curves and underlines their complexity. While this suggests that the
delays should be ascribed to intrinsic physical mechanisms, more sensitivity
and larger statistics are needed to assess whether time lags are universally
present in the early GRB emission and which dynamical time scales they trace.Comment: 9 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
The lively accretion disc in NGC 2992. III. Tentative evidence of rapid Ultra Fast Outflow variability
We report on the 2019 XMM-Newton+NuSTAR monitoring campaign of the Seyfert
galaxy NGC 2992, observed at one of its highest flux levels in the X-rays. The
time-averaged spectra of the two XMM-Newton orbits show Ultra Fast Outflows
(UFOs) absorbing structures above 9 keV with significance. A
detailed investigation of the temporal evolution on a 5 ks time scale
reveals UFO absorption lines at a confidence level 95% (2) in 8 out
of 50 XMM-Newton segments, estimated via Monte Carlo simulations. We observe a
wind variability corresponding to a length scale of 5 Schwarzschild radii
. Adopting the novel Wind in the Ionised Nuclear Environment (WINE) model,
we estimate the outflowing gas velocity (), column density
() and ionisation state ($\log(\xi_0/erg\ cm\
s^{-1})=3.7-4.7\dot{M}_{out}\simeq0.3-0.8 M_{\odot} yr^{-1}\dot{p}_{out}\simeq 20-90 L_{Bol}/c\dot{E}_K \simeq 2-25 L_{Bol}\approx\approx\approx 5 r_S, 10^{11} {cm}^{-3}$, respectively.Comment: Accepted for publication in the Astrophysical Journal. 21 pages, 11
figure
Profibrotic Effects of Endothelin-1 on Fibroblasts Are Mediated by Aldosterone in Vitro: Relevance to the Pathogenesis and Therapy of Systemic Sclerosis and Pulmonary Arterial Hypertension
Endothelin-1 (ET-1) is a vasoactive and profibrotic peptide that plays a pivotal role in diseases such as systemic sclerosis (SSc) and pulmonary arterial hypertension (PAH), by inducing fibrosis and vascular remodeling. Such effects may be sustained by the induction of aldosterone production and reactive oxygen species (ROS). We have used fibroblasts obtained from skin of healthy donors and SSc patients and commercial fibroblasts from lung to evaluate whether ET-1 is able to stimulate ROS production directly or indirectly through aldosterone induction. We found that ET-1 receptors are present in all types of fibroblasts analyzed, whereas the expression of mineralocorticoid receptor (MCR) is lower in dermal fibroblasts from healthy donors (HDFs) compared to fibroblasts derived from lung (HPFs) or from skin of SSc patients (SScHDFs). ET-1 induces ROS production in HDFs and SScHDFs after 24 h of incubation involving its receptor B (ETB), whereas aldosterone exerts its effects after 40 min of incubation. Moreover, ROS production was inhibited by the pre-incubation of cells with MCR inhibitor. Our results indicate that ET-1 induces ROS indirectly through aldosterone production suggesting that aldosterone may play a pivotal role in the pathogenesis of SSc and PAH
Calibration of the NuSTAR High Energy Focusing X-ray Telescope
We present the calibration of the \textit{Nuclear Spectroscopic Telescope
Array} (\nustar) X-ray satellite. We used the Crab as the primary effective
area calibrator and constructed a piece-wise linear spline function to modify
the vignetting response. The achieved residuals for all off-axis angles and
energies, compared to the assumed spectrum, are typically better than \%
up to 40\,keV and 5--10\,\% above due to limited counting statistics. An
empirical adjustment to the theoretical 2D point spread function (PSF) was
found using several strong point sources, and no increase of the PSF half power
diameter (HPD) has been observed since the beginning of the mission. We report
on the detector gain calibration, good to 60\,eV for all grades, and discuss
the timing capabilities of the observatory, which has an absolute timing of
3\,ms. Finally we present cross-calibration results from two campaigns
between all the major concurrent X-ray observatories (\textit{Chandra},
\textit{Swift}, \textit{Suzaku} and \textit{XMM-Newton}), conducted in 2012 and
2013 on the sources 3C\,273 and PKS\,2155-304, and show that the differences in
measured flux is within 10\% for all instruments with respect to \nustar
Polarization studies with NuSTAR
The capability of NuSTAR to detect polarization in the Compton scattering regime (>50 keV) has been investigated. The NuSTAR mission, flown on June 2012 a Low Earth Orbit (LEO), provides a unique possibility to confirm the findings of INTEGRAL on the polarization of cosmic sources in the hard X-rays. Each of the two focal plane detectors are high resolution pixellated CZT arrays, sensitive in the energy range ~ 3 - 80 keV. These units have intrinsic polarization capabilities when the proper information on the double events is transmitted on ground. In this case it will be possible to detect polarization from bright sources on timescales of the order of 10^5 s
The Nuclear Spectroscopic Telescope Array (NuSTAR)
The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer
mission that will carry the first focusing hard X-ray (5 -- 80 keV) telescope
to orbit. NuSTAR will offer a factor 50 -- 100 sensitivity improvement compared
to previous collimated or coded mask imagers that have operated in this energy
band. In addition, NuSTAR provides sub-arcminute imaging with good spectral
resolution over a 12-arcminute field of view. After launch, NuSTAR will carry
out a two-year primary science mission that focuses on four key programs:
studying the evolution of massive black holes through surveys carried out in
fields with excellent multiwavelength coverage, understanding the population of
compact objects and the nature of the massive black hole in the center of the
Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae,
and probing the nature of particle acceleration in relativistic jets in active
galactic nuclei. A number of additional observations will be included in the
primary mission, and a guest observer program will be proposed for an extended
mission to expand the range of scientific targets. The payload consists of two
co-aligned depth-graded multilayer coated grazing incidence optics focused onto
solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus
rocket into a low-inclination Earth orbit. Data will be publicly available at
GSFC's High Energy Astrophysics Science Archive Research Center (HEASARC)
following validation at the science operations center located at Caltech.Comment: 9 pages, 5 figures, to appear in Proceedings of the SPIE, Space
Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ra
NuSTAR Spectroscopy of Multi-Component X-ray Reflection from NGC 1068
We report on observations of NGC1068 with NuSTAR, which provide the best
constraints to date on its ~keV spectral shape. We find no strong
variability over the past two decades, consistent with its Compton-thick AGN
classification. The combined NuSTAR, Chandra, XMM-Newton, and Swift-BAT
spectral dataset offers new insights into the complex reflected emission. The
critical combination of the high signal-to-noise NuSTAR data and a spatial
decomposition with Chandra allow us to break several model degeneracies and
greatly aid physical interpretation. When modeled as a monolithic (i.e., a
single N_H) reflector, none of the common Compton-reflection models are able to
match the neutral fluorescence lines and broad spectral shape of the Compton
reflection. A multi-component reflector with three distinct column densities
(e.g., N_H~1.5e23, 5e24, and 1e25 cm^{-2}) provides a more reasonable fit to
the spectral lines and Compton hump, with near-solar Fe abundances. In this
model, the higher N_H components provide the bulk of the Compton hump flux
while the lower N_H component produces much of the line emission, effectively
decoupling two key features of Compton reflection. We note that ~30% of the
neutral Fe Kalpha line flux arises from >2" (~140 pc), implying that a
significant fraction of the <10 keV reflected component arises from regions
well outside of a parsec-scale torus. These results likely have ramifications
for the interpretation of poorer signal-to-noise observations and/or more
distant objects [Abridged].Comment: Submitted to ApJ; 23 pages (ApJ format); 11 figures and 3 tables;
Comments welcomed
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