1,560 research outputs found
Antimatter research in Space
Two of the most compelling issues facing astrophysics and cosmology today are
to understand the nature of the dark matter that pervades the universe and to
understand the apparent absence of cosmological antimatter. For both issues,
sensitive measurements of cosmic-ray antiprotons and positrons, in a wide
energy range, are crucial. Many different mechanisms can contribute to
antiprotons and positrons production, ranging from conventional reactions up to
exotic processes like neutralino annihilation. The open problems are so
fundamental (i.e.: is the universe symmetric in matter and antimatter ?) that
experiments in this field will probably be of the greatest interest in the next
years. Here we will summarize the present situation, showing the different
hypothesis and models and the experimental measurements needed to lead to a
more established scenario.Comment: 10 pages, 7 figures, Invited talk at the 18th European Cosmic Ray
Symposium, Moscow, July 2002, submitted to Journal of Physics
Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
We present the results of multi-year gamma-ray observations by the AGILE
satellite of the black hole binary system Cygnus X-1. In a previous
investigation we focused on gamma-ray observations of Cygnus X-1 in the hard
state during the period mid-2007/2009. Here we present the results of the
gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out
for which includes a remarkably prolonged `soft state' phase (June 2010 -- May
2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a
possible existence of a non-thermal particle component with substantial
modifications of the Comptonized emission from the inner accretion disk. Our
AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1,
provide a significant upper limit for gamma-ray emission above 100 MeV of
F_soft < 20 x 10^{-8} ph/cm^2/s, excluding the existence of prominent
non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We
discuss theoretical implications of our findings in the context of high-energy
emission models of black hole accretion. We also discuss possible gamma-ray
flares detected by AGILE. In addition to a previously reported episode observed
by AGILE in October 2009 during the hard state, we report a weak but important
candidate for enhanced emission which occurred at the end of June 2010
(2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with a
hard-to-soft state transition and before an anomalous radio flare. An appendix
summarizes all previous high-energy observations and possible detections of
Cygnus X-1 above 1 MeV.Comment: 16 pages, 12 figures, 1 table, accepted for publication in Ap
The Cosmic-Ray Proton and Helium Spectra measured with the CAPRICE98 balloon experiment
A new measurement of the primary cosmic-ray proton and helium fluxes from 3
to 350 GeV was carried out by the balloon-borne CAPRICE experiment in 1998.
This experimental setup combines different detector techniques and has
excellent particle discrimination capabilities allowing clear particle
identification. Our experiment has the capability to determine accurately
detector selection efficiencies and systematic errors associated with them.
Furthermore, it can check for the first time the energy determined by the
magnet spectrometer by using the Cherenkov angle measured by the RICH detector
well above 20 GeV/n. The analysis of the primary proton and helium components
is described here and the results are compared with other recent measurements
using other magnet spectrometers. The observed energy spectra at the top of the
atmosphere can be represented by (1.27+-0.09)x10^4 E^(-2.75+-0.02) particles
(m^2 GeV sr s)^-1, where E is the kinetic energy, for protons between 20 and
350 GeV and (4.8+-0.8)x10^2 E^(-2.67+-0.06) particles (m^2 GeV nucleon^-1 sr
s)^-1, where E is the kinetic energy per nucleon, for helium nuclei between 15
and 150 GeV nucleon^-1.Comment: To be published on Astroparticle Physics (44 pages, 13 figures, 5
tables
Sensitive detection of circulating breast cancer cells by reverse-transcriptase polymerase chain reaction of maspin gene
Background: Maspin, a recently identified protein related to the family of serpins, is believed to play a role in human breast cancer. In an effort to improve the present methods of detection, we have developed a reverse-transcriptase polymerase chain reaction (RT-PCR) assay for maspin transcript to identify small numbers of mammary carcinoma cells in the peripheral blood and bone marrow of patients with breast cancer. Patients and methods: Five non-neoplastic mammary tissue samples, 13 breast cancer specimens as well as 17 peripheral blood and 4 bone marrow samples from normal subjects were screened for the presence of maspin mRNA by RT-PCR. The same assay was applied to peripheral blood or bone marrow samples obtained from 29 patients with stages I to IV breast cancer. Results: By RT-PCR it was possible to amplify maspin mRNA in all of the primary and metastatic breast cancer specimens, but in none of the normal hemopoietic samples from healthy donors. Thus, detection of maspin transcript in the peripheral blood or marrow of a patient known to have breast cancer is indicative of the presence of mammary carcinoma cells. In reconstitution experiments, maspin RT-PCR reliably detected 10 mammary carcinoma cells in 1 million normal peripheral-blood mononuclear cells (PBMCs). None of the 9 patients with stages I, II, or III breast cancer had maspin transcript in peripheral blood. Of note, 3 of 9 patients with stage TV breast cancer receiving systemic therapy at the time of sample collection, but only I of 11 patients with stage IV not receiving therapy, had detectable maspin transcript in peripheral blood. Moreover, 3 marrow specimens from stage TV patients tested positive by this assay. Conclusions: This pilot study suggests that maspin RT-PCR assay is a sensitive, specific and sufficiently rapid method for detection of small numbers of circulating cells and marrow micrometastases in breast cancer patients. The possibility of applying this assay in the detection of tumor cell contamination of both marrow and stem-cell apheresis harvests of breast cancer patients merits further investigation
On the Angular Resolution of the AGILE gamma-ray imaging detector
We present a study of the Angular Resolution of the AGILE gamma-ray imaging
detector (GRID) that is operational in space since April 2007. The AGILE
instrument is made of an array of 12 planes each equipped with a Tungsten
converter and Silicon micros trip detectors and is sensitive in the energy
range 50 MeV - 10 GeV. Among the space instruments devoted to gamma-ray
astrophysics, AGILE uniquely exploits an analog readout system with dedicated
electronics coupled with Silicon detectors. We show the results of Monte Carlo
simulations carried out to reproduce the gamma-ray detection by the GRID, and
we compare them to in-flight data. We use the Crab (pulsar + Nebula) system for
discussion of real data performance, since its E^{-2} energy spectrum is
representative of the majority of gamma-ray sources. For Crab-like spectrum
sources, the GRID angular resolution (FWHM of ~4deg at 100 MeV; ~0.8deg at 1
GeV; ~0.9deg integrating the full energy band from 100 MeV to tens of GeV) is
stable across a large field of view, being characterized by a flat response up
to 30deg off-axis. A comparison of the angular resolution obtained by the two
operational gamma-ray instruments, AGILE-GRID and Fermi-LAT, is interesting in
view of future gamma-ray missions, that are currently under study. The two
instruments exploit different detector configurations affecting the angular
resolution: the former being optimized in the readout and track reconstruction
especially in the low-energy band, the latter in terms of converter thickness
and power consumption. We show that, despite these differences, the angular
resolution of both instruments is very similar between 100 MeV and a few GeV.Comment: 19 pages, 8 figures, accepted for publication in Ap
Episodic Transient Gamma-Ray Emission from the Microquasar Cygnus X-1
Cygnus X-1 is the archetypal black hole (BH) binary system in our Galaxy. We
report the main results of an extensive search for transient gamma-ray emission
from Cygnus X-1 carried out in the energy range 100 MeV - 3 GeV by the AGILE
satellite, during the period 2007 July - 2009 October. The total exposure time
is about 300 days, during which the source was in the "hard" X-ray spectral
state. We divided the observing intervals in 2 or 4 week periods, and searched
for transient and persistent emission. We report an episode of significant
transient gamma-ray emission detected on 2009, October 16 in a position
compatible with Cygnus X-1 optical position. This episode, occurred during a
hard spectral state of Cygnus X-1, shows that a 1-2 day time variable emission
above 100 MeV can be produced during hard spectral states, having important
theoretical implications for current Comptonization models for Cygnus X-1 and
other microquasars. Except for this one short timescale episode, no significant
gamma-ray emission was detected by AGILE. By integrating all available data we
obtain a 2 upper limit for the total integrated flux of
in the energy range
100 MeV - 3 GeV. We then clearly establish the existence of a spectral cutoff
in the energy range 1-100 MeV that applies to the typical hard state outside
the flaring period and that confirms the historically known spectral cutoff
above 1 MeV.Comment: Accepted for publication by ApJ on the 9th of Feb 2010, 5 pages, 3
figure
Direct Evidence for Hadronic Cosmic-Ray Acceleration in the Supernova Renmant IC 443
The Supernova Remnant (SNR) IC 443 is an intermediate-age remnant well known
for its radio, optical, X-ray and gamma-ray energy emissions. In this Letter we
study the gamma-ray emission above 100 MeV from IC 443 as obtained by the AGILE
satellite. A distinct pattern of diffuse emission in the energy range 100 MeV-3
GeV is detected across the SNR with its prominent maximum (source "A")
localized in the Northeastern shell with a flux F = (47 \pm 10) 10^{-8} photons
cm^{-2} s^{-1} above 100 MeV. This location is the site of the strongest shock
interaction between the SNR blast wave and the dense circumstellar medium.
Source "A" is not coincident with the TeV source located 0.4 degree away and
associated with a dense molecular cloud complex in the SNR central region. From
our observations, and from the lack of detectable diffuse TeV emission from its
Northeastern rim, we demonstrate that electrons cannot be the main emitters of
gamma-rays in the range 0.1-10 GeV at the site of the strongest SNR shock. The
intensity, spectral characteristics, and location of the most prominent
gamma-ray emission together with the absence of co-spatial detectable TeV
emission are consistent only with a hadronic model of cosmic-ray acceleration
in the SNR. A high-density molecular cloud (cloud "E") provides a remarkable
"target" for nucleonic interactions of accelerated hadrons: our results show
enhanced gamma-ray production near the molecular cloud/shocked shell
interaction site. IC 443 provides the first unambiguous evidence of cosmic-ray
acceleration by SNRs.Comment: 5 pages, 2 figures; accepted by ApJLetters on Jan 21, 201
Calibration of AGILE-GRID with In-Flight Data and Monte Carlo Simulations
Context: AGILE is a gamma-ray astrophysics mission which has been in orbit
since 23 April 2007 and continues to operate reliably. The gamma-ray detector,
AGILE-GRID, has observed Galactic and extragalactic sources, many of which were
collected in the first AGILE Catalog. Aims: We present the calibration of the
AGILE-GRID using in-flight data and Monte Carlo simulations, producing
Instrument Response Functions (IRFs) for the effective area A_eff), Energy
Dispersion Probability (EDP), and Point Spread Function (PSF), each as a
function of incident direction in instrument coordinates and energy. Methods:
We performed Monte Carlo simulations at different gamma-ray energies and
incident angles, including background rejection filters and Kalman filter-based
gamma-ray reconstruction. Long integrations of in-flight observations of the
Vela, Crab and Geminga sources in broad and narrow energy bands were used to
validate and improve the accuracy of the instrument response functions.
Results: The weighted average PSFs as a function of spectra correspond well to
the data for all sources and energy bands. Conclusions: Changes in the
interpolation of the PSF from Monte Carlo data and in the procedure for
construction of the energy-weighted effective areas have improved the
correspondence between predicted and observed fluxes and spectra of celestial
calibration sources, reducing false positives and obviating the need for
post-hoc energy-dependent scaling factors. The new IRFs have been publicly
available from the Agile Science Data Centre since November 25, 2011, while the
changes in the analysis software will be distributed in an upcoming release
AGILE Observations of the Gravitational Wave Event GW150914
We report the results of an extensive search in the AGILE data for a
gamma-ray counterpart of the LIGO gravitational wave event GW150914. Currently
in spinning mode, AGILE has the potential of covering with its gamma-ray
instrument 80 % of the sky more than 100 times a day. It turns out that AGILE
came within a minute from the event time of observing the accessible GW150914
localization region. Interestingly, the gamma-ray detector exposed about 65 %
of this region during the 100 s time intervals centered at -100 s and +300 s
from the event time. We determine a 2-sigma flux upper limit in the band 50 MeV
- 10 GeV, obtained
about 300 s after the event. The timing of this measurement is the fastest ever
obtained for GW150914, and significantly constrains the electromagnetic
emission of a possible high-energy counterpart. We also carried out a search
for a gamma-ray precursor and delayed emission over timescales ranging from
minutes to days: in particular, we obtained an optimal exposure during the
interval -150 / -30 s. In all these observations, we do not detect a
significant signal associated with GW150914. We do not reveal the weak
transient source reported by Fermi-GBM 0.4 s after the event time. However,
even though a gamma-ray counterpart of the GW150914 event was not detected, the
prospects for future AGILE observations of gravitational wave sources are
decidedly promising.Comment: 20 pages, 6 figures. Submitted to the Astrophysical Journal Letters
on April 1, 201
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