265 research outputs found
Enhanced J/Psi Production in Deconfined Quark Matter
In high energy heavy ion collisions at the Relativistic Heavy Ion Collider
(RHIC) at Brookhaven and the Large Hadron Collider (LHC) at CERN, each central
event will contain multiple pairs of heavy quarks. if a region of deconfined
quarks and gluons is formed, a new mechanism for the formation of heavy
quarkonium bound states will be activated. This is a result of the mobility of
heavy quarks in the deconfined region, such that bound states can be formed
from a quark and an antiquark which were originally produced in separate
incoherent interactions. Model estimates of this effect for J/psi production at
RHIC indicate that significant enhancements are to be expected. Experimental
observation of such enhanced production would provide evidence for
deconfinement unlikely to be compatible with competing scenarios.Comment: Added predictions with nonthermal charm quark distributions, also
with gluon dissociation replaced by screening. Accepted for publication in
Phys. Rev.
Synergetic aerosol retrieval from SCIAMACHY and AATSR onboard ENVISAT
International audienceThe synergetic aerosol retrieval method SYNAER (Holzer-Popp et al., 2002a) has been extended to the use of ENVISAT measurements. It exploits the complementary information of a radiometer and a spectrometer onboard one satellite platform to extract aerosol optical depth (AOD) and speciation (as choice from a representative set of pre-defined mixtures of water-soluble, soot, mineral dust, and sea salt components). SYNAER consists of two retrieval steps. In the first step the radiometer is used to do accurate cloud screening, and subsequently to quantify the aerosol optical depth (AOD) at 550 nm and spectral surface brightness through a dark field technique. In the second step the spectrometer is applied to choose the most plausible aerosol type through a least square fit of the measured spectrum with simulated spectra using the AOD and surface brightness retrieved in the first step. This method was developed and a first case study evaluation against few (15) multi-spectral ground-based AERONET sun photometer observations was conducted with a sensor pair (ATSR-2 and GOME) onboard ERS-2. However, due to instrumental limitations the coverage of SYNAER/ERS-2 and the AERONET network in 1997/98 is very sparse and thus only few coincidences with AERONET were found. Therefore, SYNAER was transferred to similar sensors AATSR and SCIAMACHY onboard ENVISAT. While transferring to the new sensor pair a thorough evaluation of the synergetic methodology and its information content has been conducted, which led to significant improvements in the methodology: an update of the aerosol model, an improved cloud detection, and an enhanced dark field albedo characterization. This paper describes the information content analysis and these improvements in detail and presents first results of applying the SYNAER methodology to AATSR and SCIAMACHY
B_c Meson Production in Nuclear Collisions at RHIC
We study quantitatively the formation and evolution of B_c bound states in a
space-time domain of deconfined quarks and gluons (quark-gluon plasma, QGP). At
the Relativistic Heavy Ion Collider (RHIC) one expects for the first time that
typical central collisions will result in multiple pairs of heavy (in this case
charmed) quarks. This provides a new mechanism for the formation of heavy
quarkonia which depends on the properties of the deconfined region. We find
typical enhancements of about 500 fold for the B_c production yields over
expectations from the elementary coherent hadronic B_c-meson production
scenario. The final population of bound states may serve as a probe of the
plasma phase parameters.Comment: 9 Pages, 11 Postscript Figure
VERITAS Observations of the BL Lac Object 1ES 1218+304
The VERITAS collaboration reports the detection of very-high-energy (VHE)
gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 1218+304
located at a redshift of z=0.182. A gamma-ray signal was detected with high
statistical significance for the observations taken during several months in
the 2006-2007 observing season. The photon spectrum between ~160 GeV and ~1.8
TeV is well described by a power law with an index of Gamma = 3.08 +/-
0.34(stat) +/- 0.2(sys). The integral flux above 200 GeV corresponds to ~6% of
that of the Crab Nebula. The light curve does not show any evidence for VHE
flux variability. Using lower limits on the density of the extragalactic
background light (EBL) in the near-IR to mid-IR we are able to limit the range
of intrinsic energy spectra for 1ES 1218+304. We show that the intrinsic photon
spectrum is harder than a power law with an index of Gamma = 2.32 +/- 0.37.
When including constraints from the spectra of 1ES 1101-232 and 1ES 0229+200,
the spectrum of 1ES 1218+304 is likely to be harder than Gamma = 1.86 +/- 0.37.Comment: Submitted to Proceedings of "4th Heidelberg International Symposium
on High Energy Gamma-Ray Astronomy 2008
VERITAS Observations of the gamma-Ray Binary LS I +61 303
LS I +61 303 is one of only a few high-mass X-ray binaries currently detected
at high significance in very high energy gamma-rays. The system was observed
over several orbital cycles (between September 2006 and February 2007) with the
VERITAS array of imaging air-Cherenkov telescopes. A signal of gamma-rays with
energies above 300 GeV is found with a statistical significance of 8.4 standard
deviations. The detected flux is measured to be strongly variable; the maximum
flux is found during most orbital cycles at apastron. The energy spectrum for
the period of maximum emission can be characterized by a power law with a
photon index of Gamma=2.40+-0.16_stat+-0.2_sys and a flux above 300 GeV
corresponding to 15-20% of the flux from the Crab Nebula.Comment: accepted for publication in The Astrophysical Journa
Status of the VERITAS Observatory
VERITAS, an Imaging Atmospheric Cherenkov Telescope (IACT) system for
gammma-ray astronomy in the GeV-TeV range, has recently completed its first
season of observations with a full array of four telescopes. A number of
astrophysical gamma-ray sources have been detected, both galactic and
extragalactic, including sources previously unknown at TeV energies. We
describe the status of the array and some highlight results, and assess the
technical performance, sensitivity and shower reconstruction capabilities.Comment: Submitted to Proceedings of "4th Heidelberg International Symposium
on High Energy Gamma-Ray Astronomy 2008
Evidence for long-term Gamma-ray and X-ray variability from the unidentified TeV source HESS J0632+057
HESS J0632+057 is one of only two unidentified very-high-energy gamma-ray
sources which appear to be point-like within experimental resolution. It is
possibly associated with the massive Be star MWC 148 and has been suggested to
resemble known TeV binary systems like LS I +61 303 or LS 5039. HESS J0632+057
was observed by VERITAS for 31 hours in 2006, 2008 and 2009. During these
observations, no significant signal in gamma rays with energies above 1 TeV was
detected from the direction of HESS J0632+057. A flux upper limit corresponding
to 1.1% of the flux of the Crab Nebula has been derived from the VERITAS data.
The non-detection by VERITAS excludes with a probability of 99.993% that HESS
J0632+057 is a steady gamma-ray emitter. Contemporaneous X-ray observations
with Swift XRT reveal a factor of 1.8+-0.4 higher flux in the 1-10 keV range
than earlier X-ray observations of HESS J0632+057. The variability in the
gamma-ray and X-ray fluxes supports interpretation of the ob ject as a
gamma-ray emitting binary.Comment: 8 pages, 3 figures, Accepted for publication in The Astrophysical
Journa
A connection between star formation activity and cosmic rays in the starburst galaxy M 82
Although Galactic cosmic rays (protons and nuclei) are widely believed to be
dominantly accelerated by the winds and supernovae of massive stars, definitive
evidence of this origin remains elusive nearly a century after their discovery
[1]. The active regions of starburst galaxies have exceptionally high rates of
star formation, and their large size, more than 50 times the diameter of
similar Galactic regions, uniquely enables reliable calorimetric measurements
of their potentially high cosmic-ray density [2]. The cosmic rays produced in
the formation, life, and death of their massive stars are expected to
eventually produce diffuse gamma-ray emission via their interactions with
interstellar gas and radiation. M 82, the prototype small starburst galaxy, is
predicted to be the brightest starburst galaxy in gamma rays [3, 4]. Here we
report the detection of >700 GeV gamma rays from M 82. From these data we
determine a cosmic-ray density of 250 eV cm-3 in the starburst core of M 82, or
about 500 times the average Galactic density. This result strongly supports
that cosmic-ray acceleration is tied to star formation activity, and that
supernovae and massive-star winds are the dominant accelerators.Comment: 18 pages, 4 figures; published in Nature; Version is prior to
Nature's in-house style editing (differences are minimal
Multiwavelength Observations of the Blazar Mrk 421 in December 2002 and January 2003
We report on a multiwavelength campaign on the TeV gamma-ray blazar Markarian
(Mrk) 421 performed during December 2002 and January 2003. These target of
opportunity observations were initiated by the detection of X-ray and TeV
gamma-ray flares with the All Sky Monitor (ASM) on board the Rossi X-ray Timing
Explorer (RXTE) and the 10 m Whipple gamma-ray telescope.The campaign included
observational coverage in the radio (University of Michigan Radio Astronomy
Observatory), optical (Boltwood, La Palma KVA 0.6m, WIYN 0.9m), X-ray (RXTE
pointed telescopes), and TeV gamma-ray (Whipple and HEGRA) bands.
At TeV energies, the observations revealed several flares at intermediate
flux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula.
While the time averaged spectrum can be fitted with a single power law of
photon index Gamma =2.8, we find some evidence for spectral variability.
Confirming earlier results, the campaign reveals a rather loose correlation
between the X-ray and TeV gamma-ray fluxes. In one case, a very strong X-ray
flare is not accompanied by a comparable TeV gamma-ray flare. Although the
source flux was variable in the optical and radio bands, the sparse sampling of
the optical and radio light curves does not allow us to study the correlation
properties in detail.
We present a simple analysis of the data with a synchrotron-self Compton
model, emphasizing that models with very high Doppler factors and low magnetic
fields can describe the data.Comment: Accepted for publication in the Astrophysical Journa
- …