291 research outputs found
Upper Limits on the Extragalactic Background Light from the Gamma-Ray Spectra of Blazars
The direct measurement of the extragalactic background light (EBL) is
difficult at optical to infrared wavelengths because of the strong foreground
radiation originating in the Solar System. Very high energy (VHE, E100 GeV)
gamma rays interact with EBL photons of these wavelengths through pair
production. In this work, the available VHE spectra from six blazars are used
to place upper limits on the EBL. These blazars have been detected over a range
of redshifts and a steepening of the spectral index is observed with increasing
source distance. This can be interpreted as absorption by the EBL. In general,
knowledge of the intrinsic source spectrum is necessary to determine the
density of the intervening EBL. Motivated by the observed spectral steepening
with redshift, upper limits on the EBL are derived by assuming that the
intrinsic spectra of the six blazars are . Upper limits are
then placed on the EBL flux at discrete energies without assuming a specific
spectral shape for the EBL. This is an advantage over other methods since the
EBL spectrum is uncertain.Comment: 33 pages, 14 figures, accepted by Ap
SWIFT observations of TeV BL Lac objects
Context: We present the results of a set of observations of nine TeV detected
BL Lac objects performed by the XRT and UVOT detectors on board the Swift
satellite between March and December 2005. Aims: We are mainly interested in
measuring the spectral parameters, and particularly the intrinsic curvature in
the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac
TeV objects using either a log-parabolic or a simple power-law model . Results:
We found that many of the objects in our sample do show significant spectral
curvature, whereas those having the peak of the spectral energies distribution
at energies lower than ~0.1 keV show power law spectra. In these cases,
however, the statistics are generally low thus preventing a good estimate of
the curvature. Simultaneous UVOT observations are important to verify how X-ray
spectra can be extrapolated at lower frequencies and to search for multiple
emission components. Conclusions: The results of our analysis are useful for
the study of possible signatures of statistical acceleration processes
predicting intrinsically curved spectra and for modelling the SED of BL
Lacertae objects up to TeV energies where a corresponding curvature is likely
to be present.Comment: 10 pages, 7 figures. Accepted for publication in the Astronomy and
Astrophysic
Heavy Flavor Hadrons in Statistical Hadronization of Strangeness-rich QGP
We study b, c quark hadronization from QGP. We obtain the yields of charm and
bottom flavored hadrons within the statistical hadronization model. The
important novel feature of this study is that we take into account the high
strangeness and entropy content of QGP, conserving strangeness and entropy
yields at hadronization.Comment: v2 expended: 20 pages, 23 figures, 5 tables, in press EPJ-
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
The TeV spectrum of H1426+428
The BL Lac object H1426+428 was recently detected as a high energy gamma-ray
source by the VERITAS collaboration (Horan et al. 2002). We have reanalyzed the
2001 portion of the data used in the detection in order to examine the spectrum
of H1426+428 above 250 GeV. We find that the time-averaged spectrum agrees with
a power law of the shape dF/dE = 10^(-7.31 +- 0.15(stat) +- 0.16(syst)) x
E^(-3.50 +- 0.35(stat) +- 0.05(syst)) m^(-2)s^(-1)TeV^(-1) The statistical
evidence from our data for emission above 2.5 TeV is 2.6 sigma. With 95% c.l.,
the integral flux of H1426+428 above 2.5 TeV is larger than 3% of the
corresponding flux from the Crab Nebula. The spectrum is consistent with the
(non-contemporaneous) measurement by Aharonian et al. (2002) both in shape and
in normalization. Below 800 GeV, the data clearly favours a spectrum steeper
than that of any other TeV Blazar observed so far indicating a difference in
the processes involved either at the source or in the intervening space.Comment: LaTeX, 8 pages, 4 figures, accepted for publication in Ap
Search for High Energy Gamma Rays from an X-ray Selected Blazar Sample
Our understanding of blazars has been greatly increased in recent years by
extensive multi-wavelength observations, particularly in the radio, X-ray and
gamma-ray regions. Over the past decade the Whipple 10m telescope has
contributed to this with the detection of 5 BL Lacertae objects at very high
gamma-ray energies. The combination of multi-wavelength data has shown that
blazars follow a well-defined sequence in terms of their broadband spectral
properties. Together with providing constraints on emission models, this
information has yielded a means by which potential sources of TeV emission may
be identified and predictions made as to their possible gamma-ray flux. We have
used the Whipple telescope to search for TeV gamma-ray emission from eight
objects selected from a list of such candidates. No evidence has been found for
VHE emission from the objects in our sample, and upper limits have been derived
for the mean gamma-ray flux above 390GeV. These flux upper limits are compared
with the model predictions and the implications of our results for future
observations are discussed.Comment: 15 pages, 2 figures, Accepted for publication in Ap
Search for Primordial Black Holes with SGARFACE
The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) uses the
Whipple 10 m telescope to search for bursts of rays. SGARFACE is
sensitive to bursts with duration from a few ns to 20 s and with
-ray energy above 100 MeV. SGARFACE began operating in March 2003 and
has collected 2.2 million events during an exposure time of 2267 hours. A
search for bursts of rays from explosions of primordial black holes
(PBH) was carried out. A Hagedorn-type PBH explosion is predicted to be visible
within 60 pc of Earth. Background events were caused by cosmic rays and by
atmospheric phenomena and their rejection was accomplished to a large extent
using the time-resolved images. No unambiguous detection of bursts of
rays could be made as the remaining background events mimic the expected shape
and time development of bursts. Upper limits on the PBH explosion rate were
derived from the SGARFACE data and are compared to previous and future
experiments. We note that a future array of large wide-field air-Cherenkov
telescopes equipped with a SGARFACE-like trigger would be able to operate
background-free with a 20 to 30 times higher sensitivity for PBH explosions.Comment: 18 pages, 30 figures, accepted by Astroparticle Physics, corrected
author list and Section 2.
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
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