481 research outputs found
Detection of TeV Gamma-Rays from the BL Lac 1ES1959+650 in its low states and during a major outburst in 2002
TeV gamma-rays from the BL Lac object 1ES1959+650 have been measured during
the years 2000 and 2001 with a significance of 5.2 sigma at a value of 5.3% of
the Crab flux and in May 2002 during strong outbursts with > 23 sigma at a flux
level of up to 2.2 Crab, making 1ES1959+650 the TeV Blazar with the third best
event statistics. The deep observation of 197.4 h has been performed with the
HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT
system). 1ES1959+650 is located at a redshift of z = 0.047, providing an
intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the
much more distant object H1426+428. This makes 1ES1959+650 an important member
of the class of TeV Blazars in view of the absorption of TeV photons by the
diffuse extragalactic background radiation (DEBRA). The differential energy
spectrum of 1ES1959+650 during the flares can be fitted by a power law with a
spectral index of 2.83 +- 0.14_stat +- 0.08_sys or by a power law with an
exponential cut-off at (4.2^(+0.8)_(-0.6)_stat +- 0.9_sys) TeV and a spectral
index of 1.83 +- 0.15_stat +- 0.08_sys. The low state differential energy
spectrum obtained with lower statistics can be described by a pure power law
with a spectral index of 3.18 +- 0.17_stat +- 0.08_sys.Comment: 5 pages, 3 figures, refereed version, corrected for a typ
EU External Relations: Exclusive Competence Revisited
This Article will focus on the question of exclusive competence in the field of EU external relations, especially in the light of recent developments. After a brief discussion on the origins and development of exclusive competence, a distinction will be made between common commercial policy, which has traditionally been the most important area of an explicit âa prioriâ exclusive competence, and what is often called an implicit exclusive competence, which, as it is today based on some general criteria enshrined in TFEU Article 3(2), may be called âsuperveningâ exclusive competence. With regard to both categories, the main focus will be on recent developments, notably the impact of the Treaty of Lisbon, which introduced the TFEU and its Articles 2 and 3, as well as the case law of the European Court of Justice (âECJâ or the âCourtâ) following the entry into force of the Treaty of Lisbon, on December 1, 2009
An unidentified TeV source in the vicinity of Cygnus OB2
Deep observation (âŒ113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Äerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and âŒ0.5° north of Cyg X-3. The source centre of gravity is RA αJ2000: 20hr32m07s± 9.2stats±2.2syss, Dec ÎŽJ2000: +41°30âČ30âł2.0stat±0.4âČsys. The source is steady, has a post-trial significance of +4.6Ï, indication for extension with radius 5.6âČ at the âŒ3Ï level, and has a differential power-law flux with hard photon index of - 1.9 ± 0.3stat ± 0.3sys. The integral flux above 1 TeV amounts âŒ3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage Îł-ray production via a jet-driven termination shock.F. A. Aharonian, ... G. P. Rowell, ... [et al
The unidentified TeV source (TeVJ2032+4130) and surrounding field: Final HEGRA IACT-System results
The unidentified TeV source in Cygnus is now confirmed by follow-up
observations from 2002 with the HEGRA stereoscopic system of Cherenkov
Telescopes. Using all data (1999 to 2002) we confirm this new source as steady
in flux over the four years of data taking, extended with radius 6.2 arcmin
(+-1.2 arcmin (stat) +-0.9 arcmin (sys)) and exhibiting a hard spectrum with
photon index -1.9. It is located in the direction of the dense OB stellar
association, Cygnus OB2. Its integral flux above energies E>1 TeV amounts to
\~5% of the Crab assuming a Gaussian profile for the intrinsic source
morphology. There is no obvious counterpart at radio, optical nor X-ray
energies, leaving TeVJ2032+4130 presently unidentified. Observational
parameters of this source are updated here and some astrophysical discussion is
provided. Also included are upper limits for a number of other interesting
sources in the FoV, including the famous microquasar Cygnus X-3.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
Cherenkov Telescopes as Optical Telescopes for Bright Sources: Today's Specialised Thirty Metre Telescopes?
Imaging Atmospheric Cherenkov Telescopes (IACTs) use large-aperture (~ 10 -
30 m) optical telescopes with arcminute angular resolution to detect TeV
gamma-rays in the atmosphere. I show that IACTs are well-suited for optical
observations of bright sources (V <= 8 - 10), because these sources are
brighter than the sky background. Their advantages are especially great on
rapid time-scales. Thus, IACTs are ideal for studying many phenomena optically,
including transiting exoplanets and the brightest gamma-ray bursts. In
principle, an IACT could achieve millimagnitude photometry of these objects
with second-long exposures. I also consider the potential for optical
spectroscopy with IACTs, finding that their poor angular resolution limits
their usefulness for high spectral resolutions, unless complex instruments are
developed. The high photon collection rate of IACTs is potentially useful for
precise polarimetry. Finally, I briefly discuss the broader possibilities of
extremely large, low resolution telescopes, including a 10" resolution
telescope and spaceborne telescopes.Comment: 10 pages, 3 figures, accepted by MNRA
Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000
In this paper we present the results of simultaneous observations of the TeV
blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the
Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope
system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively.
The source was monitored from February 2nd to February 16th and from May 3rd to
May 8th, 2000. We discuss in detail the temporal and spectral properties of the
source. Remarkably, the TeV observations of February 7th/8th showed
statistically significant evidence for substantial TeV flux variability on 30
min time scale. We show the results of modeling the data with a time dependent
homogeneous Synchrotron Self-Compton (SSC) model. The X-ray and TeV gamma-ray
emission strengths and energy spectra together with the rapid flux variability
strongly suggest that the emission volume is approaching the observer with a
Doppler factor of 50 or higher. The different flux variability time scales
observed at X-rays and TeV Gamma-rays indicate that a more detailed analysis
will require inhomogeneous models with several emission zones.Comment: Accepted for Publication in ApJ, 21 Pages, 5 Figure
Rejection of the hypothesis that Markarian 501 TeV photons are pure Bose-Einstein condensates
The energy spectrum of the Blazar type galaxy Markarian 501 (Mrk 501) as
measured by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cerenkov telescopes
extends beyond 16 TeV and constitutes the most energetic photons observed from
an extragalactic object. A fraction of the emitted spectrum is possibly
absorbed in interactions with low energy photons of the diffuse extragalactic
infrared radiation, which in turn offers the unique possibility to measure the
diffuse infrared radiation density by TeV spectroscopy. The upper limit on the
density of the extragalactic infrared radiation derived from the TeV
observations imposes constraints on models of galaxy formation and stellar
evolution. One of the recently published ideas to overcome severe absorption of
TeV photons is based upon the assumption that sources like Mrk 501 could
produce Bose-Einstein condensates of coherent photons. The condensates would
have a higher survival probability during the transport in the diffuse
radiation field and could mimic TeV air shower events. The powerful
stereoscopic technique of the HEGRA air Cerenkov telescopes allows to test this
hypothesis by reconstructing the penetration depths of TeV air shower events:
Air showers initiated by Bose-Einstein condensates are expected to reach the
maximum of the shower development in the atmosphere earlier than single photon
events. By comparing the energy-dependent penetration depths of TeV photons
from Mrk 501 with those from the TeV standard-candle Crab Nebula and simulated
air shower events, we can reject the hypothesis that TeV photons from Mrk 501
are pure Bose-Einstein condensates.Comment: 9 pages, 2 figures, published by ApJ Letters, revised version
(simulation results added
The TeV Energy Spectrum of Mkn 501 Measured with the Stereoscopic Telescope System of HEGRA during 1998 and 1999
During 1997, the BL Lac object Mkn 501 went into an extraordinary state of
high X-ray and TeV gamma-ray activity, lasting more than 6 months. In this
paper we report on the TeV emission characteristics of the source in the
subsequent years of 1998 and 1999 as measured with the Stereoscopic Cherenkov
Telescope System of HEGRA (La Palma, Canary Islands). Our observations reveal a
1998-1999 mean emission level at 1 TeV of 1/3 of the flux of the Crab Nebula, a
factor of 10 lower than during the year of 1997. A dataset of 122 observations
hours with the HEGRA telescope system makes it possible to assess for the first
time the Mkn 501 TeV energy spectrum for a mean flux level substantially below
that of the Crab Nebula with reasonable statistical accuracy. Excluding the
data of a strong flare, we find evidence that the 1998--1999 low-flux spectrum
is substantially softer (by 0.44+-0.1(stat) in spectral index) than the 1997
time averaged spectrum. The 500 GeV to 10 TeV energy spectrum can well be
described by a power law model with exponential cutoff: dN/dE ~ E^(-alpha)
exp(-E/E0) with alpha=2.31+-0.22(stat), and E0=5.1 (-2.3+7.8)(stat) TeV. Within
statistical accuracy, also a pure power law model gives an acceptable fit to
the data: dN/dE ~ E^(-Gamma) with Gamma=2.76+-0.08(stat). After presenting the
1998-1999 TeV characteristics of the source we discuss the implications of the
results.Comment: Accepted for publication in The Astrophysical Journal, Part 1, on
August 4th, 200
- âŠ