49 research outputs found
An unidentified TeV source in the vicinity of Cygnus OB2
Deep observation (∼113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov 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 ∼0.5° north of Cyg X-3. The source centre of gravity is RA αJ2000: 20hr32m07s± 9.2stats±2.2syss, Dec δJ2000: +41°30′30″2.0stat±0.4′sys. The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius 5.6′ at the ∼3σ 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 ∼3% 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 γ-ray production via a jet-driven termination shock.F. A. Aharonian, ... G. P. Rowell, ... [et al
The Energy Spectrum of TeV Gamma-Rays from the Crab Nebula as measured by the HEGRA system of imaging air Cherenkov telescopes
The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray
Astronomy) stereoscopic system of imaging air Cherenkov telescopes (IACTs) for
a total of about 200 hrs during two observational campaigns: from September
1997 to March 1998 and from August 1998 to April 1999. The recent detailed
studies of system performance give an energy threshold and an energy resolution
for gamma-rays of 500 GeV and ~ 18%, respectively. The Crab energy spectrum was
measured with the HEGRA IACT system in a very broad energy range up to 20 TeV,
using observations at zenith angles up to 65 degrees. The Crab data can be
fitted in the energy range from 1 to 20 TeV by a simple power-law, which yields
dJg/dE = (2.79+/-0.02 +/- 0.5) 10^{-7} E^{-2.59 +/- 0.03 +/- 0.05}, ph m^{-2}
s^{-1} TeV^{-1} The Crab Nebula energy spectrum, as measured with the HEGRA
IACT system, agrees within 15% in the absolute scale and within 0.1 units in
the power law index with the latest measurements by the Whipple, CANGAROO and
CAT groups, consistent within the statistical and systematic errors quoted by
the experiments. The pure power-law spectrum of TeV gamma-rays from the Crab
Nebula constrains the physics parameters of the nebula environment as well as
the models of photon emission.Comment: to appear in ApJ, 29 pages, 6 figure
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
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
Correlated intense X-ray and TeV activity of Mrk~501 in 1998 June
We present exactly simultaneous X-ray and TeV monitoring with {\it RXTE} and
HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial
period of very low flux at both wavelengths, the source underwent a remarkable
flare in the TeV and X-ray energy bands, lasting for about six days and with a
larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV
flare, rapid TeV flux variability on sub-hour timescales is found. Large
spectral variations are observed at X-rays, with the 3--20 keV photon index of
a pure power law continuum flattening from to on a
timescale of 2--3 days. This implies that during the maximum of the TeV
activity, the synchrotron peak shifted to energies keV, a behavior
similar to that observed during the longer-lasting, more intense flare in 1997
April. The TeV spectrum during the flare is described by a power law with
photon index and an exponential cutoff at 4 TeV; an
indication for spectral softening during the flare decay is observed in the TeV
hardness ratios. Our results generally support a scenario where the TeV photons
are emitted via inverse Compton scattering of ambient seed photons by the same
electron population responsible for the synchrotron X-rays. The simultaneous
spectral energy distributions (SEDs) can be fit with a one-zone
synchrotron-self Compton model assuming a substantial increase of the magnetic
field and the electron energy by a factor of 3 and 10, respectively.Comment: Accepted for publication in ApJ, Part
Multiwavelength Observations of Strong Flares From the TeV-Blazar 1ES 1959+650
Following the detection of strong TeV gamma-ray flares from the BL Lac object
1ES 1959+650 with the Whipple 10 m Cherenkov telescope on May 16 and 17, 2002,
we performed intensive Target of Opportunity (ToO) radio, optical, X-ray and
TeV gamma-ray observations from May 18, 2002 to August 14, 2002. Observations
with the X-ray telescope RXTE and the Whipple and HEGRA gamma-ray telescopes
revealed several strong flares, enabling us to sensitively test the
X-ray/gamma-ray flux correlation properties. Although the X-ray and gamma-ray
fluxes seemed to be correlated in general, we found an ``orphan'' gamma-ray
flare that was not accompanied by an X-ray flare. After describing in detail
the radio (UMRAO, VLA), optical (Boltwood, Abastumani), X-ray (RXTE) and
gamma-ray (Whipple, HEGRA) light curves and Spectral Energy Distributions
(SEDs) we present initial modeling of the SED with a simple Synchrotron
Self-Compton (SSC) model. With the addition of another TeV blazar with good
broadband data, we consider the set of all TeV blazars to begin to look for a
connection of the jet properties to the properties of the central accreting
black hole thought to drive the jet. Remarkably, the temporal and spectral
X-ray and gamma-ray emission characteristics of TeV blazars are very similar,
even though the masses estimates of their central black holes differ by up to
one order of magnitude.Comment: Accepted for Publication in the ApJ on October 2nd, 200
The Crab Nebula and Pulsar between 500 GeV and 80 TeV: Observations with the HEGRA stereoscopic air Cherenkov telescopes
The Crab supernova remnant has been observed regularly with the stereoscopic
system of 5 imaging air Cherenkov telescopes that was part of the High Energy
Gamma Ray Astronomy (HEGRA) experiment. In total, close to 400 hours of useful
data have been collected from 1997 until 2002. The spectrum extends up to
energies of 80 TeV and is well matched by model calculations in the framework
of inverse Compton scattering of various seed photons in the nebula including
for the first time a recently detected compact emission region at
mm-wavelengths. The observed indications for a gradual steepening of the energy
spectrum in data is expected in the inverse Compton emission model.The average
magnetic field in the emitting volume is determined to be
G. The presence of protons in
the nebula is not required to explain the observed flux and upper limits on the
injected power of protons are calculated being as low as 20 % of the total spin
down luminosity for bulk Lorentz factors of the wind in the range of
.The position and size of the emission region have been studied over
a wide range of energies. The position is shifted by 13\arcsec to the west of
the pulsar with a systematic uncertainty of 25\arcsec. No significant shift in
the position with energy is observed. The size of the emission region is
constrained to be less than 2\arcmin at energies between 1 and 10 TeV. Above 30
TeV the size is constrained to be less than 3\arcmin.No indications for pulsed
emission has been found and upper limits in differential bins of energy have
been calculated reaching typically 1-3 % of the unpulsed component.Comment: 53 pages, 12 figures. Accepted for publication in Astrophysical
Journa
Mutations in fam20b and xylt1 Reveal That Cartilage Matrix Controls Timing of Endochondral Ossification by Inhibiting Chondrocyte Maturation
Differentiating cells interact with their extracellular environment over time. Chondrocytes embed themselves in a proteoglycan (PG)-rich matrix, then undergo a developmental transition, termed “maturation,” when they express ihh to induce bone in the overlying tissue, the perichondrium. Here, we ask whether PGs regulate interactions between chondrocytes and perichondrium, using zebrafish mutants to reveal that cartilage PGs inhibit chondrocyte maturation, which ultimately dictates the timing of perichondral bone development. In a mutagenesis screen, we isolated a class of mutants with decreased cartilage matrix and increased perichondral bone. Positional cloning identified lesions in two genes, fam20b and xylosyltransferase1 (xylt1), both of which encode PG synthesis enzymes. Mutants failed to produce wild-type levels of chondroitin sulfate PGs, which are normally abundant in cartilage matrix, and initiated perichondral bone formation earlier than their wild-type siblings. Primary chondrocyte defects might induce the bone phenotype secondarily, because mutant chondrocytes precociously initiated maturation, showing increased and early expression of such markers as runx2b, collagen type 10a1, and ihh co-orthologs, and ihha mutation suppressed early perichondral bone in PG mutants. Ultrastructural analyses demonstrated aberrant matrix organization and also early cellular features of chondrocyte hypertrophy in mutants. Refining previous in vitro reports, which demonstrated that fam20b and xylt1 were involved in PG synthesis, our in vivo analyses reveal that these genes function in cartilage matrix production and ultimately regulate the timing of skeletal development
Folk medicine and integrative healing in West Asia: Analysis of historical and modern practice and perceptions
Image Distortion in 7 Tesla and its significance for high-field Amygdala neurofeedback
Introduction: Even though the higher spatial resolution in 7T imaging is a boon for real-time fMRI BOLD imaging, a primary concern at 7 Tesla for obtaining a neurofeedback signal is image distortion in difficult-to-image regions near boundaries of tissue and air. Signal variability is -more so than at 3T- altered by image distortions, due to longer readout times of the EPI sequence (to obtain higher special resolution), and shorter T2*. Image distortions can be as large as 20-30 voxels with 160x160 readout matrix, and distributed along the entire brain. Therefore, we assess effects of distortion correction for fMRI Neurofeedback outcome measures. Image distortion in EPI imaging can affect fMRI timeseries and as a consequence, could impair quality of a RT-fMRI NF experiment. In this work we present the effect of using a real-time distortion correction on the quality of the NF signal in an Amygdala neurofeedback experiment, by prospective off-line comparison of corrected and uncorrected timecourses [1] Methods: The data were obtained from a 15 subjects at 7T using an echo-planar imaging scanning sequence covering a part of the brain including the amygdala and cingulate cortex, with a resolution of 1.4x1.4x2.0 mm3. A practice run was followed by training 1,2 and 3 runs concluded with a transfer run. The experiment consisted five 8-minute runs consisting of rest (r), up-regulation (u) and counting (cnt) blocks; see Fig. 1 (C). Interleaved with the blocks, feedback(f) was given, but not during Transfer Run. During the experiment, data was real-time motion corrected, real-time distortion corrected [2] and real-time transferred to an external computer using TCP/IP which drove the experiment with RtExplorer [1]. We retrospectively examine the effect of applying distortion correction on learning effects. Learning effects is defined by the increase-over-runs of image intensity of (up - count)/count in the Amygdala Mask. Results: Our results show that the distortion correction restored the distortions in the frontal lobe (Fig 1A) by pushing back signal up to 20 voxels. We also found that 1) the average distortion-corrected timeseries have a different shape in Amygdala ROI than the uncorrected timeseries and 2) that the average image intensity within the Amygdala is higher for the uncorrected EPI images: 478 instead of 402. Within the off-line analysis of the self-regulation capabilities across runs we found similar learning effects for both distorted and undistorted data, albeit with differences in the rate of change across runs. Conclusions: The most pronounced effect of image distortion can be spotted in frontal regions but expands also in more basal regions such as the Amygdala. Exploratory analysis of the effects of distortions in EPI images show that employment or omission of a real-time distortion correction has implications regarding neurofeedback training signal and effect sizes at 7 Tesla. These differences are likely due to the distortion correction 'pushing' the signal in the right anatomical location. The impact of distortion effects even increases with higher field strengths. At this point it is difficult to state whether the distortion-corrected or uncorrected images represent the 'true' BOLD changes throughout time. However, for anatomical correctness, an on-line EPI distortion correction is a critical step in the fMRI Neurofeedback processing pipeline at 7T