41 research outputs found
High Precision Mass Measurements in and Families Revisited
High precision mass measurements in and families performed
in 1980-1984 at the VEPP-4 collider with OLYA and MD-1 detectors are revisited.
The corrections for the new value of the electron mass are presented. The
effect of the updated radiative corrections has been calculated for the
and mass measurements.Comment: 5 pages, 1 table, submitted to Phys. Lett.
New precise determination of the \tau lepton mass at KEDR detector
The status of the experiment on the precise lepton mass measurement
running at the VEPP-4M collider with the KEDR detector is reported. The mass
value is evaluated from the cross section behaviour around the
production threshold. The preliminary result based on 6.7 pb of data is
MeV. Using 0.8 pb of data
collected at the peak the preliminary result is also obtained:
eV.Comment: 6 pages, 8 figures; The 9th International Workshop on Tau-Lepton
Physics, Tau0
VHE gamma-ray emission of PKS 2155-304: spectral and temporal variability
Observations of very high energy gamma-rays from blazars provide information
about acceleration mechanisms occurring in their innermost regions. Studies of
variability in these objects allow a better understanding of the mechanisms at
play. To investigate the spectral and temporal variability of VHE (>100 GeV)
gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304
with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range of
flux states. Data collected from 2005 to 2007 are analyzed. Spectra are derived
on time scales ranging from 3 years to 4 minutes. Light curve variability is
studied through doubling timescales and structure functions, and is compared
with red noise process simulations. The source is found to be in a low state
from 2005 to 2007, except for a set of exceptional flares which occurred in
July 2006. The quiescent state of the source is characterized by an associated
mean flux level of 4.32 +/-0.09 x 10^-11 cm^-2 s^-1 above 200 GeV, or
approximately 15% of the Crab Nebula, and a power law photon index of 3.53
+/-0.06. During the flares of July 2006, doubling timescales of ~2 min are
found. The spectral index variation is examined over two orders of magnitude in
flux, yielding different behaviour at low and high fluxes,which is a new
phenomenon in VHE gamma-ray emitting blazars. The variability amplitude
characterized by the fractional r.m.s. is strongly energy-dependent and is
proportional to E^(0.19 +/- 0.01). The light curve r.m.s. correlates with the
flux. This is the signature of a multiplicative process which can be accounted
for as a red noise with a Fourier index of ~2. This unique data set shows
evidence for a low level gamma-ray emission state from PKS 2155-304, which
possibly has a different origin than the outbursts. The discovery of the light
curve lognormal behaviour might be an indicator ..Comment: 17 pages, 13 figures, Accepted for publication to Astronomy &
Astrophysic
Multi-wavelength observations of H 2356-309
AIMS: The properties of the broad-band emission from the high-frequency
peaked BL Lac H 2356-309 (z=0.165) are investigated. METHODS: Very High Energy
(VHE; E > 100 GeV) observations of H 2356-309 were performed with the High
Energy Stereoscopic System (HESS) from 2004 through 2007. Simultaneous
optical/UV and X-ray observations were made with the XMM-Newton satellite on
June 12/13 and June 14/15, 2005. NRT radio observations were also
contemporaneously performed in 2005. ATOM optical monitoring observations were
also made in 2007. RESULTS: A strong VHE signal, ~13 sigma total, was detected
by HESS after the four years HESS observations (116.8 hrs live time). The
integral flux above 240 GeV is I(>240 GeV) = (3.06 +- 0.26 {stat} +- 0.61
{syst}) x 10^{-12} cm^{-2} s^{-1}, corresponding to ~1.6% of the flux observed
from the Crab Nebula. A time-averaged energy spectrum is measured from 200 GeV
to 2 TeV and is characterized by a power law (photon index of Gamma = 3.06 +-
0.15 {stat} +- 0.10 {syst}). Significant small-amplitude variations in the VHE
flux from H 2356-309 are seen on time scales of months and years, but not on
shorter time scales. No evidence for any variations in the VHE spectral slope
are found within these data. The XMM-Newton X-ray measurements show a
historically low X-ray state, characterized by a hard, broken-power-law
spectrum on both nights. CONCLUSIONS: The broad-band spectral energy
distribution (SED) of the blazar can be adequately fit using a simple one-zone
synchrotron self-Compton (SSC) model. In the SSC scenario, higher VHE fluxes
could be expected in the future since the observed X-ray flux is at a
historically low level.Comment: 11 pages; 13 figures; Accepted for publication in Astronomy &
Astrophysics; Figure 8 (SED) updated from V1 to have erroneously plotted
optical point (without host galaxy subtracted) removed
Discovery of VHE -ray emission and multi-wavelength observations of the BL Lacertae object 1RXS J101015.9-311909
1RXS J101015.9-311909 is a galaxy located at a redshift of z=0.14 hosting an
active nucleus belonging to the class of bright BL Lac objects. Observations at
high (HE, E > 100 MeV) and very high (VHE, E > 100 GeV) energies provide
insights into the origin of very energetic particles present in such sources
and the radiation processes at work. We report on results from VHE observations
performed between 2006-10 with H.E.S.S. H.E.S.S. data have been analysed with
enhanced analysis methods, making the detection of faint sources more
significant. VHE emission at a position coincident with 1RXS J101015.9-311909
is detected with H.E.S.S. for the first time. In a total good-quality livetime
of about 49 h, we measure 263 excess counts, corresponding to a significance of
7.1\sigma. The photon spectrum above 0.2 TeV can be described by a power-law
with a photon index of \Gamma\ = 3.08\pm0.42_{stat}\pm0.20_{sys}. The integral
flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no
significant variability over the time reported. In addition, public Fermi/LAT
data are analysed to search for high energy emission from the source. The
Fermi/LAT HE emission is significant at 8.3\sigma\ in the chosen 25-month
dataset. UV and X-ray contemporaneous observations with the Swift satellite in
May 2007 are also reported, together with optical observations performed with
the ATOM telescope located at the H.E.S.S. site. Swift observations reveal an
absorbed X-ray flux of F_{0.3-7 keV} = 1.04^{+0.04}_{-0.05} \times 10^{-11}
erg.cm^{-2}.s^{-1} in the 0.3-7 keV range. Finally, all the available data are
used to study the source's multi-wavelength properties. The SED can be
reproduced using a simple one-zone SSC model with emission from a region with a
Doppler factor of 30 and a magnetic field between 0.025 and 0.16 G. These
parameters are similar to those obtained for other sources of this type.Comment: 10 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
Spectral Analysis and Interpretation of the -Ray Emission from the Starburst Galaxy NGC 253
Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100
GeV) data from \gamma-ray observations performed with the H.E.S.S. telescope
array and the Fermi-LAT instrument, respectively, are analysed in order to
investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE
\gamma-ray data can be described by a power law in energy with differential
photon index \Gamma=2.14 \pm 0.18_stat \pm 0.30_sys and differential flux
normalisation at 1 TeV of F_0 = (9.6 \pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14}
TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE \gamma-ray
spectrum reveals a photon index of \Gamma=2.24 \pm 0.14_stat \pm 0.03_sys and
an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 \pm
1.0_stat \pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral
break or turnover is found over the dynamic range of both the LAT instrument
and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE
\gamma-ray data results in a differential photon index \Gamma=2.34 \pm 0.03
with a p-value of 30%. The \gamma-ray observations indicate that at least about
20% of the energy of the cosmic rays capable of producing hadronic interactions
is channeled into pion production. The smooth alignment between the spectra in
the HE and VHE \gamma-ray domain suggests that the same transport processes
dominate in the entire energy range. Advection is most likely responsible for
charged particle removal from the starburst nucleus from GeV to multiple TeV
energies. In a hadronic scenario for the \gamma-ray production, the single
overall power-law spectrum observed would therefore correspond to the mean
energy spectrum produced by the ensemble of cosmic-ray sources in the starburst
region.Comment: paper accepted for publication in ApJ, 12 pages, 3 figures, 5 table
H.E.S.S. constraints on dark matter annihilations towards the sculptor and carina dwarf galaxies
25 pages, 12 figuresThe Sculptor and Carina Dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov telescope array between January 2008 and December 2009. The data sets consist of a total of 11.8 and 14.8 hours of high quality data, respectively. No gamma-ray signal was detected at the nominal positions of these galaxies above 220 GeV and 320 GeV, respectively. Upper limits on the gamma-ray fluxes at 95% C.L. assuming two forms for the spectral energy distribution (a power law shape and one derived from dark matter annihilation) are obtained at the level of 10^-13 to 10^-12 cm^-2s^-1 in the TeV range. Constraints on the velocity weighted dark matter particle annihilation cross section for both Sculptor and Carina dwarf galaxies range from ~ 10^-21 cm^3s^-1 down to ~ 10^-22 cm^3s^-1 depending on the dark matter halo model used. Possible enhancements of the gamma-ray flux are studied: the Sommerfeld effect, which is found to exclude some dark matter particle masses, the internal Bremsstrahlung and clumps in the dark-matter halo distributions
Discovery of hard-spectrum -ray emission from the BL Lacertae object 1ES 0414+009
1ES 0414+009 (z = 0.287) is a distant high-frequency-peaked BL Lac object,
and has long been considered a likely emitter of very-high energy (VHE, E>100
GeV) gamma-rays due to its high X-ray and radio flux. Observations in the VHE
gamma-ray band and across the electromagnetic spectrum can provide insights
into the origin of highly energetic particles present in the source and the
radiation processes at work. Because of the distance of the source, the
gamma-ray spectrum might provide further limits on the level of the
Extragalactic Background Light (EBL). We report observations made between
October 2005 and December 2009 with H.E.S.S., an array of four imaging
atmospheric Cherenkov telescopes. Observations at high energies (HE, 100 MeV -
100 GeV) with the Fermi-LAT instrument in the first 20 months of its operation
are also reported. To complete the multi-wavelength picture, archival UV and
X-ray observations with the Swift satellite and optical observations with the
ATOM telescope are also used. Based on the observations with H.E.S.S., 1ES
0414+009 is detected for the first time in the VHE band. An excess of 224
events is measured, corresponding to a significance of 7.8 sigma. The photon
spectrum of the source is well described by a power law, with photon index of
3.45 \pm 0.25stat \pm 0.20syst. The integral flux above 200 GeV is (1.88 \pm
0.20stat \pm 0.38syst) \times10-12 cm-2 s-1. Observations with the Fermi-LAT in
the first 20 months of operation show a flux between 200 MeV and 100 GeV of
(2.3 \pm 0.2stat) \times 10-9 erg cm-2 s-1, and a spectrum well described by a
power-law function with a photon index 1.85 \pm 0.18. Swift/XRT observations
show an X-ray flux between 2 and 10 keV of (0.8 - 1) \times 10-11 erg cm-2 s-1,
and a steep spectrum (2.2 - 2.3). Combining X-ray with optical-UV data, a fit
with a log-parabolic function locates the synchrotron peak around 0.1 keV. ...Comment: 9 pages, 4 figures, accepted for publication in A&