791 research outputs found
Probe of Lorentz Invariance Violation effects and determination of the distance of PG 1553+113
The high frequency peaked BL Lac object PG 1553+113 underwent a flaring event
in 2012. The High Energy Stereoscopic System (H.E.S.S.) observed this source
for two consecutive nights at very high energies (VHE, 100~GeV). The data
show an increase of a factor of three of the flux with respect to archival
measurements with the same instrument and hints of intra-night variability. The
data set has been used to put constraints on possible Lorentz invariance
violation (LIV), manifesting itself as an energy dependence of the velocity of
light in vacuum, and to set limits on the energy scale at which Quantum Gravity
effects causing LIV may arise. With a new method to combine H.E.S.S. and Fermi
large area telescope data, the previously poorly known redshift of PG 1555+113
has been determined to be close to the value derived from optical measurements.Comment: 2014 Fermi Symposium proceedings - eConf C14102.
H.E.S.S. discovery of very-high-energy gamma-ray emission of PKS 1440-389
Blazars are the most abundant class of known extragalactic very-high-energy
(VHE, E>100 GeV) gamma-ray sources. However, one of the biggest difficulties in
investigating their VHE emission resides in their limited number, since less
than 60 of them are known by now. In this contribution we report on H.E.S.S.
observations of the BL Lac object PKS 1440-389. This source has been selected
as target for H.E.S.S. based on its high-energy gamma-ray properties measured
by Fermi-LAT. The extrapolation of this bright, hard-spectrum gamma-ray blazar
into the VHE regime made a detection on a relatively short time scale very
likely, despite its uncertain redshift. H.E.S.S. observations were carried out
with the 4-telescope array from February to May 2012 and resulted in a clear
detection of the source. Contemporaneous multi-wavelength data are used to
construct the spectral energy distribution of PKS 1440-389 which can be
described by a simple one-zone synchrotron-self Compton model.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
Optical-NIR spectroscopy of the puzzling gamma-ray source 3FGL 1603.9-4903/PMN J1603-4904 with X-shooter
The Fermi/LAT instrument has detected about two thousands Extragalactic High
Energy (E > 100 MeV) gamma-ray sources. One of the brightest is 3FGL
1603.9-4903, associated to the radio source PMN J1603-4904. Its nature is not
yet clear, it could be either a very peculiar BL Lac or a CSO (Compact
Symmetric Object) radio source, considered as the early stage of a radio
galaxy. The latter, if confirmed, would be the first detection in gamma-rays
for this class of objects. Recently a redshift z=0.18 +/- 0.01 has been claimed
on the basis of the detection of a single X-ray line at 5.44 +/- 0.05 keV
interpreted as a 6.4 keV (rest frame) fluorescent line. We aim to investigate
the nature of 3FGL 1603.9-4903/PMN J1603-4904 using optical to NIR
spectroscopy. We observed PMN J1603-4904 with the UV-NIR VLT/X-shooter
spectrograph for two hours. We extracted spectra in the VIS and NIR range that
we calibrated in flux and corrected for telluric absorption and we
systematically searched for absorption and emission features. The source was
detected starting from ~6300 Ang down to 24000 Ang with an intensity comparable
to the one of its 2MASS counterpart and a mostly featureless spectrum. The
continuum lacks absorption features and thus is non-stellar in origin and
likely non-thermal. On top of this spectrum we detected three emission lines
that we interpret as the Halpha-[NII] complex, the [SII] 6716,6731 doublet and
the [SIII] 9530 line, obtaining a redshift estimate of z= 0.2321 +/- 0.0004.
The equivalent width of the Halpha-[NII] complex implies that PMN J1603-4904
does not follow the observational definition of BL Lac, the line ratios suggest
that a LINER/Seyfert nucleus is powering the emission. This new redshift
measurement implies that the X-ray line previously detected should be
interpreted as a 6.7 keV line which is very peculiar.Comment: Published in Astronomy and Astrophysic
New AGNs discovered by H.E.S.S
During the last year, six new Active Galactic Nuclei (AGN) have been
discovered and studied by H.E.S.S. at Very High Energies (VHE). Some of these
recent discoveries have been made thanks to new enhanced analysis methods and
are presented at this conference for the first time. The three blazars 1ES
0414+009, SHBL J001355.9-185406 and 1RXS J101015.9-311909 have been targeted
for observation due to their high levels of radio and X-ray fluxes, while the
Fermi/LAT catalogue of bright sources triggered the observation of PKS 0447-439
and AP Librae. Additionally, the BL Lac 1ES 1312-423 was discovered in the
field-of-view (FoV) of Centaurus A thanks to the large exposure dedicated by
H.E.S.S. to this particularly interesting source. The newly-discovered sources
are presented here and in three companion presentations at this conference.Comment: 8 pages, 3 figures, proceeding from the 25th Texas Symposium on
Relativistic Astrophysics (Heidelberg, Germany, 2010
Observations of the Crab Nebula with H.E.S.S. Phase II
The High Energy Stereoscopic System (H.E.S.S.) phase I instrument was an
array of four mirror area Imaging Atmospheric Cherenkov
Telescopes (IACTs) that has very successfully mapped the sky at photon energies
above GeV. Recently, a telescope was added to
the centre of the existing array, which can be operated either in standalone
mode or jointly with the four smaller telescopes. The large telescope lowers
the energy threshold for gamma-ray observations to several tens of GeV, making
the array sensitive at energies where the Fermi-LAT instrument runs out of
statistics. At the same time, the new telescope makes the H.E.S.S. phase II
instrument. This is the first hybrid IACT array, as it operates telescopes of
different size (and hence different trigger rates) and different field of view.
In this contribution we present results of H.E.S.S. phase II observations of
the Crab Nebula, compare them to earlier observations, and evaluate the
performance of the new instrument with Monte Carlo simulations.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
Long term variability of the blazar PKS 2155-304
International audienceTime variability of the photon flux is a known feature of active galactic nuclei (AGN) and in particular of blazars. The high frequency peaked BL Lac (HBL) object PKS 2155-304 is one of the brightest sources in the TeV band and has been monitored regularly with different instruments and in particular with the H.E.S.S. experiment above 200 GeV for more than 11 years. These data together with the observations of other instruments and monitoring programs like SMARTS (optical), Swift-XRT/RXTE/XMM-Newton (X-ray) and Fermi-LAT (100 MeV < E < 300 GeV) are used to characterize the variability of this object in the quiescent state over a wide energy range. Variability studies are made by looking at the lognormality of the light curves and at the fractional root mean square (rms) variability F var in several energy bands. Lognormality is found in every energy range and the evolution of F var with the energy shows a similar increase both in X-rays and in TeV bands
Discovery of VHE gamma-rays from RGB J0152+017
The BL Lac object RGB J0152+017 (z = 0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes.
We report recent observations of this source made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cerenkov telescopes. Contemporaneous observations were made in X-rays with the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope.
As a result, RGB J0152+017 is discovered as a source of VHE gamma-rays by
H.E.S.S. A signal of 173 gamma-ray photons corresponding to a statistical
significance of 6.6 sigmas was found in the data. The energy spectrum of the
source can be described by a powerlaw with a spectral index of = 2.95 +-
0.36stat +- 0.20syst. The integral flux above 300 GeV corresponds to ~2% of the
flux of the Crab nebula. The source spectral energy distribution (SED) can be
described using a two-component (extended jet and blob in jet) non-thermal
synchrotron self-Compton (SSC) leptonic model, plus a thermal host galaxy
component. The parameters that are found are very close to those found for TeV
blazars in similar SSC studies.
The location of its synchrotron peak, as derived from the SED in Swift data,
allows clear classification as a high-frequency-peaked BL Lac (HBL).Comment: 6 pages, 5 figures; accepted to appear in the proceedings of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, 22-25
April 2008, Palaiseau, Franc
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