1,011 research outputs found
Binaries with the eyes of CTA
The binary systems that have been detected in gamma rays have proven very
useful to study high-energy processes, in particular particle acceleration,
emission and radiation reprocessing, and the dynamics of the underlying
magnetized flows. Binary systems, either detected or potential gamma-ray
emitters, can be grouped in different subclasses depending on the nature of the
binary components or the origin of the particle acceleration: the interaction
of the winds of either a pulsar and a massive star or two massive stars;
accretion onto a compact object and jet formation; and interaction of a
relativistic outflow with the external medium. We evaluate the potentialities
of an instrument like the Cherenkov telescope array (CTA) to study the
non-thermal physics of gamma-ray binaries, which requires the observation of
high-energy phenomena at different time and spatial scales. We analyze the
capability of CTA, under different configurations, to probe the spectral,
temporal and spatial behavior of gamma-ray binaries in the context of the known
or expected physics of these sources. CTA will be able to probe with high
spectral, temporal and spatial resolution the physical processes behind the
gamma-ray emission in binaries, significantly increasing as well the number of
known sources. This will allow the derivation of information on the particle
acceleration and emission sites qualitatively better than what is currently
available.Comment: 23 pages, 13 figures, accepted for publication in Astroparticle
Physics, special issue on Physics with the Cherenkov Telescope Arra
The major upgrade of the MAGIC telescopes, Part II: A performance study using observations of the Crab Nebula
MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in
the Canary island of La Palma, Spain. During summer 2011 and 2012 it underwent
a series of upgrades, involving the exchange of the MAGIC-I camera and its
trigger system, as well as the upgrade of the readout system of both
telescopes. We use observations of the Crab Nebula taken at low and medium
zenith angles to assess the key performance parameters of the MAGIC stereo
system. For low zenith angle observations, the standard trigger threshold of
the MAGIC telescopes is ~50GeV. The integral sensitivity for point-like sources
with Crab Nebula-like spectrum above 220GeV is (0.66+/-0.03)% of Crab Nebula
flux in 50 h of observations. The angular resolution, defined as the sigma of a
2-dimensional Gaussian distribution, at those energies is < 0.07 degree, while
the energy resolution is 16%. We also re-evaluate the effect of the systematic
uncertainty on the data taken with the MAGIC telescopes after the upgrade. We
estimate that the systematic uncertainties can be divided in the following
components: < 15% in energy scale, 11-18% in flux normalization and +/-0.15 for
the energy spectrum power-law slope.Comment: 21 pages, 25 figures, accepted for publication in Astroparticle
Physic
MAGIC detection of short-term variability of the high-peaked BL Lac object 1ES 0806+524
The high-frequency-peaked BL Lac (HBL) 1ES 0806+524 (z = 0.138) was
discovered in VHE rays in 2008. Until now, the broad-band spectrum of
1ES 0806+524 has been only poorly characterized, in particular at high
energies. We analysed multiwavelength observations from rays to radio
performed from 2011 January to March, which were triggered by the high activity
detected at optical frequencies. These observations constitute the most precise
determination of the broad-band emission of 1ES 0806+524 to date. The
stereoscopic MAGIC observations yielded a -ray signal above 250 GeV of
per cent of the Crab Nebula flux with a statistical
significance of 9.9 . The multiwavelength observations showed
significant variability in essentially all energy bands, including a VHE
-ray flare that lasted less than one night, which provided
unprecedented evidence for short-term variability in 1ES 0806+524. The spectrum
of this flare is well described by a power law with a photon index of between 150 GeV and 1 TeV and an integral flux of
per cent of the Crab Nebula flux above 250 GeV. The spectrum during the
non-flaring VHE activity is compatible with the only available VHE observation
performed in 2008 with VERITAS when the source was in a low optical state. The
broad-band spectral energy distribution can be described with a one-zone
Synchrotron Self Compton model with parameters typical for HBLs, indicating
that 1ES 0806+524 is not substantially different from the HBLs previously
detected.Comment: 12 pages, 8 figures, 3 tables, accepted 2015 April 20 for publication
in Monthly Notices of the Royal Astronomical Society Main Journa
Measurement of the Strong Coupling Constant from Inclusive Jet Production at the Tevatron Collider
We report a measurement of the strong coupling constant, ,
extracted from inclusive jet production in collisions at
1800 GeV. The QCD prediction for the evolution of with
jet transverse energy is tested over the range 40<<450 GeV using
for the renormalization scale. The data show good agreement with QCD in
the region below 250 GeV. In the text we discuss the data-theory comparison in
the region from 250 to 450 GeV. The value of at the mass of the
boson averaged over the range 40<<250 GeV is found to be
. The associated theoretical uncertainties are mainly due to the choice
of renormalization scale (^{+6%}_{-4%}) and input parton distribution
functions (5%).Comment: 7 pages, 3 figures, using RevTeX. Submitted to Physical Review
Letter
Probing the very-high-energy gamma-ray spectral curvature in the blazar PG 1553+113 with the MAGIC telescopes
PG 1553+113 is a very-high-energy (VHE, ) -ray
emitter classified as a BL Lac object. Its redshift is constrained by
intergalactic absorption lines in the range . The MAGIC telescopes
have monitored the source's activity since 2005. In early 2012, PG 1553+113 was
found in a high-state, and later, in April of the same year, the source reached
its highest VHE flux state detected so far. Simultaneous observations carried
out in X-rays during 2012 April show similar flaring behaviour. In contrast,
the -ray flux at observed by Fermi-LAT is
compatible with steady emission. In this paper, a detailed study of the flaring
state is presented. The VHE spectrum shows clear curvature, being well fitted
either by a power law with an exponential cut-off or by a log-parabola. A
simple power-law fit hypothesis for the observed shape of the PG 1553+113 VHE
-ray spectrum is rejected with a high significance (fit probability
P=2.6 ). The observed curvature is compatible with the
extragalactic background light (EBL) imprint predicted by current generation
EBL models assuming a redshift . New constraints on the redshift are
derived from the VHE spectrum. These constraints are compatible with previous
limits and suggest that the source is most likely located around the optical
lower limit, , based on the detection of Ly absorption. Finally,
we find that the synchrotron self-Compton (SSC) model gives a satisfactory
description of the observed multi-wavelength spectral energy distribution
during the flare.Comment: 13 pages, 7 figures, accepted for publication in MNRA
Measurement of Rapidity Distribution for High Mass Drell-Yan ee Pairs at CDF
We report on the first measurement of the rapidity distribution dsigma/dy
over nearly the entire kinematic region of rapidity for e^+e^- pairs in the
Z-boson region of 66116 GeV/c^2.
The data sample consists of 108 pb^{-1} of ppbar collisions at \sqrt{s}=1.8 TeV
taken by the Collider Detector at Fermilab during 1992--1995. The total cross
section in the -boson region is measured to be 252 +- 11 pb. The measured
total cross section and d\sigma/dy are compared with quantum chromodynamics
calculations in leading and higher orders.Comment: 7 pages, 3 figures. Submitted to Physical Review Letter
Search for a Fourth-Generation Quark More Massive than the Z0 Boson in ppbar Collisions at sqrt(s) = 1.8 TeV
We present the results of a search for pair production of a fourth-generation
charge -1/3 quark (b') in sqrt(s)=1.8 TeV ppbar collisions using 88 pb^(-1) of
data obtained with the Collider Detector at Fermilab. We assume that both
quarks decay via the flavor-changing neutral current process b' -> bZ and that
the b' mass is greater than m_Z + m_b. We studied the decay mode b'b'bar -> ZZ
b bbar where one Z0 decays into e^+e^- or mu^+ mu^- and the other decays
hadronically, giving a signature of two leptons plus jets. An upper limit on
the cross section of ppbar -> b'b'bar times [BR (b' -> bZ)]^2 is established as
a function of the b' mass. We exclude at 95% confidence level a b' quark with
mass between 100 and 199 GeV/c^2 for BR(b' -> bZ) = 100%.Comment: 12 pages, 2 figures, submitted to Phys. Rev. Letters on 9/12/9
First broadband characterization and redshift determination of the VHE blazar MAGIC J2001+439
We aim to characterize the broadband emission from 2FGL J2001.1+4352, which
has been associated with the unknown-redshift blazar MG4 J200112+4352. Based on
its gamma-ray spectral properties, it was identified as a potential very high
energy (VHE; E > 100 GeV) gamma-ray emitter. The source was observed with MAGIC
first in 2009 and later in 2010 within a multi-instrument observation campaign.
The MAGIC observations yielded 14.8 hours of good quality stereoscopic data.
The object was monitored at radio, optical and gamma-ray energies during the
years 2010 and 2011. The source, named MAGIC J2001+439, is detected for the
first time at VHE with MAGIC at a statistical significance of 6.3 {\sigma} (E >
70 GeV) during a 1.3-hour long observation on 2010 July 16. The
multi-instrument observations show variability in all energy bands with the
highest amplitude of variability in the X-ray and VHE bands. We also organized
deep imaging optical observations with the Nordic Optical Telescope in 2013 to
determine the source redshift. We determine for the first time the redshift of
this BL Lac object through the measurement of its host galaxy during low blazar
activity. Using the observational evidence that the luminosities of BL Lac host
galaxies are confined to a relatively narrow range, we obtain z = 0.18 +/-
0.04. Additionally, we use the Fermi-LAT and MAGIC gamma-ray spectra to provide
an independent redshift estimation, z = 0.17 +/- 0.10. Using the former (more
accurate) redshift value, we adequately describe the broadband emission with a
one-zone SSC model for different activity states and interpret the few-day
timescale variability as produced by changes in the high-energy component of
the electron energy distribution.Comment: 17 pages, 15 figures, Accepted for publication in A&
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