450 research outputs found
Contextual Object Detection with a Few Relevant Neighbors
A natural way to improve the detection of objects is to consider the
contextual constraints imposed by the detection of additional objects in a
given scene. In this work, we exploit the spatial relations between objects in
order to improve detection capacity, as well as analyze various properties of
the contextual object detection problem. To precisely calculate context-based
probabilities of objects, we developed a model that examines the interactions
between objects in an exact probabilistic setting, in contrast to previous
methods that typically utilize approximations based on pairwise interactions.
Such a scheme is facilitated by the realistic assumption that the existence of
an object in any given location is influenced by only few informative locations
in space. Based on this assumption, we suggest a method for identifying these
relevant locations and integrating them into a mostly exact calculation of
probability based on their raw detector responses. This scheme is shown to
improve detection results and provides unique insights about the process of
contextual inference for object detection. We show that it is generally
difficult to learn that a particular object reduces the probability of another,
and that in cases when the context and detector strongly disagree this learning
becomes virtually impossible for the purposes of improving the results of an
object detector. Finally, we demonstrate improved detection results through use
of our approach as applied to the PASCAL VOC and COCO datasets
Long-term gamma-ray observations of the binary HESS J0632+057 with H.E.S.S., MAGIC and VERITAS
The gamma-ray binary HESS J0632+057 has been observed at very-high energies
(E 100 GeV) for more than ten years by the major systems of imaging
atmospheric Cherenkov telescopes. We present a summary of results obtained with
the H.E.S.S., MAGIC, and VERITAS experiments based on roughly 440 h of
observations in total. This includes a discussion of an unusually bright TeV
outburst of HESS J0632+057 in January 2018. The updated gamma-ray light curve
now covers all phases of the orbital period with significant detections in
almost all orbital phases. Results are discussed in context with simultaneous
observations with the X-ray Telescope onboard the Neil Gehrels Swift
Observatory.Comment: Proceedings of the 36th International Cosmic Ray Conference
(ICRC2019) Madison, WI July 24-Aug 1, 201
Detection of bridge emission above 50 GeV from the Crab pulsar with the MAGIC telescopes
The Crab pulsar is the only astronomical pulsed source detected at very high
energy (VHE, E>100GeV) gamma-rays. The emission mechanism of VHE pulsation is
not yet fully understood, although several theoretical models have been
proposed. In order to test the new models, we measured the light curve and the
spectra of the Crab pulsar with high precision by means of deep observations.
We analyzed 135 hours of selected MAGIC data taken between 2009 and 2013 in
stereoscopic mode. In order to discuss the spectral shape in connection with
lower energies, 4.6 years of {\it Fermi}-LAT data were also analyzed. The known
two pulses per period were detected with a significance of and
. In addition, significant emission was found between the two
pulses with . We discovered the bridge emission above 50 GeV
between the two main pulses. This emission can not be explained with the
existing theories. These data can be used for testing new theoretical models.Comment: 5 pages, 4 figure
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&
Measurement of the Crab Nebula spectrum over three decades in energy with the MAGIC telescopes
The MAGIC stereoscopic system collected 69 hours of Crab Nebula data between
October 2009 and April 2011. Analysis of this data sample using the latest
improvements in the MAGIC stereoscopic software provided an unprecedented
precision of spectral and night-by-night light curve determination at gamma
rays. We derived a differential spectrum with a single instrument from 50 GeV
up to almost 30 TeV with 5 bins per energy decade. At low energies, MAGIC
results, combined with Fermi-LAT data, show a flat and broad Inverse Compton
peak. The overall fit to the data between 1 GeV and 30 TeV is not well
described by a log-parabola function. We find that a modified log-parabola
function with an exponent of 2.5 instead of 2 provides a good description of
the data (). Using systematic uncertainties of red the MAGIC and
Fermi-LAT measurements we determine the position of the Inverse Compton peak to
be at (53 3stat + 31syst -13syst) GeV, which is the most precise
estimation up to date and is dominated by the systematic effects. There is no
hint of the integral flux variability on daily scales at energies above 300 GeV
when systematic uncertainties are included in the flux measurement. We consider
three state- of-the-art theoretical models to describe the overall spectral
energy distribution of the Crab Nebula. The constant B-field model cannot
satisfactorily reproduce the VHE spectral measurements presented in this work,
having particular difficulty reproducing the broadness of the observed IC peak.
Most probably this implies that the assumption of the homogeneity of the
magnetic field inside the nebula is incorrect. On the other hand, the
time-dependent 1D spectral model provides a good fit of the new VHE results
when considering a 80 {\mu}G magnetic field. However, it fails to match the
data when including the morphology of the nebula at lower wavelengths.Comment: accepted by JHEAp, 9 pages, 6 figure
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
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
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
- …