[Show abstract][Hide abstract] ABSTRACT: A survey of the inner Galaxy region of Galactic longitude l in [+15, +50]
degree and latitude b in [-4,+4] degree is performed using one-third of the
High Altitude Water Cherenkov (HAWC) Observatory operated during its
construction phase. To address the ambiguities arising from unresolved sources
in the data, we use a maximum likelihood technique to identify point source
candidates. Ten sources and candidate sources are identified in this analysis.
Eight of these are associated with known TeV sources but not all have
differential fluxes compatible with previous measurements. Three sources are
detected with significances $>5\,\sigma$ after accounting for statistical
trials, and are associated with known TeV sources.
[Show abstract][Hide abstract] ABSTRACT: The Auger Engineering Radio Array (AERA) is part of the Pierre Auger
Observatory and is used to detect the radio emission of cosmic-ray air showers.
These observations are compared to the data of the surface detector stations of
the Observatory, which provide well-calibrated information on the cosmic-ray
energies and arrival directions. The response of the radio stations in the 30
to 80 MHz regime has been thoroughly calibrated to enable the reconstruction of
the incoming electric field. For the latter, the energy density is determined
from the radio pulses at each observer position and is interpolated using a two
dimensional function that takes into account signal asymmetries due to
interference between the geomagnetic and charge excess emission components. The
spatial integral over the signal distribution gives a direct measurement of the
energy transferred from the primary cosmic ray into radio emission in the AERA
frequency range. We measure 15.8 MeV of radiation energy for a 1 EeV air shower
arriving perpendicularly to the geomagnetic field. This radiation energy --
corrected for geometrical effects -- is used as a cosmic-ray energy estimator.
Performing an absolute energy calibration against the surface-detector
information, we observe that this radio-energy estimator scales quadratically
with the cosmic-ray energy as expected for coherent emission. We find an energy
resolution of the radio reconstruction of 22% for the data set and 17% for a
high-quality subset containing only events with at least five radio stations
[Show abstract][Hide abstract] ABSTRACT: Ultra high energy photons and neutrinos are carriers of very important astrophysical information. They may be produced at the sites of cosmic ray acceleration or during the propagation of the cosmic rays in the intergalactic medium. In contrast to charged cosmic rays, photon and neutrino arrival directions point to the production site because they are not deflected by the magnetic fields of the Galaxy or the intergalactic medium. In this work we study the characteristics of the longitudinal development of showers initiated by photons and neutrinos at the highest energies. These studies are relevant for development of techniques for neutrino and photon identification by the JEM-EUSO telescope. In particular, we study the possibility of observing the multi-peak structure of very deep horizontal neutrino showers with JEM-EUSO. We also discuss the possibility to determine the flavor content of the incident neutrino flux by taking advantage of the different characteristics of the longitudinal profiles generated by different type of neutrinos. This is of grate importance for the study of the fundamental properties of neutrinos at the highest energies. Regarding photons, we discuss the detectability of the cosmogenic component by JEM-EUSO and also estimate the expected upper limits on the photon fraction which can be obtained from the future JEM-EUSO data for the case in which there are no photons in the samples.
[Show abstract][Hide abstract] ABSTRACT: Supernova remnants (SNRs) are among the most important targets for gamma-ray observatories. Being prominent non-thermal sources, they are very likely responsible for the acceleration of the bulk of Galactic cosmic rays (CRs). To firmly establish the SNR paradigm for the origin of cosmic rays, it should be confirmed that protons are indeed accelerated in, and released from, SNRs with the appropriate flux and spectrum. This can be done by detailed theoretical models which account for microphysics of acceleration and various radiation processes of hadrons and leptons. The current generation of Cherenkov telescopes has insu�cient sensitivity to constrain theoretical models. A new facility, the Cherenkov Telescope Array (CTA), will have superior capabilities and may finally resolve this long standing issue of high-energy astrophysics. We want to assess the capabilities of CTA to reveal the physics of various types of SNRs in the initial 2000 years of their evolution. During this time, the effi�ciency to accelerate cosmic rays is highest. We perform time-dependent simulations of the hydrodynamics, the magnetic fields, the cosmic-ray acceleration, and the non-thermal emission for type Ia, Ic and IIP SNRs. We calculate the CTA response to the gamma-ray emission from these SNRs for various ages and distances, and we perform a realistic analysis of the simulated data. We derive distance limits for the detectability and resolvability of these SNR types at several ages. We test the ability of CTA to reconstruct their morphological and spectral parameters as a function of their distance. Finally, we estimate how well CTA data will constrain the theoretical models.
(available on-line 28 September 2014)
[Show abstract][Hide abstract] ABSTRACT: We performed a 4.5-month multi-instrument campaign (from radio to VHE gamma
rays) on Mrk421 between January 2009 and June 2009, which included VLBA,
F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC, and Whipple, among other
instruments and collaborations. Mrk421 was found in its typical (non-flaring)
activity state, with a VHE flux of about half that of the Crab Nebula, yet the
light curves show significant variability at all wavelengths, the highest
variability being in the X-rays. We determined the power spectral densities
(PSD) at most wavelengths and found that all PSDs can be described by
power-laws without a break, and with indices consistent with pink/red-noise
behavior. We observed a harder-when-brighter behavior in the X-ray spectra and
measured a positive correlation between VHE and X-ray fluxes with zero time
lag. Such characteristics have been reported many times during flaring
activity, but here they are reported for the first time in the non-flaring
state. We also observed an overall anti-correlation between optical/UV and
X-rays extending over the duration of the campaign.
The harder-when-brighter behavior in the X-ray spectra and the measured
positive X-ray/VHE correlation during the 2009 multi-wavelength campaign
suggests that the physical processes dominating the emission during non-flaring
states have similarities with those occurring during flaring activity. In
particular, this observation supports leptonic scenarios as being responsible
for the emission of Mrk421 during non-flaring activity. Such a temporally
extended X-ray/VHE correlation is not driven by any single flaring event, and
hence is difficult to explain within the standard hadronic scenarios. The
highest variability is observed in the X-ray band, which, within the one-zone
synchrotron self-Compton scenario, indicates that the electron energy
distribution is most variable at the highest energies.
[Show abstract][Hide abstract] ABSTRACT: Context: MWC 656 has recently been established as the first observationally
detected high-mass X-ray binary system containing a Be star and a black hole
(BH). The system has been associated with a gamma-ray flaring event detected by
the AGILE satellite in July 2010. Aims: Our aim is to evaluate if the MWC 656
gamma-ray emission extends to very high energy (VHE > 100 GeV) gamma rays.
Methods. We have observed MWC 656 with the MAGIC telescopes for $\sim$23 hours
during two observation periods: between May and June 2012 and June 2013. During
the last period, observations were performed contemporaneously with X-ray
(XMM-Newton) and optical (STELLA) instruments. Results: We have not detected
the MWC 656 binary system at TeV energies with the MAGIC Telescopes in either
of the two campaigns carried out. Upper limits (ULs) to the integral flux above
300 GeV have been set, as well as differential ULs at a level of $\sim$5\% of
the Crab Nebula flux. The results obtained from the MAGIC observations do not
support persistent emission of very high energy gamma rays from this system at
a level of 2.4\% the Crab flux.
[Show abstract][Hide abstract] ABSTRACT: 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 ∼ 50
GeV. The integral sensitivity for point-like sources with Crab Nebula-like spectrum above 220 GeV is (0.66 ± 0.03)% of Crab Nebula flux in 50 h of observations. The angular resolution, defined as the σ of a 2-dimensional Gaussian distribution, at those energies is ≲ 0.07°, 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.
[Show abstract][Hide abstract] ABSTRACT: We present the third Fermi Large Area Telescope source catalog (3FGL) of sources in the 100 MeV–300 GeV range. Based on the first four years of science data from the Fermi Gamma-ray Space Telescope mission, it is the deepest yet in this energy range. Relative to the 2FGL catalog, the 3FGL catalog incorporates twice as much data as well as a number of analysis improvements, including improved calibrations at the event reconstruction level, an updated model for Galactic diffuse γ-ray emission, a refined procedure for source detection, and im- proved methods for associating LAT sources with potential counterparts at other wavelengths. The 3FGL catalog includes 3033 sources above 4σ significance, with source location regions, spectral properties, and monthly light curves for each. Of these, 78 are flagged as potentially being due to imperfections in the model for Galactic diffuse emission. Twenty-five sources are modeled explicitly as spatially extended, and overall 232 sources are considered as identified based on angular extent or correlated variability (periodic or otherwise) observed at other wavelengths. For 1009 sources we have not found plausible counterparts at other wavelengths. More than 1100 of the identified or associated sources are ac- tive galaxies of the blazar class; several other classes of non-blazar active galaxies are also represented in the 3FGL. Pulsars represent the largest Galactic source class. From source counts of Galactic sources we estimate the contribution of unresolved sources to the Galactic diffuse emission is ∼3% at 1 GeV.
The Astrophysical Journal Supplement Series 01/2015; · 11.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In these preliminary remarks we discuss our motivations for holding the San Vito di Cadore conference as well as some personal reflections on the history and current status of the origin of cosmic rays. We argue that it is time to think beyond the ‘standard model’ and contemplate the possibility of sources other than SNRs contributing to the observed cosmic ray flux even if the bulk originate in SNRs. In fact everyone tacitly assumes that at the very highest energies we do in fact see a new extra-Galactic component, but what it is and where exactly the transition occurs remain subjects of investigation. Similarly the positron excess seen by PAMELA and confirmed by AMS clearly points to an additional source of high-energy leptons in our Galactic neighbourhood. The recent observation by Agile and Fermi of the remarkable Crab gamma-ray flares point to some non-standard and very rapid form of particle acceleration which, if it occurs in other environments, may contribute to the acceleration of cosmic rays. In summary, it is clear that the origin of cosmic rays is a richer field of study than just diffusive shock acceleration in SNRs.
[Show abstract][Hide abstract] ABSTRACT: A flare from the TeV blazar Mrk 421, occurring in March 2010, was observed
for 13 consecutive days from radio to very high energy (VHE; E > 100 GeV)
gamma-rays with MAGIC, VERITAS, Whipple, FermiLAT, MAXI, RXTE, Swift,
GASP-WEBT, and several optical and radio telescopes. We model the day-scale
SEDs with one-zone and two-zone synchrotron self-Compton (SSC) models,
investigate the physical parameters, and evaluate whether the observed
broadband SED variability can be associated to variations in the relativistic
Flux variability was remarkable in the X-ray and VHE bands while it was minor
or not significant in the other bands. The one-zone SSC model can describe
reasonably well the SED of each day for the 13 consecutive days. This flaring
activity is also very well described by a two-zone SSC model, where one zone is
responsible for the quiescent emission while the other smaller zone, which is
spatially separated from the first one, contributes to the daily-variable
emission occurring in X-rays and VHE gamma-rays.
Both the one-zone SSC and the two-zone SSC models can describe the daily SEDs
via the variation of only four or five model parameters, under the hypothesis
that the variability is associated mostly to the underlying particle
population. This shows that the particle acceleration and cooling mechanism
producing the radiating particles could be the main one responsible for the
broadband SED variations during the flaring episodes in blazars. The two-zone
SSC model provides a better agreement to the observed SED at the narrow peaks
of the low- and high-energy bumps during the highest activity, although the
reported one-zone SSC model could be further improved by the variation of the
parameters related to the emitting region itself ($\delta$, $B$ and $R$), in
addition to the parameters related to the particle population.