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Aleksic J.,IFAE | Alvarez E.A.,Complutense University of Madrid | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 165 more authors.
Journal of Cosmology and Astroparticle Physics | Year: 2011

We report the results of the observation of the nearby satellite galaxy Segue 1 performed by the MAGIC-I ground-based gamma-ray telescope between November 2008 and March 2009 for a total of 43.2 hours. No significant gamma-ray emission was found above the background. Differential upper limits on the gamma-ray flux are derived assuming various power-law slopes for the possible emission spectrum. Integral upper limits are also calculated for several power-law spectra and for different energy thresholds. The values are of the order of 10-11 ph cm-2 s-1 above 100 GeV and 10-12 ph cm-2 s-1 above 200 GeV. Segue 1 is currently considered one of the most interesting targets for indirect dark matter searches. In these terms, the upper limits have been also interpreted in the context of annihilating dark matter particles. For such purpose, we performed a grid scan over a reasonable portion of the parameter space for the minimal SuperGravity model and computed the flux upper limit for each point separately, taking fully into account the peculiar spectral features of each model. We found that in order to match the experimental upper limits with the model predictions, a minimum flux boost of 103 is required, and that the upper limits are quite dependent on the shape of the gamma-ray energy spectrum predicted by each specific model. Finally we compared the upper limits with the predictions of some dark matter models able to explain the PAMELA rise in the positron ratio, finding that Segue 1 data are in tension with the dark matter explanation of the PAMELA spectrum in the case of a dark matter candidate annihilating into τ+τ-. A complete exclusion however is not possible due to the uncertainties in the Segue 1 astrophysical factor. © 2011 IOP Publishing Ltd and SISSA.

Aleksic J.,IFAE | Ansoldi S.,University of Udine | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 169 more authors.
Astroparticle Physics | Year: 2016

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. © 2015 Elsevier B.V.

Aleksic J.,IFAE | Ansoldi S.,University of Udine | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 169 more authors.
Astroparticle Physics | Year: 2016

Abstract The MAGIC telescopes are two Imaging Atmospheric Cherenkov Telescopes (IACTs) located on the Canary island of La Palma. The telescopes are designed to measure Cherenkov light from air showers initiated by gamma rays in the energy regime from around 50 GeV to more than 50 TeV. The two telescopes were built in 2004 and 2009, respectively, with different cameras, triggers and readout systems. In the years 2011-2012 the MAGIC collaboration undertook a major upgrade to make the stereoscopic system uniform, improving its overall performance and easing its maintenance. In particular, the camera, the receivers and the trigger of the first telescope were replaced and the readout of the two telescopes was upgraded. This paper (Part I) describes the details of the upgrade as well as the basic performance parameters of MAGIC such as raw data treatment, linearity in the electronic chain and sources of noise. In Part II, we describe the physics performance of the upgraded system. © 2015 Elsevier B.V.

Evans D.F.,Keele University | Southworth J.,Keele University | Maxted P.F.L.,Keele University | Skottfelt J.,Open University Milton Keynes | And 38 more authors.
Astronomy and Astrophysics | Year: 2016

Context. Wide binaries are a potential pathway for the formation of hot Jupiters. The binary fraction among host stars is an important discriminator between competing formation theories, but has not been well characterised. Additionally, contaminating light from unresolved stars can significantly affect the accuracy of photometric and spectroscopic measurements in studies of transiting exoplanets. Aims. We observed 101 transiting exoplanet host systems in the Southern hemisphere in order to create a homogeneous catalogue of both bound companion stars and contaminating background stars, in an area of the sky where transiting exoplanetary systems have not been systematically searched for stellar companions. We investigate the binary fraction among the host stars in order to test theories for the formation of hot Jupiters. Methods. Lucky imaging observations from the Two Colour Instrument on the Danish 1.54 m telescope at La Silla were used to search for previously unresolved stars at small angular separations. The separations and relative magnitudes of all detected stars were measured. For 12 candidate companions to 10 host stars, previous astrometric measurements were used to evaluate how likely the companions are to be physically associated. Results. We provide measurements of 499 candidate companions within 20 arcsec of our sample of 101 planet host stars. 51 candidates are located within 5 arcsec of a host star, and we provide the first published measurements for 27 of these. Calibrations for the plate scale and colour performance of the Two Colour Instrument are presented. Conclusions. We find that the overall multiplicity rate of the host stars is 38-13 +17%, consistent with the rate among solar-type stars in our sensitivity range, suggesting that planet formation does not preferentially occur in long period binaries compared to a random sample of field stars. Long period stellar companions (P > 10 yr) appear to occur independently of short period companions, and so the population of close-in stellar companions is unconstrained by our study. © ESO, 2016.

Aleksic J.,IFAE | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | Backes M.,TU Dortmund | And 156 more authors.
Astronomy and Astrophysics | Year: 2011

Context. 3C 279, the first quasar discovered to emit VHE γ-rays by the MAGIC telescope in 2006, was reobserved by MAGIC in January 2007 during a major optical flare and from December 2008 to April 2009 following an alert from the Fermi space telescope on an exceptionally high γ-ray state. Aims. The January 2007 observations resulted in a detection on January 16 with significance 5.4σ, corresponding to a F (>150 GeV) (3.8 ± 0.8) × 10-11 ph cm-2 s-1 while the overall data sample does not show significant signal. The December 2008-April 2009 observations did not detect the source. We study the multiwavelength behaviour of the source at the epochs of MAGIC observations, collecting quasi-simultaneous data at optical and X-ray frequencies and for 2009 also γ-ray data from Fermi. Methods. We study the light curves and spectral energy distribution of the source. The spectral energy distributions of three observing epochs (including the February 2006, which has been previously published) are modelled with one-zone inverse Compton models and the emission on January 16, 2007 also with two zone model and with a lepto-hadronic model. Results. We find that the VHE γ-ray emission detected in 2006 and 2007 challenges standard one-zone model, based on relativistic electrons in a jet scattering broad line region photons, while the other studied models fit the observed spectral energy distribution more satisfactorily. © 2011 ESO.

Aleksic J.,IFAE | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | Backes M.,TU Dortmund | And 158 more authors.
Astrophysical Journal Letters | Year: 2011

Very high energy (VHE) γ-ray emission from the flat spectrum radio quasar (FSRQ) PKS1222+21 (4C21.35, z = 0.432) was detected with the MAGIC Cherenkov telescopes during a short observation (0.5hr) performed on 2010 June 17. The MAGIC detection coincides with high-energy MeV/GeV γ-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The VHE spectrum measured by MAGIC extends from about 70GeV up to at least 400GeV and can be well described by a power-law dN/dE E -Γ with a photon index Γ = 3.75 0.27stat 0.2syst. The averaged integral flux above 100GeV is (4.6 0.5) × 10-10 cm-2 s-1 (1 Crab Nebula flux). The VHE flux measured by MAGIC varies significantly within the 30 minute exposure implying a flux doubling time of about 10 minutes. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light (EBL), can be described by a single power law with photon index 2.72 0.34 between 3GeV and 400GeV, and is consistent with emission belonging to a single component in the jet. The absence of a spectral cutoff constrains the γ-ray emission region to lie outside the broad-line region, which would otherwise absorb the VHE γ-rays. Together with the detected fast variability, this challenges present emission models from jets in FSRQs. Moreover, the combined Fermi/LAT and MAGIC spectral data yield constraints on the density of the EBL in the UV-optical to near-infrared range that are compatible with recent models. © 2011. The American Astronomical Society. All rights reserved.

Aleksic J.,IFAE | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | Backes M.,TU Dortmund | And 163 more authors.
Astrophysical Journal Letters | Year: 2010

We report on the detection with the MAGIC telescopes of very high energy (VHE) γ -rays from IC 310, a head-tail radio galaxy in the Perseus galaxy cluster, observed during the interval 2008 November to 2010 February. The Fermi satellite has also detected this galaxy. The source is detected by MAGIC at a high statistical significance of 7.6σ in 20.6 hr of stereo data. The observed spectral energy distribution is flat with a differential spectral index of -2.00 ± 0.14. The mean flux above 300 GeV, between 2009 October and 2010 February, (3.1 ± 0.5)×10-12 cm-2 s -1, corresponds to (2.5 ± 0.4)% of Crab Nebula units. Only an upper limit, of 1.9% ofCrab Nebula units above 300 GeV,was obtained with the 2008 data. This, together with strong hints (>3σ) of flares in themiddle of 2009 October and November, implies that the emission is variable. TheMAGIC results favor a scenario with the VHE emission originating from the inner jet close to the central engine. More complicated models than a simple one-zone synchrotron self-Compton (SSC) scenario, e.g., multi-zone SSC, external Compton, or hadronic, may be required to explain the very flat spectrum and its extension overmore than three orders ofmagnitude in energy. © 2010. The American Astronomical Society. All rights reserved.

Aleksic J.,IFAE | Ansoldi S.,University of Udine | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 163 more authors.
Journal of Cosmology and Astroparticle Physics | Year: 2014

We present the results of stereoscopic observations of the satellite galaxy Segue 1 with the MAGIC Telescopes, carried out between 2011 and 2013. With almost 160 hours of good-quality data, this is the deepest observational campaign on any dwarf galaxy performed so far in the very high energy range of the electromagnetic spectrum. We search this large data sample for signals of dark matter particles in the mass range between 100 GeV and 20 TeV. For this we use the full likelihood analysis method, which provides optimal sensitivity to characteristic gamma-ray spectral features, like those expected from dark matter annihilation or decay. In particular, we focus our search on gamma-rays produced from different final state Standard Model particles, annihilation with internal bremsstrahlung, monochromatic lines and box-shaped signals. Our results represent the most stringent constraints to the annihilation cross-section or decay lifetime obtained from observations of satellite galaxies, for masses above few hundred GeV. In particular, our strongest limit (95% confidence level) corresponds to a ∼ 500 GeV dark matter particle annihilating into τ+τ-, and is of order 〈σ annv〉 ≃ 1.2 × 10-24 cm3 s -1 - a factor ∼ 40 above the 〈σannv〉 ≃ thermal value.

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