Major Atmospheric Gamma-ray Imaging Cherenkov Research Articles

The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission

A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source spectral energy distribution (SED). In the optical we found a general bluer-when-brighter trend. The X-ray spectrum remained stable during 2013, but a comparison with previous observations suggests that it becomes harder when the X-ray flux increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display a softer shape that is confirmed by previous HST-COS and IUE observations. Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely lies in the 4-30 eV energy range, with a general shift towards higher frequencies for increasing X-ray brightness. However, the UV and X-ray spectra do not connect smoothly. Possible interpretations include: i) orientation effects, ii) additional absorption, iii) multiple emission components, and iv) a peculiar energy distribution of relativistic electrons. We discuss the first possibility in terms of an inhomogeneous helical jet model.

Black hole physics. Black hole lightning due to particle acceleration at subhorizon scales.

Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.

Indirect detection of dark matter with γ rays

The details of what constitutes the majority of the mass that makes up dark matter in the Universe remains one of the prime puzzles of cosmology and particle physics today—80 y after the first observational indications. Today, it is widely accepted that dark matter exists and that it is very likely composed of elementary particles, which are weakly interacting and massive [weakly interacting massive particles (WIMPs)]. As important as dark matter is in our understanding of cosmology, the detection of these particles has thus far been elusive. Their primary properties such as mass and interaction cross sections are still unknown. Indirect detection searches for the products of WIMP annihilation or decay. This is generally done through observations of γ-ray photons or cosmic rays. Instruments such as the Fermi large-area telescope, high-energy stereoscopic system, major atmospheric gamma-ray imaging Cherenkov, and very energetic radiation imaging telescope array, combined with the future Cherenkov telescope array, will provide important complementarity to other search techniques. Given the expected sensitivities of all search techniques, we are at a stage where the WIMP scenario is facing stringent tests, and it can be expected that WIMPs will be either be detected or the scenario will be so severely constrained that it will have to be rethought. In this sense, we are on the threshold of discovery. In this article, I will give a general overview of the current status and future expectations for indirect searches of dark matter (WIMP) particles.

The MAGIC telescopes: performance, results and future perspectives

MAGIC (Major Atmospheric Gamma–ray Imaging Cherenkov) is a two 17 meter diameter telescope system, located at the Roque de los Muchachos Observatory in the Canarian island La Palma (Spain) at a height of 2240 m.a.s.l., operating in stereoscopic mode since fall 2009. We report on the performance of the instrument and highlight selected recent results from observations. The future perspectives are also illustrated.

Mrk 421 active state in 2008: the MAGIC view, simultaneous multi-wavelength observations and SSC model constrained

Context: The blazar Markarian 421 is one of the brightest TeV gamma-ray sources of the northern sky. From December 2007 until June 2008 it was intensively observed in the very high energy (VHE, E > 100 GeV) band by the single-dish Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC-I). Aims: We aimed to measure the physical parameters of the emitting region of the blazar jet during active states. Methods: We performed a dense monitoring of the source in VHE with MAGIC-I, and also collected complementary data in soft X-rays and optical-UV bands; then, we modeled the spectral energy distributions (SED) derived from simultaneous multi-wavelength data within the synchrotron self--compton (SSC) framework. Results: The source showed intense and prolonged gamma-ray activity during the whole period, with integral fluxes (E > 200 GeV) seldom below the level of the Crab Nebula, and up to 3.6 times this value. Eight datasets of simultaneous optical-UV (KVA, Swift/UVOT), soft X-ray (Swift/XRT) and MAGIC-I VHE data were obtained during different outburst phases. The data constrain the physical parameters of the jet, once the spectral energy distributions obtained are interpreted within the framework of a single-zone SSC leptonic model. Conclusions: The main outcome of the study is that within the homogeneous model high Doppler factors (40 <= delta <= 80) are needed to reproduce the observed SED; but this model cannot explain the observed short time-scale variability, while it can be argued that inhomogeneous models could allow for less extreme Doppler factors, more intense magnetic fields and shorter electron cooling times compatible with hour or sub-hour scale variability.

RESULTS FROM MAGIC OBSERVATIONS OF EXTRAGALACTIC RELATIVISTIC SOURCES

The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) experiment is an array of two 17-meter telescopes located in the Canary Island of La Palma that observes the very-high energy (VHE) gamma-ray sky in stereoscopic mode since 2009. MAGIC is distinguished by its low-energy threshold of approximately 50 GeV, which grants the system a unique potential in the study of distant extragalactic sources whose gamma-ray emission is significantly attenuated due to absorption by the extragalactic background light (EBL). The observation of non-thermal gamma rays in the GeV-TeV range from extragalactic sources is a characteristic signature of their relativistic nature and therefore fundamentally important for our understanding of the physics of these objects. Since the beginning of its stereo operation, MAGIC has observed a large number of active galactic nuclei (AGN) of different classes, including several blazars and distant quasars. In this paper we will review some of the most important results of these observations.

IMPLICATIONS OF THE VERY HIGH ENERGY GAMMA-RAY DETECTION OF THE QUASAR 3C279

The Major Atmospheric Gamma-Ray Imaging Cherenkov Telescope very high energy (VHE) γ-ray astronomy collaboration recently reported the detection of the quasar 3C279 at >100 GeV γ-ray energies. Here, we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snapshot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton (SSC) origin of the entire spectral energy distribution (SED), including the VHE γ-ray emission is highly problematic as it would require an unrealistically low magnetic field. The measured level of VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions, BLRs). However, such an interpretation would require either an unusually low magnetic field of B ∼ 0.03 G, or (in order to achieve approximate equipartition between magnetic field at B ∼ 0.25 G and relativistic electrons) an unrealistically high Doppler factor of Γ ∼ 140. In addition, such a model fails to reproduce the observed X-ray flux. Furthermore, both versions of leptonic one-zone models produce intrinsic VHE γ-ray spectra steeper than measured, even in the case of the lowest plausible extragalactic γγ absorption. We therefore conclude that a simple one-zone, homogeneous leptonic jet model is not able to plausibly reproduce the SED of 3C279 including the recently detected VHE γ-ray emission. This as well as the lack of correlated variability in the optical with the VHE γ-ray emission and the substantial γγ opacity of the BLR radiation field to VHE γ-rays suggests a multi-zone model in which the optical emission is produced in a different region than the VHE γ-ray emission. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray–VHE γ-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well, both in scenarios in which only the internal synchrotron field serves as targets for pγ pion production, and with a substantial contribution from external photons, e.g., from the BLR. However, either version of the hadronic model requires a rather extreme jet power of up to Lj ∼ 1049 erg s−1, compared to a requirement of Lj ∼ 2 × 1047 erg s−1 for a multi-zone leptonic model.

3C 66B as a TeV radio galaxy

Abstract The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescope collaboration reported the detection of a new very high energy source, MAGIC J0223+430, located close to the position of the blazar 3C 66A, considered a candidate TeV blazar since a long time. A careful analysis showed that the events with energies above 150 GeV are centred on the position of the Fanaro–Riley type I radio galaxy 3C 66B (at 6 arcmin from 3C 66A), with a probability of 95.4 per cent (85.4 per cent including systematic uncertainties) that the source is not related to 3C 66A. We present a model for the possible emission of 3C 66B based on the structured jet model already used to interpret the TeV emission of the radio galaxy M87. The model requires parameters similar to those used for M87 but a larger luminosity for the layer, to account for the more luminous TeV emission. We also show that the spectrum obtained by MAGIC can be interpreted as the combined emission of 3C 66B, dominating above ∼200 GeV, and 3C 66A. The high-energy emission from the latter source, being strongly attenuated by the interaction with the extragalactic background light, can only contribute at low energies. If we were to see the jet emission of 3C 66B at small viewing angles, we would see a spectral energy distribution closely resembling the one of S5 0716+714, a typical blazar.

The drive system of the major atmospheric gamma-ray imaging Cherenkov telescope

The MAGIC telescope is an imaging atmospheric Cherenkov telescope, designed to observe very high energy gamma-rays while achieving a low energy threshold. One of the key science goals is fast follow-up of the enigmatic and short lived gamma-ray bursts. The drive system for the telescope has to meet two basic demands: (1) During normal observations, the 72-ton telescope has to be positioned accurately, and has to track a given sky position with high precision at a typical rotational speed in the order of one revolution per day. (2) For successfully observing GRB prompt emission and afterglows, it has to be powerful enough to position to an arbitrary point on the sky within a few ten seconds and commence normal tracking immediately thereafter. To meet these requirements, the implementation and realization of the drive system relies strongly on standard industry components to ensure robustness and reliability. In this paper, we describe the mechanical setup, the drive control and the calibration of the pointing, as well as present measurements of the accuracy of the system. We show that the drive system is mechanically able to operate the motors with an accuracy even better than the feedback values from the axes. In the context of future projects, envisaging telescope arrays comprising about 100 individual instruments, the robustness and scalability of the concept is emphasized.

A method to correct IACT data for atmospheric absorption due to the Saharan Air Layer

Context. Using the atmosphere as a detector volume, Imaging Air Cherenkov Telescopes (IACTs) depend highly on the properties and the condition of the air mass above the telescope. On the Canary Island of La Palma, where the Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC) is situated, the Saharan Air Layer (SAL) can cause strong atmospheric absorption affecting the data quality and resulting in a reduced gamma flux.

FADC signal reconstruction for the MAGIC telescope

Until April 2007 the Major Atmospheric Gamma ray Imaging Cherenkov (MAGIC) telescope used a 300 MSamples/s flash analog-to-digital converter (FADC) system to sample the shaped photomultiplier tube (PMT) signals produced by the captured Cherenkov photons of air showers. Different algorithms to reconstruct the signal from the read-out samples (extractors) have been implemented and are described and compared. Criteria based on the obtained charge and time resolution/bias are defined and used to judge the different extractors, by applying them to calibration, cosmic and pedestal signals. The achievable charge and time resolution have been derived as functions of the number of incident photo-electrons.

The proton low-mass microquasar: high-energy emission

Context. A population of unidentified gamma-ray sources is forming a structure resembling a halo around the Galactic center. These sources are highly variable, and hence they should be associated with compact objects. Microquasars are objects undergoing accretion with relativistic jets; if such an object has a low-mass, evolved, donor star, it might be found in the Galactic halo. If these low-mass microquasars can generate detectable gamma-ray emission, then they are natural candidates to account for the halo high-energy sources. Aims. We aim to construct models for high-energy emission of low-mass microquasars, which could produce a significant luminosity in the gamma-ray domain. Methods. We consider that a significant fraction of the relativistic particles in the jets of low-mass microquasars are protons and then we study the production of high-energy emission through proton synchrotron radiation and photopion production. Photopair production and leptonic processes are considered as well. We compute a number of specific models with different parameters to explore the possibilities of this scenario. Results. We find that important luminosities, in the range of 10 34 -10 37 erg s -1 , can be achieved by proton synchrotron radiation in the Gamma-Ray Large Area Space Telescope (GLAST) energy range, and lower, but still significant luminosities at higher energies for some models. Conclusions. We conclude that the proton microquasar model offers a very interesting alternative to account for the halo gamma-ray sources and presents a variety of predictions that might be tested in the near future by instruments like GLAST, the High-Energy Stereoscopic System II (HESS II), the Major Atmospheric Gamma-ray Imaging Cherenkov telescope II (MAGIC II), and neutrino telescopes like IceCube.

Testbench to characterize pixels of the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope

Photomultipliers have a wide range of applications, from nuclear medicine to nuclear physics. In particular, they are commonly used in high energy physics and astrophysics. The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope is an example of their applications. This work presents a test system for the characterization of the transient response of the photomultipliers used as fast light detectors in the telescope. The bias of the photomultiplier is described, taking into account its effects in the response to fast light pulses. Two methods are used to generate these pulses: 1. a specifically designed pulse generator, and 2. a plastic scintillator. For the former case, we also calculate the anode charge we expect in this setup to cross-check it with the actual measurements. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

The trigger system of the MAGIC telescope: on-line selection strategies for Cherenkov telescopes

The Major Atmospheric Gamma-ray Imaging Cherenkov Telescope aims at the detection of very low energy γ-rays ( E>10 GeV ) through the atmospheric emission of Cherenkov light. The high background rate originated by various sources sets a serious challenge to this goal. Application of topological selection cuts at trigger level can contribute to the background reduction, allowing the telescope to operate at lower thresholds and reducing the minimum detectable energy.

The MAGIC Telescope - prospects for GRB research

The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescope collaboration is constructing a large Cherenkov telescope (17 m diameter) for the exploration of the gamma-ray energy regime above 10 GeV with high sensitivity. One of the highlights in the science program of this future observatory are the plans for fast follow-up observations of of GRBs. By ``fast'' we mean delays of less than 30 s between notification and the beginning of observations. The expected gamma counting rates are of the order of 100 Hz for a EGRET counting rate of 0.1 Hz above 100 MeV and a spectral index of 2.0 (this would correspond to a fluence of 10^{-5} erg cm^{-2} for 60 s burst duration). The good photon statistics will permit determination of spectra, search for cutoffs and measurement of light-curves with a time resolution of the order of 1 s.