Context.
High-energy photons emitted by flaring active galactic nuclei (AGNs) have been used for many years to constrain modified dispersion relations in vacuum encountered in the context of quantum ...gravity phenomenology. In such studies, done in the GeV–TeV range, energy-dependent delays (spectral lags) are searched for, usually neglecting any source-intrinsic time delay.
Aims.
With the aim being to distinguish Lorentz invariance violation (LIV) effects from lags generated at the sources themselves, a detailed investigation into intrinsic spectral lags in flaring AGNs above 100 GeV is presented in the frame of synchrotron-self-Compton scenarios for their very-high-energy (VHE) emission.
Methods.
A simple model of VHE flares in blazars is proposed, allowing to explore the influence of the main physical parameters describing the emitting zones on intrinsic delays.
Results.
For typical conditions expected in TeV blazars, significant intrinsic lags are obtained, which can dominate over LIV effects, especially at low redshifts, and should therefore be carefully disentangled from any extrinsic lags. Moreover, two main regimes are identified with characteristic spectral lags, corresponding to long-lasting and fast particle acceleration.
Conclusions.
Such intrinsic spectral lags should be detected with new-generation instruments at VHE such as the Cherenkov Telescope Array which begins operation in a few years. This will provide original constraints on AGN flare models and open a new era for LIV searches in the photon sector.
A possible violation of Lorentz Invariance (LIV) in the form of Modified Dispersion Relations (MDR) for photons in vacuum appeared in the late 90s as a possible outcome of some models developed with ...the goal to provide a full theory of Quantum Gravity (QG). Since then, several ways to probe quantum spacetime from high-energy gamma-ray observations of distant sources have been followed and provide stringent limits on the characteristic energy scale of these QG-related phenomena. In this paper, the use of astrophysical sources for constraining MDR will be described. In particular, strengths and weaknesses of each category of sources as well as temporal effects due to emission mechanisms will be discussed. The latest results obtained from observations of Gamma-Ray Bursts, flaring Active Galactic Nuclei and pulsars will be briefly reviewed. Then, efforts on-going to get more robust constraints on MDR and LIV will be discussed and put in context with the beginning of the Cherenkov Telescope Array operations in the next few years.
We analyze the MeV/GeV emission from four bright gamma-ray bursts (GRBs) observed by the Fermi Large Area Telescope to produce robust, stringent constraints on a dependence of the speed of light in ...vacuo on the photon energy (vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by some quantum gravity (QG) theories. First, we use three different and complementary techniques to constrain the total degree of dispersion observed in the data. Additionally, using a maximally conservative set of assumptions on possible source-intrinsic, spectral-evolution effects, we constrain any vacuum dispersion solely attributed to LIV. We then derive limits on the QG energy scale (the energy scale where LIV-inducing QG effects become strong, E sub(QG)) and the coefficients of the Standard Model Extension. For the subluminal case (where high-energy photons propagate more slowly than lower-energy photons) and without taking into account any source-intrinsic dispersion, our most stringent limits (at 95% C.L.) are obtained from GRB 090510 and are E sub(QG,1) > 7.6 times the Planck energy (E sub(P1)) and E sub(QG,2) > 1.3 x 10 super(11) GeV for linear and quadratic leading-order LIV-induced vacuum dispersion, respectively. These limits improve the latest constraints by Fermi and H.E.S.S. by a factor of ~ 2. Our results disfavor any class of models requiring E sub(QG,1) <, ~ E sub(P1).
Lorentz Invariance Violation (LIV) may be a good observational window on Quantum Gravity physics. Within last few years, all major gamma-ray experiments have published results from the search for LIV ...with variable astrophysical sources: gamma-ray bursts with detectors on-board satellites and Active Galactic Nuclei with ground-based experiments. In this paper, the recent time-of-flight studies with unpolarized photons published from the space and ground based observations are reviewed. Various methods used in the time delay searches are described, and their performance discussed. Since no significant time-lag value was found within experimental precision of the measurements, the present results consist of 95% confidence level limits on the Quantum Gravity scale on the linear and quadratic terms in the standard photon dispersion relations.
The aim of the present study is to explore how to disentangle energy-dependent time delays due to a possible Lorentz invariance violation (LIV) at Planck scale from intrinsic delays expected in ...standard blazar flares. We first characterised the intrinsic time delays in BL Lacs and flat-spectrum radio quasars in standard one-zone time-dependent synchrotron self-Compton or external Compton models, during flares produced by particle acceleration and cooling processes. We simulated families of flares with both intrinsic and external LIV-induced energy-dependent delays. Discrimination between intrinsic and LIV delays is then investigated in two different ways. A technique based on Euclidean distance calculation between delays obtained in the synchrotron and in the inverse-Compton spectral bumps is used to assess their degree of correlation. A complementary study is performed using spectral hardness versus intensity diagrams in both energy ranges. We show that the presence of non-negligible LIV effects, which essentially act only at very high energy (VHE), can drastically reduce the strong correlation expected between the X-ray and the VHE gamma-ray emission in leptonic scenarios. The LIV phenomenon can then be hinted at measuring the Euclidean distance $d_ E $ from simultaneous X-ray and gamma-ray flare monitoring. Large values of minimum distance $d_ E,min $ would directly indicate the influence of non-intrinsic time delays possibly due to LIV in SSC flares. LIV effects can also significantly modify the VHE hysteresis patterns in hardness-intensity diagrams and even change their direction of rotation compared to the X-ray behaviour. Both observables could be used to discriminate between LIV and intrinsic delays, provided high-quality flare observations are available.
The study of time lags between spikes in gamma-ray burst light curves in different energy bands as a function of redshift may lead to the detection of effects due to quantum gravity. We present an ...analysis of 15 gamma-ray bursts with measured redshift, detected by the HETE-2 mission, in order to measure time lags related to astrophysical effects and search for a quantum gravity signature in the framework of an extradimensional string model. The wavelet transform method is used both for denoising the light curves and for detecting sharp transitions. The use of photon-tagged data allows us to consider various energy ranges and to evaluate systematic effects due to selections and cuts. The analysis of maxima and minima of the light curves leads to no significant quantum gravity effect. A lower limit at the 95% confidence level on the quantum gravity scale parameter of image GeV is set.
Science with the Cherenkov Telescope Array The Cta Consortium, The Cta Consortium
The Astrophysical journal. Supplement series,
01/2019, Letnik:
240, Številka:
2
eBook, Journal Article, Book
Recenzirano
Odprti dostop
This book summarizes the science to be carried out by the upcoming Cherenkov Telescope Array, a major ground-based gamma-ray observatory that will be constructed over the next six to eight years. The ...major scientific themes, as well as core program of key science projects, have been developed by the CTA Consortium, a collaboration of scientists from many institutions worldwide. CTA will be the major facility in high-energy and very high-energy photon astronomy over the next decade and beyond. CTA will have capabilities well beyond past and present observatories. Thus, CTA's science program is expected to be rich and broad and will complement other major multiwavelength and multimessenger facilities. This book is intended to be the primary resource for the science case for CTA and it thus will be of great interest to the broader physics and astronomy communities. The electronic version (e-book) is available in open access.
Because they are bright and distant, Gamma-ray Bursts (GRBs) have been used for more than a decade to test propagation of photons and to constrain relevant Quantum Gravity (QG) models in which the ...velocity of photons in vacuum can depend on their energy. With its unprecedented sensitivity and energy coverage, the Fermi satellite has provided the most constraining results on the QG energy scale so far. In this talk, the latest results obtained from the analysis of four bright GRBs observed by the Large Area Telescope will be reviewed. These robust results, cross-checked using three different analysis techniques set the limit on QG energy scale at EQG,1>7.6 times the Planck energy for linear dispersion and EQG,2>1.3×1011GeV for quadratic dispersion (95% CL). After describing the data and the analysis techniques in use, results will be discussed and confronted to latest constraints obtained with Active Galactic Nuclei.
Abstract
Gamma-ray astronomy has become one of the main experimental ways to test the modified dispersion relations (MDRs) of photons in vacuum, obtained in some attempts to formulate a theory of ...quantum gravity. The MDRs in use imply time delays that depend on the energy and that increase with distance following some function of redshift. The use of transient, or variable, distant and highly energetic sources already allows us to set stringent limits on the energy scale related to this phenomenon, usually thought to be of the order of the Planck energy, but robust conclusions on the existence of MDR-related propagation effects still require the analysis of a large population of sources. In order to gather the biggest sample of sources possible for MDR searches at teraelectronvolt energies, the H.E.S.S., MAGIC, and VERITAS collaborations enacted a joint task force to combine all their relevant data to constrain the quantum gravity energy scale. In the present article, the likelihood method used to combine the data and provide a common limit is described in detail and tested through simulations of recorded data sets for a gamma-ray burst, three flaring active galactic nuclei, and two pulsars. Statistical and systematic errors are assessed and included in the likelihood as nuisance parameters. In addition, a comparison of two different formalisms for distance dependence of the time lags is performed for the first time. In a second article, to appear later, the method will be applied to all relevant data from the three experiments.
NECTAr: New electronics for the Cherenkov Telescope Array Vorobiov, S.; Bolmont, J.; Corona, P. ...
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,
05/2011, Letnik:
639, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
The European astroparticle physics community aims to design and build the next generation array of Imaging Atmospheric Cherenkov Telescopes (IACTs), that will benefit from the experience of the ...existing H.E.S.S. and MAGIC detectors, and further expand the very-high energy astronomy domain. In order to gain an order of magnitude in sensitivity in the 10
GeV to
>
100
TeV
range, the Cherenkov Telescope Array (CTA) will employ 50–100 mirrors of various sizes equipped with 1000–4000 channels per camera, to be compared with the 6000 channels of the final H.E.S.S. array. A 3-year program, started in 2009, aims to build and test a demonstrator module of a generic CTA camera. We present here the NECTAr design of front-end electronics for the CTA, adapted to the trigger and data acquisition of a large IACTs array, with simple production and maintenance. Cost and camera performances are optimized by maximizing integration of the front-end electronics (amplifiers, fast analog samplers, ADCs) in an ASIC, achieving several GS/s and a few
μ
s
readout dead-time. We present preliminary results and extrapolated performances from Monte Carlo simulations.
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