Long-duration γ-ray bursts (GRBs) are the most luminous sources of electromagnetic radiation known in the Universe. They arise from outflows of plasma with velocities near the speed of light that are ...ejected by newly formed neutron stars or black holes (of stellar mass) at cosmological distances
. Prompt flashes of megaelectronvolt-energy γ-rays are followed by a longer-lasting afterglow emission in a wide range of energies (from radio waves to gigaelectronvolt γ-rays), which originates from synchrotron radiation generated by energetic electrons in the accompanying shock waves
. Although emission of γ-rays at even higher (teraelectronvolt) energies by other radiation mechanisms has been theoretically predicted
, it has not been previously detected
. Here we report observations of teraelectronvolt emission from the γ-ray burst GRB 190114C. γ-rays were observed in the energy range 0.2-1 teraelectronvolt from about one minute after the burst (at more than 50 standard deviations in the first 20 minutes), revealing a distinct emission component of the afterglow with power comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the teraelectronvolt and X-ray bands points to processes such as inverse Compton upscattering as the mechanism of the teraelectronvolt emission
. By contrast, processes such as synchrotron emission by ultrahigh-energy protons
are not favoured because of their low radiative efficiency. These results are anticipated to be a step towards a deeper understanding of the physics of GRBs and relativistic shock waves.
The mechanisms producing fast variability of the γ-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E > 100 GeV) γ-ray ...flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5 ± 0.3) × 10−10 photons cm−2 s−1 and halving time of 26 ± 8 min. The MAGIC observations were triggered by a high state in the optical and high-energy (HE, E > 100 MeV) γ-ray bands. In this paper we present the MAGIC VHE γ-ray data together with multi-wavelength data from radio, optical, X-rays, and HE γ rays from 2015 May 1 to July 31. Well-sampled multi-wavelength data allow us to study the variability in detail and compare it to the other epochs when fast, VHE γ-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays, and HE γ-rays is very similar to two other observed VHE γ-ray flares. In particular, also during this flare there was an indication of rotation of the optical polarization angle and of activity at the 43 GHz core. These repeating patterns indicate a connection between the three events. We also test modelling of the spectral energy distribution based on constraints from the light curves and VLBA observations, with two different geometrical setups of two-zone inverse Compton models. In addition we model the γ-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE γ-ray flare, but all have some tension with the multi-wavelength observations.
The Geminga pulsar, one of the brighest gamma-ray sources, is a promising candidate for emission of very-high-energy (VHE > 100 GeV) pulsed gamma rays. Also, detection of a large nebula has been ...claimed by water Cherenkov instruments. We performed deep observations of Geminga with the MAGIC telescopes, yielding 63 h of good-quality data, and searched for emission from the pulsar and pulsar wind nebula. We did not find any significant detection, and derived 95% confidence level upper limits. The resulting upper limits of 5.3 × 10-13 TeV cm-2 s-1 for the Geminga pulsar and 3.5 × 10-12 TeV cm-2 s-1 for the surrounding nebula at 50 GeV are the mostconstraining ones obtained so far at VHE. To complement the VHE observations, we also analyzed 5 yr of Fermi-LAT data from Geminga, finding that the sub-exponential cut-off is preferred over the exponential cut-off that has been typically used in the literature. We also find that, above 10 GeV, the gamma-ray spectra from Geminga can be described with a power law with index softer than 5. The extrapolation of the power-law Fermi-LAT pulsed spectra to VHE goes well below the MAGIC upper limits, indicating that the detection of pulsed emission from Geminga with the current generation of Cherenkov telescopes is very difficult.
Long-duration gamma-ray bursts (GRBs) are the most luminous sources of electromagnetic radiation known in the Universe. They arise from outflows of plasma with velocities near the speed of light that ...are ejected by newly formed neutron stars or black holes (of stellar mass) at cosmological distances.sup.1,2. Prompt flashes of megaelectronvolt-energy gamma-rays are followed by a longer-lasting afterglow emission in a wide range of energies (from radio waves to gigaelectronvolt gamma-rays), which originates from synchrotron radiation generated by energetic electrons in the accompanying shock waves.sup.3,4. Although emission of gamma-rays at even higher (teraelectronvolt) energies by other radiation mechanisms has been theoretically predicted.sup.5-8, it has not been previously detected.sup.7,8. Here we report observations of teraelectronvolt emission from the gamma-ray burst GRB 190114C. gamma-rays were observed in the energy range 0.2-1 teraelectronvolt from about one minute after the burst (at more than 50 standard deviations in the first 20 minutes), revealing a distinct emission component of the afterglow with power comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the teraelectronvolt and X-ray bands points to processes such as inverse Compton upscattering as the mechanism of the teraelectronvolt emission.sup.9-11. By contrast, processes such as synchrotron emission by ultrahigh-energy protons.sup.10,12,13 are not favoured because of their low radiative efficiency. These results are anticipated to be a step towards a deeper understanding of the physics of GRBs and relativistic shock waves.
The Cherenkov Telescope Array (CTA) will be the next-generation observatory in the field of very-high-energy (20 GeV to 300 TeV) gamma-ray astroparticle physics. Classically, data analysis in the ...field maximizes sensitivity by applying quality cuts on the data acquired. These cuts, optimized using Monte Carlo simulations, select higher quality events from the initial dataset. Subsequent steps of the analysis typically use the surviving events to calculate one set of instrument response functions (IRFs). An alternative approach is the use of event types, as implemented in experiments such as the Fermi-LAT. In this approach, events are divided into sub-samples based on their reconstruction quality, and a set of IRFs is calculated for each sub-sample. The sub-samples are then combined in a joint analysis, treating them as independent observations. This leads to an improvement in performance parameters such as sensitivity, angular and energy resolution. Data loss is reduced since lower quality events are included in the analysis as well, rather than discarded. In this study, machine learning methods will be used to classify events according to their expected angular reconstruction quality. We will report the impact on CTA high-level performance when applying such an event-type classification, compared to the classical procedure.
The statistics of X-ray flares in the afterglow of gamma-ray bursts (GRBs) have been studied extensively without considering the possible different origins of each flare. By satisfying six ...observational criteria, we find a sample composed of \(16\) long GRBs observed by \textit{Swift} satellite may share a same origin. By applying the Markov chain Monte Carlo iteration and the machine learning algorithms (locally weighted regression and Gaussian process regression), impressively, the flares in these GRBs show strong correlations with the energy released in the prompt emission. These correlations were never discovered in previous papers, and they could not be well explained by previous models. These correlations imply that the prompt emission and the X-ray flare are not independent, they may be originated following a same sequence. The new THESUS satellite will provide us a larger sample and more detailed spectra to refine the results we obtained in this article.
Nature 575 (2019) 455-458 Gamma-ray bursts (GRBs) of the long-duration class are the most luminous
sources of electromagnetic radiation known in the Universe. They are generated
by outflows of plasma ...ejected at near the speed of light by newly formed
neutron stars or black holes of stellar mass at cosmological distances. Prompt
flashes of MeV gamma rays are followed by longer-lasting afterglow emission
from radio waves to GeV gamma rays, due to synchrotron radiation by energetic
electrons in accompanying shock waves. Although emission of gamma rays at even
higher, TeV energies by other radiation mechanisms had been theoretically
predicted, it had never been detected previously. Here we report the clear
detection of GRB 190114C in the TeV band, achieved after many years of
dedicated searches for TeV emission from GRBs. Gamma rays in the energy range
0.2--1 TeV are observed from about 1 minute after the burst (at more than 50
standard deviations in the first 20 minutes). This unambiguously reveals a new
emission component in the afterglow of a GRB, whose power is comparable to that
of the synchrotron component. The observed similarity in the radiated power and
temporal behaviour of the TeV and X-ray bands points to processes such as
inverse Compton radiation as the mechanism of the TeV emission, while processes
such as synchrotron emission by ultrahigh-energy protons are disfavoured due to
their low radiative efficiency.
A&A 642, A190 (2020) Aims: $\gamma$ rays can be used as a tracer in the search of sources of
Galactic cosmic rays (CRs). We present deep observations of the Galactic Centre
(GC) region with the MAGIC ...telescopes, which we use for inferring the
underlying CR distribution. Methods: We observed the GC region for
${\approx}100$ hours with the MAGIC telescopes from 2012 to 2017, at high
zenith angles (58-70~deg). This implies a larger energy threshold, but also an
increased effective collection area compared to low zenith observations. Using
new software, we derive instrument response and background models, enabling us
to study the diffuse emission in the region. We use pre-existing data of the
gas distribution in the GC region to derive the underlying distribution of CRs.
Results: We obtain a significant detection for all four model components used
to fit our data (Sgr~A*, ``Arc'', G0.9+0.1, and an extended component for the
Galactic Ridge). We find that the diffuse component is best described as a
power-law with index 2 and an exponential cut-off at around 20~TeV with the
significance of the cut-off being only 2~$\sigma$. The derived cosmic-ray
profile hints to a peak at the GC position, with a measured profile index of
$1.2 \pm 0.3$, supporting the hypothesis of a CR accelerator at the GC. We
argue that the measurements of this profile are presently limited by our
knowledge of the gas distribution in the GC vicinity.
PDF
