A&A 623, A175 (2019) The mechanisms producing fast variability of the $\gamma$-ray emission in
active galactic nuclei are under debate. The MAGIC telescopes detected a fast
very high energy (VHE, ...E$>100$ GeV) $\gamma$-ray flare from BL Lacertae on 2015
June 15. The flare had a maximum flux of $(1.5\pm 0.3)\times 10^{-10}$ photons
cm$^{-2}$ s$^{-1}$ and halving time of $26\pm8$ minutes. The MAGIC observations
were triggered by a high state in the optical and high energy (HE, E$>100$ MeV)
$\gamma$-ray bands. In this paper we present the MAGIC VHE $\gamma$-ray data
together with multiwavelength data from radio, optical, X-rays, and HE $\gamma$
rays from 2015 May 1 to July 31. Well-sampled multiwavelength data allow us to
study the variability in detail and compare it to the other epochs when fast
VHE $\gamma$-ray flares have been detected from this source. Interestingly, we
find that the behaviour in radio, optical, X-rays and HE $\gamma$-rays is very
similar to two other observed VHE $\gamma$-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 $\gamma$-ray data with the
star-jet interaction model. We find that all of the tested emission models are
compatible with the fast VHE $\gamma$-ray flare, but all have some tension with
the multiwavelength observations.
We report on observations of the pulsar / Be star binary system PSR J2032+4127 / MT91 213 in the energy range between 100 GeV and 20 TeV with the VERITAS and MAGIC imaging atmospheric Cherenkov ...telescope arrays. The binary orbit has a period of approximately 50 years, with the most recent periastron occurring on 2017 November 13. Our observations span from 18 months prior to periastron to one month after. A new, point-like, gamma-ray source is detected, coincident with the location of PSR J2032+4127 / MT91 213. The gamma-ray light curve and spectrum are well-characterized over the periastron passage. The flux is variable over at least an order of magnitude, peaking at periastron, thus providing a firm association of the TeV source with the pulsar / Be star system. Observations prior to periastron show a cutoff in the spectrum at an energy around 0.5 TeV. This result adds a new member to the small population of known TeV binaries, and it identifies only the second source of this class in which the nature and properties of the compact object are firmly established. We compare the gamma-ray results with the light curve measured with the X-ray Telescope (XRT) on board the Neil Gehrels \textit{Swift} Observatory and with the predictions of recent theoretical models of the system. We conclude that significant revision of the models is required to explain the details of the emission we have observed, and we discuss the relationship between the binary system and the overlapping steady extended source, TeV J2032+4130.
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.
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.
The mechanisms producing fast variability of the \(\gamma\)-ray emission in active galactic nuclei are under debate. The MAGIC telescopes detected a fast very high energy (VHE, E\(>100\) GeV) ...\(\gamma\)-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of \((1.5\pm 0.3)\times 10^{-10}\) photons cm\(^{-2}\) s\(^{-1}\) and halving time of \(26\pm8\) minutes. The MAGIC observations were triggered by a high state in the optical and high energy (HE, E\(>100\) MeV) \(\gamma\)-ray bands. In this paper we present the MAGIC VHE \(\gamma\)-ray data together with multiwavelength data from radio, optical, X-rays, and HE \(\gamma\) rays from 2015 May 1 to July 31. Well-sampled multiwavelength data allow us to study the variability in detail and compare it to the other epochs when fast VHE \(\gamma\)-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays and HE \(\gamma\)-rays is very similar to two other observed VHE \(\gamma\)-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 \(\gamma\)-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE \(\gamma\)-ray flare, but all have some tension with the multiwavelength observations.
