Context. The origin of the γ -ray emission of the blazar Mrk 421 is still a matter of debate. Aims. We used 5.5 years of unbiased observing campaign data, obtained using the FACT telescope and the ...Fermi -LAT detector at TeV and GeV energies, the longest and densest so far, together with contemporaneous multi-wavelength observations, to characterise the variability of Mrk 421 and to constrain the underlying physical mechanisms. Methods. We studied and correlated light curves obtained by ten different instruments and found two significant results. Results. The TeV and X-ray light curves are very well correlated with a lag of < 0.6 days. The GeV and radio (15 Ghz band) light curves are widely and strongly correlated. Variations of the GeV light curve lead those in the radio. Conclusions. Lepto-hadronic and purely hadronic models in the frame of shock acceleration predict proton acceleration or cooling timescales that are ruled out by the short variability timescales and delays observed in Mrk 421. Instead the observations match the predictions of leptonic models.
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.
Aims.
In the presence of a sufficient amount of target material,
γ
-rays can be used as a tracer in the search for sources of Galactic cosmic rays (CRs). Here we present deep observations of the ...Galactic center (GC) region with the MAGIC telescopes and use them to infer the underlying CR distribution and to study the alleged PeV proton accelerator at the center of our Galaxy.
Methods.
We used data from ≈100 h observations of the GC region conducted with the MAGIC telescopes over five years (from 2012 to 2017). Those were collected at high zenith angles (58−70 deg), leading to a larger energy threshold, but also an increased effective collection area compared to low zenith observations. Using recently developed software tools, we derived the instrument response and background models required for extracting the diffuse emission in the region. We used existing measurements of the gas distribution in the GC region to derive the underlying distribution of CRs. We present a discussion of the associated biases and limitations of such an approach.
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 observe no significant difference between the
γ
-ray spectra of the immediate GC surroundings, which we model as a point source (Sgr A*) and the Galactic Ridge. The latter can be 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
σ
. The derived cosmic-ray profile hints to a peak at the GC position and with a measured profile index of 1.2 ± 0.3 is consistent with the 1/
r
radial distance scaling law, which supports 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.
We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these ...energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of
Fermi
-LAT data above 100 MeV were also analysed. From the two pulses per rotation seen by
Fermi
-LAT, only the second one,
P
2, is detected in the MAGIC energy range, with a significance of 6.3
σ
. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index Γ = 5.62 ± 0.54, which smoothly extends the
Fermi
-LAT spectrum. A joint fit to MAGIC and
Fermi
-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6
σ
significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.
ABSTRACT
By studying the variability of blazars across the electromagnetic spectrum, it is possible to resolve the underlying processes responsible for rapid flux increases, so-called flares. We ...report on an extremely bright X-ray flare in the high-peaked BL Lacertae object Markarian 421 (Mrk 421) that occurred simultaneously with enhanced γ-ray activity detected at very high energies by First G-APD Cherenkov Telescope on 2019 June 9. We triggered an observation with XMM–Newton, which observed the source quasi-continuously for 25 h. We find that the source was in the brightest state ever observed using XMM–Newton, reaching a flux of 2.8 × 10−9 $\mathrm{erg\, cm^{-2}\, s^{-1}}$ over an energy range of 0.3–10 keV. We perform a spectral and timing analysis to reveal the mechanisms of particle acceleration and to search for the shortest source-intrinsic time-scales. Mrk 421 exhibits the typical harder-when-brighter behaviour throughout the observation and shows a clock-wise hysteresis pattern, which indicates that the cooling dominates over the acceleration process. While the X-ray emission in different sub-bands is highly correlated, we can exclude large time lags as the computed z-transformed discrete correlation functions are consistent with a zero lag. We find rapid variability on time-scales of 1 ks for the 0.3–10 keV band and down to 300 s in the hard X-ray band (4–10 keV). Taking these time-scales into account, we discuss different models to explain the observed X-ray flare, and find that a plasmoid-dominated magnetic reconnection process is able to describe our observation best.
ABSTRACT The occasional hardening of the GeV-to-TeV spectrum observed from the blazar Mrk 501 has reopened the debate on the physical origin of radiation and particle acceleration processes in TeV ...blazars. We have used the ∼7 years of Fermi-LAT data to search for the time intervals with unusually hard spectra from the nearby TeV blazar Mrk 501. We detected hard spectral components above 10 GeV with photon index <1.5 at a significance level of more than 5 sigma on 17 occasions, each with 30 day integration time. The photon index of the hardest component reached a value of 0.89 0.29. We interpret these hard spectra as signatures of intermittent injection of sharply peaked and localized particle distributions from the base of the jet.
Starburst galaxies and star-forming active galactic nuclei are among the candidate sources thought to contribute appreciably to the extragalactic gamma-ray and neutrino backgrounds. NGC 1068 is the ...brightest of the star-forming galaxies found to emit gamma-rays from 0.1 to 50 GeV. Precise measurements of the high-energy spectrum are crucial to study the particle accelerators and probe the dominant emission mechanisms. We have carried out 125 hr of observations of NGC 1068 with the MAGIC telescopes in order to search for gamma-ray emission in the very-high-energy band. We did not detect significant gamma-ray emission, and set upper limits at the 95% confidence level to the gamma-ray flux above 200 GeV f < 5.1 × 10−13 cm−2 s−1. This limit improves previous constraints by about an order of magnitude and allows us to put tight constraints on the theoretical models for the gamma-ray emission. By combining the MAGIC observations with the Fermi-LAT spectrum we limit the parameter space (spectral slope, maximum energy) of the cosmic ray protons predicted by hadronuclear models for the gamma-ray emission, while we find that a model postulating leptonic emission from a semi-relativistic jet is fully consistent with the limits. We provide predictions for IceCube detection of the neutrino signal foreseen in the hadronic scenario. We predict a maximal IceCube neutrino event rate of 0.07 yr−1.
ABSTRACT
We investigate the physical nature and origin of the gamma-ray emission from the extended source HESS J1841−055 observed at TeV and GeV energies. We observed HESS J1841−055 at TeV energies ...for a total effective time of 43 h with the MAGIC telescopes, in 2012 and 2013. Additionally, we analysed the GeV counterpart making use of about 10 yr of Fermi-LAT data. Using both Fermi-LAT and MAGIC, we study both the spectral and energy-dependent morphology of the source for almost four decades of energy. The origin of the gamma-ray emission from this region is investigated using multiwaveband information on sources present in this region, suggested to be associated with this unidentified gamma-ray source. We find that the extended emission at GeV–TeV energies is best described by more than one source model. We also perform the first energy-dependent analysis of the HESS J1841−055 region at GeV–TeV. We find that the emission at lower energies comes from a diffuse or extended component, while the major contribution of gamma rays above 1 TeV arises from the southern part of the source. Moreover, we find that a significant curvature is present in the combined observed spectrum of MAGIC and Fermi-LAT. The first multiwavelength spectral energy distribution of this unidentified source shows that the emission at GeV–TeV energies can be well explained with both leptonic and hadronic models. For the leptonic scenario, bremsstrahlung is the dominant emission compared to inverse Compton. On the other hand, for the hadronic model, gamma-ray resulting from the decay of neutral pions (π0) can explain the observed spectrum. The presence of dense molecular clouds overlapping with HESS J1841−055 makes both bremsstrahlung and π0-decay processes the dominant emission mechanisms for the source.