The relentless variability of Mrk 421 from the TeV to the radio

The origin of the gamma-ray emission of the blazar Mrk 421 is still a matter of debate. 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. We studied and correlated light curves obtained by ten different instruments and found two significant 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. 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.Comment: 10 pages, 8 figures, 1 tabl

Fractional variability—a tool to study blazar variability

Active Galactic Nuclei emit radiation over the whole electromagnetic spectrum up to TeV energies. Blazars are one subtype with their jets pointing towards the observer. One of their typical features is extreme variability on timescales, from minutes to years. The fractional variability is an often used parameter for investigating the degree of variability of a light curve. Different detection methods and sensitivities of the instruments result in differently binned data and light curves with gaps. As they can influence the physics interpretation of the broadband variability, the effects of these differences on the fractional variability need to be studied. In this paper, we study the systematic effects of completeness in time coverage and the sampling rate. Using public data from instruments monitoring blazars in various energy ranges, we study the variability of the bright TeV blazars Mrk 421 and Mrk 501 over the electromagnetic spectrum, taking into account the systematic effects, and compare our findings with previous results. Especially in the TeV range, the fractional variability is higher than in previous studies, which can be explained by the much longer (seven years compared to few weeks) and more complete data sample

MAGIC and H.E.S.S. detect VHE gamma rays from the blazar OT081 for the first time: a deep multiwavelength study

https://pos.sissa.it/395/815/pdfPublished versio

The Broadband Behaviour of Bright TeV Gamma-Ray Emitting Blazars

This thesis presents a characterisation of the broadband behaviour of TeV gamma-ray emitting blazars that belong to the family of active galactic nuclei. Their high bolometric luminosity as well as their bright non-thermal emission up to at least TeV energies make them among the most energetic persistent sources in the universe. Blazars are also identified by large relativistic plasma jets that are aligned at a small angle to the observer’s line of sight. These jets have a size of order ∼kpc-Mpc and radiate non-thermal emission. The emission is strongly variable over the entire spectrum on all time scales and one can observe flux changes by an order of magnitude on sub-hour time scale in the most extreme cases. The origins of these phenomena are still poorly understood and the underlying mechanisms for the gamma-ray emission in blazars are still debated. These violent and extreme properties make blazars candidate sources of ultra-high-energy cosmic rays (E > 10^18 eV), whose origins are still unknown. Blazars are also potential factories of PeV neutrinos, which diffuse flux has been detected by the IceCube detector. The goal of this thesis is to provide a detailed characterisation of the broadband emission properties of blazars with the ultimate aim to understand their emission mechanisms. This work focuses on two TeV emitting blazars, belonging to the category of BL Lac type objects: 1ES 2344+514 and Mrk 421. These two objects exhibit on a temporary basis a synchrotron energy flux peaking at energies around or above 1 keV. Only a handful objects detected in the TeV band are known to exhibit such peculiar and extreme properties; they are dubbed as extreme high-frequency BL Lacs (EHBL). 1ES 2344+514 and Mrk 421 are also among the brightest and the closest TeV gamma-ray emitting blazars. Thanks to their brightness, their broadband evolution can be characterised on relatively short time scales by current instruments. Thanks to their proximity, their gamma-ray emission is not strongly absorbed by the extragalactic background light. Therefore, 1ES 2344+514 and Mrk 421 are ideal targets to better understand the most extreme phenomena observed in the jets of blazars. In order to achieve an optimal energy and temporal coverage from radio to TeV, the present work makes use of data from a large number of instruments: MAGIC, ACT, Fermi-LAT, Swift-BAT, Swift-XRT, Swift-UVOT, instruments from the Whole Earth Blazar Telescope network, NOT, KAIT, OVRO, Metsähovi and Medicina. This thesis also makes use of observations from the NuSTAR instrument, which is currently the most sensitive telescope in the hard X-ray regime (> 10 keV) and the only instrument capable of precisely measuring the flux on a sub-hour time scale at these energies. The NuSTAR data are crucial to constrain models given that within leptonic scenarios, the hard X-ray emission is expected to originate from electrons with similar energies than those responsible for the TeV emission measured by MAGIC and FACT. Based on this extensive multiwavelength coverage, several atypical results are reported and the complex behaviour of 1ES 2344+514 and Mrk 421 are characterised with a high precision. The emission is interpreted using leptonic scenarios set up for this work. The blazar 1ES 2344+514 was observed during a period of strong flux activity in summer 2016. The measured very-high-energy (VHE; E > 100 GeV) flux reached a similar level as the historical maximum. A high activity was also measured in the X-rays. The spectral properties during this peculiar event are investigated in detail. One finds that the X-ray spectrum is hard, indicating a low-energy component of the spectral energy distribution (SED) peaking around or above 10 18 Hz (∼4 keV). This value is about two orders of magnitude higher than during archival observations of 1ES 2344+514. 1ES 2344+514 showed an EHBL behaviour on a temporary basis in August 2016. The SED is interpreted with a leptonic and a proton-synchrotron scenario for the gamma-ray emission. Thanks to the wealth of multiwavelength data that has been collected, these models can be better constrained compared to previous works. One finds that both theoretical models are able to describe the SED but require a system out of equipartition by several orders of magnitude, challenging current acceleration process thought to occur in relativistic plasma jets. Mrk 421 was observed from radio to TeV over a ∼7-month period between November 2016 and June 2017. Based on the dense temporal and energy coverage, the correlation patterns of the source are characterised in order to better constrain the emission mechanisms. The VHE versus X-ray flux correlation is studied by binning the data into several sub-energy bands. The correlation is characterised up to the hard X-ray band. The fluxes are strongly correlated, in agreement with leptonic models, which predict that the X-ray and VHE fluxes are emitted by a single electron population. Nonetheless, the present work unveils that the correlation characteristics strongly vary across the sub-energy bands: the correlation ranges from a linear to a cubic trend. Over the multiwavelength campaign, several nights appear as outliers and follow different correlation patterns. They show an enhanced gamma-ray activity without a strong X-ray emission. Using simultaneous MAGIC/NuSTAR/Swift-XRT observations, this peculiar behaviour can be explained by an emitting region in the jet that is changing its size over time. The correlation between the X-ray and UV/optical emission is also studied. An anti-correlation at the significance level above 3σ is found. A hint of this pattern was measured only once until now and only at a marginal significance. The second occurrence of an anti-correlation reported in this work is an important result as it strongly supports the hypothesis that this behaviour has a real physical origin. It may be indicative of changes in the cooling and acceleration efficiencies of electrons in the jet. Finally, a short and bright VHE flare of Mrk 421 is investigated in detail. Interestingly, this VHE flare is identified by an absence of clear X-ray flux increase. This particular day appears as an evident outlier in the VHE versus X-ray correlation study. Using time-resolved SEDs and a leptonic modelling, this work proposes that such a behaviour may be caused by the appearance of a second electron population (which adds up to the electron population responsible for the quiescent state) with a narrow energy distribution located in a compact blob

Measurement of the extragalactic background light using MAGIC and Fermi-LAT gamma-ray observations of blazars up to z=1

We present a measurement of the extragalactic background light (EBL) based on a joint likelihood analysis of 32 gamma-ray spectra for 12 blazars in the redshift range z = 0.03–0.944, obtained by the MAGIC telescopes and Fermi-LAT. The EBL is the part of the diffuse extragalactic radiation spanning the ultraviolet, visible, and infrared bands. Major contributors to the EBL are the light emitted by stars through the history of the Universe, and the fraction of it that was absorbed by dust in galaxies and re-emitted at longer wavelengths. The EBL can be studied indirectly through its effect on very high energy photons that are emitted by cosmic sources and absorbed via γγ interactions during their propagation across cosmological distances. We obtain estimates of the EBL density in good agreement with state-of-the-art models of the EBL production and evolution. The 1σ upper bounds, including systematic uncertainties, are between 13 per cent and 23 per cent above the nominal EBL density in the models. No anomaly in the expected transparency of the Universe to gamma-rays is observed in any range of optical depth. We also perform a wavelength-resolved EBL determination, which results in a hint of an excess of EBL in the 0.18–0.62 μm range relative to the studied models, yet compatible with them within systematics.ISSN:0035-8711ISSN:1365-296

Deep observations of the globular cluster M15 with the MAGIC telescopes

A population of globular clusters (GCs) has been recently established by the Fermi-LAT telescope as a new class of GeV γ-ray sources. Leptons accelerated to TeV energies, in the inner magnetospheres of MSPs or in their wind regions, should produce γ-rays through the inverse Compton scattering in the dense radiation field from the huge population of stars. We have conducted deep observations of the GC M15 with the MAGIC telescopes and used 165 h in order to search for γ-ray emission. A strong upper limit on the TeV γ-ray flux <3.2×10−¹³cm−²s−¹ above 300 GeV (<0.26 per cent of the Crab nebula flux) has been obtained. We interpret this limit as a constraint on the efficiency of the acceleration of leptons in the magnetospheres of the MSPs. We constrain the injection rate of relativistic leptons, ηₑ, from the MSPs magnetospheres and their surrounding. We conclude that ηₑ must be lower than expected from the modelling of high-energy processes in MSP inner magnetospheres. For leptons accelerated with the power-law spectrum in the MSP wind regions, ηₑ is constrained to be much lower than derived for the wind regions around classical pulsars. These constraints are valid for the expected range of magnetic field strengths within the GC and for the range of likely energies of leptons injected from the inner magnetospheres, provided that the leptons are not removed from the GC very efficiently due to advection process. We discuss consequences of these constraints for the models of radiation processes around millisecond pulsars.ISSN:0035-8711ISSN:1365-296

MAGIC very large zenith angle observations of the Crab Nebula up to 100 TeV

Aims. We measure the Crab Nebula γ-ray spectral energy distribution in the ~100 TeV energy domain and test the validity of existing leptonic emission models at these high energies. Methods. We used the novel very large zenith angle observations with the MAGIC telescope system to increase the collection area above 10 TeV. We also developed an auxiliary procedure of monitoring atmospheric transmission in order to assure proper calibration of the accumulated data. This employs recording optical images of the stellar field next to the source position, which provides a better than 10% accuracy for the transmission measurements. Results. We demonstrate that MAGIC very large zenith angle observations yield a collection area larger than a square kilometer. In only ~ 56 h of observations, we detect the γ-ray emission from the Crab Nebula up to 100 TeV, thus providing the highest energy measurement of this source to date with Imaging Atmospheric Cherenkov Telescopes. Comparing accumulated and archival MAGIC and Fermi/LAT data with some of the existing emission models, we find that none of them provides an accurate description of the 1 GeV to 100 TeV γ-ray signal.ISSN:0004-6361ISSN:1432-074

Monitoring of the radio galaxy M 87 during a low -emission state from m 2012 to 2015 with MAGIC

ISSN:0035-8711ISSN:1365-296

MAGIC observations of the diffuse γ-ray emission in the vicinity of the Galactic center

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 2σ. 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.ISSN:0004-6361ISSN:1432-074