Due to the limited field of view (FoV) of Cherenkov telescopes, the time
needed to achieve target sensitivity for surveys of the extragalactic and
Galactic sky is large. To optimize the time spent to ...perform such surveys, a
so-called "divergent mode" of the Cherenkov Telescope Array Observatory (CTAO)
was proposed as an alternative observation strategy to the traditional parallel
pointing. In the divergent mode, each telescope points to a position in the sky
that is slightly offset, in the outward direction, from the original center of
the field of view. This bring the advantage of increasing the total
instantaneous arrays' FoV. From an enlarged field of view also benefits the
search for very-high-energy transient sources, making it possible to cover
large sky regions in follow-up observations, or to quickly cover the
probability sky map in case of Gamma Ray Bursts (GRB), Gravitational Waves
(GW), and other transient events. In this contribution, we present the proposed
implementation of the divergent pointing mode and its first preliminary
performance estimation for the southern CTAO array.
The Lorentz Invariance Violation (LIV), a proposed consequence of certain quantum gravity (QG) scenarios, could instigate an energy-dependent group velocity for ultra-relativistic particles. This ...energy dependence, although suppressed by the massive QG energy scale \(E_\mathrm{QG}\), expected to be on the level of the Planck energy \(1.22 \times 10^{19}\) GeV, is potentially detectable in astrophysical observations. In this scenario, the cosmological distances traversed by photons act as an amplifier for this effect. By leveraging the observation of a remarkable flare from the blazar Mrk\,421, recorded at energies above 100 GeV by the MAGIC telescopes on the night of April 25 to 26, 2014, we look for time delays scaling linearly and quadratically with the photon energies. Using for the first time in LIV studies a binned-likelihood approach we set constraints on the QG energy scale. For the linear scenario, we set \(95\%\) lower limits \(E_\mathrm{QG}>2.7\times10^{17}\) GeV for the subluminal case and \(E_\mathrm{QG}> 3.6 \times10^{17}\) GeV for the superluminal case. For the quadratic scenario, the \(95\%\) lower limits for the subluminal and superluminal cases are \(E_\mathrm{QG}>2.6 \times10^{10}\) GeV and \(E_\mathrm{QG}>2.5\times10^{10}\) GeV, respectively.
In recent years, a new generation of optical intensity interferometers has emerged, leveraging the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC telescopes ...host the MAGIC-SII system (Stellar Intensity Interferometer), implemented to investigate the feasibility and potential of this technique on IACTs. After the first successful measurements in 2019, the system was upgraded and now features a real-time, dead-time-free, 4-channel, GPU-based correlator. These hardware modifications allow seamless transitions between MAGIC's standard very-high-energy gamma-ray observations and optical interferometry measurements within seconds. We establish the feasibility and potential of employing IACTs as competitive optical Intensity Interferometers with minimal hardware adjustments. The measurement of a total of 22 stellar diameters are reported, 9 corresponding to reference stars with previous comparable measurements, and 13 with no prior measurements. A prospective implementation involving telescopes from the forthcoming Cherenkov Telescope Array Observatory's northern hemisphere array, such as the first prototype of its Large-Sized Telescopes, LST-1, is technically viable. This integration would significantly enhance the sensitivity of the current system and broaden the UV-plane coverage. This advancement would enable the system to achieve competitive sensitivity with the current generation of long-baseline optical interferometers over blue wavelengths.
We present the first multi-wavelength study of Mrk 501 including
very-high-energy (VHE) gamma-ray observations simultaneous to X-ray
polarization measurements from the Imaging X-ray Polarimetry ...Explorer (IXPE).
We use radio-to-VHE data from a multi-wavelength campaign organized between
2022-03-01 and 2022-07-19. The observations were performed by MAGIC, Fermi-LAT,
NuSTAR, Swift (XRT and UVOT), and several instruments covering the optical and
radio bands. During the IXPE pointings, the VHE state is close to the average
behavior with a 0.2-1 TeV flux of 20%-50% the emission of the Crab Nebula.
Despite the average VHE activity, an extreme X-ray behavior is measured for the
first two IXPE pointings in March 2022 with a synchrotron peak frequency >1
keV. For the third IXPE pointing in July 2022, the synchrotron peak shifts
towards lower energies and the optical/X-ray polarization degrees drop. The
X-ray polarization is systematically higher than at lower energies, suggesting
an energy-stratification of the jet. While during the IXPE epochs the
polarization angle in the X-ray, optical and radio bands align well, we find a
clear discrepancy in the optical and radio polarization angles in the middle of
the campaign. We model the broad-band spectra simultaneous to the IXPE
pointings assuming a compact zone dominating in the X-rays and VHE, and an
extended zone stretching further downstream the jet dominating the emission at
lower energies. NuSTAR data allow us to precisely constrain the synchrotron
peak and therefore the underlying electron distribution. The change between the
different states observed in the three IXPE pointings can be explained by a
change of magnetization and/or emission region size, which directly connects
the shift of the synchrotron peak to lower energies with the drop in
polarization degree.
Axion-like particles (ALPs) are pseudo-Nambu-Goldstone bosons that emerge in various theories beyond the standard model. These particles can interact with high-energy photons in external magnetic ...fields, influencing the observed gamma-ray spectrum. This study analyzes 41.3 hrs of observational data from the Perseus Galaxy Cluster collected with the MAGIC telescopes. We focused on the spectra the radio galaxy in the center of the cluster: NGC 1275. By modeling the magnetic field surrounding this target, we searched for spectral indications of ALP presence. Despite finding no statistical evidence of ALP signatures, we were able to exclude ALP models in the sub-micro electronvolt range. Our analysis improved upon previous work by calculating the full likelihood and statistical coverage for all considered models across the parameter space. Consequently, we achieved the most stringent limits to date for ALP masses around 50 neV, with cross sections down to \(g_{a\gamma} = 3 \times 10^{-12}\) GeV\(^{-1}\).
We perform the first broadband study of Mrk421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE. The data were collected within an extensive ...multiwavelength campaign organized between May and June 2022 using MAGIC, Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE. During the IXPE exposures, the measured 0.2-1 TeV flux is close to the quiescent state and ranges from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the VHE and X-ray emission are positively correlated at a \(4\sigma\) significance level. The IXPE measurements unveil a X-ray polarization degree that is a factor of 2-5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the Swift-XRT monitoring unveils an X-ray flux increase with a clear spectral hardening. It suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counter-clockwise), implying important changes in the particle acceleration efficiency on \(\sim\)hour timescales.
Monthly Notices of the Royal Astronomical Society, Volume 527,
Issue 3, January 2024, Pages 5856-5867 Gamma-ray bursts (GRBs) are explosive transient events occurring at
cosmological distances, ...releasing a large amount of energy as electromagnetic
radiation over several energy bands. We report the detection of the long
GRB~201216C by the MAGIC telescopes. The source is located at $z=1.1$ and thus
it is the farthest one detected at very high energies. The emission above
\SI{70}{\GeV} of GRB~201216C is modelled together with multi-wavelength data
within a synchrotron and synchrotron-self Compton (SSC) scenario. We find that
SSC can explain the broadband data well from the optical to the
very-high-energy band. For the late-time radio data, a different component is
needed to account for the observed emission. Differently from previous GRBs
detected in the very-high-energy range, the model for GRB~201216C strongly
favors a wind-like medium. The model parameters have values similar to those
found in past studies of the afterglows of GRBs detected up to GeV energies.
The BL Lac 1ES 2344+514 is known for temporary extreme properties (e.g., a shift of the synchrotron SED peak energy \(\nu_{synch,p}\) above 1keV). While those extreme states were so far observed only ...during high flux levels, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to VHE performed so far, focusing on a systematic characterisation of the intermittent extreme states. While our results confirm that 1ES 2344+514 typically exhibits \(\nu_{synch,p}>\)1keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase of the electron acceleration efficiency without a change in the electron injection luminosity. We also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of \(\nu_{synch,p}\). During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. During a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions contribute significantly to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3-2keV band. Using a time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model.
LHAASO J2108+5157 is one of the few known unidentified Ultra-High-Energy (UHE) gamma-ray sources with no Very-High-Energy (VHE) counterpart, recently discovered by the LHAASO collaboration. We ...observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its High-Energy (HE) counterpart 4FGL J2108.0+5155. We found an excess (3.7 sigma) in the LST-1 data at energies E > 3 TeV. Further analysis in the whole LST-1 energy range assuming a point-like source, resulted in a hint (2.2 sigma) of hard emission which can be described with a single power law with photon index Gamma = 1.6 +- 0.2 between 0.3 - 100 TeV. We did not find any significant extended emission which could be related to a Supernova Remnant (SNR) or Pulsar Wind Nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. The LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of \(100^{+70}_{-30}\) TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a PWN or a TeV halo. The lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHE gamma rays can also be explained as \(\pi^0\) decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. The hard spectrum in the LST-1 band is compatible with protons escaping a shock around a middle-aged SNR because of their high low-energy cut-off.
