Vitaly Neustroev

Aims . The Large High Altitude Air Shower Observatory (LHAASO) Collaboration has recently reported the first detection at TeV energies of a low-luminosity radio galaxy, NGC 4278. The aim of this work is to investigate the high-energy properties of NGC 4278 during the flaring and subsequent quasi-quiescent states with the Florian Goebel Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. Methods . NGC 4278 is located in the field of view of two blazars, 1ES 1215+303 and 1ES 1218+304, previously observed by the MAGIC telescopes. Therefore, we reanalyzed MAGIC observations made between 2010 and 2024 on these sources. We also modeled the broadband spectral energy distribution of the source during and after the flaring state at TeV energies. Results . We did not detect any statistically significant γ-ray emission from NGC 4278 with MAGIC. The corresponding upper limits obtained using the entire MAGIC dataset ( F _{UL,>150 GeV} = 1.5 × 10 ⁻¹² ph s ⁻¹ cm ⁻² ) are consistent with the LHAASO results. The best-fit models obtained for both emission states suggest that the emitting region is strongly particle-dominated, and an efficient acceleration mechanism has to be in action in order to reach TeV energies. The transition between the flaring and quasi-quiescent state cannot be explained by a simple radiative cooling of the emitting particles. The inferred jet power, on the order of L _jet ∼ 10 ⁴² erg s ⁻¹ , is dominated by the kinetic component in both states and it is in a good agreement with previous, time-averaged observational estimates, supporting the idea that such high-energy flares might be recurrent. The jet, however, remains too weak to break the host-galaxy confinement.

We present very-high-energy gamma-ray observations of two binary black hole merger candidates, GW240615_113620 and GW241125_010116, performed with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the first Large-Sized Telescope of the Cherenkov Telescope Array Observatory’s (CTAO LST-1). GW240615_113620 was the best localized event of the fourth observing run of the LIGO-Virgo-KAGRA gravitational waves interferometers. GW241125_010116 was temporally and spatially coincident with a sub-threshold short-duration burst detected with the Swift-Burst Alert Telescope, the Swift-X-Ray Telescope and the Einstein Probe Follow-up X-ray Telescope. We observed the two events in stereoscopic mode, taking advantage of the improved sensitivity of joint MAGIC+LST-1 observations. No detection was achieved in the GeV–TeV gamma-ray band for any of the two sources. The unfavourable observing conditions of both events posed a challenge for a standard analysis and therefore a non standard analysis was necessary for both objects. Owing to the small localization area and the association with a GRB-like burst respectively, these events represented an unprecedented opportunity to study in details the electromagnetic emission from binary black holes merger events and, in particular, we discussed two theoretical models that predict a detectable gamma-ray emission and the possible future applications.

Flat spectrum radio quasars (FSRQs) often remain undetected at very-high-energy (VHE, E > 100 GeV) γ rays due to their typically soft spectra and large distance. In December 2023, the Large-Sized Telescope prototype (LST-1) detected for the first time VHE γ-ray emission from the FSRQ OP 313 during an extraordinarily bright flare, becoming the furthest blazar ever observed at VHE with z=0.997. We aim to characterize the γ-ray emission of OP 313 during this flare, comparing it with its average emission state in order to understand the processes leading to this detection. Its remarkable distance also enables studies on the extragalactic gackground light (EBL), with the goal of evaluating the attenuation of VHE γ-ray photons. Thanks to an intensive multiwavelength campaign, we also study in detail the broadband emission of the source. We characterize the γ-ray emission during the flare in December 2023 and the low emission state observed in January 2024 thanks to the LST-1 and MAGIC data and quasi-simultaneous -LAT observations. Fermi This dataset also enables us to evaluate the EBL attenuation by systematically exploring the EBL intensity over the γ-ray spectrum. . Finally, we study the multiwavelength emission and interpret the broadband spectral energy distribution (SED) within blazar radiative models including the thermal contributions from the accretion disk, dusty torus and broad line region. We measured significant VHE γ-ray emission during the December 2023 flare at a level of 0.3 Crab units above 100 GeV . We also characterize the flare brightness in the high-energy (HE, E>100 MeV) γ-ray band, which was found to be a factor of 50 above the average emission seen by -LAT. The HE and VHE observations allow us to set constrains on the EBL density, resulting in an upper limit at 0.6 μm of łambda I_ Fermi łambda <8.74 nW m 2 $ sr^-1 with LST-1 data, and łambda I_ łambda <14.7 nW m^2 sr^-1 with MAGIC data, including systematic uncertainties. Finally, thanks to the extensive multiwavelength campaign organized, we are able to construct and model the broadband SED of OP 313 within the framework of a two-zone leptonic model where the γ-ray emission is produced via inverse-Compton scattering of the broad line region, accretion disk, and dusty torus photon fields. However, the dominant external photon field remains unknown, as several combinations are able to successfully explain the $γ-ray emission observed .

Extremely high-peaked BL Lac objects—also named extreme blazars —are among the most energetic and persistent extragalactic accelerators in the Universe, defined by a synchrotron emission peaking above 10 ¹⁷ Hz in X-rays. Such emission is then reprocessed and produces radiation extending deeply into very-high-energy (VHE; energy E > 100 GeV) gamma-rays. Observations in this energy band—optimally investigated by the Imaging Air-Shower Cherenkov telescopes—are crucial for probing the physical processes that drive their extreme behavior. This study extends our investigation of extreme blazars in the VHE gamma-ray range, providing a second new mini-catalog of sources observed by the MAGIC telescopes. We report on the monitoring of seven targets between 2017 and 2025, including four newly observed sources and three that have been part of long-term observation campaigns, for a total of approximately 338 hr of observations. The analysis of MAGIC data reveals two new VHE detections of extreme blazars, along with three additional sources showing hints of VHE emission. Joint observations of MAGIC and the first Large-Sized Telescope (LST-1) also confirmed a new VHE extreme blazar. Our results are complemented by simultaneous multiwavelength observations in other energy bands, including optical-UV, X-rays, and high-energy gamma-rays (100 MeV < E < 100 GeV). We confirm typical behavior of extreme blazars, such as a modest variability and a “harder-when-brighter” trend in X-rays across the sample. This new set increases the population of extreme blazars, and their broadband analysis confirms the physical properties of these extreme sources.

Aims . Magnetic white dwarfs (MWDs) are key to understanding the origin and evolution of magnetic fields in compact stars. While large spectroscopic surveys such as SDSS have greatly expanded the known sample, the potential of LAMOST has not yet been fully explored. Our aim is to identify and characterize isolated MWDs in the LAMOST data release ten (DR10). Methods . We crossmatched LAMOST DR10 spectra with white dwarf candidates from Gaia early data release three (EDR3) and with recent SDSS-based catalogs of MWDs. Zeeman splitting in Balmer and helium absorption lines was used as the primary diagnostic to identify magnetic fields and to estimate their strengths. Reference objects from the SDSS catalogs were used to test the detectability of MWDs in LAMOST low-resolution spectra. Results . We identified 63 isolated MWDs in LAMOST DR10, 32 of which are new discoveries. Surface magnetic field strengths were measured from Zeeman splitting, covering a range from a few megagauss up to several tens. For previously known SDSS MWDs, our LAMOST-based field measurements mostly show agreement with published values. Conclusions . This work demonstrates the capability of LAMOST low-resolution spectroscopy to identify and characterize isolated MWDs. The newly discovered objects expand the known population and provide valuable targets for future high-resolution spectroscopic and polarimetric follow-up studies. Our results highlight the potential of combining LAMOST with Gaia and other large surveys to build a more complete census of MWDs.

Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive currently operating gamma-ray telescopes, namely: the satellite-borne Fermi -LAT telescope; the ground-based imaging atmospheric Cherenkov telescope arrays H.E.S.S., MAGIC, and VERITAS; and the HAWC water Cherenkov detector. Individual datasets were analyzed using a common statistical approach. Results were subsequently combined via a global joint likelihood analysis. We obtain constraints on the velocity-weighted cross section 〈 σv 〉 for DM self-annihilation as a function of the DM particle mass. This five-instrument combination allows the derivation of up to 2-3 times more constraining upper limits on 〈 σv 〉 than the individual results over a wide mass range spanning from 5 GeV to 100 TeV. Depending on the DM content modeling, the 95% confidence level observed limits reach 1.5×10 ^-24 cm ³ s ^-1 and 3.2×10 ^-25 cm ³ s ^-1 , respectively, in the τ ⁺ τ ^- annihilation channel for a DM mass of 2 TeV.

In 2013 April, the TeV blazar Markarian 421 underwent one of its most powerful emission outbursts recorded to date. An extensive multi-instrument campaign featuring MAGIC, VERITAS, and NuSTAR provided comprehensive very high-energy (VHE; E > 100 GeV) and X-ray coverage over nine consecutive days. The VHE flux peaked at approximately 15 times that of the Crab Nebula, with rapid variability detected on timescales down to 15 minutes in both X-ray and VHE bands. This rich data set, characterized by its dense temporal coverage and high photon statistics, offers an unparalleled opportunity to probe the broadband emission dynamics in blazars. In this work, we perform a detailed spectral analysis of the X-ray and VHE emissions on subhour timescales throughout the flare. We identify several clockwise spectral hysteresis loops in the X-rays, revealing a spectral evolution more complex than a simple harder-when-brighter trend. The VHE spectrum extends beyond 10 TeV, and its temporal evolution closely mirrors the behavior in the X-rays. Crucially, we report the first evidence of VHE spectral hysteresis occurring simultaneously with the X-ray loops. To interpret these findings, we apply a time-dependent leptonic model to 240 broadband spectral energy distributions (SEDs) binned on a 15 minute scale, allowing us to self-consistently track the particle distribution’s history. Our modeling shows that the majority of the subhour flux and spectral variations are driven by changes in the luminosity and slope of the injected electron distribution. The required variations in the electron slope are difficult to reconcile with magnetic reconnection, but they are consistent with a shock-acceleration scenario where the shock compression ratio evolves by a factor of ∼2. The model also points to a relatively stable magnetic field and emitting region size, favoring a scenario where the emission originates from a stationary feature in the jet, such as a recollimation shock. However, this scenario requires a jet Lorentz factor that significantly exceeds values from VLBI measurements to account for the high minimum electron energy implied by the lack of variability in the optical band.

The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multimessenger approach, which seeks to detect the γ -rays that accompany neutrinos as they are produced at their sources. Multimessenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in high-energy ( E > 1 GeV) and very-high-energy (VHE; E > 100 GeV) γ -rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: follow-up observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known γ -ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of follow-up observations of high-energy neutrino events observed by the four IACTs between 2017 September (after the IceCube-170922A event) and 2021 January. Our study found no associations between γ -ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE γ -ray flux.

The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multi-messenger approach, which seeks to detect the gamma rays that accompany neutrinos as they are produced at their sources. Multi-messenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in highenergy (HE, E > 1 GeV) and very-high-energy (VHE, E > 100 GeV) gamma rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: followup observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known gamma-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of follow-up observations of high-energy neutrino events observed by the four IACTs between September 2017 (after the IceCube-170922A event) and January 2021. Our study found no associations between gamma-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE gamma-ray flux.