High-energy Bump Research Articles

Oscillations between axion-like particles (ALPs) and photons in astrophysical magnetic fields can lead to irregularities in the high energy gamma ray spectra of blazars. The magnetic field within the blazar jet plays a crucial role in shaping these effects, with its strength in the emission region being an important parameter determined by multi-wavelength observations. However, the origin of the high energy bump observed in the spectral energy distribution of some blazars is a topic of debate, with both leptonic and hadronic scenarios providing plausible explanations that result in different magnetic field strengths in the emission region. In this study, we investigate the impact of magnetic field configurations on the constraints of ALP parameters. We consider both leptonic and hadronic emission scenarios for the blazar Mrk 501 and derive the corresponding jet magnetic field strengths. Additionally, we explore two jet magnetic field models: one with a toroidal component and the other with helical and tangled components. By analyzing the spectra of Mrk 501 observed by MAGIC and Fermi-LAT from 2017-06-17 to 2019-07-23, which are well-described by both emission scenarios, we derive constraints on the ALP parameters. Our results demonstrate that both the emission scenario and the magnetic field structure play a significant role in deriving these constraints, with the hadronic model leading to much more stringent limits compared to the leptonic model.

JWST has revealed a population of low-luminosity active galactic nuclei at z > 4 in compact, red hosts (the “Little Red Dots,” or LRDs), which are largely undetected in X-rays. We investigate this phenomenon using General Relativistic Radiation Magnetohydrodynamics simulations of super-Eddington accretion onto a supermassive black hole (SMBH) with M • = 107 M ⊙ at z ∼ 6, representing the median population; the spectral energy distributions (SEDs) that we obtain are intrinsically X-ray weak. The highest levels of X-ray weakness occur in SMBHs accreting at mildly super-Eddington rates (1.4 < f Edd < 4) with zero spin, viewed at angles >30° from the pole. X-ray bolometric corrections in the observed 2–10 keV band reach ∼104 at z = 6, ∼5 times higher than the highest constraint from X-ray stacking. Most SEDs are extraordinarily steep and soft in the X-rays (median photon index Γ = 3.1, mode of Γ = 4.4). SEDs strong in the X-rays have harder spectra with a high-energy bump when viewed near the hot (>108 K) and highly relativistic jet, whereas X-ray weak SEDs lack this feature. Viewing an SMBH within 10° of its pole, where beaming enhances the X-ray emission, has a ∼1.5% probability, matching the LRD X-ray detection rate. Next-generation observatories like AXIS will detect X-ray-weak LRDs at z ∼ 6 from any viewing angle. Although many SMBHs in the LRDs are already estimated to accrete at super-Eddington rates, our model explains 50% of their population by requiring that their masses are overestimated by a mere factor of ∼3. In summary, we suggest that LRDs host slowly spinning SMBHs accreting at mildly super-Eddington rates, with large covering factors and broad emission lines enhanced by strong winds, providing a self-consistent explanation for their X-ray weakness and complementing other models.

Magnetic reconnection driven by a capacitor coil target is an innovative way to investigate low-β magnetic reconnection in the laboratory, where β is the ratio of particle thermal pressure to magnetic pressure. Low-β magnetic reconnection frequently occurs in the Earth’s magnetosphere, where the plasma is characterized by β ≲ 0.01. In this paper, we analyze electron acceleration during magnetic reconnection and its effects on the electron energy spectrum via particle-in-cell simulations informed by parameters obtained from experiments. We note that magnetic reconnection starts when the current sheet is down to about three electron inertial lengths. From a quantitative comparison of the different mechanisms underlying the electron acceleration in low-β reconnection driven by coil targets, we find that the electron acceleration is dominated by the betatron mechanism, whereas the parallel electric field plays a cooling role and Fermi acceleration is negligible. The accelerated electrons produce a hardened power-law spectrum with a high-energy bump. We find that injecting electrons into the current sheet is likely to be essential for further acceleration. In addition, we perform simulations for both a double-coil co-directional magnetic field and a single-coil one to eliminate the possibility of direct acceleration of electrons beyond thermal energies by the coil current. The squeeze between the two coil currents can only accelerate electrons inefficiently before reconnection. The simulation results provide insights to guide future experimental improvements in low-β magnetic reconnection driven by capacitor coil targets.

Abstract Using 2D particle-in-cell plasma simulations, we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top sources in solar flares. We focus on the long-term effect of T e,⊥ &gt; T e,∥ instabilities by driving the anisotropy growth during the entire simulation time through imposing a shearing or a compressing plasma velocity (T e,⊥ and T e,∥ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes T e,⊥/T e,∥ grow due to electron magnetic moment conservation, and amplifies the ratio ω ce/ω pe from ∼0.53 to ∼2 (ω ce and ω pe are the electron cyclotron and plasma frequencies, respectively). In the regime ω ce/ω pe ≲ 1.2–1.7, the instability is dominated by oblique, quasi-electrostatic modes, and the acceleration is inefficient. When ω ce/ω pe has grown to ω ce/ω pe ≳ 1.2–1.7, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z modes. After ω ce/ω pe reaches ∼2, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power law of index α s ∼ 2.9 plus a high-energy bump reaching ∼300 keV. In the compressing runs, α s ∼ 3.7 with a spectral break above ∼500 keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting that a critical role is played by the type of anisotropy driving, ω ce/ω pe, and the electron temperature in the efficiency of the acceleration.

ABSTRACT We report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016–2020. Optical, X-ray, and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very high energy (VHE; ≳100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by The Owens Valley Radio Observatory indicates the multizone nature of the emission from this object. We constrain the VHE duty cycle of the source to be &amp;lt;16 2014-like flares per year (95 per cent confidence). For the first time for this source, a broad-band low-state spectral energy distribution is constructed with a deep exposure up to the VHE range. A flux upper limit on the low-state VHE gamma-ray emission of an order of magnitude below that of the 2014 flare is determined. The X-ray data are used to fit the column density of (8.10 ± 0.93stat) × 1021 cm−2 of the dust in the lensing galaxy. VLBI observations show a clear radio core and jet components in both lensed images, yet no significant movement of the components is seen. The radio measurements are used to model the source-lens-observer geometry and determine the magnifications and time delays for both components. The quiescent emission is modelled with the high-energy bump explained as a combination of synchrotron-self-Compton and external Compton emission from a region located outside of the broad-line region. The bulk of the low-energy emission is explained as originating from a tens-of-parsecs scale jet.

Several natural threats characterize hard coal mining in Poland. The coexistence of methane and rock-burst hazards lowers the safety level during exploration. The most dangerous are high-energy bumps, which might cause rock-burst. Additionally, created during exploitation, safety pillars, which protect openings, might be the reason for the formation of so-called gas traps. In this part, rock mass is usually not disturbed and methane in seams that form the safety pillars is not dangerous as long as they remain intact. Nevertheless, during a rock-burst, a sudden methane outflow can occur. Preventing the existing hazards increases mining costs, and employing inadequate measures threatens the employees’ lives and limbs. Using two longwalls as examples, the authors discuss the consequences of the two natural hazards’ coexistence. In the area of longwall H-4 in seam 409/4, a rock-burst caused a release of approximately 545,000 cubic meters of methane into the excavations, which tripled methane concentration compared to the values from the period preceding the burst. In the second longwall (IV in seam 703/1), a bump was followed by a rock-burst, which reduced the amount of air flowing through the excavation by 30 percent compared to the airflow before, and methane release rose by 60 percent. The analyses presented in this article justify that research is needed to create and implement innovative methods of methane drainage from coal seams to capture methane more effectively at the stage of mining.

Heavy sterile neutrinos with masses \U0001d4aa(100) MeV mixing with active neutrinos can be produced in the core of a collapsing supernova (SN). In order to avoid an excessive energy loss, shortening the observed duration of the SN 1987A neutrino burst, we show that the active-sterile neutrino mixing angle should satisfy sin2 θ ≲ 5 × 10−7. For a mixing with tau flavour, this bound is much stronger than the ones from laboratory searches. Moreover, we show that in the viable parameter space the decay of such “heavy” sterile neutrinos in the SN envelope would lead to a very energetic flux of daughter active neutrinos; if not too far below current limits, this would be detectable in large underground neutrino observatories, like Super-Kamiokande, as a (slightly time-delayed) high-energy bump in the spectrum of a forthcoming Galactic SN event.

After a long low-activity period, a gamma-ray flare from the narrow-line Seyfert 1 PKS 1502+036 (z=0.4089) was detected by the Large Area Telescope (LAT) on board Fermi in 2015. On 2015 December 20 the source reached a daily peak flux, in the 0.1-300 GeV band, of (93 $\pm$ 19) $\times$10$^{-8}$ ph cm$^{-2}$ s$^{-1}$, attaining a flux of (237 $\pm$ 71) $\times$10$^{-8}$ ph cm$^{-2}$ s$^{-1}$ on 3-hr time-scales, which corresponds to an isotropic luminosity of (7.3 $\pm$ 2.1) $\times$10$^{47}$ erg/s. The gamma-ray flare was not accompanied by significant spectral changes. We report on multi-wavelength radio-to-gamma-ray observations of PKS 1502+036 during 2008 August-2016 March by Fermi-LAT, Swift, XMM-Newton, Catalina Real-Time Transient Survey, and the Owens Valley Radio Observatory (OVRO). An increase in activity was observed on 2015 December 22 by Swift in optical, UV, and X-rays. The OVRO 15 GHz light curve reached the highest flux density observed from this source on 2016 January 12, indicating a delay of about three weeks between the gamma-ray and 15 GHz emission peaks. This suggests that the gamma-ray emitting region is located beyond the broad line region. We compared the spectral energy distribution (SED) of an average activity state with that of the flaring state. The two SED, with the high-energy bump modelled as an external Compton component with seed photons from a dust torus, could be fitted by changing the electron distribution parameters as well as the magnetic field. The fit of the disc emission during the average state constrains the black hole mass to values lower than 10$^8$ solar masses. The SED, high-energy emission mechanisms, and gamma-ray properties of the source resemble those of a flat spectrum radio quasar.

1ES 1011+496 $(z=0.212)$ was discovered in very high energy (VHE, E >100 GeV) $\gamma$-rays with MAGIC in 2007. The absence of simultaneous data at lower energies led to a rather incomplete characterization of the broadband spectral energy distribution (SED). We study the source properties and the emission mechanisms, probing whether a simple one-zone synchrotron-self-Compton (SSC) scenario is able to explain the observed broadband spectrum. We analyzed VHE to radio data from 2011 and 2012 collected by MAGIC, $Fermi$-LAT, $Swift$, KVA, OVRO, and Mets\"ahovi in addition to optical polarimetry data and radio maps from the Liverpool Telescope and MOJAVE. The VHE spectrum was fit with a simple power law with a photon index of $3.69\pm0.22$ and a flux above 150 GeV of $(1.46\pm0.16)\times10^{-11}$ ph cm$^{-2}$ s$^{-1}$. 1ES 1011+496 was found to be in a generally quiescent state at all observed wavelengths, showing only moderate variability from radio to X-rays. A low degree of polarization of less than 10% was measured in optical, while some bright features polarized up to 60% were observed in the radio jet. A similar trend in the rotation of the electric vector position angle was found in optical and radio. The radio maps indicated a superluminal motion of $1.8\pm0.4\,c$, which is the highest speed statistically significantly measured so far in a high-frequency-peaked BL Lac. For the first time, the high-energy bump in the broadband SED of 1ES 1011+496 could be fully characterized from 0.1 GeV to 1 TeV which permitted a more reliable interpretation within the one-zone SSC scenario. The polarimetry data suggest that at least part of the optical emission has its origin in some of the bright radio features, while the low polarization in optical might be due to the contribution of parts of the radio jet with different orientations of the magnetic field to the optical emission.

A flare from the TeV blazar Mrk 421, occurring in March 2010, was observed for 13 consecutive days from radio to very high energy (VHE, E > 100 GeV) gamma-rays with MAGIC, VERITAS, Whipple, FermiLAT, MAXI, RXTE, Swift, GASP-WEBT, and several optical and radio telescopes. We model the day-scale SEDs with one-zone and two-zone synchrotron self-Compton (SSC) models, investigate the physical parameters, and evaluate whether the observed broadband SED variability can be associated to variations in the relativistic particle population. Flux variability was remarkable in the X-ray and VHE bands while it was minor or not significant in the other bands. The one-zone SSC model can describe reasonably well the SED of each day for the 13 consecutive days. This flaring activity is also very well described by a two-zone SSC model, where one zone is responsible for the quiescent emission while the other smaller zone, which is spatially separated from the first one, contributes to the daily-variable emission occurring in X-rays and VHE gamma-rays. Both the one-zone SSC and the two-zone SSC models can describe the daily SEDs via the variation of only four or five model parameters, under the hypothesis that the variability is associated mostly to the underlying particle population. This shows that the particle acceleration and cooling mechanism producing the radiating particles could be the main one responsible for the broadband SED variations during the flaring episodes in blazars. The two-zone SSC model provides a better agreement to the observed SED at the narrow peaks of the low- and high-energy bumps during the highest activity, although the reported one-zone SSC model could be further improved by the variation of the parameters related to the emitting region itself ($\delta$, $B$ and $R$), in addition to the parameters related to the particle population.

One of the most important unresolved issues in gamma-ray burst physics is the origin of the prompt gamma-ray spectrum. Its general non-thermal character and the softness in the X-ray band remain unexplained. We tackle these issues by performing Monte Carlo simulations of radiation-matter interactions in a scattering dominated photon-lepton plasma. The plasma -- initially in equilibrium -- is driven to non-equilibrium conditions by a sudden energy injection in the lepton population, mimicking the effect of a shock wave or the dissipation of magnetic energy. Equilibrium restoration occurs due to energy exchange between the photons and leptons. While the initial and final equilibrium spectra are thermal, the transitional photon spectra are characterized by non-thermal features such as power-law tails, high energy bumps, and multiple components. Such non-thermal features are observed at infinity if the dissipation occurs at small to moderate optical depths, and the spectrum is released before thermalization is complete. We model the synthetic spectra with a Band function and show that the resulting spectral parameters are similar to observations for a frequency range of 2-3 orders of magnitude around the peak. In addition, our model predicts correlations between the low-frequency photon index and the peak frequency as well as between the low- and high-frequency indices. We explore baryon and pair dominated fireballs and reach the conclusion that baryonic fireballs are a better model for explaining the observed features of gamma-ray burst spectra.

Estimates of magnetic field strength in relativistic jets of active galactic nuclei (AGN), obtained by measuring the frequency-dependent radio core location, imply that the total magnetic fluxes in those jets are consistent with the predictions of the magnetically-arrested disk (MAD) scenario of jet formation. On the other hand, the magnetic field strength determines the luminosity of the synchrotron radiation, which forms the low-energy bump of the observed blazar spectral energy distribution (SED). The SEDs of the most powerful blazars are strongly dominated by the high-energy bump, which is most likely due to the external radiation Compton (ERC) mechanism. This high Compton dominance may be difficult to reconcile with the MAD scenario, unless 1) the geometry of external radiation sources (broad-line region, hot-dust torus) is quasi-spherical rather than flat, or 2) most gamma-ray radiation is produced in jet regions of low magnetization, e.g., in magnetic reconnection layers or in fast jet spines.

Accreting black holes in galactic X-ray sources are surrounded by hot plasma. The innermost part of these systems is likely a corona with different temperatures for ions and electrons. In the so-called low-hard state, hot electrons Comptonize soft X-ray photons from the disk that partially penetrates the corona, producing emission up to $\sim 150$ keV, well beyond the expectations for an optically thick disk of maximum temperature $\sim 10^{7}$ K. However, sources such as Cygnus X-1 produce steady emission up to a few MeV, which is indicative of a non-thermal contribution to the spectral energy distribution. We study the radiative output produced by the injection of non-thermal (both electron and proton) particles in a magnetized corona around a black hole. Energy losses and maximum energies are estimated for all types of particles in a variety of models, characterized by different kinds of advection and relativistic proton content. Transport equations are solved for primary and secondary particles, and spectral energy distributions are determined and corrected by internal absorption. We show that a local injection of non-thermal particles can account for the high energy excess observed in some sources, and we predict the existence of a high-energy bump at energies above 1 TeV, and typical luminosities of $\sim 10^{33}$ erg s$^{-1}$. High-energy instruments such as the future Cherenkov Telescope Array (CTA) can be used to probe the relativistic particle content of the coronae around galactic black holes.

The extragalactic background (EGB) of diffuse gamma rays can be determined by subtracting the Galactic contribution from the data. This requires a Galactic model (GM) and we include for the first time the contribution of dark matter annihilation (DMA), which was previously proposed as an explanation for the EGRET excess of diffuse Galactic gamma rays above 1 GeV. In this paper it is shown that the newly determined EGB shows a characteristic high energy bump on top of a steeply falling soft contribution. The bump is shown to be compatible with a contribution from an extragalactic DMA signal from weakly interacting massive particles (WIMPs) with a mass between 50 and 100 GeV in agreement with the EGRET excess of the Galactic diffuse gamma rays and in disagreement with earlier analysis. The remaining soft contribution of the EGB is shown to resemble the spectra of the observed point sources in our Galaxy.

We present the broad band analysis of two Beppo SAX observations of the Seyfert 1 Mkn 509. In 2000 the source was in a typical flux state, $F_{2-10~\rm keV}=2.7\times 10^{-11}~ \;\rm erg~cm^{-2}\,s^{-1}$, while in 1998 it was found in a high flux state, $F_{2-10~\rm keV}=5.7\times 10^{-11}~ \;\rm erg~cm^{-2}\,s^{-1}$. A comparison between the two states shows a energy–dependent flux variation of about a factor of three and a factor of two in the LECS (0.15-3 keV) and MECS (1.5-10 keV), respectively, while in the PDS (13-200 keV) the difference is marginal. A soft excess, a narrow iron line and a Compton reflection hump above 10 keV are clearly apparent in the residuals after fitting the spectra with a simple power law. We tested two alternative models. In the first the iron line and the high energy bump are well reproduced by reprocessing in a cold and Compton thick material. The intensity of the iron line (also consistent with a Chandra measurement) as well as the normalization of the reflection hump are consistent with a constant in the two epochs: this, combined with the fact that the line is narrow as observed by Chandra , suggests a common origin from distant and optically thick matter. This model further requires a component to model the soft excess: the empirical choice of two black bodies accounts well for the excess in both observations; their combined strength was a factor of about three higher in the high than in the low flux state defined above. However, the relative contribution of the soft excess is higher in the low flux state. In the second model we attempted to reproduce all spectral features, except for the narrow cold line, with reflection from an ionized disc. This model is successful only in the high flux state, but it fails in the low flux state, when the soft excess is only partially accounted for. In either model, the slope of the power law is greater in the high than in the low flux state, ($\Delta\Gamma \sim0.2$), in agreement with a behaviour known to be shared by several objects of the same type.

Abstract A simple model of cosmic ray electron acceleration at the jet boundary yields a power law particle energy distribution of ultra-relativistic electrons with an energy cut-off growing with time, and, finally, a growing particle bump at the energy where energy gains equal radiation losses. For such electron distribution, in tens-of-kpc scale jets, we derived the observed time-varying spectra of synchrotron and inverse Compton radiation, including Comptonisation of synchrotron and cosmic microwave background photons. Slowly varying spectral index along the jet in the ‘low frequency’ spectral range is a natural consequence of boundary layer acceleration. Variations of the high energy bump of the electron distribution can give rise to anomalous behaviour in the X-ray band in comparison to the lower frequencies.

We consider effects on an (ultra)relativistic jet and its ambient medium caused by high-energy cosmic rays accelerated at the jet side boundary. As illustrated by simple models, during the acceleration process a flat cosmic ray distribution can be created, with gyro-radii for the highest particle energies reaching scales comparable to the jet radius or energy density comparable to the pressure of the ambient medium. In the case of efficient radiative losses, a high-energy bump in the spectrum can dominate the cosmic ray pressure. In extreme cases, the cosmic rays are able to push the ambient medium off, providing a ‘cosmic ray cocoon’ separating the jet from the surrounding medium. The considered cosmic rays provide an additional jet braking force and lead to a number of consequences for the jet structure and its radiative output. In particular, the dynamic and acceleration time-scales involved are in the range observed in variable active galactic nuclei.

Single particle states in the continuum

The physical phenomena of the ECR-microwave discharge are numerically studied by a one-dimensional hybrid model of the fluid electrons and particle ions. The present model includes both the ECR heating phenomena and the transport of ions along divergent axial magnetic field lines, microwave is considered as an energy flow attenuated by the thermal electron fluid. Individual ion motion is determined by ambipolar electric field and Monte-Carlo collisions together with the /spl nabla/B force. In the fluid description of electrons, electron motions are coupled to the ions through ambipolarity, and the energy transport is treated with the temperature equation. The simulation results for argon discharges show the two characteristic features of the measurements for the ion energy distribution: the low energy peak as found in energy analyzer measurements, and the high energy bump as found in LIF measurements. Also the strong effect of the distributed ionization on the ion energy distribution is observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Over the last few years, GINGA observations of several bright Seyfert galaxies have revealed the existence of a new flat component in the X-ray spectrum of these objects above 8 keV. This component (the «high-energy bump») is probably produced by the reprocessing of the intrinsic emission (through Compton scattering and photoelectric absorption) by a very thick and cold medium in the environment of the central source, such as an accretion disk. An alternative explanation is provided by partial covering and reprocessing by thick clouds, whose existence is supported by the latest results on the broad line region obtained by line reverberation measurements in the optical-ultraviolet band. The presence of iron fluorescent lines at 6.4 keV with an equivalent width ≅ (100÷200) eV in all those galaxies supports both scenarios. The implications of the presence of the high-energy bump on the spectral shape of the intrinsic continuum, the level of the soft X-ray excess and the X-ray background are briefly pointed out.