Search

We introduce a simple parametric model of the radio–infrared correlation (i.e., the ratio between the IR luminosity and the 1.4 GHz radio luminosity, q IR) by considering the energy loss rate of high-energy cosmic-ray (CR) electrons governed by radiative cooling (synchrotron, bremsstrahlung, inverse Compton scattering), ionization, and adiabatic expansion. Each process of CR electron energy loss is explicitly computed and compared to each other. We rewrite the energy loss rate of each process to be dependent on the gas surface density and redshift using the relevant scaling relations. By combining each energy loss rate, the fraction of the synchrotron energy loss rate is computed as a function of gas surface density and redshift and used to extrapolate the well-established "local" radio–infrared correlation to the high-redshift Universe. The locally established q IR is reformulated to be dependent upon the redshift and the gas surface density and applied for understanding the observed distribution of the radio–infrared correlation of high-redshift galaxies in I. Delvecchio et al. Our model predicts that the q IR value is anticorrelated with gas surface density and the redshift dependency of the q IR value changes by the gas surface density of galaxies, which captures the observed trend of q IR values for stellar-mass-selected star-forming galaxies with a minimal impact of radio–infrared selection bias. [ABSTRACT FROM AUTHOR]

The detection of high-flux gamma beams from laser plasma interactions is always hampered by signal pileup. In this study, a gamma threshold detector based on the bubble chamber is designed to detect high-flux gamma beams. Through simulations, it has been demonstrated that this detector can detect gamma rays with energies above 5 MeV through photonuclear reactions, even at fluences as high as 1011 photons/cm2. In addition, by setting the detection threshold at 0.1 MeV/μm, a yield of up to 10−2 bubbles per gamma interaction can be achieved. Finally, the bubble chamber in the detection of a gamma ray from the inverse Compton scattering process is also discussed. [ABSTRACT FROM AUTHOR]

A high efficiency simulation method for propagation-based phase-contrast imaging, called directional macro-wavefront (DMWF), is developed with the aim of simulating high-energy phase-contrast imaging. This method takes both Monte Carlo and wave optical propagation into consideration. Traditional wave-optics-based simulation methods for phase-contrast imaging encounter unacceptable computational complexity when high-energy radiation is used. In contrast, this method effectively addresses this issue by using macro-wavefront integration. Several simulation examples using typical parameters of inverse Compton scattering sources are presented to illustrate the excellent energy adaptability and efficiency of the DMWF method. This method provides a more efficient approach for phase-contrast imaging simulations, which will drive the advancement of high-energy phase-contrast imaging. [ABSTRACT FROM AUTHOR]

This paper proposed an X-band phase shifter for the very compact inverse Compton scattering gamma-ray source program at Tsinghua University and conducted its structure design, numerical simulation, fabrication, cold test, and high-power testing. This phase shifter is composed of a polarizer, circular waveguide, and a piston with a choke structure. The simulation results show that the reflection coefficient of the phase shifter is under −40 dB and the insertion coefficient exceeds −0.06 dB at the operating frequency of 11.424 GHz. The phase variation is 20°/mm, and a linear phase change from 0° to 360° can be achieved with a piston displacement of 18 mm. The manufactured phase shifter has exhibited good performance in the cold test by using the vector network analyzer. After 16 h conditioning in the Tsinghua X-band high-power test stand, the phase shifter reached a peak power of 72 MW at 230 ns pulse width and a peak power of 82 MW at 130 ns pulse width. After processing signals from the high-speed oscilloscope, it was found that the transmission phase variations were in good agreement with the simulation results. [ABSTRACT FROM AUTHOR]

In the last few decades, galactic stellar black hole X-ray binary systems (BHXRBs) have aroused intense observational and theoretical research efforts specifically focusing on their multi-messenger emissions (radio waves, X-rays, γ -rays, neutrinos, etc.). In this work, we investigate jet emissions of high-energy neutrinos and gamma-rays created through several hadronic and leptonic processes taking place within the jets. We pay special attention to the effect of the black hole's spin (Kerr black holes) on the differential fluxes of photons originating from synchrotron emission and inverse Compton scattering and specifically on their absorption due to the accretion disk's black-body radiation. The black hole's spin (dimensionless spin parameter a * ) enters into the calculations through the radius of the innermost circular orbit around the black hole, the R I S C O parameter, assumed to be the inner radius of the accretion disk, which determines its optical depth τ d i s k . In our results, the differential photon fluxes after the absorption effect are depicted as a function of the photon energy in the range 1 GeV ≤ E ≤ 10 3 GeV. It is worth noting that when the black holes' spin ( α * ) increases, the differential photon flux becomes significantly lower. [ABSTRACT FROM AUTHOR]

We present the generation of attosecond gamma rays via inverse Compton scattering within the framework of laser wakefield acceleration through 2D Particle-In-Cell simulations. Utilizing the near-threshold ionization injection mechanism, an attosecond micro-bunched electron beam characterized by a comb-like current density profile can be achieved with a linearly polarized laser at an intensity of a0 = 1.5. The micro-bunched beam provides a beam energy of approximately 300 MeV and achieves a minimum relative energy spread of about 1.64% after undergoing 2 mm of acceleration. In the inverse Compton scattering scheme, these attosecond electron micro-bunches interact with the reflected driving laser pulse, resulting in the attosecond gamma-ray radiation exhibiting similar structures. Individual spatial-separated gamma-ray pulses exhibit a length of approximately 260–300 as, with a critical energy of 2.0 ± 0.2 MeV. The separated attosecond gamma-ray source owns a peak brilliance of ~1022 photons s−1 mm−2 mrad−2 0.1% BW. This brilliance is competitive in a laboratory for multi-MeV γ-ray sources with a laser intensity of I = 5 × 1018 W/cm2. Such attosecond gamma-ray radiation offers promising applications requiring ultrashort X-ray/gamma ray sources. [ABSTRACT FROM AUTHOR]

A timing method for experiments on the interaction of a near-infrared laser and an ultra-relativistic electron beam via a semiconductor plasma switch is experimentally validated. As an intermediate medium, a thin Si plate is excited by the energetic, intense electron beam to produce a semiconductor plasma, which in turn deflects counter-colliding laser light having 1 μm wavelength. An electron beam of sub-nC charge sufficiently induces the needed electron number density gradient of 1 × 1020 cm−3 per tens of μm length at the interaction point. Demonstration during an inverse Compton scattering experiment by a counter-colliding electron beam of 300 pC and 70 MeV with an Nd: YAG laser at a wavelength of 1 μm is reported. [ABSTRACT FROM AUTHOR]

In this work, we explore the connection of three jetted |$\gamma -$| loud AGNs classes: Compact steep-spectrum sources (CSS), Narrow-line Seyfert 1 (NLS1), and flat-spectrum radio quasars, through the modelling of the spectral energy distribution (SED). We selected two sources identified as CSS/NLS1 hybrids, PKS 2004-440 and 3C 286. Additionally, we included the source PKS 0440-00, initially classified as an FSRQ in the first Fermi -LAT catalogue, but recently reclassified as an NLS1. We present the results of their broadband SED modelling using a one-zone leptonic synchrotron-self Compton (SSC) + external Compton (EC) model. By exploring the parameter space and investigating the disc–jet connection in these sources, we analyse their classification in a model-dependent way. Our findings reveal that modeling PKS 2004-447 at relatively large angles, as expected for CSS, results in an SSC-dominated inverse Compton emission. In contrast, at low-observing angles, the inverse Compton emission is dominated by external photon fields. Both scenarios result in a jet with a low-radiative power. For 3C 286, we found that using a one-zone model limits the jet viewing angle to |$\sim 7^{\circ }$|⁠ , mainly due to its impact on the |$\gamma$| -ray emission. Our model results show a magnetically dominated jet, consistent with |$\gamma$| -CSS sources. Our results suggest that PKS 0440-00, can be classified as a powerful |$\gamma -$| NLS1, characterized by high accretion power and a jet dominated by bulk motion, similar to FSRQs. [ABSTRACT FROM AUTHOR]

Production of hard X-ray via inverse Compton scattering at photon energies below 100 keV range aimed at potential applications in medicine and material research is reported. Experiments have been performed at the Brookhaven National Laboratory, Accelerator Test Facility, employing the counter collision of a 70 MeV, 0.3 nC electron beam with a near infra-red Nd: YAG laser (1064 nm wavelength) pulse containing ~ 100 mJ in a single shot basis. The radiation distribution of the scattered photon beam is assessed to be sufficiently quasi monochromatic to produce clear contrast from the Au K- edge at 80.7 keV. [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis of approximately 15 years (2006–2021) of X-ray observations of UGC 6728, a low-mass bare AGN, for the first time. Our study encompasses both spectral and temporal aspects of this source. The spectral properties of this source are studied using various phenomenological and physical models. We conclude that (a) the observed variability in X-ray luminosity is not attributed to the hydrogen column density (N H) as UGC 6728 exhibits a bare nucleus, implying a negligible N H contribution along the line of sight, and (b) the spectral slope in the X-ray band demonstrates a systematic variation over time, indicating a transition from a relatively hard state to a comparatively soft state. We propose that the underlying accretion dynamics around the central object account for this behaviour. By performing X-ray spectral fitting, we estimate the mass of the central supermassive black hole (SMBH) in UGC 6728 to be |$M_{\rm BH}=(7.13\pm 1.23)\times 10^5$|  M |$_\odot$|⁠. Based on our spectral and temporal analysis, we suggest that UGC 6728 lacks a prominent Compton hump or exhibits a very subtle hump that remains undetectable in our analysis. Furthermore, the high-energy X-ray photons in this source are likely to originate from the low-energy X-ray photons through inverse Compton scattering in a Compton cloud, highlighting a connection between the emission in two energy ranges. We noticed a strong soft excess component in the initial part of our observations, which was later reduced substantially. This variation of soft excess is explained in view of accretion dynamics. [ABSTRACT FROM AUTHOR]

NGC 1068 is a nearby, widely studied Seyfert II galaxy presenting radio, infrared, X-ray, and γ-ray emission, along with strong evidence for high-energy neutrino emission. Recently, the evidence for neutrino emission was explained in a multimessenger model, whereby the neutrinos originate from the corona of the active galactic nucleus. In this environment, γ-rays are strongly absorbed, so that an additional contribution is necessary, for instance, from the circumnuclear starburst ring. In this work, we discuss whether the radio jet can be an alternative source of the γ-rays between about 0.1 and 100 GeV, as observed by Fermi-LAT. In particular, we include both leptonic and hadronic processes, namely, accounting for inverse Compton emission and signatures from pp as well as pγ interactions. In order to constrain our calculations, we used VLBA and ALMA observations of the radio knot structures, which are spatially resolved at different distances from the supermassive black hole. Our results show that the best leptonic scenario for the prediction of the Fermi-LAT data is provided by the radio knot closest to the central engine. For that to be the case, a magnetic field strength of ∼1 mG is needed as well as a strong spectral softening of the relativistic electron distribution at (1 − 10) GeV. However, we show that neither such a weak magnetic field strength, nor such a strong softening is expected for that knot. A possible explanation for the ∼10 GeV γ-rays could potentially be provided by hadronic pion production in case of a gas density ≳104 cm−3. Nonetheless, this process is not found to contribute significantly to the low-energy end of the Fermi-LAT range. We conclude that the emission sites in the jet are not sufficient to explain the γ-rays across the whole Fermi-LAT energy band. [ABSTRACT FROM AUTHOR]

Pulsations from the Crab pulsar have been detected by the MAGIC telescopes at energies up to 1.5 TeV, and the pulsed emission from the Vela pulsar was detected by H.E.S.S. reaching tens of TeV. These discoveries, along with the proposed additional emission due to inverse Compton scattering at TeV energies, lead us to consider suitable candidates for detection with current and future extensive air show (EAS) experiments at very-high-energy (VHE; 0.1 |$-$| 100 TeV) ranges. Leveraging energy spectrum data from pulsars as observed by Fermi and Imaging Atmospheric Cherenkov Telescopes (IACTs) and considering the sensitivities of both LHAASO and SWGO, this study evaluates their detectability and estimates the time required for their significant detection. Our results indicate that LHAASO could detect the Crab's pulsed signal within six years, while SWGO might detect Vela's signal within one year. Observations of the most energetic Fermi pulsars with EAS experiments will provide insight into the nature of VHE pulsar emissions, helping us to clarify the primary characteristics of VHE pulsars. [ABSTRACT FROM AUTHOR]

Pulsar halos (also termed "TeV halos") are a new class of γ -ray sources in the Galaxy, which manifest as extended γ -ray emission around middle-aged pulsars, as discovered around the Geminga pulsar, the Monogem pulsar, and PSR J0622+3749 by the High-Altitude Water Cherenkov Observatory and the Large High-Altitude Air Shower Observatory. A consensus has been reached that the teraelectronvolt emission comes from the inverse Compton scattering of escaping electrons/positrons from the pulsar wind nebula of the soft background radiation field, while the particle transport mechanism in the halo is still in dispute. Currently, there are mainly three interpretations: the isotropic, suppressed diffusion model; the isotropic, unsuppressed diffusion model that considers the ballistic propagation of newly injected particles; and the anisotropic diffusion model. While the predicted γ -ray surface brightness profiles of all three models can be more or less consistent with the observations, the implications of the three models for cosmic-ray transport mechanisms and the properties of the interstellar magnetic field are quite different. In this study, we calculate the anticipated X-ray emission of pulsar halos under the three models. We show that the synchrotron radiation of these escaping electrons/positrons can produce a corresponding X-ray halo around the pulsar and that the expected surface brightness profiles are distinct in the three models. We suggest that sensitive X-ray detectors of a large field of view (such as eROSITA and the Einstein Probe) with a reasonably long exposure time are crucial to understanding the formation mechanism of pulsar halos and can serve as a probe of the properties of interstellar turbulence. [ABSTRACT FROM AUTHOR]

The possibility of the breaking of Lorentz symmetry has been discussed in many models of quantum gravity. In this paper we follow the Lorentz violation model in Ref. [14] (i.e., our previous work) to discuss the Doppler frequency shift of photons and the Compton scattering process between photons and electrons, pointing out that following the idea in Ref. [14] we have to modify the usual quantum relation of photons in the Doppler effect. But due to the current limited information and knowledge, we could not yet determine the specific expression for the correction coefficient in the modified quantum relation of photons. However, the phenomenon called spontaneous radiation in a cyclotron maser give us an opportunity to see what the expression for this correction coefficient might look like. Therefor, under some necessary constraints, we construct a very concise expression for this correction coefficient through the discussion of different cases. And then we use this expression to analyze the wavelength of radiation in the cyclotron maser, which tends to a limited value at v → c, rather than to 0 as predicted by the Lorentz model. And the inverse Compton scattering phenomenon is also discussed and we find that there is a limit to the maximum energy that can be obtained by photons in the collision between extremely relativistic particles and low-energy photons, which conclusion is also very different from that obtained from the Lorentz model, in which the energy that can be obtained by the photon tends to be infinite as the velocity of particle is close to c. This paper still follows the purpose in Ref. [14] that the energy and momentum of particles (i.e., any particles, including photons) cannot be infinite, otherwise it will make some physical scenarios invalid. When the parameter Q characterizing the degree of deviation from the Lorentz model is equal to 0, all the results and conclusions in this paper will return to the case as in the Lorentz model, so this paper also provides us with a possible experimental scheme to determine the value of Q in Ref. [14], although it still requires extremely high experimental energy.

Highly radio-loud quasars (HRLQs; log  R > 2.5) at z ≳ 4 show apparent enhanced X-ray emission compared to matched HRLQs at lower redshifts, perhaps due to a redshift-dependent fractional contribution to the X-ray luminosity from inverse-Compton scattering of cosmic microwave background photons (IC/CMB). Using new Chandra observations and archival X-ray data, we investigate this phenomenon with an optically flux-limited sample of 41 HRLQs at z  = 4–5.5 all with sensitive X-ray coverage, the largest sample utilized to date by a wide margin. X-ray enhancements are assessed using X-ray-to-optical flux ratios and spectral energy distributions. We confirm the presence of X-ray enhancements at a 4.9–5.3σ significance level, finding that the median factor of enhancement is ≈1.8 at our sample median redshift of z ≈ 4.4. Under a fractional IC/CMB model, the expected enhancement at lower redshifts is modest; e.g. ≈4 per cent at z ≈ 1.5. We also investigate a sample of seven radio-loud quasars (RLQs; log  R > 1) at even higher redshifts of z  = 5.6–6.8, using new and archival X-ray data. These RLQs also show evidence for X-ray enhancements by a median factor of ≈2.7 at a 3.7–4.9σ significance level. The X-ray spectral and other properties of these z  = 5.6–6.8 RLQs, however, pose challenges for a straightforward fractional IC/CMB interpretation of their enhancements. [ABSTRACT FROM AUTHOR]

Low-emittance photoinjector-enabled cutting-edge scientific instruments, such as free-electron lasers, inverse Compton scattering light sources, and ultrafast electron diffraction, will greatly benefit from the improved repetition rate. In this paper, we proposed a specifically designed S-band radio frequency (RF) photoinjector to obtain low emittance and kilohertz (kHz) high-repetition rates simultaneously. By lowering the gradient, much lower RF power is needed to feed the electron gun, and then the heat problem is much easier to handle. Meanwhile, by optimizing the length of the gun's first cell from the normal case of 0.6-cell to 0.4-cell, the launch phase and the extraction field are significantly improved, thus ensuring the generation of low-emittance electron beams. In our design, the proposed 1.4-cell RF gun can work effectively under different field gradients ranging from 30 MV/m to 100 MV/m. For a standard case of 60 MV/m, 2.5 MW peak RF power with ys level pulse width is sufficient, thus offering the feasibility of improving the repetition rate to kHz level with a standard 5 MW irradiation klystron. In addition, simulated electron beams with a low emittance of 0.29 mm.mrad@200 pC can be generated by this proposed photoinjector, showing that this high-repetition rate injector holds the potential to deliver high-quality beams comparable to those of state-of-the-art S-band photoinjectors. Combining the merits of low emittance and high-repetition rate, this proposed photoinjector will provide a new possibility for future free-electron laser facilities operating at repetition rates ranging from kHz to tens of kHz. [ABSTRACT FROM AUTHOR]

The indirect search for dark matter is typically restricted to individual photon bands and instruments. In the context of multiwavelength observations, finding a weak signal in large foreground and background at only one wavelength band is hampered by systematic uncertainties dominating the signal strength. Dark matter particle annihilation is producing Standard Model particles of which the prompt photon emission is searched for in many studies. However, also the secondary emission of charged particles from dark matter annihilation in the TeV range results in comparable or even stronger fluxes in the GHz–GeV range. In this study, we calculate the prompt and secondary emission of a scotogenic weakly interacting massive particle (WIMP) with a mass of 1 TeV in 27 dwarf galaxies of the Milky Way. For the secondary emission, we include inverse Compton scattering, bremsstrahlung, and synchrotron radiation, which results in a 'triple hump' structure characteristic for only dark matter and no other astrophysical source. In order to determine the best candidates for multi-instrument analyses, we estimate the diffuse emission component of the Milky Way itself, including its own dark matter halo from the same scotogenic WIMP model. We find signal-to-background ratios of individual sources on the order of 10−3 to 10−2 across X-ray to γ-ray assuming J factors for the cold dark matter distribution inferred from observations and no additional boosting due to small-scale clumping. We argue that a joint multiwavelength analysis of all nearby galaxies and the extension towards the cosmic gamma-ray background is required to disentangle possible dark matter signals from astrophysical background and foreground. [ABSTRACT FROM AUTHOR]

The paper, like the seminar where it was presented, is dedicated to the memory of the outstanding physicist and nuclear scientist, Professor of Moscow State University B.S. Ishkhanov, more precisely, to his contribution to the research and development of LEXG—laser–electron X-ray generators. The relevance and prospects of this direction are associated with the "gap" between two existing types of X-ray generators: on the one hand, X-ray tubes, and on the other, electron accelerators, storage rings, and free electron lasers. This involves a vast difference in beam characteristics, cost, scale, energy consumption, etc. The history of LEXG, modern projects abroad, as well as the Lomonosov Moscow State University–Jilin University project, carried out under the supervision of B.S. Ishkhanov, are briefly presented. From the point of view of their applications, LEXG designs can be divided into two types and, accordingly, devices of different scales. In the first case, it is the application area of common X-ray tubes, and in the second, it is radiation medicine, nuclear technologies, and nuclear physics research. International conferences on LEXG have been held since 2008. Their list is provided in the Appendix to the article. [ABSTRACT FROM AUTHOR]

High redshift blazars are among the most powerful non-explosive sources in the Universe and play a crucial role in understanding the evolution of relativistic jets. To understand these bright objects, we performed a detailed investigation of the multiwavelength properties of 79 γ-ray blazars with redshifts ranging from z = 2.0 to 2.5, using data from Fermi LAT, Swift XRT / UVOT , and NuSTAR observations. In the γ-ray band, the spectral analysis revealed a wide range of flux and photon indices, from 5.32 × 10−10 to 3.40 × 10−7 photon cm−2 s−1 and from 1.66 to 3.15, respectively, highlighting the diverse nature of these sources. The detailed temporal analysis showed that flaring activities were observed in 31 sources. Sources such as 4C+71.07, PKS 1329-049, and 4C + 01.02, demonstrated significant increase in the γ-ray luminosity and flux variations, reaching peak luminosity exceeding 1050 erg s−1. The temporal analysis extended to X-ray and optical/ultraviolet (UV) bands, showed clear flux changes in some sources in different observations. The time-averaged properties of high redshift blazars were derived through modeling the spectral energy distributions with a one-zone leptonic scenario, assuming the emission region is within the broad-line region (BLR) and the X-ray and γ-ray emissions are due to inverse Compton scattering of synchrotron and BLR-reflected photons. This modeling allowed us to constrain the emitting particle distribution, estimate the magnetic field inside the jet, and evaluate the jet luminosity, which is discussed in comparison with the disc luminosity derived from fitting the excess in the UV band. [ABSTRACT FROM AUTHOR]

We present the generation of attosecond gamma rays via inverse Compton scattering within the framework of laser wakefield acceleration through 2D Particle-In-Cell simulations. Utilizing the near-threshold ionization injection mechanism, an attosecond micro-bunched electron beam characterized by a comb-like current density profile can be achieved with a linearly polarized laser at an intensity of a0 = 1.5. The micro-bunched beam provides a beam energy of approximately 300 MeV and achieves a minimum relative energy spread of about 1.64% after undergoing 2 mm of acceleration. In the inverse Compton scattering scheme, these attosecond electron micro-bunches interact with the reflected driving laser pulse, resulting in the attosecond gamma-ray radiation exhibiting similar structures. Individual spatial-separated gamma-ray pulses exhibit a length of approximately 260–300 as, with a critical energy of 2.0 ± 0.2 MeV. The separated attosecond gamma-ray source owns a peak brilliance of ~1022 photons s−1 mm−2 mrad−2 0.1% BW. This brilliance is competitive in a laboratory for multi-MeV γ-ray sources with a laser intensity of I = 5 × 1018 W/cm2. Such attosecond gamma-ray radiation offers promising applications requiring ultrashort X-ray/gamma ray sources.

From 16 years of INTEGRAL/SPI γ-ray observations, we derive bounds on annihilating light dark matter particles in the halo of the Milky Way up to masses of about 300 MeV. We test four different spatial templates for the dark matter halo, including a Navarro–Frenk–White (NFW), Einasto, Burkert, and isothermal sphere profile, as well as three different models for the underlying diffuse inverse compton emission. We find that the bounds on the s-wave velocity-averaged annihilation cross sections for both the electron-positron and the photon–photon final states are the strongest to date from γ-ray observations alone in the mass range ≲6 MeV. We provide fitting formulae for the upper limits and discuss their dependences on the halo profile. The bounds on the two-photon final state are superseding the limits from the cosmic microwave background in the range of 50 keV up to ∼3 MeV, showing the great potential future MeV mission will have in probing light dark matter. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) comprise short, bright, energetic flashes of emission from extragalactic sources followed by a longer afterglow phase of decreased brightness. Recent discoveries of very high energy (VHE, ≳100 GeV) afterglow emission from GRB 180720B and GRB 190829A by the High Energy Stereoscopic System have raised questions regarding the emission mechanism responsible. We interpret this observed late-time emission to be the result of inverse Compton emission of ultrarelativistic electrons in the GRB blast wave in an external radiation field, i.e. external Compton (EC), considering both the wind and interstellar medium scenarios. We present predictions of multiwavelength light curves and energy spectra, ranging from optical to VHE, and include the synchrotron and synchrotron self-Compton (SSC) radiation mechanisms as well. We corrected the EC and SSC models for the gamma-ray attenuation by absorption of photons through their interaction with the extragalactic background light. We compared our results to multiwavelength data and found that EC gives a satisfactory fit for a given set of fixed model parameters for GRB 180720B, whereas SSC results in a better fit for GRB 190829A. For both GRBs, a wind environment is preferred over constant-density interstellar medium, and the cosmic microwave background as the external radiation field. However, with more data and an effective optimization tool we can find a more robust fit of the model, implying better constraints on the GRB environment and the particle energy requirements for the emission observed at late times. This has consequences for future observations of GRBs at these extreme energies. [ABSTRACT FROM AUTHOR]

We develop a lepton–hadron model for the possible origin of hard very high energy (VHE) spectra from a distant blazar. The model includes synchrotron self-Compton (SSC) and hadronic components. The lepton components include synchrotron radiation and inverse Compton scattering of relativistic electrons. For the hadronic components, we consider proton synchrotron radiation and investigate the interaction of protons with the synchrotron emission soft photons or cosmic microwave background (CMB) photons. Upon adopting the parametrization of the observed spectrum of 1ES 1218+304, we obtain the following results: (1) the model is able to match the spectral energy distribution of 1ES 1218+304; (2) we find that in E p ≈ 10 10 ∼ 10 17 eV , the π 0 → γ -ray process contributes the majority of the secondary photons; and (3) the interaction of protons with the low-energy photons may occur in or outside the jet. [ABSTRACT FROM AUTHOR]

We construct simulated galaxy data sets based on the High Energy X-ray Probe (HEX-P) mission concept to demonstrate the significant advances in galaxy science that will be yielded by the HEX-P observatory. The combination of high spatial resolution imaging (<20 arcsec FWHM), broad spectral coverage (0.2-80 keV), and sensitivity superior to current facilities (e.g., XMM-Newton and NuSTAR) will enable HEX-P to detect hard (4-25 keV) X-ray emission from resolved point-source populations within ~800 galaxies and integrated emission from ~6,000 galaxies out to 100 Mpc. These galaxies cover wide ranges of galaxy types (e.g., normal, starburst, and passive galaxies) and properties (e.g., metallicities and star-formation histories). In such galaxies, HEX-P will: 1) provide unique information about X-ray binary populations, including accretor demographics (black hole and neutron stars), distributions of accretion states and state transition cadences; 2) place order-of-magnitude more stringent constraints on inverse Compton emission associated with particle acceleration in starburst environments; and 3) put into clear context the contributions from X-ray emitting populations to both ionizing the surrounding interstellar medium in low-metallicity galaxies and heating the intergalactic medium in the z > 8 Universe. [ABSTRACT FROM AUTHOR]

Fast Radio Bursts (FRBs) are extra-galactic origin milli-second duration bright radio bursts. Theoretically, FRBs may produce optical counterparts with durations from milliseconds to hours. The FRB optical counterparts may be detectable in future large field telescopes, including the China Space Station Telescope (CSST), the 2.5-meter Wide Field Survey Telescope (WFST) lead by the University of Science and Technology of China (USTC) and the Purple Mountain Observatory (PMO), and the Earth 2.0 (ET). The fast radio burst optical counterparts are grouped into millisecond time-scale optical counterparts, hourly time-scale optical counterparts, and optical afterglow for our study. The first two can be generated by the high-energy extension of the radio radiation of fast radio bursts and the inverse Compton scattering of high-energy electrons. The event rates highly depend on the optical-to-radio flux ratio η ν. For millisecond duration optical counterparts, the detection rate of WFST, CSST, and ET can reach hundreds per year in an ideal case. If η ν ∼ 10 − 3 , the corresponding annual detection rates of WFST and CSST are in the order of 1, and the annual detection rate of ET is 19.5. For the hourly timescale optical counterparts, ideally, the age of the supernova remnant is 5 years, η ν is about 10 − 6 , and the annual detection rates are above 100. The X-ray counterpart of FRB 200428 indicates that FRBs may produce relativistic outflow, which will interact with the interstellar medium to produce optical afterglows. Combined with the standard afterglow model, the detectability of optical afterglow is explored with a simulation of fast radio bursts following the redshift and energy distribution from the literature. With a total energy-radio energy ratio similar to FRB 200428, (ζ = 10 5), the estimated annual detection rates of CSST, WFST, and ET are 1.3, 1.0, and 67, respectively. [ABSTRACT FROM AUTHOR]

The structure of a compact X-ray source based on inverse Compton scattering and the beam dynamics in the linac and the storage ring is considered. The features of collective effects in the storage rings of such installations are studied in compaаrison with fourth-generation SR sources. The thresholds of longitudinal microwave instability and transverse instability of coupled modes are calculated. [ABSTRACT FROM AUTHOR]

The Compton scattering effect of electromagnetic waves (photons) on free electrons has been known since the beginning of the twentieth century. A photon moving toward a fast electron can be scattered in the opposite direction, significantly increasing its energy because of the loss of it by the electron. When the scattering electron is not free, but moves in a single crystal in the channeling mode, periodic harmonics of the potential of atomic planes or axes along which the channeled electron moves can serve as the scattered photon. In the so-called accompanying reference system moving at the velocity equal to the longitudinal component of the channeled particle velocity, these harmonics can be considered as electromagnetic waves (equivalent photons) interacting with the electron making finite orbital or oscillatory motion in the averaged potential of the atomic axis or plane. The result of equivalent photon backscattering is a real photon, observable as a high-energy photon in the laboratory system. Because of the discreteness of the transverse motion energy spectrum of the channeled electron, the emitted photon spectrum will also be discrete. This work continues a series of our previous studies [3–5] of effects associated with electromagnetic processes during the passage of fast charged particles through crystal structures within a mixed quantum-classical approach. The backscattering kinematics of photons and equivalent photons by the ultrarelativistic electrons has been analyzed in detail in the general case and in the channeling mode. The characteristic frequencies have been calculated and the intensity of the resulting radiation has been estimated. [ABSTRACT FROM AUTHOR]

In 2017 the Event Horizon Telescope (EHT) observed the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz (λ = 1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT data sets. The high observing frequency means that pulsars—which typically exhibit steep emission spectra—are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope, and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the fast folding algorithm, and single-pulse searches targeting both pulsars and burst-like transient emission. We use the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction (≲2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region. [ABSTRACT FROM AUTHOR]

We construct simulated galaxy data sets based on the High Energy X-ray Probe (HEX-P) mission concept to demonstrate the significant advances in galaxy science that will be yielded by the HEX-P observatory. The combination of high spatial resolution imaging (<20 arcsec FWHM), broad spectral coverage (0.2-80 keV), and sensitivity superior to current facilities (e.g., XMM-Newton and NuSTAR) will enable HEX-P to detect hard (4-25 keV) X-ray emission from resolved point-source populations within ~800 galaxies and integrated emission from ~6,000 galaxies out to 100 Mpc. These galaxies cover wide ranges of galaxy types (e.g., normal, starburst, and passive galaxies) and properties (e.g., metallicities and star-formation histories). In such galaxies, HEX-P will: 1) provide unique information about X-ray binary populations, including accretor demographics (black hole and neutron stars), distributions of accretion states and state transition cadences; 2) place order-of-magnitude more stringent constraints on inverse Compton emission associated with particle acceleration in starburst environments; and 3) put into clear context the contributions from X-ray emitting populations to both ionizing the surrounding interstellar medium in low-metallicity galaxies and heating the intergalactic medium in the z > 8 Universe. [ABSTRACT FROM AUTHOR]

Presented here are the first results of commissioning of the S-Band hybrid photoinjector and laser systems at the new accelerator and light source facility, MITHRA, at UCLA. The radiation bunker and capabilities of the facility are described with motivation for detailed measurement of beam parameters explained. Following thorough characterization of the photoinjector, a 1.5 m linac is to be installed and experiments up to 30 MeV will begin. These will include experiments in basic plasma physics, space plasma, terahertz production in dielectric structures, and inverse Compton scattering and applications for the X-rays produced. [ABSTRACT FROM AUTHOR]

Dark matter is a popular candidate to a new source of primary-charged particles, especially positrons in cosmic rays, which are proposed to account for observable anomalies. While this hypothesis of decaying or annihilating DM is mostly applied for our Galaxy, it could possibly lead to some interesting phenomena when applied for the other ones. In this work, we look into the hypothetical asymmetry in gamma radiation from the upper and lower hemisphere of the dark matter halo of the Andromeda galaxy due to inverse Compton scattering of starlight on the DM-produced electrons and positrons. While our 2D toy model raises expectations for the possible effect, a more complex approach gives negligible effect for the dark halo case, but shows some prospects for a dark disk model. [ABSTRACT FROM AUTHOR]

Laser–plasma interactions (LPIs) are an emerging source of a range of energetic radiation. LPI experiments drive ultra-short (< ps) and brilliant sources of X-rays from keV to MeV energies. Designing detectors to maximise the sensitivity and resolution achievable with these sources is paramount to optimising laser-driven accelerators. In this article, we explore the key parameters associated with laser-driven X-ray sources and the detector systems required to characterise them. We present a concise approach to modelling the sensitivity and resolution for indirect detector systems factoring in both the optical collection and the X-ray attenuation within the scintillator.

N 157B located in the Large Magellanic Cloud is the first pulsar wind nebula detected outside of the Galaxy in γ-rays. In this paper, we analyse the emission above 30 GeV from N 157B using ∼14.3 yr of Fermi -LAT data. The γ-ray spectrum between 30 and 500 GeV is well described by a single power-law function with a photon index of 1.83 ± 0.26, and its integral photon flux is (4.10 ± 0.83) × 10−11 photons cm−2 s−1. We adopt a one-zone leptonic model to investigate whether the multiband non-thermal emission of the target source can be generated by synchrotron radiation and inverse-Compton scattering of the electrons/positrons. Assuming the electrons/positrons in the nebula have a broken power-law spectrum with two breaks, the model can reproduce the observed fluxes in the radio, X-ray, and γ-ray bands. This result indicates that the γ-ray emission from N 157B can be explained by the leptonic process of electrons/positrons via inverse-Compton scattering. [ABSTRACT FROM AUTHOR]

BACKGROUND: Inverse Compton scattering (ICS) source can produce quasi-monoenergetic micro-focus X-rays ranging from keV to MeV level, with potential applications in the field of high-resolution computed tomography (CT) imaging. ICS source has an energy-angle correlated feature that lower photon energy is obtained at larger emission angle, thus different photon energies are inherently contained in each ICS pulse, which is especially advantageous for dual- or multi-energy CT imaging. OBJECTIVE: This study proposes a dual-energy micro-focus CT scheme based on the energy-angle correlation of ICS source and tests its function using numerical simulations. METHODS: In this scheme, high- and low-energy regions are chosen over the angular direction of each ICS pulse, and dual-energy projections of the object are obtained by an angularly-splicing scanning method. The field-of-view (FOV) of ICS source is extended simultaneously through this scanning method, thus the scale of the imaging system can be efficiently reduced. A dedicated dual-energy CT algorithm is developed to reconstruct the monoenergetic attenuation coefficients, electron density, and effective atomic number distributions of the object. RESULTS: A test object composed of different materials (carbon, aluminium, titanium, iron and copper) and line pairs with different widths (15/24/39/60 μm) is imaged by the proposed dual-energy CT scheme using numerical simulations, and high-fidelity monoenergetic attenuation coefficient, electron density, and effective atomic number distributions are obtained. All the line pairs are well identified, and the contrast ratio of the 15 μm lines is 22%, showing good accordance with the theoretical predictions. CONCLUSIONS: The proposed dual-energy CT scheme can reconstruct fine inner structures and material compositions of the object simultaneously, opening a new possibility for the application of ICS source in the field of non-destructive testing. [ABSTRACT FROM AUTHOR]

We use Fermi-LAT data to analyse the faint gamma-ray source located at the centre of the Sagittarius (Sgr) dwarf spheroidal galaxy. In the 4FGL-DR3 catalogue, this source is associated with the globular cluster, M54. We investigate the spectral energy distribution and spatial extension of this source, with the goal of testing two hypotheses: (1) the emission is due to millisecond pulsars within M54, or (2) the emission is due to annihilating dark matter from the Sgr halo. For the pulsar interpretation, we consider a two-component model which describes both the lower-energy magnetospheric emission and possible high-energy emission arising from inverse Compton scattering. We find that this source has a point-like morphology at low energies, consistent with magnetospheric emission, and find no evidence for a higher-energy component. For the dark matter interpretation, we find the signal favours a dark matter mass of m χ = 29.6 ± 5.8 GeV and an annihilation cross section of |$\sigma v = (2.1 \pm 0.59) \times 10^{-26} \, \text{cm}^3$|  s−1 for the |$b \bar{b}$| channel (or m χ = 8.3 ± 3.8 GeV and |$\sigma v = (0.90 \pm 0.25) \times 10^{-26} \, \text{cm}^3$|  s−1 for the τ+τ− channel), when adopting a J -factor of |$J=10^{19.6} \, \text{GeV}^2 \, \text{cm}^{-5}$|⁠. This parameter space is consistent with gamma-ray constraints from other dwarf galaxies and with dark matter interpretations of the Galactic Centre Gamma-Ray Excess. [ABSTRACT FROM AUTHOR]

The detection of hard X-ray spectra (spectral index <2) from the kiloparsec-scale jet of active galactic nuclei cannot be accounted for by the synchrotron emission mechanism from the electron distribution responsible for the radio/optical emission. Alternate explanations are the inverse Compton scattering of cosmic microwave background photons (IC/CMB) or synchrotron emission from a second electron population. When the X-ray emission is interpreted as an IC/CMB process, the Compton spectrum often peaks at GeV energy and many sources are predicted to be Fermi candidate sources. The absence of significant gamma-ray flux from some of these galaxies by Fermi disfavours the IC/CMB interpretation of the high-energy emission. We extend this study to predict the very-high-energy (VHE) gamma-ray emission due to the IC/CMB model, which can be investigated by the Cherenkov Telescope Array Observatory (CTAO). The model parameters deciding the broad-band spectral energy distribution are estimated using an analytical approximation of the emissivity functions. The emission model is extrapolated to VHE and then compared with the CTAO sensitivity. For this particular study, we have selected 18 knots with harder X-ray spectra and for which the IC/CMB model for X-ray emission has been suggested. [ABSTRACT FROM AUTHOR]

The progressive development of high power lasers over the last several decades enables the study of γ-photon generation when an intense laser beam interacts with matter, mainly via inverse Compton scattering at the high intensity limit. γ-ray flashes are a phenomenon of broad interest, drawing the attention of researchers working in topics ranging from cosmological scales to elementary particle scales. Over the last few years, a plethora of studies predict extremely high laser energy to γ-photon energy conversion using various target and/or laser field configurations. The aim of this article is to discuss several recently proposed γ-ray flash generation schemes, as a guide for upcoming γ-photon related experiments and for further evolution of the presently available theoretical schemes. [ABSTRACT FROM AUTHOR]

Enhanced radiotherapy and X-ray fluorescence computed tomography (XFCT) using nanoparticles have been widely investigated. However, the integration of enhanced radiotherapy and XFCT is challenging as their optimal induced photon energies are different requiring a shift for the corresponding mode. Inverse Compton Scattering (ICS) source, with a compact footprint, provides quasi-monochromatic and continuously energy-tuneable X-rays, enabling the potential clinical usage of fluorescence image-guided radiation-enhanced radiotherapy. A simulation of enhanced radiotherapy and XFCT was conducted using a combination of Geant4-DNA and Geant4-Livermore[AC10][LB11]. A compact arrangement tumor model was established to mimic the real rapid growth of cancer cells. The ICS source was set to be tunable from 10 KeV to 2 MeV with 2*109 photons. The radiation enhancement effect of nanoparticles, as radiosensors, was evaluated by the dose enhancement factor (DEF). The efficiency of XFCT was assessed by the ratio of fluorescence signal to noise (the ratio of fluorescence to Compton scattering photons multiplying the ratio of fluorescence to incident photons). The optimal energies for enhanced radiotherapy and XFCT were determined and the performances were compared with conventional clinical radiation sources. The optimal energy and corresponding efficiency of enhanced therapy and XFCT were evaluated under geometry and concentration of our simulation. Gold nanoparticles had the highest efficiency of enhanced therapy at 28 KeV, and that of XFCT at 97 KeV, correspondingly. The optimal energy depends on the specified condition. The concentration of nanoparticles also influenced the optimal energy of XFCT but had less influence on that of DEF. With the same type and concentration of nanoparticle, the ICS source had a higher DEF and quality of XFCT imaging compared to the conventional radiation sources due to the tunable photon energy allowing the highest efficiency for both modes. The optimal energies for enhanced therapy and XFCT were different and changed with the properties of nanoparticles. The implementation of the ICS source could achieve the integration of enhanced radiotherapy and XFCT with the highest therapeutic efficiency by changing the photon energy. Further experiment is needed for transferring from the simulation to the clinical applications. [ABSTRACT FROM AUTHOR]

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Fast radio bursts (FRBs) are still a mystery in contemporary astrophysics. Unlike many other astronomical objects whose basic physical mechanism is already identified and the research on which focuses mainly on refining details, FRBs are still largely unknown regarding their source(s) and radiation mechanism(s). To make progress in the field, a "top-down" or "detective's approach" is desirable. I will summarize how some key observational facts have narrowed down the options to interpret FRBs and show that at least some FRBs are produced from the magnetospheres of highly magnetized neutron stars (or magnetars). I will also argue that the current data seem to favor a type of coherent inverse Compton scattering process by relativistic particle bunches off a low-frequency wave propagating in the magnetosphere. This brief contribution is a shorter version of an extended review to be published in Reviews of Modern Physics, and it was written as a tribute to the 80th anniversary of Remo Ruffini. [ABSTRACT FROM AUTHOR]

The paper deals with kinematic conditions for the inverse Compton scattering of photons by relativistic electrons and polarizations of the colliding particles, which affect the value of the differential cross section of the process. A significant influence of the electron and photon helicity on the value of the cross section has been found. In the ultrarelativistic case, a surprising effect of an almost twofold increase in the cross section of scattering in the direction of the initial electron momentum has also been discovered when the initial photon momentum is transverse to that of the initial electron. [ABSTRACT FROM AUTHOR]

γ-ray-emitting narrow-line Seyfert 1 galaxies (γ-NLS1) constitute an intriguing small population of active galactic nuclei with γ-ray emission resembling low-power flat-spectrum radio quasars (FSRQ), but with differing physical properties. They are jetted, γ/radio-loud Seyfert galaxies, with relatively low black hole masses, accreting at exceptionally high, near-Eddington rates. Certain of these sources exhibit highly variable emission states on relatively short time-scales, the physical origin of which remains elusive. In this work, varying emission states of two bona fide NLS1s, 1H 0323+342 and PMN J0948+0022, and one little-studied FSRQ/intermediate object, B2 0954+25A, are examined. For each source, we analysed quasi-simultaneous multiwavelength data for different states of γ-ray activity and present the results of their broad-band emission modelling, taking into account all available physical constraints to limit the range of the model parameters. Two different scenarios are discussed, in the framework of a one-zone leptonic model, where the high-energy emission is due to the inverse Compton scattering of the disc and broad line region (BLR) or torus photons by relativistic electrons within the jet. The transition from low to high state is well described by variations of the jet parameters, leaving the external photon fields unchanged. The parameterization favours an emission scenario with particle injection on a stationary shock inside the jet. When considering all physical constraints, the disc and BLR scenario is preferred for all three sources. We use the multi-epoch modelling to characterize total jet powers and discuss the intrinsic nature of γ-NLS1 galaxies and FSRQs. [ABSTRACT FROM AUTHOR]

X-ray binary systems (XRBs) exhibit similar dynamics and multimessenger emission mechanisms to active galactic nuclei (AGNs) with the benefit of shorter time scaling. Those systems produce rich spectral energy distributions (SEDs) ranging from the radio band to the very high energy gamma rays. The emission origin varies between the system's accretion disk (X-rays) to the corona and, most notably, to the two twin plasma ejections (jets) that often meet the interstellar medium forming highly observable radio lobes. Modeling of the jets offers an excellent opportunity to understand the intrinsic mechanisms and the jet particles, such as electrons, positrons, and protons. In this work, we employ a lepto-hadronic jet model that assumes particle acceleration through shock waves over separate zonal regions of the jet. The hadronic models consider proton–proton collisions that end up in gamma-ray photons through neutral pion decays. The main leptonic mechanisms involve synchrotron radiation (from both electrons and protons) and inverse Compton scattering of ambient photons (coming from the disk, the corona, and the companion star) on jet electrons. The emissions from the disk, the corona, and the donor star are also included in the SED calculations, along with the photon absorption effects due to their interaction with higher-energy jet photons. We apply the model on a 10 M ⊙ black hole accreting at the Eddington rate out of a 20 M ⊙ companion star. One of our goals is to investigate and determine an optimal frame concerning the values for the free parameters that enter our calculations to produce higher integral fluxes. [ABSTRACT FROM AUTHOR]

As cosmic microwave background (CMB) photons traverse the universe, anisotropies can be induced via Thomson scattering (proportional to the electron density; optical depth) and inverse Compton scattering (proportional to the electron pressure; thermal Sunyaev–Zel'dovich effect). Measurements of anisotropy in optical depth τ and Compton y parameters are imprinted by the galaxies and galaxy clusters and are thus sensitive to the thermodynamic properties of the circumgalactic medium and intergalactic medium. We use an analytic halo model to predict the power spectrum of the optical depth (τ τ), the cross-correlation between the optical depth and the Compton y parameter (τ y), and the cross-correlation between the optical depth and galaxy clustering (τ g), and compare this model to cosmological simulations. We constrain the optical depths of halos at z ≲ 3 using a technique originally devised to constrain patchy reionization at a higher redshift range. The forecasted signal-to-noise ratio is 2.6, 8.5, and 13, respectively, for a CMB-S4-like experiment and a Vera C. Rubin Observatory–like optical survey. We show that a joint analysis of these probes can constrain the amplitude of the density profiles of halos to 6.5% and the pressure profiles to 13%. These constraints translate to astrophysical parameters, such as the gas mass fraction, f g, which can be constrained to 5.3% uncertainty at z ∼ 0. The cross-correlations presented here are complementary to other CMB and galaxy cross-correlations since they do not require spectroscopic galaxy redshifts and are another example of how such correlations are a powerful probe of the astrophysics of galaxy evolution. [ABSTRACT FROM AUTHOR]

With tantalizing evidence of the recent e-Rosita mission, re-discovering very soft X-rays and Extreme-Ultraviolet (EUV) radiation from a cluster of galaxies or its environment, the question of the origin of cluster EUV excess is revisited in this work. It will be shown that the gas temperature, density, and frozen-in magnetic field of the intracluster medium, collectively support the emission and propagation of coherent C̆erenkov radiation, which is low frequency and large amplitude radiation capable of accelerating charged particles to relativistic speeds. Owing to the spectrum of C̆erenkov radiation, most of the incipient relativistic electrons undergo inverse-Compton scattering with the cosmic microwave background. It turns out the scattered radiation has observable ramifications only in the EUV band, of photon energy 70–100 eV, having a luminosity ≈1044 ergs s−1. This luminosity is on par with the EUV excess level detected from Abell 1795 and the Coma cluster. It should be stressed, as caveat emptor , that although the main subject is the putative large amplitude coherent C̆erenkov modes which are highly non-linear, the results presented were derived using a quasi-linear approach to highlight the observable features of the phenomenon, namely, the EUV emission. [ABSTRACT FROM AUTHOR]

Despite advances in our understanding of low-luminosity active galactic nuclei (LLAGNs), the fundamental details about the mechanisms of radiation and flare/outburst in hot accretion flow are still largely missing. We have systematically analysed the archival Chandra and NuSTAR X-ray data of the nearby LLAGN M81*, whose L bol ∼ 10−5 L Edd. Through a detailed study of X-ray light curve and spectral properties, we find that the X-ray continuum emission of the power-law shape more likely originates from inverse Compton scattering within the hot accretion flow. In contrast to Sgr A*, flares are rare in M81*. Low-amplitude variation can only be observed in soft X-ray band (amplitude usually ≲2). Several simple models are tested, including sinusoidal-like and quasi-periodical. Based on a comparison of the dramatic differences of flare properties among Sgr A*, M31*, and M81*, we find that, when the differences in both the accretion rate and the black hole mass are considered, the flares in LLAGNs can be understood universally in a magnetohydrodynamical model. [ABSTRACT FROM AUTHOR]