Bright Γ-ray Source Research Articles

Revisiting the TeV flare of PKS 2155-304 in 2006

ABSTRACT Blazars, a subclass of active galactic nuclei (AGN), are known to be bright γ-ray sources, frequently exhibiting active (flaring) periods. Blazar PKS 2155-304 is a high synchrotron-peaked BL Lac object located at redshift z = 0.116. On 2006 July 28, an extremely remarkable outburst of VHE γ-ray emission from this blazar was reported by the H.E.S.S. experiment, with an average flux more than 10 times the low-state level. The variability time-scale of this extraordinary flare was as short as approximately 200 s. In order to guarantee the transparency of the emission region for TeV photons, the fast variability demands an extremely high Doppler factor δD > 50 of the jet within the classical one-zone model, leading to the so-called ‘Doppler factor crisis’. Here, we demonstrate that the stochastic dissipation model, which is a multiblob scenario for blazars, can self-consistently explain the giant TeV flares of PKS 2155-304 and the low-state emission before and after the flares, in terms of both multiwavelength spectral and variability characteristics. The required Doppler factor in this model can be as low as 20, which is a reasonable and typical value for blazar jets. The obtained model parameters may shed some light on the physical properties of the relativistic jet.

Efficient generation of collimated multi-GeV gamma-rays along solid surfaces

Bright γ -ray sources are of great significance for fundamental research, medicine, and industry. However, γ -ray production by bremsstrahlung, Compton scattering, and synchrotron radiation is often subjected to large divergence, wide size, and/or low efficiency, making it difficult to achieve high-brightness γ -ray sources. Here, we have found an efficient mechanism to generate γ -rays with, to our knowledge, unprecedented brilliance by use of an ultrarelativistic electron beam with nC charge incident onto a solid surface at grazing incidence. With this interaction configuration, extreme high quasistatic magnetic fields up to the gigagauss level or effective electric fields up to 10 14 V / m are induced by the background electron backflows at the target surface. Subsequently, the electron beam is strongly focused by such fields by over an order of magnitude to submicrometer diameter, and its density is increased beyond the solid density; the induced effective fields are high enough to trigger quantum electrodynamics (QED) effects. These result in the production of extremely brilliant dense γ -ray beams with photon energy reaching multi-GeV, where the electron-to-photon energy conversion efficiency can exceed 60%. This offers a promising route to push the beam–target interaction to the QED regime, leading to ultrabright γ -ray sources for various applications.

Optical/γ-ray blazar flare correlations: understanding the high-energy emission process using ASAS-SN and Fermi light curves

ABSTRACT Using blazar light curves from the optical All-Sky Automated Survey for Supernovae (ASAS-SN) and the γ-ray Fermi-LAT telescope, we performed the most extensive statistical correlation study between both bands, using a sample of 1180 blazars. This is almost an order of magnitude larger than other recent studies. Blazars represent more than 98 per cent of the AGNs detected by Fermi-LAT and are the brightest γ-ray sources in the extragalactic sky. They are essential for studying the physical properties of astrophysical jets from central black holes. However, their γ-ray flare mechanism is not fully understood. Multiwavelength correlations help constrain the dominant mechanisms of blazar variability. We search for temporal relationships between optical and γ-ray bands. Using a Bayesian Block Decomposition, we detect 1414 optical and 510 γ-ray flares, we find a strong correlation between both bands. Among all the flares, we find 321 correlated flares from 133 blazars, and derive an average rest-frame time delay of only 1.1$_{-8.5}^{+7.1}$ d, with no difference between the flat-spectrum radio quasars, BL Lacertae-like objects or low, intermediate, and high-synchrotron peaked blazar classes. Our time-delay limit rules out the hadronic proton-synchrotron model as the driver for non-orphan flares and suggests a leptonic single-zone model. Limiting our search to well-defined light curves and removing 976 potential but unclear ‘orphan’ flares, we find 191 (13 per cent) and 115 (22 per cent) clear ‘orphan’ optical and γ-ray flares. The presence of ‘orphan’ flares in both bands challenges the standard one-zone blazar flare leptonic model and suggests multizone synchrotron sites or a hadronic model for some blazars.

Diffuse non-thermal emission in the disks of the Magellanic Clouds

Context. The Magellanic Clouds, two dwarf galaxy companions to the Milky Way, are among the Fermi Large Area Telescope (LAT) brightest γ-ray sources. Aims. We present comprehensive modeling of the non-thermal electromagnetic and neutrino emission in both Clouds. Methods. We self-consistently model the radio and γ-ray spectral energy distribution from their disks based on recently published Murchison Widefield Array and Fermi/LAT data. All relevant radiative processes involving relativistic and thermal electrons (synchrotron, Compton scattering, and bremsstrahlung) and relativistic protons (neutral pion decay following interaction with thermal protons) are considered, using exact emission formulae. Results. Joint spectral analyses indicate that radio emission in the Clouds has both primary and secondary electron synchrotron and thermal bremsstrahlung origin, whereas γ rays originate mostly from π0 decay with some contributions from relativistic bremsstrahlung and Comptonized starlight. The proton spectra in both galaxies are modeled as power laws in energy with similar spectral indices, ~2.4, and energy densities, ~1 eV cm−3. The predicted 0.1–10 GeV neutrino flux is too low for detection by current and upcoming experiments. Conclusions. We confirm earlier suggestions of a largely hadronic origin of the γ-ray emission in both Magellanic Clouds.

Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a 0, which scales as . A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10−14 kg m2 s−1 is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 1024 photons s−1 mm−2 mrad−2 per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.

η Carinae with Fermi-LAT: two full orbits and the third periastron

Context. Colliding-wind binaries are massive stellar systems featuring strong, interacting winds. These binaries may be actual particle accelerators, making them variable γ-ray sources due to changes in the wind collision region along the orbit. However, only two of these massive stellar binary systems have been identified as high-energy sources. The first and archetypical system of this class is η Carinae, a bright γ-ray source with orbital variability peaking around its periastron passage. Aims. The origin of the high-energy emission in η Carinae is still unclear, with both lepto-hadronic and hadronic scenarios being under discussion. Moreover, the γ-ray emission seemed to differ between the two periastrons previously observed with the Fermi-Large Area Telescope. Continuing observations might provide highly valuable information for understanding the emission mechanisms in this system. Methods. We have used almost 12 yr of data from the Fermi-Large Area Telescope. We studied both low- and high-energy components, searching for differences and similarities between both orbits, and we made use of this large dataset to search for emission from nearby colliding-wind binaries. Results. We show how the energy component above 10 GeV of η Carinae peaks months before the 2014 periastron, while the 2020 periastron is the brightest one to date. Additionally, upper limits are provided for the high-energy emission in other particle-accelerating colliding-wind systems. Conclusions. Current γ-ray observations of η Carinae strongly suggest that the wind collision region of this system is perturbed from orbit to orbit, affecting particle transport within the shock.

Ornstein-Uhlenbeck parameter extraction from light curves of Fermi-LAT observed blazars

Context. Monthly binned γ-ray light curves of 236 bright γ-ray sources, particularly blazars, selected from a sample of 2278 high-galactic latitude objects observed with Fermi-LAT show flux variability characterized by power spectral densities consisting of a single power-law component, ranging from Brownian to white noise. Aims. The main goal here is to assess the Ornstein-Uhlenbeck (OU) model by studying the range of its three parameters that reproduces these statistical properties. Methods. We develop procedures for extracting values of the three OU model parameters (mean flux, correlation length, and random amplitude) from time series data and apply them to compare numerical integrations of the OU process with the Fermi-LAT data. Results. The OU process fully describes the statistical properties of the flux variations of the 236 blazars. The distributions of the extracted OU parameters are narrowly peaked around well-defined values (σ, μ, θ) = (0.2, −8.4, 0.5) with variances (0.004, 0.07, 0.13). The distributions of rise and the decay time scales of flares in the numerical simulations, meaning major flux variations fulfilling pre-defined criteria, are in agreement with the observed ones. The power spectral densities of the synthetic light curves are statistically indistinguishable from those of the measured light curves. Conclusions. The long-term γ-ray flux variability of blazars on monthly time scales is well described by a stochastic model that involves only three parameters. The methods described here are powerful tools for studying randomness in light curves and thereby for constraining the physical mechanisms responsible for the observed flux variations.

Inferring the origins of the pulsed γ-ray emission from the Crab pulsar with ten-year Fermi-LAT data

Context. The Crab pulsar is a bright γ-ray source, which has been detected at photon energies up to ∼1 TeV. Its phase-averaged and phase-resolved γ-ray spectra below 10 GeV exhibit exponential cutoffs, while those above 10 GeV apparently follow simple power laws. Aims. We re-visit the γ-ray properties of the Crab pulsar with ten-year Fermi Large Area Telescope (LAT) data in the range of 60 MeV–500 GeV. With the phase-resolved spectra, we investigate the origins and mechanisms responsible for the emissions. Methods. The phaseograms were reconstructed for different energy bands and further analysed using a wavelet decomposition. The phase-resolved energy spectra were combined with the observations of ground-based instruments, such as MAGIC and VERITAS, to achieve a larger energy converage. We fitted power-law models to the overlapping energy spectra from 10 GeV to ∼1 TeV. In the fit, we included a relative cross-calibration of energy scales between air-shower-based gamma-ray telescopes with the orbital pair-production telescope from the Fermi mission. Results. We confirm the energy-dependence of the γ-ray pulse shape and, equivalently, the phase-dependence of the spectral shape for the Crab pulsar. A relatively sharp cutoff at a relatively high energy of ∼8 GeV is observed for the bridge-phase emission. The E > 10 GeV spectrum observed for the second pulse peak is harder than those for other phases. Conclusions. In view of the diversity of phase-resolved spectral shapes of the Crab pulsar, we tentatively propose a multi-origin scenario where the polar-cap, outer-gap, and relativistic-wind regions are involved.

Collimated GeV attosecond electron–positron bunches from a plasma channel driven by 10 PW lasers

High-energy positrons and bright γ-ray sources are of great importance both in fundamental research and for practical applications. However, collimated GeV electron–positron pair jets and γ-ray flashes are still rarely produced in the laboratory. Here, we demonstrate that by irradiating a near-critical-density plasma channel with two 10 PW-scale laser pulses, highly directional GeV electron–positron pairs and bright γ-ray beams can be efficiently generated. Three-dimensional particle-in-cell simulations show the formation of GeV positron jets with high density (8×1021/ cm3), attosecond duration (400 as), and a divergence angle of 14°. Additionally, ultrabright [2×1025 photons s−1 mm−2 mrad−2 (0.1% bandwidth)−1] collimated attosecond (370 as) γ-ray flashes with a laser energy conversion efficiency of 5.6% are emitted. These features show the significant advantage of using a plasma channel as compared with a uniform plasma and thus open up new possibilities for a wide variety of applications.

All-optical bright γ-ray and dense positron source by laser driven plasmas-filled cone.

An all-optical scheme for bright γ-rays and dense e-e+ pair source is proposed by irradiating a 1022 W/cm2 laser onto a near-critical-density plasmas filled Al cone. Two-dimensional (2D) QED particle-in-cell (PIC) simulations show that, a dense electron bunch is confined in the laser field due to the radiation reaction and the trapped electrons oscillate transversely, emitting bright γ-rays forward in two ways: (1) nonlinear Compton scattering due to oscillation of electrons in the laser field, and (2) Compton backwardscattering resulting from the bunch colliding with the reflected laser by the cone tip. Finally, the multi-photon Breit-Wheeler process is initiated, producing abundant e-e+ pairs with a density of ∼ 1027m-3. The scheme is further demonstrated by full 3D PIC simulations, which indicates a positron number up to 2 × 109. This compact γ-rays and e-e+ pair source may have many potential applications, such as the laboratory study of astrophysics and nuclear physics.

High energy emission from the Galactic Center: Theory and observations

Galactic Centre is a bright γ-ray source with the GeV-TeV band spectrum composed of two distinct components in the 1-10GeV and 1-10TeV energy ranges. The nature of the two components is not clearly understood. We report the analysis of the data of 74 months of observations of the Galactic Center by Fermi/LAT γ-ray telescope with the goal to constrain the morphology of the source and the nature of the two components. Spatially the Galactic Center is consistent with a point source with 0.13 ◦ 3σ upper limit on its radius. We use the Fermi/LAT data in the energy band below 100MeV to show that the γ-ray emission from the Galactic Centre source can't be explained within the pure hadronic model and discuss the necessary modifications to make the model to be consistent with the observational data. We also discuss an alternative self-consistent interpretation of the 60MeV-30TeV spectrum of the source by a model in which the signal is produced via inverse Compton scattering of the ambient infrared radiation field.

Fermi LARGE AREA TELESCOPE OBSERVATIONS OF BLAZAR 3C 279 OCCULTATIONS BY THE SUN.

Observations of occultations of bright γ-ray sources by the Sun may reveal predicted pair halos around blazars and/or new physics, such as, e.g., hypothetical light dark matter particles-axions. We use Fermi Gamma-Ray Space Telescope (Fermi) data to analyze four occultations of blazar 3C 279 by the Sun on October 8 each year from 2008 to 2011. A combined analysis of the observations of these occultations allows a point-like source at the position of 3C 279 to be detected with significance of ≈3σ, but does not reveal any significant excess over the flux expected from the quiescent Sun. The likelihood ratio test rules out complete transparency of the Sun to the blazar γ-ray emission at a 3σ confidence level.

The red blazar PMN J2345−1555 becomes blue

Abstract The flat spectrum radio quasar PMN J2345−1555 is a bright γ-ray source, that recently underwent a flaring episode in the infrared (IR), ultraviolet (UV) and γ-ray bands. The flux changed quasi-simultaneously at different frequencies, suggesting that it was produced by a single population of emitting particles, hence by a single and well-localized region of the jet. While the overall spectral energy distribution (SED) before the flare was typical of powerful blazars (namely two broad humps peaking in the far-IR and below 100 MeV bands, respectively), during the flare the peaks moved to the optical–UV and to energies larger than 1 GeV, to resemble low power BL Lac objects, even if the observed bolometric luminosity increased by more than one order of magnitude. We interpret this behaviour as due to a change of the location of the emission region in the jet, from within the broad-line region, to just outside. The corresponding decrease of the radiation energy density as seen in the comoving frame of the jet allowed the relativistic electrons to be accelerated to higher energies, and thus produce a ‘bluer’ SED.

Potential neutrino signals in a northern hemisphere neutrino telescope from galactic gamma-ray sources

Neutrino energy spectra have been calculated based on the recently measured energy spectra of Galactic very high energy γ-ray sources. Based on these neutrino spectra the expected event rates in the ANTARES neutrino telescope and KM3NeT, a future neutrino telescope in the Mediterranean Sea with an instrumented volume of one km3, have been calculated. For the brightest γ-ray sources we find event rates of the order of one neutrino per year. Although the neutrino event rates are comparable to the background from atmospheric neutrinos the detection of individual sources seems possible.

Potential neutrino signals from galactic γ-ray sources

Recent measurements of the energy spectra of Galactic TeV γ-ray sources with the H.E.S.S. instrument have been used to calculate the expected high energy neutrino fluxes from these sources. Based on the results the expected event rates in a next generation neutrino telescope in the Mediterranean Sea, KM3NeT, have been estimated, assuming an instrumented volume of 1 km3. We find that for energies above 1TeV event rates of a few neutrinos per year can be expected from the brightest γ-ray sources. Although these rates are comparable to those of the background from atmospheric neutrinos a detection of individual sources seems possible.

A model of object Geminga as a degenerate white dwarf orbiting around a black hole

Geminga (2CG195+04) is a very bright γ-ray source with the flux Fγ = 2.4×10−9 erg s−1 cm−2 for E≥50 MeV (refs 1–3). It has been recently identified with X-ray4 and optical5 sources. The optical object is the bluest for its magnitude5, indicating compactness of the emitting region. The high ratios L/LX≈103, LX/Lopt≈200 and the absence of radio emission are very unusual. The observed periodicity is P≈1 min and it changes at a rate corresponding to a doubling time of about several hundred years. I propose here a model which quantitatively explains the values of P and Ṗ. It consists of a degenerate dwarf (Md=0.6 M⊙) filling its Roche lobe and orbiting a black hole (Mbh=4.4 M⊙) with a period P=1 min. Gravitational radiation dissipates the angular momentum, which leads to mass transfer with a current rate ∼10−3 M⊙ yr−1 and a period change Ṗ which agrees with the observed value. At a distance of 100 pc the flux of this radiation would be ∼5×10−3 erg s−1 cm−2 (at ν=1/30 Hz). This is about seven orders of magnitude greater than from any other known stationary source.

An identification for GEMINGA: A review of all the available data

Dedicated X-ray, optical and radio observations aimed at the identification of the bright γ-ray source 2CG195+04 (GEMINGA) are presented. A very promising candidate is found and its properties are discussed in the context of possible astrophysical scenarios.