Bright Γ-ray Research Articles

Gamma-ray flares and broad-band spectral study of PKS 0402-362

ABSTRACTWe study the long-term behaviour of the bright gamma-ray blazar PKS 0402-362. We collected approximately 13 yr of Fermi-LAT data between August 2008 to January 2021 and identified three bright γ-ray activity epochs. The second was found to be the brightest epoch ever seen in this source. We observed most of the γ-ray flare peaks to be asymmetric in profile suggesting a slow cooling time of particles or the varying Doppler factor as the main cause of these flares. The γ-ray spectrum is fitted with PL and LP spectral models, and in both cases, the spectral index is very steep. The γ-ray spectrum does not extend beyond 10 GeV energy suggesting the emission is produced within the BLR. The absence of time lags between optical-IR and γ-ray suggest one zone emission model. Using the above information, we have modelled the broad-band SED with a simple one-zone emission model using the publicly available code ‘GAMERA’. The particle distribution index is found to be the same as expected in diffusive shock acceleration suggesting it as the main mechanism of particle acceleration to very high energy up to 4–6 GeV. Throughout the various flux phases, we noticed that the optical emission is dominated by the thermal disc, suggesting it to be a good source to examine the disc-jet coupling. We postulate that the observed broad-band flares could be linked with perturbation produced in the disc, which propagates to the jet and interacts with the standing shock. However, a more detailed examination is required.

Limits on the non-thermal emission of the WR–WR system Apep

Colliding-wind binaries (CWBs) constitute an emerging class of γ-ray sources powered by strong, dense winds in massive stellar systems. The most powerful of them are those binaries hosting a Wolf-Rayet (WR) star. Following the recent discovery of Apep – the closest known Galactic WR–WR binary – we discuss the non-detection of its putative high-energy emission by the Fermi Large Area Telescope (Fermi-LAT) in this Letter. The limits reported in the GeV regime can be used to set a lower limit on the magnetic field pressure density within the shocked wind-collision region (WCR), and to exclude Apep as a bright γ-ray emitting binary. Given that this WR–WR system is the most luminous CWB identified until now at radio wavelengths, this result proves unambiguously that non-thermal synchrotron emission is not a suitable identifier for the subset of γ-ray emitters in this class of particle accelerators. Rather, Apep could be an interesting case of study for magnetic field amplification in shocked stellar winds.

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.

A bright γ-ray flare interpreted as a giant magnetar flare in NGC253.

Soft γ-ray repeaters exhibit bursting emission in hard X-rays and soft γ-rays. During the active phase, they emit random short (milliseconds to several seconds long), hard-X-ray bursts, with peak luminosities1 of 1036 to 1043 erg per second. Occasionally, a giant flare with an energy of around 1044 to 1046 erg is emitted2. These phenomena are thought to arise from neutron stars with extremely high magnetic fields (1014 to 1015 gauss), called magnetars1,3,4. A portion of the second-long initial pulse of a giant flare in some respects mimics short γ-ray bursts5,6, which have recently been identified as resulting from the merger of two neutron stars accompanied by gravitational-wave emission7. Two γ-ray bursts, GRB051103 and GRB070201, have been associated with giant flares2,8-11. Here we report observations of the γ-ray burst GRB200415A, which we localized to a 20-square-arcmin region of the starburst galaxy NGC253, located about 3.5 million parsecs away. The burst had a sharp, millisecond-scale hard spectrum in the initial pulse, which was followed by steady fading and softening over 0.2 seconds. The energy released (roughly 1.3×1046 erg) is similar to that of the superflare5,12,13 from the Galactic soft γ-ray repeater SGR 1806-20 (roughly 2.3×1046 erg). We argue that GRB200415A is a giant flare from a magnetar in NGC253.

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.

Time-dependent Electron Acceleration in Pulsar Wind Termination Shocks: Application to the 2007 September Crab Nebula Gamma-Ray Flare

Abstract In 2007 September, the Crab Nebula exhibited a bright γ-ray flare in the GeV energy range that was detected by AGILE. The observed emission at ≳160 MeV indicates that the radiating electrons had energies above the classical synchrotron radiation-reaction limit, thus presenting a serious challenge to classical models for electron acceleration in astrophysical environments. In this paper, we apply our recently developed time-dependent self-similar analytical model describing electrostatic acceleration in the explosive reconnection region around the pulsar wind termination shock to the 2007 September flare. This event was unique in that it displayed both long-duration “wave” and short-duration “subflare” features. The unusual temporal variation makes this flare an especially interesting test for our model. We demonstrate that our model can reproduce the time-dependent γ-ray spectrum for this event, as well as the associated γ-ray light curve, obtained by integrating the spectrum for photon energies ≥100 MeV. This establishes that our time-dependent electrostatic acceleration model can explain both wave and subflare transients, which lends further support to the theoretical framework we have developed. We also further examine the validity of the self-similar electric and magnetic field evolution implied by our model. We conclude that strong electrostatic acceleration driven by shock-induced magnetic reconnection is able to power the Crab Nebula γ-ray flares by energizing the electrons on sub-Larmor timescales.

The Bright γ-ray Flare of 3C 279 in 2015 June: AGILE Detection and Multifrequency Follow-up Observations

Abstract We report the AGILE detection and the results of the multifrequency follow-up observations of a bright γ-ray flare of the blazar 3C 279 in 2015 June. We use AGILE and Fermi gamma-ray data, together with Swift X-ray andoptical-ultraviolet data, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution during the γ-ray flare. The γ-ray flaring data, compared with as yet unpublished simultaneous optical data that will allow constraints on the big blue bump disk luminosity, show very high Compton dominance values of ∼100, with the ratio of γ-ray to optical emission rising by a factor of three in a few hours. The multiwavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the 2015 June flare are also compared with already published data and nonsimultaneous historical 3C 279 archival data.

High-energy gamma-ray observations of the accreting black hole V404 Cygni during its 2015 June outburst

Abstract We report on Fermi/Large Area Telescope observations of the accreting black hole low-mass X-ray binary V404 Cygni during its outburst in 2015 June–July. Detailed analyses reveal a possible excess of γ-ray emission on 2015 26 June, with a very soft spectrum above 100 MeV, at a position consistent with the direction of V404 Cyg (within the 95 per cent confidence region and a chance probability of 4 × 10−4). This emission cannot be associated with any previously known Fermi source. Its temporal coincidence with the brightest radio and hard X-ray flare in the light curve of V404 Cyg, at the end of the main active phase of its outburst, strengthens the association with V404 Cyg. If the γ-ray emission is associated with V404 Cyg, the simultaneous detection of 511 keV annihilation emission by INTEGRAL reqires that the high-energy γ-rays originate away from the corona, possibly in a Blandford–Znajek jet. The data give support to models involving a magnetically arrested disc where a bright γ-ray jet can re-form after the occurrence of a major transient ejection seen in the radio.

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.

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.

γ-ray emission region located in the parsec scale jet of OJ287

We report on the location of the γ-ray emission region in flares of the BL Lacertae object OJ287 at > 14 pc from the central supermassive black hole. We employ data from multi-spectral range (total flux and linear polarization) monitoring programs combined with sequences of ultra-high-resolution 7 mm VLBA images. The correlation between the brightest γ-ray and mm flares is found to be statistically significant. The two γ-ray peaks, detected by Fermi-LAT, that we report here happened at the rising phase of two exceptionally bright mm flares accompanied by sharp linear polarization peaks. The VLBA images show that these mm flares in total flux and polarization degree occurred in a jet region at > 14 pc from the innermost jet region. The time coincidence of the brighter γ-ray flare and its corresponding mm linear polarization peak evidences that both the γ-ray and mm outbursts occur > 14 pc from the central black hole. We find two sharp optical flares occurring at the peak times of the two reported γ-ray flares. This is interpreted as the γ-ray flares being produced by synchrotron self-Compton scattering of optical photons from the flares triggered by the interaction of moving knots with a stationary conical shock in the jet.

The puzzling case of GRB 990123: prompt emission and broad-band afterglow modeling

We report on BeppoSAX simultaneous X- andγ-ray observations of the bright γ-ray burst (GRB) 990123. We present the broad-band spectrum of the prompt emission, including optical, X- andγ-rays, confirming the suggestion that the emission mechanisms at low and high frequencies must have different physical origins. In the framework of the standard fire ball model, we discuss the X-ray afterglow observed by the Narrow Field Instruments (NFIs) on board BeppoSAX and its hard X- ray emission up to 60 keV several hours after the burst, detected for about 20 ks by the Phoswich Detection System (PDS). Considering the 2− 10 keV and optical light curves, the 0.1− 60 keV spectrum during the 20 ks in which the PDS signal was present and the 8.46 GHz upper limits, we find that the multi-wavelength observ ations cannot be readily accommodated by basic afterglow models. While the temporal and spectral behavior of the optical afterglow is possibly explained by a synchrotron cooling frequency between the optical and the X-ray energy band during the NFIs observations, in X-rays this assumption only accounts for the slope of the 2− 10 keV light curve, but not for the flatness of the 0.1− 60 keV spectrum. Including the contribution of Inverse Compton (IC) scattering, we solve the problem of the flat X-ray spectrum and justify the hard X-ra y emission; we also suggest that the lack of a significant detec tion of 15− 60 keV emission in the following 75 ks and last 70 ks spectra, should be related to poorer statistics rather than to an important suppression of IC contribution. However, considering also the radio band data, we find the 8.46 GHz upper limits violated. On the other hand, leaving unchanged the emission mechanism requires modifying the hydrodynamics by invoking an ambient medium whose density rises rapidly with radius and by having the shock losing energy. Thus we are left with an open puzzle which requires further inspection.

The X-ray Spectral Properties for Gamma-ray Loud Blazars

We selected a sample of bright γ-ray (E>100 MeV) blazars whose X-rays in 0.1–10.0 keV band are observed by ROSAT and other X-ray satellites, and make a detailed spectral analysis between the soft (0.1–2.0 keV) and hard band (2.0–10.0keV) as well as between X-ray and γ-ray properties using both un-weighted and weighted analysis methods. Our results indicate: (i) there are significant anti-correlations of the spectral indices between soft X-rays and γ-rays, and between hard X-ray and γ-ray of flat-spectrum radio quasars (FSRQs) and BL Lac objects; (ii) in the X-ray band, the correlation is confusing, there is positive correlation of the spectral indices between soft and hard X-ray band using weighted correlation analysis, but this correlation is negative using un-weighted correlation analysis.

The afterglow, redshift and extreme energetics of the γ-ray burst of 23 January 1999

Long-lived emission, known as afterglow, has now been detected from about a dozen γ-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ∼1 to 3. The energy required to produce these bright γ-ray flashes is enormous: up to ∼1053 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z ⩾ 1.6. This is to date the brightest γ-ray burst with a well-localized position and if the γ-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.

On the Soft X‐Ray Spectra of γ‐Loud Blazars

ROSAT observations of a large sample of bright γ-ray (E > 100 MeV) blazars are presented. Results of a detailed spectral analysis in the soft approximately 0.1-2.0 keV energy range are discussed in relation to the overall energy distribution with particular emphasis on the relation between X-ray and γ-ray properties. A significant anticorrelation between X-ray and γ-ray spectral shapes of flat-spectrum radio quasars (FSRQs) and BL Lac objects has been discovered. A different shape in the overall energy distributions from radio to γ-ray energies between FSRQs and BL Lac objects is also implied by the correlation of their broadband spectral indices αro and αxγ. Both the above correlations can be explained if both the infrared to ultraviolet emission and the hard X-ray to γ-ray emission originate from the same electron population, via, respectively, the synchrotron process and the inverse Compton mechanism. We suggest that a key parameter for understanding the overall energy distributions of both classes of objects is the energy at which the synchrotron emission peaks in a ν-νF(ν) representation.

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.