Γγ Absorption Research Articles

Constraining the Jet Composition of GRB 221009A with the Prompt TeV Emission Limit

Recent LHAASO observations of the prompt emission phase of the brightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratio between the TeV and MeV emissions, F TeV/F MeV ≤ 2 × 10−5, during the period 220–230 s after the trigger. This period covers the peak of the main MeV burst and is just before the TeV afterglow emerges. Within the framework of internal shocks, we study the internal γγ absorption in GRB 221009A by generating a set of synthetic bursts in a simulation that reproduces the observed feature of GRB 221009A. We find that the γγ absorption does not lead to an exponential cutoff, but rather a power-law spectrum, consistent with previous works. We further find that the attenuation due to γγ absorption alone cannot explain the flux limit ratio of GRB 221009A, suggesting a low ratio between synchrotron self-Compton (SSC) and synchrotron emission outputs. This requires the magnetic field energy density to be much larger than the synchrotron photon energy density so that the SSC flux is greatly suppressed. This indicates that the jet composition of GRB 221009A is likely Poynting flux dominated.

Core-collapse supernovae in dense environments – particle acceleration and non-thermal emission

ABSTRACT Supernova remnants (SNRs) are known to accelerate cosmic rays from the detection of non-thermal emission in radio waves, X-rays, and gamma-rays. However, the ability to accelerate cosmic rays up to PeV energies has yet to be demonstrated. The presence of cut-offs in the gamma-ray spectra of several young SNRs led to the idea that PeV energies might only be achieved during the first years of a remnant’s evolution. We use our time-dependent acceleration-code RATPaC to study the acceleration of cosmic rays in supernovae expanding into dense environments around massive stars. We performed spherically symmetric one-dimensional (1D) simulations in which we simultaneously solve the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. We investigated typical circumstellar-medium (CSM) parameters expected around red supergiant (RSG) and luminous blue variable (LBV) stars for freely expanding winds and accounted for the strong γγ absorption in the first days after explosion. The maximum achievable particle energy is limited to below $600\,$TeV even for the largest considered values of the magnetic field and mass-loss rates. The maximum energy is not expected to surpass $\approx 200\,$ and $\approx 70\,$TeV for LBVs and RSGs that experience moderate mass-loss prior to the explosion. We find gamma-ray peak-luminosities consistent with current upper limits and evaluate that current-generation instruments are able to detect the gamma-rays from Type-IIP explosions at distances up to $\approx 60\,$ kpc and Type-IIn explosions up to $\approx 1.0\,$ Mpc. We also find a good agreement between the thermal X-ray and radio synchrotron emission predicted by our models with a range of observations.

Very-high-energy Emission and Cascade Radiation of Gamma-Ray Burst Afterglows: Homogeneous versus Wind External Media

Abstract Recent detection of sub-TeV emission from gamma-ray bursts (GRBs) represents a breakthrough in the GRB study. The multiwavelength data of the afterglows of GRB 190114C support the synchrotron self-Compton (SSC) origin for its sub-TeV emission. We present a comparative analysis on the SSC emission of GRB afterglows in the homogeneous and wind environments in the framework of the forward shock model. The γ γ absorption of very-high-energy photons due to pair production within the source and the Klein–Nishina effect on the inverse Compton scattering are considered. Generally a higher SSC flux is expected for a larger circum-burst density due to a larger Compton parameter, but meanwhile the internal γ γ absorption is more severe for sub-TeV emission. The flux ratio between the SSC component and the synchrotron component decreases more quickly with time in the wind medium case than in the homogeneous density medium case. The light curves of the SSC emission are also different for the two types of media. We also calculate the cascade emission resulting from the absorbed high-energy photons. In the ISM environment with n ≳ 1 cm−3, the cascade synchrotron emission could be comparable to the synchrotron emission of the primary electrons in the optical band, which may flatten the optical afterglow light curve at an early time (t < 1 hr). In the wind medium with A * ≳ 0.1, the cascade emission in the eV–GeV band is comparable or even larger than the emission of the primary electrons at the early time.

On the Minimum Jet Power of TeV BL Lac Objects in the p–γ Model

Abstract We study the requirement of the jet power in the conventional p–γ models (photopion production and Bethe–Heitler pair production) for TeV BL Lac objects. We select a sample of TeV BL Lac objects whose spectral energy distributions are difficult to explain by the one-zone leptonic model. Based on the relation between the p–γ interaction efficiency and the opacity of γγ absorption, we find that the detection of TeV emission poses upper limits on the p–γ interaction efficiencies in these sources and hence minimum jet powers can be derived accordingly. We find that the obtained minimum jet powers exceed the Eddington luminosity of the supermassive black holes (SMBHs). Implications for the accretion mode of the SMBHs in these BL Lac objects and the origin of their TeV emissions are discussed.

The Blazar TXS 0506+056 Associated with a High-energy Neutrino: Insights into Extragalactic Jets and Cosmic-Ray Acceleration

Abstract A neutrino with energy ∼290 TeV, IceCube-170922A, was detected in coincidence with the BL Lac object TXS 0506+056 during enhanced gamma-ray activity, with chance coincidence being rejected at ∼3σ level. We monitored the object in the very-high-energy (VHE) band with the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes for ∼41 hr from 1.3 to 40.4 days after the neutrino detection. Day-timescale variability is clearly resolved. We interpret the quasi-simultaneous neutrino and broadband electromagnetic observations with a novel one-zone lepto-hadronic model, based on interactions of electrons and protons co-accelerated in the jet with external photons originating from a slow-moving plasma sheath surrounding the faster jet spine. We can reproduce the multiwavelength spectra of TXS 0506+056 with neutrino rate and energy compatible with IceCube-170922A, and with plausible values for the jet power of . The steep spectrum observed by MAGIC is concordant with internal γγ absorption above ∼100 GeV entailed by photohadronic production of a ∼290 TeV neutrino, corroborating a genuine connection between the multi-messenger signals. In contrast to previous predictions of predominantly hadronic emission from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton upscattering of external photons by accelerated electrons. The X-ray and VHE bands provide crucial constraints on the emission from both accelerated electrons and protons. We infer that the maximum energy of protons in the jet comoving frame can be in the range ∼1014 – 1018 eV.

Gamma–Gamma Absorption in the γ-ray Binary System PSR B1259-63/LS 2883

Abstract The observed TeV light curve from the γ-ray binary PSR B1259-63/LS 2883 shows a decrease in the flux at periastron that has not been fully explained by emission mechanisms alone. This observed decrease can, however, be explained by γγ absorption due to the stellar and disk photons. We calculate the γγ absorption in PSR B1259-63/LS 2883 taking into account photons from both the circumstellar disk and star, assuming that the γ-rays originate at the position of the pulsar. The γγ absorption due to the circumstellar disk photons produces a ≈14% decrease in the flux, and there is a total decrease of ≈52% (>1 TeV) within a few days before periastron, accompanied by a hardening of the γ-ray photon index. While the γγ absorption alone is not sufficient to explain the full complexity of the H.E.S.S. γ-ray light curve, it results in a significant decrease in the predicted flux, which is coincident with the observed decrease. In addition, we have calculated an upper limit on the γγ absorption, assuming that the emission is produced at the apex of the bow shock. Future observations with CTA during the 2021 periastron passage may be able to confine the location of the emission based on the degree of γγ absorption, as well as measure the hardening of the spectrum around periastron.

GAMMA–GAMMA ABSORPTION IN THE BROAD LINE REGION RADIATION FIELDS OF GAMMA-RAY BLAZARS

ABSTRACT The expected level of γγ absorption in the Broad Line Region (BLR) radiation field of γ-ray loud Flat Spectrum Radio Quasars (FSRQs) is evaluated as a function of the location of the γ-ray emission region. This is done self-consistently with parameters inferred from the shape of the spectral energy distribution (SED) in a single-zone leptonic EC-BLR model scenario. We take into account all geometrical effects both in the calculation of the γγ opacity and the normalization of the BLR radiation energy density. As specific examples, we study the FSRQs 3C279 and PKS 1510-089, keeping the BLR radiation energy density at the location of the emission region fixed at the values inferred from the SED. We confirm previous findings that the optical depth due to γγ absorption in the BLR radiation field exceeds unity for both 3C279 and PKS 1510-089 for locations of the γ-ray emission region inside the inner boundary of the BLR. It decreases monotonically, with distance from the central engine and drops below unity for locations within the BLR. For locations outside the BLR, the BLR radiation energy density required for the production of GeV γ-rays rapidly increases beyond observational constraints, thus making the EC-BLR mechanism implausible. Therefore, in order to avoid significant γγ absorption by the BLR radiation field, the γ-ray emission region must therefore be located near the outer boundary of the BLR.

High-energy neutrino emission from low-mass microquasars

In this paper we study the high-energy neutrino emission from low-mass microquasars (LMMQs) by assuming that an acceleration region for particles is located in the inner jet, based on the framework of a hadronic jet model. Adopting the parametrized formulae for inelastic pp interactions and the recent pγ parametrizations, we calculate the spectral energy distributions (SEDs) of various photon and neutrino spectra. Furthermore, we also consider the electromagnetic cascade processes due to γγ absorption. We then apply the jet model to the microquasar (MQ) GX 339-4, and calculate the expected neutrino event rate for three years of observations using the newest effective area of the KM3NeT detector in the Mediterranean Sea. The resulting results indicate that (1) photon spectra can roughly reproduce observations from radio to X-ray energies, and can also predict emission fluxes at high and very high energies (VHE); (2) GX 339-4 is a potential neutrino source, neutrinos from which are likely to be identified with some years of observations from the next-generation km 3 -scale neutrino telescopes.

IMPLICATIONS OF THE VERY HIGH ENERGY GAMMA-RAY DETECTION OF THE QUASAR 3C279

The Major Atmospheric Gamma-Ray Imaging Cherenkov Telescope very high energy (VHE) γ-ray astronomy collaboration recently reported the detection of the quasar 3C279 at >100 GeV γ-ray energies. Here, we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snapshot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton (SSC) origin of the entire spectral energy distribution (SED), including the VHE γ-ray emission is highly problematic as it would require an unrealistically low magnetic field. The measured level of VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions, BLRs). However, such an interpretation would require either an unusually low magnetic field of B ∼ 0.03 G, or (in order to achieve approximate equipartition between magnetic field at B ∼ 0.25 G and relativistic electrons) an unrealistically high Doppler factor of Γ ∼ 140. In addition, such a model fails to reproduce the observed X-ray flux. Furthermore, both versions of leptonic one-zone models produce intrinsic VHE γ-ray spectra steeper than measured, even in the case of the lowest plausible extragalactic γγ absorption. We therefore conclude that a simple one-zone, homogeneous leptonic jet model is not able to plausibly reproduce the SED of 3C279 including the recently detected VHE γ-ray emission. This as well as the lack of correlated variability in the optical with the VHE γ-ray emission and the substantial γγ opacity of the BLR radiation field to VHE γ-rays suggests a multi-zone model in which the optical emission is produced in a different region than the VHE γ-ray emission. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray–VHE γ-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well, both in scenarios in which only the internal synchrotron field serves as targets for pγ pion production, and with a substantial contribution from external photons, e.g., from the BLR. However, either version of the hadronic model requires a rather extreme jet power of up to Lj ∼ 1049 erg s−1, compared to a requirement of Lj ∼ 2 × 1047 erg s−1 for a multi-zone leptonic model.

GAMMA-RAY STUDIES OF BLAZARS: SYNCHRO-COMPTON ANALYSIS OF FLAT SPECTRUM RADIO QUASARS

We extend a method for modeling synchrotron and synchrotron self-Compton radiations in blazar jets to include external Compton (EC) processes. The basic model assumption is that the blazar radio through soft X-ray flux is nonthermal synchrotron radiation emitted by isotropically distributed electrons in the randomly directed magnetic field of outflowing relativistic blazar jet plasma. Thus, the electron distribution is given by the synchrotron spectrum, depending only on the Doppler factor δD and the mean magnetic field B, given that the comoving emission region size scale R'b ≲ cδDtv/(1 + z), where tv is the variability time and z is the source redshift. Generalizing the approach of Georganopoulos, Kirk, & Mastichiadis to arbitrary anisotropic target radiation fields, we use the electron spectrum implied by the synchrotron component to derive accurate Compton-scattered γ-ray spectra throughout the Thomson and Klein–Nishina regimes for EC scattering processes. We derive and calculate accurate γ-ray spectra produced by relativistic electrons that Compton-scatter (1) a point source of radiation located radially behind the jet, (2) photons from a thermal Shakura–Sunyaev accretion disk, and (3) target photons from the central source scattered by a spherically symmetric shell of broad-line region gas. The calculations of broadband spectral energy distributions from the radio through γ-ray regimes are presented, which include self-consistent γγ absorption on the same radiation fields that provide target photons for Compton scattering. The application of this baseline flat spectrum radio/γ-ray quasar model is considered in view of data from γ-ray telescopes and contemporaneous multiwavelength campaigns.

Pulsar wind zone processes in LS 5039

Several γ-ray binaries have been recently detected by the high-energy stereoscopy array (HESS) and the major atmospheric imaging cerenkov (MAGIC) telescope. In at least two cases, their nature is unknown. In this paper we aim to provide the details of a theoretical model of close γ-ray binaries containing a young energetic pulsar as compact object, earlier presented in recent letters. This model includes a detailed account of the system geometry, the angular dependence of processes such as Klein–Nishina inverse Compton and γγ absorption in the anisotropic radiation field of the massive star, and a Monte Carlo simulation of leptonic cascading. We present and derive the used formulae and give all details about their numerical implementation, particularly, on the computation of cascades. In this model, emphasis is put in the processes occurring in the pulsar wind zone of the binary, since, as we show, opacities in this region can be already important for close systems. We provide a detailed study on all relevant opacities and geometrical dependencies along the orbit of binaries, exemplifying with the case of LS 5039. This is used to understand the formation of the very high-energy lightcurve and phase dependent spectrum. For the particular case of LS 5039, we uncover an interesting behavior of the magnitude representing the shock position in the direction to the observer along the orbit, and analyze its impact in the predictions. We show that in the case of LS 5039, the HESS phenomenology is matched by the presented model, and explore the reasons why this happens while discussing future ways of testing the model.

The Hard VHE γ-Ray Emission in High-Redshift TeV Blazars: Comptonization of Cosmic Microwave Background Radiation in an Extended Jet?

Observations of very-high-energy (VHE; E > 250 GeV) γ-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting the observed VHE spectrum for γγ absorption, even by the lowest plausible level of the IIBR, provides evidence for a very hard (photon spectral index Γph < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE γ-ray spectrum poses a serious challenge to the conventional synchrotron self-Compton (SSC) interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of cosmic microwave background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101–232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields (B ∼ 10 μG) and if electrons are still being accelerated out to TeV energies (γ ≳ 4 × 106) on kiloparsec scales along the jet.

Intergalactic absorption and blazar γ-ray spectra

The distribution of TeV spectral slopes versus redshift for currently known TeV blazars (16 sources with z ≤ 0.21, and one with z > 0.25) is essentially a scatter plot with hardly any hint of a global trend. We suggest that this is the outcome of two combined effects of intergalactic γγ absorption, plus an inherent feature of the SSC (synchro-self-Compton) process of blazar emission. First, flux dimming introduces a bias that favors detection of progressively more flaring sources at higher redshifts. According to mainstream SSC models, more flaring source states imply sources with flatter TeV slopes. This results in a structured relation between intrinsic TeV slope and redshift. The second effect, spectral steepening by intergalactic absorption, affects sources progressively with distance and effectively wipes out the intrinsic slope-redshift correlation.

Constraints on the geometry of the VHE emission in LS 5039 from photon–photon deabsorption

Evidence for orbital modulation of the very high energy (VHE) γ-ray emission from the high-mass X-ray binary and microquasar LS 5039 has recently been reported by the HESS collaboration. The observed flux modulation was found to go in tandem with a change in the GeV–TeV spectral shape, which may partially be a result of γγ absorption in the intense radiation field of the massive companion star. However, it was suggested that γγ absorption effects alone cannot be the only cause of the observed spectral variability since the flux at ∼200 GeV, which is near the minimum of the expected γγ absorption trough, remained essentially unchanged between superior and inferior conjunction of the binary system. In this paper, a detailed parameter study of the γγ absorption effects in this system is presented. For a range of plausible locations of the VHE γ-ray emission region and the allowable range of viewing angles, the deabsorbed, intrinsic VHE γ-ray spectra and total VHE photon fluxes and luminosities are calculated and compared to luminosity constraints based on Bondi-Hoyle limited wind accretion onto the compact object in LS 5039. Based on these arguments, it is found that (1) it is impossible to choose the viewing angle and location of the VHE emission region in a way that the intrinsic (deabsorbed) fluxes and spectra in superior and inferior conjunction are identical; consequently, the intrinsic VHE luminosities and spectral shapes must be fundamentally different in different orbital phases, (2) if the VHE luminosity is limited by wind accretion from the companion star and the system is viewed at an inclination angle of i ≳ 40°, the emission is most likely beamed by a larger Doppler factor than inferred from the dynamics of the large-scale radio outflows, (3) the still poorly constrained viewing angle between the line of sight and the jet axis is most likely substantially smaller than the maximum of ∼64° inferred from the lack of eclipses. (4) Consequently, the compact object is more likely to be a black hole rather than a neutron star. (5) There is a limited range of allowed configurations for which the expected VHE neutrino flux would actually anti-correlate with the observed VHE γ-ray emission. If hadronic models for the γ-ray production in LS 5039 apply, a solid detection of the expected VHE neutrino flux and its orbital modulation with km 3 scale water-Cherenkov neutrino detectors might require the accumulation of data over more than 3 years.

Models of Very-High-Energy Gamma-Ray Emission from the Jets of Microquasars: Orbital Modulation

The recent detection of very-high-energy (GeV — TeV) γ-ray emission from the Galactic black-hole candidate and microquasar LS 5039 has sparked renewed interest in jet models for the high-energy emission in those objects. In this work, we have focused on models in which the high-energy emission results from synchrotron and Compton emission by relativistic electrons in the jet (leptonic jet models). Particular attention has been paid to a possible orbital modulation of the high-energy emission due to azimuthal asymmetries caused by the presence of the companion star. Both orbitalphase dependent γγ absorption and Compton scattering of optical/UV photons from the companion star may lead to an orbital modulation of the gamma-ray emission. We make specific predictions which should be testable with refined data from HESS and the upcoming GLAST mission.

Photon-Photon Absorption of Very High Energy Gamma Rays from Microquasars: Application to LS 5039

Very high energy (VHE) γ-rays have recently been detected from the Galactic black hole candidate and microquasar LS 5039. A plausible site for the production of these VHE γ-rays is the region close to the base of the mildly relativistic outflow. However, at distances comparable to the binary separation, the intense photon field of the stellar companion leads to substantial γγ absorption of VHE γ-rays. If the system is viewed at a substantial inclination (i ≠ 0), this absorption feature will be modulated on the orbital period of the binary as a result of a phase-dependent stellar-radiation intensity and pair-production threshold. We apply our results to LS 5039 and find that (1) γγ absorption effects will be substantial if the photon production site is located at a distance from the central compact object of the order of the binary separation (≈ 2.5 × 1012 cm) or less; (2) the γγ absorption depth will be largest at a few hundred GeV, leading to a characteristic absorption trough; (3) the γγ absorption feature will be strongly modulated on the orbital period, characterized by a spectral hardening accompanying periodic dips of the VHE γ-ray flux; and (4) γ-rays can escape virtually unabsorbed, even from within ≈1012 cm, when the star is located behind the production site as seen by the observer.

Multiwavelength Observations of Markarian 501 during the 1997 High State

During the observation period 1997, the nearby blazar Mrk 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray, and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The nonlinear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a synchrotron self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for γγ absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for γ-ray flares of the flat-spectrum radio quasar PKS 0528+134.