Annihilation Features Research Articles

Hybrid Comptonization and Electron–Positron Pair Production in the Black-hole X-Ray Binary MAXI J1820+070

Abstract We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform an analysis of two joint spectra from NuSTAR and INTEGRAL, covering the range of 3–650 keV, and of an average joint spectrum over the rise of the hard state, covering the 3–2200 keV range. The spectra are well modeled by Comptonization of soft seed photons. However, the distributions of the scattering electrons are not purely thermal; we find they have substantial high-energy tails, well modeled as power laws. The photon tail in the average spectrum is detected well beyond the threshold for electron–positron pair production, 511 keV. This allows us to calculate the rate of the electron–positron pair production and put a lower limit on the size of the source from pair equilibrium. At the fitted Thomson optical depth of the Comptonizing plasma, the limit is about 4 gravitational radii. If we adopt the sizes estimated by us from the reflection spectroscopy of >20 gravitational radii, the fractional pair abundance becomes much less than unity. The low pair abundance is confirmed by the lack of both an annihilation feature and of a pair absorption cutoff above 511 keV in the average spectrum.

FIRST INTEGRAL OBSERVATIONS OF V404 CYGNI DURING THE 2015 OUTBURST: SPECTRAL BEHAVIOR IN THE 20–650 KeV ENERGY RANGE

In June 2015, the source V404 Cygni (= GS2023+38) underwent an extraordinary outburst. We present the results obtained during the first revolution dedicated to this target by the INTEGRAL mission, and focus on the spectral behavior in the hard X-ray domain, using both SPI and IBIS instruments. The source exhibits extreme variability, and reaches fluxes of several tens of Crab. However, the emission between 20 and 650 keV can be understood in terms of two main components, varying on all the observable timescales, similar to what is observed in the persistent black hole system Cyg X-1. The low energy component (up to ~ 200 keV) presents a rather unusual shape, probably due to the intrinsic source variability. Nonetheless, a satisfactory description is obtained with a Comptonization model, if an unusually hot population of seed photons ($kT_0$ ~ 7 keV) is introduced. Above this first component, a clear excess extending up to 400-600 keV leads us to investigate a scenario where an additional (cutoff) power law could correspond to the contribution of the jet synchrotron emission, as proposed in Cyg X-1. A search for an annihilation feature did not provide any firm detection, with an upper limit of 2 x $10^{-4} ph/cm^2 s$ (2 \sigma) for a narrow line centered at 511 keV, on the averaged obtained spectrum.

On the peculiarities of positron annihilation features in silicalite-1 and Y-zeolites

Conditions that facilitate high Ps formation and interactions that modify the o-Ps lifetime were investigated by positron annihilation techniques in silicalite-1 and various Y-zeolites. Long lifetimes, up to 135 ns, and o-Ps fractions as high as 40% were found. The influence of heat treatment (in the range of 90–520 K), capillary condensation of N 2 (0.085 MPa) and correlation with water removal was examined in Y-zeolites and in silicalite-1, respectively. The latter matrix was also studied in the presence of liquids. In various samples unexpected features were found (peculiar changes in the 2γ/3γ ratio, inversion in the trend of lifetime variations, disappearance of specific components, etc.), denoting the complexity of the processes governing the fate of e + and Ps.

Gamma-Ray Burst Spectral Features: Interpretation as X-Ray Emission from a Photoionized Plasma

Numerous reports have been made of features, either in emission or absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally interpreted in the context of Galactic neutron star models as cyclotron line emission and $e^+ - e^-$ annihilation features, the recent demonstration that the majority of GRBs lie at cosmological distances make these explanations unlikely. In this letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal rich blobs or filaments of plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, similar to those detected, can be observed. We find a limited region of parameter space capable of reproducing the observed GRB spectra. Finally, we discuss possible constraints further high-energy spectral observations could place on fireball model parameters.

Spectral distinction between black holes and neutron stars: The contribution of the SIGMA telescope

After more than four years of operation, the imaging γ - ray telescope SIGMA has accumulated several days of observation toward black holes and neutron stars. So far, more than twenty five of them have been detected with SIGMA. Although all these sources have spectra dominated by comptonisation, it is remarkable that suspected black hole systems are particularly numerous among the most luminous sources above 150 keV. Also, two black hole candidates have shown a strong transient emission around 511 keV, which has been interpreted as electron-positron annihilation features. We will discuss in this paper the reliability of these two phenomena, a strong luminosity above 150 keV and the emission of an annihilation line, as potential black hole criteria.

Electron-positron pair production and annihilation spectral features from compact sources

We study the efficiency of e± pair production in compact γ-ray sources, as measured by the fraction of power supplied to a source that is converted into pair annihilation radiation emitted by the source (the pair yield). We focus mainly on thermal plasmas, in contrast to previous studies of the pair yield in nonthermal plasmas. We calculate the pair yield in thermal plasmas where bremsstrahlung is the only source of soft photons as well in plasmas in which there is a copious source of soft photons. We find that the pair yield reaches its maximum of ~0.3 in Comptonized bremsstrahlung plasmas. The corresponding maximum fraction of source power converted into pair rest mass is ~0.2. Addition of ambient soft photons to the source reduces the pair yield. The maximum yield is still close to ~0.3 in plasmas where the soft photons are Comptonized into hard power laws of energy spectral index α ≃ 0.1–0.2, but decreases with increasing α and becomes < 0.01 for α ≳ 0.7. Even when the pair yield is large, we never find an observable annihilation feature from a thermal plasma. To have a high pair yield, typically either the plasma temperature must be relativistic or photons must undergo many Compton scatterings before escaping the source. Hence, the emergent annihilation spectrum is either intrinsically broad or strongly smeared by repeated Compton scatterings and is difficult to distinguish from the underlying Compton scattering and bremsstrahlung continuum. This is in contrast to the nonthermal case where strong, narrow annihilation features can be produced by pairs which have cooled in the source. We summarize the conditions when such strong features are produced, and calculate the pair yield when primary electrons are injected with power-law energy distributions. While observable annihilation features are not produced directly in thermal sources, strong pair outflows, e.g., electron-positron jets, may be created in sources with hard spectra. The outflowing pairs may cool and annihilate outside the source region, in which case a visible annihilation feature is possible. However, our results applied to Nova Muscae show that neither thermal nor nonthermal pair models can explain the large strength of the annihilation feature observed from that source (unless a complex obscuration scenario is invoked). On the other hand, the annihilation feature of the 1979 March 5 γ-ray burst can be explained by our models provided the source is located at a distance much less than that to the LMC.

Models of pair annihilation in 1E 1740.7-2942 and the HEAO 1 A-4 annihilation source

We study possible models of two Galactic sources of transient pair annihilation radiation, 1E 1740.7-2942 and a source observed by High Energy Astronomy Observatory (HEAO) 1 A-4. We fit the observed spectral features by thermal annihilation spectra and find that the redshifts obtained by us are much larger than those obtained from fitting Caussian lines centered on 511 keV. This effect, which is due to the net blueshift (with respect to 511 keV) of the annihilation spectrum due to the thermal energies of pairs, puts strong constraints on models of sources. We consider those constraints first without considering the mechanism of positron production. From the shape of the observed spectra, we are able to rule out both spherical clouds and layers above cold matter as possible source geometries. The observed spectra are compatible with two source geometries: (1) a nearly face-on disk in the Kerr metric and (2) a jet close to a black hole. We consider, then, the origin of the pairs. Theories of both thermal and nonthermal pair equilibria predict that photon-pair production is unable to produce annihilation features that contain as much as half of the bolometric luminosity, which is observed. A possible solution to this problem is obscuration of a nonthermal source (in which pairs are produced by photon-photon collisions) and an outflow of pairs to an unobscured region. This makes annihilation in a jet the most likely model of the considered sources.

Gamma-ray properties of active galactic nuclei

Recent observations by the Compton Observatory (CGRO) have increased our knowledge about the gamma-ray emission from Active Galactic Nuclei (AGN) considerably. The three most important findings of CGRO with respect to AGNs are: first, no Seyfert 1 galaxy has been found to show emission above 500 keV. The by far strongest Seyfert 1 galaxy NGC 4151 shows a spectrum which falls off exponentially with an e-folding energy of 39 keV between 65 and 500 keV. OSSE so far has detected or has indications of detections for seven additional Seyfert 1 galaxies, which, however, all show very weak hard X-ray emission compared to NGC 4151. No annihilation feature has been seen from any Seyfert galaxy to this date. Second, the radio galaxy Cen A shows a power-law energy spectrum from hard X-ray energies of about 150 keV to at least 3 MeV. It has not been seen at EGRET-energies. Third, a new class of AGN was discovered at energies above 100 MeV by EGRET. The power of these objects in gamma-rays can dominate the luminosity in other spectral ranges. These objects are associated with extragalactic sources that have blazar properties. The high-energy gamma-ray emission is probably produced in relativistically outflowing jets. At hard X-ray energies the objects are rather weak. Spectral breaks at MeV energies were found by COMPTEL for three of these objects (3C 273, 3C 279, and PKS 0528+134).

Baryon isocurvature fluctuations at small scales and baryonic dark matter.

A model of baryogenesis is described which gives rise to fluctuations in baryon number that are large on small scales but of low amplitude on large scales. This provides a mechanism for primordial blackhole formation, and allows the possibility of a critical density of dark matter in baryonic (and antibaryonic) form. Since high-density regions naturally possess both signs of baryonic excess, our model also predicts a small fraction of the mass density of the Universe to be in the form of compact antibaryonic regions. These objects may be observable via a redshifted annihilation feature in the diffuse extragalactic $\ensuremath{\gamma}$-ray background, as both steady sources of $\ensuremath{\gamma}$-ray radiation and annihilation-powered $\ensuremath{\gamma}$-ray bursters at cosmological distances, by a distortion of the spectrum of the 3K background radiation, or by the presence of antihelium nuclei as a rare component of high-energy cosmic rays.

The suppression of pulsar and gamma-ray burst annihilation lines by magnetic photon splitting

Neutron stars, relativistic and compact by nature, show great potential for the copious creation of electron-positron pairs in the magnetospheres; these rapidly cool, thermalize, and then annihilate. It is therefore expected that many neutron sources might display evidence of pair annihilation lines in the 400-500 keV range. It is shown that magnetic photon splitting, which operates effectively at these energies and in the enormous neutron star magnetic fields, can destroy an annihilation feature by absorbing line photons and reprocessing them to lower energies. In so doing, photon splitting creates a soft gamma-ray bump and a broad quasi-power-law contribution to the X-ray continuum, which is too flat to conflict with the observed X-ray paucity in gamma-ray bursts. The destruction of the line occurs in neutron stars with surface fields of 5 x 10 exp 12 G or maybe even less, depending on the size of the emission region.

Nonthermal electron-positron pairs and the break in the hard X-ray spectrum of NGC 4151

The recent observation by the detectors on board Granat of a spectral steepening above about 50 keV imposes a constraint on possible emission models for NGC 4151. This steepening, for example, is not well fitted by an exponential rollover characteristic of a purely thermal model, or by a Compton-downscattered power law. We find that this spectral behavior is consistent with that produced by a photon-starved nonthermal pair plasma with high compactness. This is the first quantitative test of the nonthermal pair plasma model against broad-band X-ray/gamma-ray observations. Above 200 keV or so, the nonthermal pair plasma model predicts an upturn in the spectrum, and a thermal annihilation feature around 511 keV. Such spectral behavior should be looked for with GRO.

Dispersionless vibrational mode of a chemisorbed adsorbate observed with inelastic helium scattering: Hg on Cu(001)

Dispersionless single phonon creation and annihilation features have been observed across the entire Brillouin zone for the low density Hg monolayer adsorbed on Cu(001). This uncorrelated vibration of Hg atoms perpendicular to the surface contrasts sharply with the vibrational activity of most other chemisorbed metal atoms on metal substrates which exhibit correlated vibrations between groups of adsorbate atoms. The measured phonon energy has been used to build an interaction potential which incorporates both dynamical as well as static structural and thermodynamic properties of this system.

Search for spin effect in positron interaction with optically active camphor

The depolarization, relative to the direction of a magnetic field, which accompanies the slowing-down of positrons in the two enantiomers of camphor, down to the positronium formation energy, has been found to be greater (27%) in L-camphor. The measuring method was based on the Ps relative yield, in the magnetic field, in triplet and singlet state. All other annihilation features, as well as the intrinsic properties of Ps once formed, do not display significant differences between the two enantiomers.

Resonant Compton cooling and annihilation line production in gamma-ray bursts

view Abstract Citations (2) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Resonant Compton Cooling and Annihilation Line Production in Gamma-Ray Bursts Preece, R. D. ; Harding, A. K. Abstract Attention is given to a synchrotron self-Compton emission model for gamma-ray bursts which produces narrow annihilation features for a variety of field strengths, primary electron injection energies, and injection rates. In this model, primary electrons are injected and cooled by synchrotron emission in a strong, homogeneous magnetic field, resulting in a pair cascade. Multiple resonant scattering with cyclotron photons efficiently traps and cools pairs in the ground state to an average energy where the Compton energy loss rate is zero, which is in agreement with previous estimates of a Compton temperature. The particle distributions in the ground state are determined by numerically solving the Fokker-Planck equation in the steady state. In the case of isotropic injection of primary electrons, a significant narrow-line feature appears in the overall emission. In the case of beamed injection, the annihilation line is broadened to the extent that it would not be observable. Publication: The Astrophysical Journal Pub Date: February 1992 DOI: 10.1086/171017 Bibcode: 1992ApJ...386..308P Keywords: Annihilation Reactions; Compton Effect; Gamma Ray Bursts; Line Spectra; Astronomical Models; Astronomical Spectroscopy; Computational Astrophysics; Electron Distribution; Pair Production; Scattering Cross Sections; Space Radiation; GAMMA RAYS: BURSTS; LINE: FORMATION; RADIATION MECHANISMS: COMPTON AND INVERSE COMPTON full text sources ADS | data products SIMBAD (1)

Annihilation radiation from a power-law distributed electron-positron plasma on the ground Landau level: the case of low magnetic fields

Intensity, polarization, and cooling rate of the two-photon annihilation radiation are studied in detail in the case of one-dimensional power-law distributions of electrons and positrons, assuming that they occupy the ground Landau level in a strong magnetic fieldB∼1010–1012 G. Simple analytical expressions for limiting cases are obtained and results of numerical calculations of radiation characteristics are presented. Power-lawe ± distributionsη ± ∝ e ± are shown to generate power-law spectra of the annihilation radiation atE≫mc 2 andE≪mc 2, with indices depending on the direction of radiation. The annihilation spectra at ϑ=0 show the largest blue-shifts of their maxima and the hardest high-energy tailsI(E≫mc 2, ϑ=0)∝E −(k−1). The blue-shifts reduce, and the hard tials steepen, with increasing ϑ. At ϑ>(2mc 2/E)1/2 the slopes of the high-energy tails rapidly transform to that at ϑ=π2,I(E≫mc 2, ϑ=π/2)∝E −(2k+3). The direction-integrated spectraS(E) also display the power-law tials at low and high energies,S(E≫mc 2)∝E −(k+1). The total annihilation rate and energy losses decrease with decreasingk, being higher than for the isotropice ± power-law distributions at the samek. The radiation is linearly polarized in the plane formed by the magnetic field and wave-vector. The polarization degreeP is maximum atE≫mc 2:P max≃0.6 for ϑ=π/2. Annihilation features and power-law-like hard tails observed in many gamma-ray burst spectra may be associated with the annihilation radiation of the magnetized power-law distributed plasma near neutron stars. Comparison of the observed and theoretical spectra allows one to estimate the power-law index of thee − e +-distribution and the gravitational redshift factor in the radiating region.

Three spectral states of 1E 1740.7 - 2942 - From standard Cygnus X-1 type spectrum to the evidence of electron-positron annihilation feature

The source 1E 1740.7-2942 is known to be the brightest hard X-ray source close to the dynamic center of our Galaxy. Three apparently different spectral states of this source were detected by the GRANAT observatory during 1990-1991 observations of the Galactic Center (GC) region. In almost all 1990 observations the source had Cyg X-1-like spectrum with nearly constant flux. The hardest of the states (observed on 1990 October 13-14) exhibits a prominent high-energy bump on the spectrum at 300-600 keV, probably related to the annihilation processes in relatively cold electron-positron plasma

Nonthermal synchrotron radiation and pair annihilation in gamma-ray bursts

A self-consistent Monte Carlo simulation of quantized synchrotron emission, pair production, and pair annihilation in strong (2 x 10 to the 12th G or greater) magnetic fields is presented as a model of the production of annihilation lines in gamma-ray burst sources. The calculation determines the self-consistent photon spectrum and pair distribution functions resulting from a nonthermal injection of electrons into a region of homogeneous, strong magnetic field. The results indicate that the appearance of observable annihilation features in gamma-ray burst spectra requires isotropic or fan-beamed injection of electrons with energies of at least 5 MeV. The lines are narrow because higher energy electrons escape annihilation and do not contribute to Doppler-broadening. Injection beamed along the field did not produce observable annihilation lines in any of the cases studied. As a result, emission at energies much greater than 1 MeV produced through beaming seems to be incompatible with observable annihilation in this model. These results are discussed in relation to observed features in gamma-ray burst spectra.

Search for a narrow annihilation feature correlated with the MeV emission of Cygnus X-1

Strong gamma-ray emission near 1 MeV from Cygnus X-1 observed in the fall of 1979 during the 'gamma 1' period has been interpreted by Liang and Dermer (1988) as thermal emission from a relativistic, pair-dominated region surrounding the black hole. The observed gamma-rays should interact with each other and with X-rays to form electron/positron pairs inside and outside the accretion disk. Because the Eddington limit is reduced by a factor of 1836 in the presence of positrons, some pairs formed inside the disk may also escape the system under the influence of radiation pressure. If these pairs stop in the cold surrounding medium, a narrow 511 keV annihilation feature should be produced. The high resolution gamma 1 spectrum has been examined for such a feature, and some evidence at 1.9 sigma statistical significance for the line is observed with a flux which is generally consistent with theoretical expectations.

Redshifts derived from gamma-ray burst spectra are inconclusive

If the reported large widths of the 'annihilation' features in gamma-ray burst spectra are real, the inferred value of the redshift can be as high as z = 0.8, depending on the emission mechanism. A model is presented in which a broad annihilation line peaking at 400 keV corresponds to a Doppler blueshift of the emitting region of z = -0.3. The prevalent view that the observed line centers of burst annihilation features imply emission from the surface of a nonrotating neutron star is not supported by the available data. However, emission from rapidly rotating neutron stars is a viable possibility.

Monte Carlo spectra from pair-equilibrium, weakly magnetized thermal plasmas

The results of Monte Carlo simulations of weakly magnetized, relativistic thermal plasmas are reported, including such physical processes as thermal synchrotron emission and absorption, bremsstrahlung, and Compton scattering, as well as pair-annihilation and pair-production. The rates of the latter two processes are kept approximately equal to each other, i.e., the plasmas are in pair equilibrium. The resulting spectra are of power-law form, and can be well described by a set of simple analytical formulae. Neither a pair annihilation feature nor a Wien hump is seen in the spectra. The possible relevance of the results to active galactic nuclei and gamma-bursts is discussed.