MeV Emission Research Articles

Lepto-hadronic model for the broadband emission of Cygnus X-1

Cygnus X-1 is a well observed microquasar. Broadband observations at all wavelengths have been collected over the years. The origin of the MeV tail observed with COMPTEL and INTEGRAL is still under debate and it has mostly been attributed to the corona, although its high degree of polarization suggests it is synchrotron radiation from a jet. The origin of the transient emission above $\sim 100$ GeV is also unclear. We aim to disentangle the origin of the broadband spectral energy distribution (SED) of Cygnus X-1, focusing particularly on the gamma-ray emission, and to gain information on the physical conditions inside the jets. We develop and apply a lepto-hadronic, inhomogeneous jet model to the non-thermal SED of Cygnus X-1. We calculate the contributions to the SED of both protons and electrons accelerated in an extended region of the jet. We also estimate the radiation of charged secondaries produced in hadronic interactions, through several radiative processes. Absorption effects are considered. We produce synthetic maps of the jets at radio wavelengths. We find two sets of model parameters that lead to good fits of the SED. One of the models fits all the observations, including the MeV tail. This model also predicts hadronic gamma-ray emission slightly below the current upper limits. The flux predicted at 8.4 GHz is in agreement with the observations available in the literature, although the synthetic source is more compact than the imaged radio jet. Our results show that the MeV emission in Cygnus X-1 may be jet synchrotron radiation. This depends mainly on the strength of the jet magnetic field and the location of the injection region of the relativistic particles. Our calculations show that there must be energetic electrons in the jets quite far from the black hole.

Modelling the high-energy emission from gamma-ray binaries using numerical relativistic hydrodynamics

Detailed modeling of the high-energy emission from gamma-ray binaries has been propounded as a path to pulsar wind physics. Fulfilling this ambition requires a coherent model of the flow and its emission in the region where the pulsar wind interacts with the stellar wind of its companion. We developed a code that follows the evolution and emission of electrons in the shocked pulsar wind based on inputs from a relativistic hydrodynamical simulation. The code is used to model the well-documented spectral energy distribution and orbital modulations from LS 5039. The pulsar wind is fully confined by a bow shock and a back shock. The particles are distributed into a narrow Maxwellian, emitting mostly GeV photons, and a power law radiating very efficiently over a broad energy range from X-rays to TeV gamma rays. Most of the emission arises from the apex of the bow shock. Doppler boosting shapes the X-ray and VHE lightcurves, constraining the system inclination to $i\approx 35^{\rm o}$. There is a tension between the hard VHE spectrum and the level of X-ray to MeV emission, which requires differing magnetic field intensities that are hard to achieve with a constant magnetisation $\sigma$ and Lorentz factor $\Gamma_{p}$ of the pulsar wind. Our best compromise implies $\sigma\approx 1$ and $\Gamma_{p}\approx 5\times 10^{3}$, respectively higher and lower than the typical values in pulsar wind nebulae. The high value of $\sigma$ derived here, where the wind is confined close to the pulsar, supports the classical picture that has pulsar winds highly magnetised at launch. However, such magnetisations will require further investigations to be based on relativistic MHD simulations.

Estimate of (n,p) Cross Section for Radionuclide 55Fe Using EMPIRE and TALYS

The radionuclide 55Fe (t1/2 = 2.73 years) is one of the radionuclides produced in large quantities inside a fusion reactor. The excitation function of the (n,p) reaction from threshold to 20 MeV and proton emission spectra from the 55Fe target at 14-MeV neutron energy are calculated using optimized input parameters in the nuclear reaction modular codes EMPIRE-3.1 and TALYS-1.4. The codes account for the major nuclear reaction mechanisms, including direct, preequilibrium, and compound nucleus contributions. The present results of 55Fe(n,p)55Mn are compared with the existing evaluated nuclear data libraries ROSFOND-2010, JEFF-3.1, and EAF-2010 along with systematics around 14-MeV neutron energy. The prediction accuracy of the present calculation is considered to satisfy the requirement for fusion reactor applications. The theoretical nuclear model calculations with a reliable parameter set up to 20 MeV are recommended to estimate the cross section of radionuclides or unstable targets in the mass region A ˜ 50 to 60. The present work is an important step to study the cross section of the 55Fe(n,p)55Mn reaction by a surrogate method.

Double Differential Alpha, Proton and Deuteron Emission Cross Section Calculations for the Structural Fusion Materials 46,48Ti

Double differential light charged particle emission cross section is a fundamental value to determine nuclear heating and material damages in structural fusion material development. In this study, double differential alpha, proton and deuteron emission cross sections for 46Ti and 48Ti target nuclei have been calculated by the TALYS 1.6 code at 15 MeV incident neutron energy. The light charged particles are emitted due to mainly three different processes—compound, direct and pre-equilibrium. The compound nucleus formation process dominates the emission of light charged particle up to 8 MeV emission energy. Pre-equilibrium and direct reaction contribution become dominant in higher particle emission energies. The calculated double differential cross sections have been compared with the available experimental data taken from the literature.

Populations of sublevels 24Mg(2+) nucleus formed in the reaction 24Mg(d, d1γ)24Mg at E d = 15.3 MeV

Angular dependences of density matrix spin tensors and sublevel populations are experimentally reconstructed for the 24Mg(2+, 1.369 MeV) nucleus formed in the inelastic scattering of deuterons by 24Mg at Ed = 15.3 MeV and deuteron emission angles from 25° to 165° (lab). The experimental results are compared to those from calculations using different versions of the coupled-channel method.

Systematic search for high-energy gamma-ray emission from bow shocks of runaway stars

Context. It has been suggested that the bow shocks of runaway stars are sources of high-energy gamma rays (E > 100 MeV). Theoretical models predicting high-energy gamma-ray emission from these sources were followed by the first detection of non-thermal radio emission from the bow shock of BD+43$^\deg$ 3654 and non-thermal X-ray emission from the bow shock of AE Aurigae. Aims. We perform the first systematic search for MeV and GeV emission from 27 bow shocks of runaway stars using data collected by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi). Methods. We analysed 57 months of Fermi-LAT data at the positions of 27 bow shocks of runaway stars extracted from the Extensive stellar BOw Shock Survey catalogue (E-BOSS). A likelihood analysis was performed to search for gamma-ray emission that is not compatible with diffuse background or emission from neighbouring sources and that could be associated with the bow shocks. Results. None of the bow shock candidates is detected significantly in the Fermi-LAT energy range. We therefore present upper limits on the high-energy emission in the energy range from 100 MeV to 300 GeV for 27 bow shocks of runaway stars in four energy bands. For the three cases where models of the high-energy emission are published we compare our upper limits to the modelled spectra. Our limits exclude the model predictions for Zeta Ophiuchi by a factor $\approx$ 5.

LS 5039 – the counterpart of the unidentified MeV source GRO J1823-12

The COMPTEL experiment on CGRO observed the gamma-ray sky at energies from 0.75 MeV to 30 MeV between April 1991 and June 2000. COMPTEL detected many gamma-ray sources, among them an unidentified one labeled GRO J1823-12, which is positionally consistent with the prominent high-mass X-ray binary LS 5039. Because LS 5039 was established as gamma-ray emitter during recent years, whose gamma-radiation radiation is modulated along its binary orbit, we reanalysed the COMPTEL data of GRO J1823-12 including an orbital resolved analysis. We find a significant MeV source, showing evidence for a modulated MeV flux corresponding to the orbital period of LS 5039 of about 3.9 days. We show that its MeV emission is stronger at the orbital part around the inferior conjuction than at the part of the superior conjunction, being in phase with X-rays and TeV gamma-rays, however being in anti-phase with GeV gamma-rays. We conclude that the COMPTEL source GRO J1823-12 is the counterpart of the microquasar candidate LS 5039, at least for the majority of its MeV emission. The COMPTEL fluxes, put into multifrequency perspective, provide new constraints on the modelling of the high-energy emission of LS 5039.

THE STAR-FORMING GALAXY CONTRIBUTION TO THE COSMIC MeV AND GeV GAMMA-RAY BACKGROUND

While star-forming galaxies could be major contributors to the cosmic GeV $\gamma$-ray background, they are expected to be MeV-dim because of the "pion bump" falling off below ~100 MeV. However, there are very few observations of galaxies in the MeV range, and other emission processes could be present. We investigate the MeV background from star-forming galaxies by running one-zone models of cosmic ray populations, including Inverse Compton and bremsstrahlung, as well as nuclear lines (including $^{26}$Al), emission from core-collapse supernovae, and positron annihilation emission, in addition to the pionic emission. We use the Milky Way and M82 as templates of normal and starburst galaxies, and compare our models to radio and GeV--TeV $\gamma$-ray data. We find that (1) higher gas densities in high-z normal galaxies lead to a strong pion bump, (2) starbursts may have significant MeV emission if their magnetic field strengths are low, and (3) cascades can contribute to the MeV emission of starbursts if they emit mainly hadronic $\gamma$-rays. Our fiducial model predicts that most of the unresolved GeV background is from star-forming galaxies, but this prediction is uncertain by an order of magnitude. About ~2% of the claimed 1 MeV background is diffuse emission from star-forming galaxies; we place a firm upper limit of <~10% based on the spectral shape of the background. The star-formation contribution is constrained to be small, because its spectrum is peaked, while the observed background is steeply falling with energy through the MeV-GeV range.

THE VERY BRIGHT AND NEARBY GRB130427A: THE EXTRA HARD SPECTRAL COMPONENT AND IMPLICATIONS FOR VERY HIGH-ENERGY GAMMA-RAY OBSERVATIONS OF GAMMA-RAY BURSTS

The extended high-energy gamma-ray (&gt;100 MeV) emission occurring after the prompt gamma-ray bursts (GRBs) is usually characterized by a single power-law spectrum, which has been explained as the afterglow synchrotron radiation. We report on the Fermi Large Area Telescope (LAT) observations of the &gt;100 MeV emission from the very bright and nearby GRB 130427A, up to ~100 GeV. By performing time-resolved spectral fits of GRB 130427A, we found a strong evidence of an extra hard spectral component above a few GeV that exists in the extended high-energy emission of this GRB. This extra spectral component may represent the first clear evidence of the long sought-after afterglow inverse Compton emission. Prospects for observations at the very high-energy gamma-rays, i.e., above 100 GeV, are described.

A LINGERING NON-THERMAL COMPONENT IN THE GAMMA-RAY BURST PROMPT EMISSION: PREDICTING GeV EMISSION FROM THE MeV SPECTRUM

The high energy GeV emission of gamma-ray bursts (GRBs), detected by \emph{Fermi}/LAT, has a significantly different morphology compared to the lower energy MeV emission, detected by \emph{Fermi}/GBM. Though the late time GeV emission is believed to be synchrotron radiation produced via an external shock, this emission as early as the prompt phase is puzzling. Meaningful connection between these two emissions can be drawn only by an accurate description of the prompt MeV spectrum. We perform a time-resolved spectroscopy of the GBM data of long GRBs having significant GeV emission, using a model consisting of 2 blackbodies and a power-law. We examine in detail the evolution of the spectral components and found that GRBs having high GeV emission (GRB 090902B and GRB 090926A) have a delayed onset of the power-law component, in the GBM spectrum, which lingers at the later part of the prompt emission. This behaviour mimics the flux evolution in LAT. In contrast, bright GBM GRBs with an order of magnitude lower GeV emission (GRB 100724B and GRB 091003) show a coupled variability of the total and the power-law flux. Further, by analyzing the data for a set of 17 GRBs, we find a strong correlation between the power-law fluence in the MeV and the LAT fluence (Pearson correlation: r=0.88 and Spearman correlation: $\rho=0.81$). We demonstrate that this correlation is not influenced by the correlation between the total and the power-law fluences at a confidence level of 2.3$\sigma$. We speculate the possible radiation mechanisms responsible for the correlation.

GRB 090510: A DISGUISED SHORT GAMMA-RAY BURST WITH THE HIGHEST LORENTZ FACTOR AND CIRCUMBURST MEDIUM

GRB 090510, observed both by Fermi and AGILE satellites, is the first bright short-hard Gamma-Ray Burst (GRB) with an emission from the keV up to the GeV energy range. Within the Fireshell model, we interpret the faint precursor in the light curve as the emission at the transparency of the expanding e+e- plasma: the Proper-GRB (P-GRB). From the observed isotropic energy we assume a total energy Ee+e-=(1.10+-0.06)*10^53 erg and derive a Baryon load B=(1.45+-0.28)*10^(-3) and a Lorentz factor at transparency Gamma=(6.7+-1.6)*10^2. The main emission 0.4 s after the initial spike is interpreted as the extended afterglow, due to the interaction of the ultrarelativistic baryons with the CircumBurst Medium (CBM). Using the condition of fully radiative regime, we infer a CBM average spherically symmetric density of nCBM=(1.85+-0.14)10^3 cm^(-3), one of the highest found in the Fireshell model. The value of the filling factor, 1.5*10^(-10)<R<3.8*10^(-8), leads to the estimate of filaments with densities nfil=nCBM/R=(10^6 - 10^14) cm^(-3). The sub-MeV and the MeV emissions are well reproduced. When compared to the canonical GRBs with nCBM=1 cm^(-3) and to the disguised short GRBs with nCBM=10^(-3) cm^(-3), the case of GRB 090510 leads to the existence of a new family of bursts exploding in an over-dense galactic region with hn nCBM=10^3 cm^(-3). The joint effect of the high Lorentz factor and the high density compresses in time and "inflates" in intensity the extended afterglow, making it appear as a short burst, which we here define as "disguised short GRB by excess". The determination of the above parameters values may represent an important step towards the explanation of the GeV emission.

DISCOVERY OF AN EXTRA HARD SPECTRAL COMPONENT IN THE HIGH-ENERGY AFTERGLOW EMISSION OF GRB 130427A

The extended high-energy gamma-ray (>100 MeV) emission occurred after the prompt gamma-ray bursts (GRBs) is usually characterized by a single power-law spectrum, which has been explained as the afterglow synchrotron radiation. The afterglow inverse-Compton emission has long been predicted to be able to produce a high-energy component as well, but previous observations have not revealed such a signature clearly, probably due to the small number of >10 GeV photons even for the brightest GRBs known so far. In this Letter, we report on the Fermi Large Area Telescope (LAT) observations of the >100 MeV emission from the very bright and nearby GRB 130427A. We characterize the time-resolved spectra of the GeV emission from the GRB onset to the afterglow phase. By performing time-resolved spectral fits of GRB 130427A, we found a strong evidence of an extra hard spectral component that exists in the extended high-energy emission of this GRB. We argue that this hard component may arise from the afterglow inverse Compton emission.

SINGLE- AND TWO-COMPONENT GAMMA-RAY BURST SPECTRA IN THEFERMIGBM-LAT ENERGY RANGE

Most Fermi GRB spectra appear as either a broken power law extending to GeV energies or as a broken power with a separate GeV power law component. Here we show that such spectra can be understood in terms of magnetically dominated relativistic jets where a dissipative photosphere produces the prompt MeV emission, which is extended into the GeV range by inverse Compton scattering in the external shock, with possible contributions from a reverse shock as well. The bulk Lorentz factors required in these models are in the range of 300-600, and the MeV-GeV time delays arise naturally. In some cases an optical flash and a sub-dominant thermal component are also present.

Positron annihilation as a cosmic ray probe

ABSTRACT Recently, the gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma-rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic processes). However, observations do not rule out contributions from bremsstrahlung emission due to CR electrons. TeV gamma-ray telescopes also observed many SNRs and discovered many unidentified sources. It is still unclear whether the TeV gamma-ray emission is produced via leptonic processes or hadronic processes. In this Letter, we propose that annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma-ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, synchrotron emission from secondary positrons and electrons and bremsstrahlung emission from CR protons can be also observed by the future X-ray telescope NuSTAR and Astro-H.

FERMIDETECTION OF γ-RAY EMISSION FROM THE M2 SOFT X-RAY FLARE ON 2010 JUNE 12

The GOES M2-class solar flare, SOL2010-06-12T00:57, was modest in many respects yet exhibited remarkable acceleration of energetic particles. The flare produced an ~50 s impulsive burst of hard X- and \gamma-ray emission up to at least 400 MeV observed by the Fermi GBM and LAT experiments. The remarkably similar hard X-ray and high-energy \gamma-ray time profiles suggest that most of the particles were accelerated to energies >300 MeV with a delay of ~10 s from mildly relativistic electrons, but some reached these energies in as little as ~3 s. The \gamma-ray line fluence from this flare was about ten times higher than that typically observed from this modest GOES class of X-ray flare. There is no evidence for time-extended >100 MeV emission as has been found for other flares with high-energy \gamma rays.

Do Fermi Large Area Telescope observations imply very large Lorentz factors in gamma-ray burst outflows?

Recent detections of GeV photons in a few gamma-ray bursts (GRBs) by Fermi Large Area Telescope (LAT) imply huge bulk Lorentz factors to avoid a large γγ optical depth at high energy. Estimates can be as high as Γ≃ 103 in the most extreme cases. This puts severe constraints on models of the central engine and the jet acceleration in GRBs. These estimates are, however, obtained from a simplified single-zone model. We present here a more realistic calculation of the γγ opacity which takes into account the time, space and direction dependent photon field existing in an outflow with several relativistically moving emitting zones. The formalism is very general and can be applied to many models of the prompt GRB emission. We present results obtained for a numerical implementation in the framework of the internal shock model. We show the following results in this paper. (i) The minimum Lorentz factor Γmin in bright Fermi-LAT GRBs is reduced by a factor of ≃2–3 compared to previous estimates if the GeV and MeV emission are produced in the same region, and by an additional factor of ≃2–8 if the GeV emission is produced at larger radii. We provide an improved approximate formula for Γmin which is in good agreement with our numerical results and which can be directly applied to Fermi-LAT GRB data. (ii) A delayed onset of the GeV emission can be due to the time evolution of the opacity in a GRB outflow. As an illustration of these first two results, we present a synthetic GRB that reproduces most features of GRB 080916C with a mean Lorentz factor of ≃340, an optically thin regime for γγ opacity at 3 GeV in time bin ‘b’, a variability time-scale of ≃0.5 s in the MeV light curve and a delayed onset of ≃5 s of the GeV emission. (iii) The γγ opacity can smooth the short time-scale variability in the GeV light curve. This last result implies that the observed variability at high energy is not necessarily a good test to distinguish between an internal and an external origin for the GeV emission in GRBs.

THE EMISSION OF CYGNUS X-1: OBSERVATIONS WITHINTEGRALSPI FROM 20 keV TO 2 MeV

We report on Cyg X-1 observations performed by the SPI telescope onboard the INTEGRAL mission and distributed over more than 6 years. We investigate the variability of the intensity and spectral shape of this peculiar source in the hard X-rays domain, and more particularly up to the MeV region. We first study the total averaged spectrum which presents the best signal to noise ratio (4 Ms of data). Then, we refine our results by building mean spectra by periods and gathering those of similar hardness. Several spectral shapes are observed with important changes in the curvature between 20 and 200 keV, even at the same luminosity level. In all cases, the emission decreases sharply above 700 keV, with flux values above 1 MeV (or upper limits) well below the recently reported polarised flux (Laurent et al. 2011), while compatible with the MeV emission detected some years ago by CGRO/COMPTEL (McConnell et al., 2002). Finally, we take advantage of the spectroscopic capability of the instrument to seek for spectral features in the 500 keV region with negative results for any significant annihilation emission on 2 ks and days timescales, as well as in the total dataset.

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.

Photosphere-internal shock model of gamma-ray bursts: case studies of Fermi/LAT bursts

Radially inhomogeneous gamma-ray burst (GRB) jets release variable photospheric emission and can have internal shocks occurring above the photosphere. We generically formulate a photospheric emission model of GRBs including Compton up-scattered photospheric (UP) emission off the electrons (and positrons) in the internal shocks, and find that the photospheric emission may correspond to the traditional (band) component at ≲1 MeV and the UP emission to the high-energy emission observed by Fermi/LAT for some GRBs at ≳10 MeV. The two components can be separate from each other in the spectrum in some cases or can mimic a smooth broad-band spectrum in other cases. We apply our formulation to the well-studied long and short LAT GRBs, GRB 080916C, GRB 090902B and GRB 090510, and typically find reasonable parameters for fitting the time-binned spectra, although fine-tuning of several parameters is required. The observed delays of the high-energy emission with respect to the MeV emission which are large compared to the variability times are unlikely to be due to simple kinematic effects of a non-evolving jet. These delays may instead be attributed to the temporal evolution of the physical parameters of the jet, and thus the delay time-scales could provide a potential tool for investigating the structures of GRB jets themselves and their progenitors. The difference of the delay time-scales of long and short GRBs inferred from the Fermi data might be due to the differences in the progenitors of long and short GRBs. Some other properties and consequences of this model are discussed, including temporal correlations among the prompt optical, the soft X-ray and the distinct high-energy component as well as the band component.

The upstream magnetic field of collisionless GRB shocks: constraint by Fermi-LAT observations

Long-lived > 100 MeV emission has been a common feature of mostFermi-LAT detected gamma-ray bursts (GRBs), e.g., detected up to ∼ 103s in long GRBs 080916C and 090902B and ∼ 102s inshort GRB 090510. This emission is consistent with being producedby synchrotron emission of electrons accelerated to high energy bythe relativistic collisionless shock propagating into the weaklymagnetized medium. Here we show that this high-energy afterglowemission constrains the preshock magnetic field to satisfy100n09/8mG < Bu < 102n03/8mG, where n0 is thepreshock density in unit of 1 cm−3, more stringent thanthe previous constraint by X-ray afterglow observations on dayscale. This suggests that the preshock magnetic field is stronglyamplified, most likely by the streaming of high energy shockaccelerated particles.