Jeffrey G. Skibo

Gamma-Ray Spectral States of Galactic Black Hole Candidates

OSSE has observed seven transient black hole candidates: GRO J0422+32, GX 339-4, GRS 1716-249, GRS 1009-45, 4U 1543-47, GRO J1655-40, and GRS 1915+105. Two gamma-ray spectral states are evident, and based on a limited number of contemporaneous X-ray and gamma-ray observations, these states appear to be correlated with X-ray states. The former three objects show hard spectra below 100 keV (photon number indices Γ < 2) that are exponentially cut off with folding energy ~100 keV, a spectral form that is consistent with thermal Comptonization. This breaking gamma-ray state is the high-energy extension of the X-ray low, hard state. In this state, the majority of the luminosity is above the X-ray band, carried by photons of energy ~100 keV. The latter four objects exhibit a power-law gamma-ray state, with a relatively soft spectral index (Γ ~ 2.5-3) and no evidence for a spectral break. For GRO J1655-40, the lower limit on the break energy is 690 keV. GRS 1716-249 exhibits both spectral states, with the power-law state having significantly lower gamma-ray luminosity. The power-law gamma-ray state is associated with the presence of a strong ultrasoft X-ray excess (kT ~ 1 keV), the signature of the X-ray high, soft (or perhaps very high) state. The physical process responsible for the unbroken power law is not well understood, although the spectra are consistent with bulk-motion Comptonization in the convergent accretion flow.

Temporal Evolution of Nonthermal Spectra from Supernova Remnants

Assuming that supernova shocks accelerate nonthermal particles, we model the temporally evolving nonthermal particle and photon spectra at different stages in the lifetime of a standard shell-type supernova remnant (SNR). A characteristic νFν spectrum of an SNR consists of a peak at radio through optical energies from nonthermal electron synchrotron emission and another high-energy gamma-ray peak due primarily to secondary pion production, nonthermal electron bremsstrahlung, and Compton scattering. We find that supernova remnants are capable of producing maximum gamma-ray luminosities 1035 ergs s-1 if the density of the local interstellar medium is 10 cm-3. This emission will persist for 105 yr after the supernova explosion because of the long energy loss timescales for electrons with kinetic energy ~1 GeV. This long gamma-ray lifetime implies that SNRs with a wide range of ages could be gamma-ray sources and could constitute some of the unidentified EGRET sources.

OSSE Mapping of Galactic 511 keV Positron Annihilation Line Emission

The Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Galactic plane and Galactic center region have been combined with scanning observations by the Transient Gamma-Ray Spectrometer (TGRS) and Solar Maximum Mission (SMM) instruments to produce maps of the Galactic narrow 511 keV positron annihilation line radiation. Two different mapping methods, singular value decomposition and maximum entropy, have been applied to the data. In both cases, the resulting maps show evidence for three distinct features: (1) a central bulge, (2) emission in the Galactic plane, and (3) an enhancement or extension of emission at positive latitudes above the Galactic center. Modeling of the data confirmed the existence of these features. The derived distribution is found to be in good qualitative agreement with nearly all of the historical observations of narrow 511 keV line emission from the Galactic center region. No evidence of time variability is found. Various possible production mechanisms for the observed positrons, including the positive-latitude enhancement, are presented. It is found that supernovae are capable of producing positrons at the required rate to account for the intensity and morphology of the observed 511 keV line emission.

The Physics of Positron Annihilation in the Solar Atmosphere

We compile the most recently measured and calculated cross sections for processes relevant to annihilation of positrons in solar flares and present the final evaluated cross sections. We calculate the 511 keV annihilation line spectrum, the number of 2γ line photons produced per positron, and the relative strength of the 3γ continuum for each of the annihilation processes, using a Monte Carlo code that simulates the thermalization of and positronium production by ~MeV positrons. We calculate the thermally averaged annihilation and positronium production rates and the positronium quenching rates. We apply the results to four specific environments (fully ionized, neutral, partially ionized, and a non-local thermodynamic equilibrium model of the quiet solar atmosphere), calculating the relative strengths of each process and the combined total annihilation line spectrum. The results are compared with data obtained recently from the high spectral resolution detectors of RHESSI. We find that positron annihilation in solar flares can occur in a wide variety of environments. The cross sections presented here are also useful for evaluation of positron annihilation in other environments such as the interstellar medium.

High-Resolution Observation of the Solar Positron-Electron Annihilation Line

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with annihilation in two vastly different solar environments. It can be produced by formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ~6000 K. However, the measured upper limit to the 3γ/2γ ratio (ratio of annihilation photons in the positronium continuum to the number in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fitted by a thermal Gaussian having a width of 8.1 ± 1.1 keV (FWHM), indicating temperatures of ~(4-7) × 105 K. The measured 3γ/2γ ratio does not constrain the density when the annihilation takes place in such an ionized medium, although the density must be high enough to slow down the positrons. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during the flare.

IS THE HIGH-ENERGY EMISSION FROM CENTAURUS A COMPTON-SCATTERED JET RADIATION ?

Abstract : We consider whether the hard X-ray and soft gamma-ray emission from Centaurus A is beamed radiation from the active nucleus which is Compton-scattered into our line-of-sight. We derive the spectrum and degree of polarization of scattered radiation when incident beamed radiation is scattered from a cold (kappa Tau << mec2) electron cloud moving with bulk relativistic motion along the jet axis, and calculate results for an unpolarized, highly-beamed incident power-law photon source. The spectra of the scattered radiation exhibit a cut-off at gamma-ray energies due to electron recoil. The cut off energy depends on the observers viewing angle and the bulk Lorentz factor of the scattering medium. We fit the OSSE data from Centaurus A with this model and find that if the scatterers are not moving relativistically, then the angle the jet makes with respect to our line-of-sight is 61 degrees plus or minus 5 degrees. We predict a high degree of polarization of the scattered radiation below ~300 keV. Future measurements with X-ray and gamma-ray polarimeters could be used to constrain or rule out such a scenario.

Spallation of Iron in Black Hole Accretion Flows

In the local Galactic interstellar medium there is approximate energy equipartition between cosmic rays, magnetic fields, and radiation. If this holds in the central regions of active galactic nuclei (AGNs), in particular Seyfert galaxies, then considerable nuclear spallation of Fe occurs, resulting in enhanced abundances of the sub-Fe elements Ti, V, Cr, and Mn. These elements produce a cluster of X-ray fluorescence lines at energies just below the 6.4 keV Fe Kα line. It is suggested that the red wings on the Fe lines observed with ASCA from various Seyfert AGNs are due to the unresolved line emission from these elements. Future observations with more sensitive X-ray instruments should resolve these lines. The estimated gamma-ray emission from nuclear de-excitation and neutral pion production is calculated and found to be below the sensitivities of any current instruments. However, very luminous nearby Seyferts displaying Fe lines with red wings could have >100 MeV continuum emissions detectable by future instruments such as the Gamma-Ray Large Area Space Telescope.

A Maximum Entropy Map of the 511 keV Positron Annihilation Line Emission Distribution Near the Galactic Center

We have applied the maximum entropy method to a large data set based on Compton Gamma Ray Observatory (CGRO)/OSSE, Solar Maximum Mission, Transient Gamma-Ray Spectrometer, Gamma-Ray Imaging Spectrometer, HEXAGONE, and FIGARO observational results of the Galactic center 511 keV radiation in order to produce a map of the 511 keV line emission. This map suggests two components: a central bulge with a flux of 5.7 ± 0.29 × 10-4 photons cm-2 s-1 and a Galactic plane (GP) component with a flux of 2.2 ± 0.4 × 10-4 photons cm-2 s-1. The central bulge is located at l = -047 ± 024, b = 01 ± 018, and FWHM = 528 ± 048. We note that the position of the GP component coincides with a strong hot spot in the COMPTEL map of 1.8 MeV26Al line emission. A comparison between CGRO/OSSE and other instruments with larger field of view suggests an extended diffuse 511 keV emission. An interesting hot spot at l = -4°, b = 7° with a flux of ~2 × 10-4 photons cm-2 s-1 is shown on our map. A bootstrap test indicates that the significance level of this feature is 3.5 σ.

OSSE Observations of the Soft Gamma-Ray Continuum from the Galactic Plane at Longitude 95°

We present the results of OSSE observations of the soft gamma-ray continuum emission from the Galactic plane at longitude 95°. Emission is detected between 50 and 600 keV where the spectrum is fitted well by a power law with photon index -2.6 ± 0.3 and flux (4.0 ± 0.5) × 10-2 photons s-1 cm-2 rad-1 MeV-1 at 100 keV. This spectral shape in this range is similar to that found for the continuum emission from the inner Galaxy, but the amplitude is lower by a factor of 4. This emission is due to either unresolved and previously unknown point sources, or diffuse electron bremsstrahlung, or a combination of the two. Simultaneous observations with OSSE and smaller field-of-view instruments operating in the soft gamma-ray energy band, such as X-ray Timing Explorer or Beppo-SAX, would help resolve this issue. If it is primarily diffuse emission due to nonthermal electron bremsstrahlung, as is the >1 MeV Galactic ridge continuum, then the power in low-energy cosmic-ray electrons exceeds that of the nuclear component of the cosmic rays by an order of magnitude. This would have profound implications for the origin of cosmic rays and the energetics of the interstellar medium. Alternatively, if the emission is diffuse and thermal, then there must be a component of the interstellar medium at temperatures ~109 K.

Annihilation Fountain in the Galactic Center Region

Two different model-independent mapping techniques have been applied to Compton Gamma Ray Observatory OSSE, SMM, TGRS, and balloon data and reveal a feature in the 0.511 MeV e+-e- annihilation radiation pattern of our Galaxy centered at l ~ -2° and b ~ 10° with a flux of approximately 5 × 10-4 0.511 MeV photons cm-2 s-1. If near the Galactic center, then positron (e+) sources are producing about 1042 e+ s-1, which annihilate ≈ 1-2 kpc above the Galactic plane. A starburst episode within the inner few hundred parsecs of our Galaxy would drive hot pair-laden gas into the halo, with the one-sidedness pointing to the site of initial pressure release at the onset of the starburst activity. Positrons lose energy and annihilate as they are convected upward with the gas flow, and we calculate high-latitude annihilation patterns and fluxes in accord with the observations. Changes in the ionization state when the escaping gas cools could give annihilation radiation substructure. The fountain of hot (~106-107 K) gas rising into the Galactic halo would be seen through its enhanced dispersion measure, thermal emission, and recombination radiation.