Compton Scattering in Jets: A Mechanism for ∼\sim 0.4 and \lsim 0.2 Mev Line Production

We show that gamma ray line emission at $\sim$0.4 MeV and \lsim 0.2 MeV can be produced by Compton scattering of beamed radiation in the jets of Galactic black hole candidates. This mechanism has the novel feature of not invoking the presence of $e^+$--$e^-$ pairs. To produce the two lines we employ a symmetric double sided jet with bulk flow velocity of about 0.5 c and incident beam radiation with a hard energy spectrum. We show that the two lines can be seen at viewing angle cosines relative to the jet ranging from 0.2 to 0.6. This comprises 40\% of the total solid angle. In addition, the line radiation is approximately 10\% polarized. Depending on the bulk flow and viewing angle the model can produce lines at other energies as well. In particular a broad feature near 1 MeV can be seen by viewing the jet close to its axis. Our model can also accommodate single line spectra if the beamed gamma ray emission or the jets themselves are asymmetric.Comment: 10 pages, uuencoded compressed postscript with figures, NRL94-05-0

Is the High-Energy Emission from Centaurus A Compton-Scattered Jet Radiation?

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 ($kT<<m_ec^2$) electron cloud moving with bulk relativistic motion along the jet axis, and calculate results for an unpolarized, highly-beamed incident power-law photon source. 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 $61^\circ\pm 5^\circ$. We predict a high degree of polarization of the scattered radiation below $\sim300$ keV. Future measurements with X-ray and gamma-ray polarimeters could be used to constrain or rule out such a scenario.Comment: 12 pages, Postscript file with 3 Figures, NRL 017-331-09

Thermal Comptonization in Mildly Relativistic Pair Plasmas

We use a Monte Carlo simulation to calculate the spectra of mildly relativistic thermal plasmas in pair balance. We use the exact integral expression for the electron-positron thermal annihilation spectrum, and provide accurate expressions for the Gaunt factors of electron-ion, electron-electron, and electron-positron thermal bremsstrahlung in the transrelativistic temperature regime. The particles are assumed to be uniformly distributed throughout a sphere, and the pair opacity is self-consistently calculated from the energy and angular distribution of scattered photons. The resultant photon spectra are compared with the nonrelativistic diffusion treatment of Sunyaev and Titarchuk, the bridging formulas of Zdziarski, and the relativistic corrections proposed by Titarchuk. We calculate allowed pair-balanced states of thermal plasmas with no pair escape which include bremsstrahlung and internal soft photons. The results are presented in the spectral index/compactness plane, and can be directly compared with observations of spectra from AGNs and Galactic black hole candidates. By comparing with X-ray spectral indices of Seyfert AGNs and compactnesses inferred from X-ray variability data, we find that the allowed solutions for pair equilibrium plasma imply that the temperatures of Seyfert galaxies are $\lesssim 300$ keV. This prediction can be tested with more sensitive gamma-ray observations of Seyfert galaxies. We find that if the X-ray variability time scale gives an accurate measure of the compactness, pair-dominated solutions are inconsistent with the data.Comment: 32 pages with 9 figures, compressed and uuencoded postscrip

The Beaming Pattern of Doppler Boosted Thermal Annihilation Radiation: Application to MeV Blazars

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei which Compton scatter photons from an external isotropic radiation field. Thus blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles theta > 1/Gamma, where Gamma is the bulk Lorentz factor of the outflowing plasma. This effect may account for the spectral features of MeV blazars discovered with the Compton Telescope on the Compton Gamma Ray Observatory. Coordinated gamma-ray observations of annihilation line radiation to infer Doppler factors and VLBI radio observations to measure transverse angular speeds of outflowing plasma blobs can be used to determine the Hubble constant.Comment: 15 pages including 3 figures, requires AAS Latex macros, accepted for publication in The Astrophysical Journa

Annihilation Fountain in the Galactic Center Region

Two different model-independent mapping techniques have been applied to OSSE, SMM, TGRS and balloon data and reveal a feature in the 0.511 MeV electron-positron annihilation radiation pattern of our galaxy centered roughly at l=-2 deg. and b=10 deg. with a flux of 5x10^(-4) 0.511 MeV ph/cm^2/s. If near the galactic center, then positron sources are producing approximately 10^42 positrons/s which annihilate 1-2 kpc above the galactic plane. A starburst episode within the inner few hundred pc 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 (10^6-10^7 K) gas rising into the galactic halo would be seen through its enhanced dispersion measure, thermal emission, and recombination radiation.Comment: 11 pages, Latex, requires AASTEX macros and psfig.tex, 2 postscript figures, Submitted to Astrophysical Journal Letter

High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres

We show that Compton scattering of soft photons with energies near 100 eV in thermally stratified black-hole accretion plasmas with temperatures in the range 100 keV - 1 MeV can give rise to an X-ray spectral hardening near 10 keV. This could produce the hardening observed in the X-ray spectra of black holes, which is generally attributed to reflection or partial covering of the incident continuum source by cold optically thick matter. In addition, we show that the presence of very hot (kT=1 MeV) cores in plasmas leads to spectra exibiting high energy tails similar to those observed from Galactic black-hole candidates.Comment: 11 pages, uuencoded gziped postscript, ApJ Letters in pres

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

We present the results of OSSE observations of the soft gamma ray continuum emission from the Galactic plane at longitude 95 degrees. Emission is detected between 50 and 600 keV where the spectrum is fit well by a power law with photon index -2.6+-0.3 and flux (4.0+-0.5) 10^{-2} photons/s/cm^2/rad/MeV 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 four. This emission is either due to unresolved and previously unknown point sources or it is of diffuse origin, 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 XTE or 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 near 10^9 K.Comment: 11 pages, Latex, requires AASTEX macros and psfig.tex, 2 postscript figures, Accepted for publication in the Astrophysical Journal Letter

INTEGRAL and Swift/XRT observations of the source PKS 0208-512

The active galaxy PKS 0208-512, detected at lower energies by COMPTEL, has been claimed to be a MeV blazar from EGRET. We report on the most recent INTEGRAL observations of the blazar PKS 0208-512, which are supplemented by Swift ToO observations. The high energy X-ray and gamma-ray emission of PKS 0208-512 during August - December 2008 has been studied using 682 ks of INTEGRAL guest observer time and ~ 56 ks of Swift/XRT observations. These data were collected during the decay of a gamma-ray flare observed by Fermi/LAT. At X-ray energies (0.2 - 10 keV) PKS 0208-512 is significantly detected by Swift/XRT, showing a power-law spectrum with a photon index of ~ 1.64. Its X-ray luminosity varied by roughly 30% during one month. At hard X-/soft gamma-ray energies PKS 0208-512 shows a marginally significant (~ 3.2 sigma) emission in the 0.5-1 MeV band when combining all INTEGRAL/SPI data. Non-detections at energies below and above this band by INTEGRAL/SPI may indicate intrinsic excess emission. If this possible excess is produced by the blazar, one possible explanation could be that its jet consists of an abundant electron-positron plasma, which may lead to the emission of an annihilation radiation feature. Assuming this scenario, we estimate physical parameters of the jet of PKS 0208-512.Comment: accepted for publication in A&

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 AGN, in particular Seyfert galaxies, then consideral 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 flourescence lines at energies just below the 6.4 keV Fe-K$\alpha$ line. It is suggested that the red wings on the Fe lines observed with ASCA from various Seyfert AGN 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 deexcitation 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 would have $>100$ MeV continuum emission detectable by future instruments such as GLAST.Comment: 11 pages, Latex, requires AASTEX macros, 5 postscript figures, Astrophysical Journal in pres

C II Radiative Cooling of the Diffuse Gas in the Milky Way

The heating and cooling of the interstellar medium allow the gas in the ISM to coexist at very different temperatures in thermal pressure equilibrium. The heating cannot be directly determined, but the cooling can be inferred from observations of C II*, which is an important coolant in different environments. The amount of cooling can be measured through either the intensity of the 157.7 \micron [C II] emission line or the C II* absorption lines at 1037.018 \AA\ and 1335.708 \AA, observable with FUSE and HST/STIS, respectively. We present the results of a survey of these far-UV absorption lines in 43 objects situated at |b|>30. We derive the cooling rates and analyze the ionization structure, the depletion, and metallicity content from the column densities of C II*, S II, P II, Fe II, and H I 21-cm emission for the low-, intermediate-, and high-velocity clouds (LVCs, IVCs, and HVCs) along the different sightlines. Based on the depletion and the ionization structure, the LVCs, IVCs, and HVCs consist mostly of warm neutral and ionized clouds. For the LVCs, the mean cooling rate in erg\,s^{-1} per H atom is -25.70^{+0.19}_{-0.36} dex. The corresponding total Galactic C II luminosity in the 157.7 \micron emission line is L~2.6x10^7 L_\sun. Combining N(C II*) with the intensity of H$\alpha$ emission, we derive that ~50% of the C II* radiative cooling comes from the warm ionized medium (WIM). The large dispersion in the cooling rates is certainly due to a combination of differences in the ionization fraction, in the dust-to-gas fraction, and physical conditions between sightlines. For the IVC IV Arch at z~1 kpc we find that on average the cooling is a factor 2 lower than in the LVCs that probe gas at lower z. For an HVC (Complex C, at z > 6 kpc) we find the much lower rate of -26.99^{+0.21}_{-0.53} dex. [Abridged]Comment: Accepted for publication in The Astrophysical Journal. Resolution of Figs. 9 & 12 has been reduced. Full resolution available on request ([email protected]