keV Line Emission Research Articles

PROBING THE ORIGINS OF THE C IV AND Fe Kα BALDWIN EFFECTS

We use UV/optical and X-ray observations of 272 radio-quiet Type 1 AGNs and quasars to investigate the CIV Baldwin Effect (BEff). The UV/optical spectra are drawn from the Hubble Space Telescope, International Ultraviolet Explorer and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra and XMM-Newton archives. We apply correlation and partial-correlation analyses to the equivalent widths, continuum monochromatic luminosities, and alpha_ox, which characterizes the relative X-ray to UV brightness. The equivalent width of the CIV 1549 emission line is correlated with both alpha_ox and luminosity. We find that by regressing l_UV with EW(CIV) and alpha_ox, we can obtain tighter correlations than by regressing l_UV with only EW(CIV). Both correlation and regression analyses imply that l_UV is not the only factor controlling the changes of EW(CIV); alpha_ox (or, equivalently, the soft X-ray emission) plays a fundamental role in the formation and variation of CIV. Variability contributes at least 60% of the scatter of the EW(CIV)-l_UV relation and at least 75% of the scatter of the of the EW(CIV)-alpha_ox relation. In our sample, narrow Fe Kalpha 6.4 keV emission lines are detected in 50 objects. Although narrow Fe Kalpha exhibits a BEff similar to that of CIV, its equivalent width has almost no dependence on either alpha_ox or EW(CIV). This suggests that the majority of narrow Fe Kalpha emission is unlikely to be produced in the broad emission-line region. We do find suggestive correlations between the emission-line luminosities of CIV and Fe Kalpha, which could be potentially used to estimate the detectability of the Fe Kalpha line of quasars from rest-frame UV spectroscopic observations.

THE GALACTIC POSITRON ANNIHILATION RADIATION AND THE PROPAGATION OF POSITRONS IN THE INTERSTELLAR MEDIUM

The ratio of the luminosity of diffuse 511 keV annihilation radiation, measured by INTEGRAL in its four years, from a Galactic positron ( 3.5 kpc), the cascade of the magnetic turbulence, which scatters the positrons, is damped by ion-neutral friction, allowing positrons to stream along magnetic flux tubes. We find that nearly 1/2 of the positrons produced in the disk escape from it into the halo. On the other hand, we show that within the extended, or interstellar, bulge (<3.5 kpc), essentially all of the positrons are born in the hot plasmas which fill that volume. We find that the diffusion mean free path is long enough that only a negligible fraction annihilate there and ~80% of them escape down into the H II and H I envelopes of molecular clouds that lie within 1.5 kpc before they slow down and annihilate, while the remaining ~20% escape out into the halo and the disk beyond. This propagation accounts for the low observed annihilation radiation luminosity of the disk compared to the bulge. In addition, we show that the primary annihilation sites of the propagating positrons in both the bulge and the disk are in the warm ionized phases of the interstellar medium. Such annihilation can also account for those two basic properties of the emission, the fraction (~93% ± 7%) of annihilation via positronium and the ratio (~0.5) of broad (~5.4 keV) to narrow (~1.3 keV) components of the bulge 511 keV line emission. Moreover, we expect that the bulk of this broad line emission comes from the tilted disk region (0.5 < R < 1.5 kpc) with a very large broad/narrow flux ratio of ~6, while much of the narrow line emission comes from the inner bulge (R < 0.5 kpc) with a negligible broad/narrow flux ratio. Separate spectral analyses of the 511 keV line emission from these two regions should be able to test this prediction, and further probe the structure of the interstellar medium. Lastly, we show that the asymmetry in the inner disk annihilation line flux, which has been suggested as added evidence for new sources, can also be fully understood from propagation and the asymmetry in the inner spiral arms as viewed from our solar perspective without any additional sources.

Radiation hardness studies of CdTe and [formula omitted] for the SIXS particle detector on-board the BepiColombo spacecraft

We report of the radiation hardness measurements that were performed in the developing work of a particle detector on-board ESA's forthcoming BepiColombo spacecraft. Two different high- Z semiconductor compounds, cadmium telluride (CdTe) and mercuric iodide (HgI 2), were irradiated with 22 MeV protons in four steps to attain the estimated total dose of 10 12 p / cm 2 for the mission time. The performance of the detectors was studied before and after every irradiation with radioactive 55Fe source Mn K α 5.9 keV emission line. We studied the impact of the proton beam exposure on detector leakage current, energy resolution and charge collection efficiency (CCE). Also the reconstructive effect of annealing on radiation induced damage was tested for CdTe detector.

Iron Emission Lines on the Galactic Ridge Observed with Suzaku

In order to elucidate the origin of galactic ridge X-ray emission, we analyzed Suzaku data taken at various regions along the galactic plane, and studied their Fe-K emission line features. Suzaku resolved the Fe line complex into three narrow lines at $\sim$ 6.4 keV, $\sim$ 6.7 keV and $\sim$ 6.97 keV, which are K-lines from neutral (or low-ionized), He-like, and H-like iron respectively. The 6.7 keV line is clearly seen in all of the observed regions, and its longitudinal distribution is consistent with that determined from previous observations. The 6.4 keV emission line was also found in various galactic plane regions ($b\sim0^{\circ}$). Differences in the flux ratios of the 6.4 keV$/$6.7 keV and 6.97 keV$/$6.7 keV lines between the galactic plane and the galactic center regions were studied and its implication was discussed.

A Celestial Gamma‐Ray Foreground Due to the Albedo of Small Solar System Bodies and a Remote Probe of the Interstellar Cosmic‐Ray Spectrum

We calculate the gamma-ray albedo flux from cosmic-ray (CR) interactions with the solid rock and ice in Main Belt asteroids (MBAs), Jovian and Neptunian Trojan asteroids, and Kuiper Belt objects (KBOs) using the Moon as a template. We show that the gamma-ray albedo for the Main Belt, Trojans, and Kuiper Belt strongly depends on the small-body size distribution of each system. Based on an analysis of the Energetic Gamma Ray Experiment Telescope (EGRET) data we infer that the diffuse emission from the MBAs, Trojans, and KBOs has an integrated flux of less than ~6x10^{-6} cm^{-2} s^{-1} (100-500 MeV), which corresponds to ~12 times the Lunar albedo, and may be detectable by the forthcoming Gamma Ray Large Area Space Telescope (GLAST). If detected by GLAST, it can provide unique direct information about the number of small bodies in each system that is difficult to assess by any other method. Additionally, the KBO albedo flux can be used to probe the spectrum of CR nuclei at close-to-interstellar conditions. The orbits of MBAs, Trojans, and KBOs are distributed near the ecliptic, which passes through the Galactic center and high Galactic latitudes. Therefore, the asteroid gamma-ray albedo has to be taken into account when analyzing weak gamma-ray sources close to the ecliptic, especially near the Galactic center and for signals at high Galactic latitudes, such as the extragalactic gamma-ray emission. The asteroid albedo spectrum also exhibits a 511 keV line due to secondary positrons annihilating in the rock. This may be an important and previously unrecognized celestial foreground for the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the Galactic 511 keV line emission including the direction of the Galactic center.

γ-RAYS FROM LIGHT DARK MATTER

The idea of light dark matter provides a plausible explanation for the observed 511 keV line emission from the galactic bulge. But the processes involved also emit the continuous spectrum of γ-rays. We present a detailed calculation of continuous spectrum for the decay model of light dark matter. Confrontation with the observed spectrum is challenging. We get the conservative mass bound mLDM ≲ 20 MeV for the decay model, and find that better understanding is needed for the diffuse γ-rays from the galactic bulge in the energy range 20 keV – 20 MeV.

SU-GG-T-218: Measurements of X-Ray Spectra Emergent From Low-Energy Photon- Emitting Brachytherapy Sources

Purpose: To investigate changes in the emergent spectra of low‐energy photon‐emitting brachytherapy sources as a function of angle in the plane of the source long axis. Method and Materials: X‐ray spectrometry was performed utilizing a high‐purity germanium detector, having a 1 cm diameter aperture, located at a distance of 177 cm from the center of the source. The source, with the long axis parallel to the floor of the laboratory, was mounted on a rotating vertical post. Emergent spectra were measured at rotation intervals that varied from 5 degrees to 15 degrees for a total of forty spectra per source. The intensity ratio of Kβ to Kαx rays from the decay of the radionuclide, the intensity ratio of Ag Kαfluorescencex rays to the 35 keV emission line from I‐125 sources that contained silver, and the air‐kerma strength were all calculated from the measuredspectra as a function of source rotation angle. Results: The degree of anisotropy about the axis perpendicular to the mid‐point of the long axis of the source was characterized by calculating the ratio of the air‐kerma strength of the source positioned perpendicular to and parallel to the detector face. Values of this “air‐anisotropy ratio”, αS, varied from 0.05 for a Pd‐103 source with thick end caps to about 1 for an I‐125 source with a uniform encapsulation thickness. Sources with low values of αS showed a significant increase in the Kβ‐to‐Kα intensity ratio for emissions from the seed ends. Conclusion: Variations in the designs of brachytherapy sources significantly influence their emergent spectra.Measuring the spectra around these sources has lead to an understanding of the response of well‐ionization chambers, used in therapy clinics to verify source air‐kerma strength prior to treatment, relative to the NIST Wide‐Angle Free‐Air Chamber (WAFAC) primary standard.

Correction of the charge sharing in photon-counting pixel detector data

Although photon-counting pixel detector arrays provide energy discrimination capabilities, their energy resolution is limited by the diffusion of the photoelectric charge between neighboring pixels. This effect often referred to as charge sharing induces a low-energy tail in the measured spectra and is particularly significant for small pixel sizes. In order to correct this effect, an iterative deconvolution method has been developed and applied to data collected with the Medipix2 photon-counting chip. The obtained spectra are corrected from charge sharing and moreover show a better energy resolution than the raw spectra. For instance the Kα (22.1 keV) and Kβ (25 keV) emission lines of a 109Cd source are reconstructed as two well-separated peaks. The principle of the method will be exposed. Various results obtained with synchrotron beams, X-ray tubes and radioactive sources will be shown.

Astrophysical and cosmological implications of large volume string compactifications

We study the spectrum, couplings and cosmological and astrophysical implications of themoduli fields for the class of Calabi–Yau IIB string compactifications for which modulistabilisation leads to an exponentially large volume and an intermediate string scalems∼1011 GeV, with TeV-scale observable supersymmetry breaking. All Kähler moduli except for theoverall volume are heavier than the susy (supersymmetry) breaking scale, withm∼ln(MP/m3/2)m3/2∼(ln(MP/m3/2))2msusy∼500 TeV and, contrary to standard expectations, have matter couplings suppressedonly by the string scale rather than the Planck scale. These decay tomatter early in the history of the universe, with a reheat temperatureT∼107 GeV, and are free from the cosmological moduli problem (CMP).The heavy moduli have a branching ratio to gravitino pairs of10−30 and do not suffer from the gravitino overproduction problem. Theoverall volume modulus is a distinctive feature of these models and is anMplanck-coupledscalar of mass m∼1 MeV and subject to the CMP. A period of thermal inflation may help relax this problem. This field has alifetime τ∼1024 s and can contribute to dark matter. It may be detected through its decays toγγ ore+e−. If accessiblethe e+e− decay mode dominates, with suppressed by a factor (ln(MP/m3/2))2. We consider the potential for detection of this field through different astrophysicalsources: the Milky Way halo, the diffuse cosmic background and nearby galaxy clusters andfind that the observed gamma ray background constrains . The decays of this field may generate the 511 keV emission line from the galactic centreobserved by INTEGRAL/SPI.

On the iron interpretation of the 6.4 keV emission line from SGR 1900+14

A 6.4 keV emission line was discovered in an unusual burst from the soft gamma repeater SGR 1900+14 with the Rossi X-ray Timing Explorer (RXTE). The line was detected in part of a complex multipeak precursor that preceded the unusual burst of 1998 August 29, i.e. two days after the giant flare of August 27 from the source. The origin of the line was not firmly identified and two possible interpretations were equally plausible including (a) Kα fluorescence from a small iron rich material that was ejected to the magnetosphere during the August 27 flare, and (b) proton or α-particle cyclotron resonance. If the iron scenario was correct, we expect to find evidence for the line during the intervening interval between the flare and the August 29 burst, i.e. on August 28. Here we present the results of the August 28 burst observation, taken with RXTE. We detect a total of seven bursts whose individual and joint spectra do not show evidence for spectral lines. We also investigated a sample of nine bursts before and after the August 29 burst (from 1998 June to December) that do not reveal evidence for a spectral line near 6.4 keV or elsewhere. These results disfavor the iron scenario and make the proton/α-particle cyclotron resonance interpretation more plausible. The appearance of the emission line in part of a complex burst and its absence from the studied sample indicate that the line is likely due to a transient phenomenon that may depend on the burst morphology, energetics and the properties of the emission region.

The Origin of Diffuse X‐Ray and γ‐Ray Emission from the Galactic Center Region: Cosmic‐Ray Particles

The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of synchrotron-emitting gas. Many of best studied sources in this region exhibit a mixture of 6.4 keV Fe Kα emission, molecular line emission, and nonthermal radio continuum radiation. The spatial correlation between fluorescent Fe Kα line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A*. Here we present a multiwavelength study of this region and find a correlation between nonthermal radio filaments and X-ray features. This correlation, when combined with distribution of molecular gas, suggests against irradiation model. Instead, we account for this distribution in terms of impact of relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both nonthermal bremsstrahlung X-ray continuum emission and diffuse 6.4 keV line emission. The production rate of Fe Kα photons associated with injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with synchrotron-emitting radio filaments or extended features is estimated to be in range between 20 and ~103 eV cm-3 for Sgr C, Sgr B1, Sgr B2, and the 45 and -30 km s-1 clouds. We also generalize this idea to explain cosmic-ray heating of molecular gas, interstellar cosmic-ray ionization, pervasive production of diffuse Kα line, and TeV emission from Galactic center molecular clouds. In particular, we suggest that inverse Compton scattering of submillimeter radiation from dust by relativistic electrons may contribute substantially to large-scale diffuse TeV emission observed toward central regions of Galaxy.

A Suzaku Observation of the Low-Ionization Fe-Line Emission from RCW 86

Abstract The newly operational X-ray satellite Suzaku observed the southwestern quadrant of the supernova remnant RCW 86 in 2006 February to study the nature of the 6.4 keV emission line first detected with the Advanced Satellite for Cosmology and Astronomy (ASCA). The new data confirm the existence of the line, localizing it for the first time; most of the line emission is adjacent and interior to the forward shock, and not at the locus of the continuum hard emission. We also report the first detection of a 7.1 keV line, which we interpret as $\mathrm{K}\beta$ emission from low-ionization iron. The Fe K line features are consistent with a non-equilibrium plasma of Fe-rich ejecta with $n_{\mathrm{e}} \, t \lesssim 10^{9} \,\mathrm{cm}^{-3} \,\mathrm{s}$ and $kT_{\mathrm{e}} \sim 5 \,\mathrm{keV}$. This combination of low $n_{\mathrm{e}} \, t$ and high $k T_{\mathrm{e}}$ suggests collisionless electron heating in an SNR shock. The Fe $\mathrm{K}\alpha$ line shows evidence for intrinsic broadening, with a width of 47 (34–59) eV (99% error region). The difference in the spatial distributions of the hard continuum above 3 keV and the Fe K line emission supports a synchrotron origin for the hard continuum.

A Suzaku Observation of the Low-Ionization Fe-Line Emission from RCW 86

The newly operational X-ray satellite Suzaku observed the southwestern quadrant of the supernova remnant (SNR) RCW 86 in February 2006 to study the nature of the 6.4 keV emission line first detected with the Advanced Satellite for Cosmology and Astronomy (ASCA). The new data confirm the existence of the line, localizing it for the first time; most of the line emission is adjacent and interior to the forward shock and not at the locus of the continuum hard emission. We also report the first detection of a 7.1 keV line that we interpret as the K-beta emission from low-ionization iron. The Fe-K line features are consistent with a non-equilibrium plasma of Fe-rich ejecta with n_{e}t <~ 10^9 cm^-3 s and kT_{e} ~ 5 keV. This combination of low n_{e}t and high kT_{e} suggests collisionless electron heating in an SNR shock. The Fe K-alpha line shows evidence for intrinsic broadening, with a width of 47 (34--59) eV (99% error region). The difference of the spatial distributions of the hard continuum above 3 keV and the Fe-K line emission support a synchrotron origin for the hard continuum.

A RIAF Interpretation for the Past Higher Activity of the Galactic Center Black Hole and the 511 keV Annihilation Emission

Abstract There are several lines of evidence to show that the supermassive black hole at the Galactic center had higher activities in the past than directly observed at present. It is shown here that these lines of evidence can quantitatively and consistently be explained if the mean accretion rate during the past $\sim 10^7 \,\mathrm{yr}$ has been $\sim 10^{3\hbox{--}4}$ times higher than the current rate, based on the picture of radiatively inefficient accretion flow (RIAF) and associated outflow that has been successfully applied to Sgr $\mathrm{A}^{*}$. We argue that this increased rate and its duration are theoretically reasonable in the Galactic center environment, while the accretion rate suddenly dropped about 300 yr ago, most likely because of the shell passage of the supernova remnant Sgr A East. The chance probability of witnessing Sgr $\mathrm{A}^{*}$ in such a low state is not extremely small ($\sim 0.5\%$). The outflow energetics is sufficient to keep the hot ($\sim 8 \,\mathrm{keV}$) diffuse gas observed in the Galactic center region. It is then shown that a significant amount of positrons should have been created around the event horizon during the higher activity phase, and injected into interstellar medium by the outflow. The predicted positron production rate and propagation distance are close to those required to explain the observed 511 keV annihilation line emission from the Galactic bulge, giving a natural explanation for the large bulge-to-disk ratio of the emission. The expected injection energy into interstellar medium is $\sim \mathrm{MeV}$, which is also favorable for an explanation of the 511 keV line emission.

X-ray broad-band study of the symbiotic X-ray binary 4U 1954+31

(Abridged) We present results of several X-ray observations of the X-ray binary 4U 1954+31 performed with the satellites BeppoSAX, EXOSAT, ROSAT, RXTE, and Swift. We also studied the RXTE ASM data over a period of more than 10 years. Light curves of all observations show an erratic behaviour with sudden increases in the source emission on timescales variable from hundreds to thousands of seconds. There are no indications of changes in the source spectral hardness, with the possible exception of the RXTE pointed observation. Timing analysis does not reveal the presence of coherent pulsations or periodicities either in the pointed observations in the range from 2 ms to 2000 s or in the long-term RXTE ASM light curve on timescales from days to years. The 0.2-150 keV spectrum, obtained with BeppoSAX, is the widest for this source available to date in terms of spectral coverage and is well described by a model consisting of a lower-energy thermal component (hot diffuse gas) plus a higher-energy (Comptonization) emission, with the latter modified by a partially-covering cold absorber plus a warm (ionized) absorber. A blackbody modelization of our BeppoSAX low-energy data is ruled out. The presence of a complex absorber local to the source is also supported by the 0.1-2 keV ROSAT spectrum. RXTE, EXOSAT and Swift X-ray spectroscopy is consistent with the above results, but indicates variations in the density and the ionization of the local absorber. A 6.5 keV emission line is possibly detected in the BeppoSAX and RXTE spectra. All this information suggests that the scenario that better describes 4U 1954+31 consists of a binary system in which a neutron star orbits in a highly inhomogeneus medium from a stellar wind coming from its optical companion, an M-type giant star.

Development of p-Channel Charge-Coupled Device for NeXT, the Next Japanese X-ray Astronomical Satellite Mission

We are developing an X-ray charge-coupled device (CCD) for the next Japanese X-ray astronomical satellite mission, NeXT (New X-ray Telescope/Nonthermal energy eXploration Telescope). We developed a trial product of the p-channel CCD fabricated on an n-type silicon wafer. It is possible to have a thick depletion layer of ∼300 µm with a p-channel CCD because it is easy to obtain high resistivity using an n-type silicon wafer compared with a p-type silicon wafer. We evaluated the performance of the p-channel CCD. The imaging area of the CCD consists of 512×512 pixels with a pixel size of 24×24 µm2. The horizontal charge transfer inefficiency (CTI) of the CCD can be improved by reducing the operating temperature and increasing the readout frequency. We obtained the best horizontal CTI of (0.98±0.09)×10-5 with an energy resolution of (202±6) eV full width at half maximum (FWHM) for 5.9 keV X-rays and a readout noise of 18 e- (rms) when the CCD was operated at a temperature of -110 °C and a readout frequency of 67 kHz. We measured the thickness of the depletion layer to be (290±33) µm from the detection efficiency of the 22.4 and 24.9 keV emission lines from 109Cd.

Recent results from a Si/CdTe semiconductor Compton telescope

We are developing a Compton telescope based on high-resolution Si and CdTe detectors for astrophysical observations in sub-MeV/MeV gamma-ray region. Recently, we constructed a prototype Compton telescope which consists of six layers of double-sided Si strip detectors (DSSDs) and CdTe pixel detectors to demonstrate the basic performance of this new technology. By irradiating the detector with gamma rays from radio isotope sources, we have succeeded in Compton reconstruction of images and spectra. The obtained angular resolution is 3.9°(FWHM) at 511 keV, and the energy resolution is 14 keV (FWHM) at the same energy. In addition to the conventional Compton reconstruction, i.e., drawing cones in the sky, we also demonstrated a full reconstruction by tracking Compton recoil electrons using the signals detected in successive Si layers. By irradiating 137Cs source, we successfully obtained an image and a spectrum of 662 keV line emission with this method. As a next step, development of larger DSSDs with a size of 4 cm × 4 cm is under way to improve the effective area of the Compton telescope. We are also developing a new low-noise analog ASIC to handle the increasing number of channels. Initial results from these two new technologies are presented in this paper as well.

The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL

We present a measurement of the sky distribution of positronium (Ps) annihilation continuum emission obtained with the SPI spectrometer on board ESA’s INTEGRAL observatory. The only sky region from which significant Ps continuum emission is detected is the Galactic bulge. The Ps continuum emission is circularly symmetric about the Galactic centre, with an extension of about 8 ◦ FWHM. Within measurement uncertainties, the sky distribution of the Ps continuum emission is consistent with that found by us for the 511 keV electron-positron annihilation line using SPI. Assuming that 511 keV line and Ps continuum emission follow the same spatial distribution, we derive a Ps fraction of 0.92±0.09. These results strengthen our conclusions regarding the origin of positrons in our Galaxy based on observations of the 511 keV line. In particular, they suggest that the main source of Galactic positrons is associated with an old stellar population, such as Type Ia supernovae, classical novae, or low-mass X-ray binaries. Light dark matter is a possible alternative source of positrons.

Searching for Annihilation Radiation from SN 1006 with SPI on INTEGRAL

Historical Type Ia supernovae are a leading candidate as the source of positrons observed through diffuse annihi- lation emission in the Galaxy. However, a search for annihilation emission from individual Type Ia supernovae was not possible before the improved sensitivity of INTEGRAL. The total 511 keV annihilation flux from individ- ual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction of positrons produced in 56 Co decays. Late optical light curves suggest that this fraction may be as high as 5%. We have searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPI instrument on INTEGRAL. We did not detect significant 511 keV line emission, with a 3 j flux upper limit of 0.59 #10 4 photons cm 2 s 1 for ∼1 Ms exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that Type Ia supernovae produce all of the positrons in the Galaxy (∼12% escape fraction), if the mean positron lifetime is less than 10 5 yr. Future observations with INTEGRALwill provide stronger limits on the escape fraction of positrons, the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy. Subject headings: gamma rays: observations — ISM: individual (SN 1006) — supernova remnants

The all-sky distribution of 511 keV electron-positron annihilation emission

We present a map of 511 keV electron-positron annihilation emission, based on data accumulated with the SPI spectrometer aboard ESA's INTEGRAL gamma-ray observatory, that covers approximately ~ of the celestial sphere. Within the exposed sky area, 511 keV line emission is significantly detected towards the galactic bulge region and, at a very low level, from the galactic disk. The bulge emission is highly symmetric and is centred on the galactic centre with an extension of ~ (FWHM). The emission is equally well described by models that represent the stellar bulge or halo populations. The detection significance of the bulge emission is ~, that of the galactic disk is ~. The disk morphology is only weakly constrained by the present data, being compatible with both the distribution of young and old stellar populations. The 511 keV line flux from the bulge and disk components is ph cm-2 s-1 and ph cm-2 s-1 respectively, corresponding to a bulge-to-disk flux ratio in the range . Assuming a positronium fraction of this translates into annihilation rates of s-1and s-1, respectively. The ratio of the bulge luminosity to that of the disk is in the range . We find no evidence for a point-like source in addition to the diffuse emission, down to a typical flux limit of ~10-4 ph cm-2 s-1. We also find no evidence for the positive latitude enhancement that has been reported from OSSE measurements; our upper flux limit for this feature is ph cm-2 s-1. The disk emission can be attributed to the -decay of the radioactive species 26 Al and 44Ti. The bulge emission arises from a different source which has only a weak or no disk component. We suggest that Type Ia supernovae and/or low-mass X-ray binaries are the prime candidates for the source of the galactic bulge positrons. Light dark matter annihilation could also explain the observed 511 keV bulge emission characteristics.