The signature of positron annihilation, namely the 511 keV γ-ray line, was first detected coming from the direction of the Galactic center in the 1970s, but the source of Galactic positrons still remains a puzzle. The measured flux of the annihilation corresponds to an intense steady source of positron production, with an annihilation rate on the order of ∼1043 . The 511 keV emission is the strongest persistent Galactic γ-ray line signal, and it shows a concentration toward the Galactic center region. An additional low-surface brightness component is aligned with the Galactic disk; however, the morphology of the latter is not well constrained. The Compton Spectrometer and Imager (COSI) is a balloon-borne soft γ-ray (0.2–5 MeV) telescope designed to perform wide-field imaging and high-resolution spectroscopy. One of its major goals is to further our understanding of Galactic positrons. COSI had a 46-day balloon flight in 2016 May–July from Wanaka, New Zealand, and here we report on the detection and spectral and spatial analyses of the 511 keV emission from those observations. To isolate the Galactic positron annihilation emission from instrumental background, we have developed a technique to separate celestial signals using the COMPTEL Data Space. With this method, we find a 7.2σ detection of the 511 keV line. We find that the spatial distribution is not consistent with a single point source, and it appears to be broader than what has previously been reported.

We present results for the first observed outburst from the transient X-ray binary source MAXI J0637-430. This study is based on eight observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and six observations from the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT) collected from 2019 November 19 to 2020 April 26 as the 3-79 keV source flux declined from 8.2 × 10 −10 to 1.4 × 10 −12 erg cm −2 s −1 . We see the source transition from a soft state with a strong disk-blackbody component to a hard state dominated by a power-law or thermal Comptonization component. NuSTAR provides the first reported coverage of MAXI J0637-430 above 10 keV, and these broadband spectra show that a two-component model does not provide an adequate description of the soft-state spectrum. As such, we test whether blackbody emission from the plunging region could explain the excess emission. As an alternative, we test a reflection model that includes a physical Comptonization continuum. Finally, we also test a spectral component based on reflection of a blackbody illumination spectrum, which can be interpreted as a simple approximation to the reflection produced by returning disk radiation due to the bending of light by the strong gravity of the black hole. We discuss the physical implications of each scenario and demonstrate the value of constraining the source distance.Unified Astronomy Thesaurus concepts: Accretion (14); Low-mass x-ray binary stars (939); Black hole physics (159)

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The balloon-borne Compton Spectrometer and Imager (COSI) had a successful 46-day flight in 2016. The instrument is sensitive to photons in the energy range 0.2–5 MeV. Compton telescopes have the advantage of a unique imaging response and provide the possibility of strong background suppression. With its high-purity germanium detectors, COSI can precisely map γ-ray line emission. The strongest persistent and diffuse γ-ray line signal is the 511 keV emission line from the annihilation of electrons with positrons from the direction of the Galactic center. While many sources have been proposed to explain the amount of positrons, , the true contributions remain unsolved. In this study, we aim at imaging the 511 keV sky with COSI and pursue a full-forward modeling approach, using a simulated and binned imaging response. For the strong instrumental background, we describe an empirical approach to take the balloon environment into account. We perform two alternative methods to describe the signal: Richardson–Lucy deconvolution, an iterative method toward the maximum likelihood solution, and model fitting with predefined emission templates. Consistently with both methods, we find a 511 keV bulge signal with a flux between 0.9 and , confirming earlier measurements, and also indications of more extended emission. The upper limit we find for the 511 keV disk, , is consistent with previous detections. For large-scale emission with weak gradients, coded aperture mask instruments suffer from their inability to distinguish isotropic emission from instrumental background, while Compton telescopes provide a clear imaging response, independent of the true emission.

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