Aluminum-26 (Al-26) is a radioactive isotope produced in massive star processes. High-resolution spectroscopy of its 1.809 MeV gamma-ray (γ-ray) decay signature constrains the dynamics of its emission as it is ejected from its progenitor sites and incorporated into the interstellar medium of the Milky Way Galaxy. Imaging reveals dominant emission in the Inner Galaxy and emission in localized regions of massive star activity. Taken together, spectroscopy and imaging of Al-26 shed light on the chemical evolution of the Galaxy over millions of years.

The Compton Spectrometer and Imager (COSI) is a compact Compton telescope designed to measure astrophysical γ-rays of energy 0.2–5 MeV. Its high-purity germanium detectors track incident photons as they Compton scatter throughout the detector volume. In 2016, COSI flew on a NASA ultra-long duration balloon for 46 days. This dissertation details the first analysis of Al-26 in the flight data and reports a measurement of 3.7σ significance above background and an Inner Galaxy flux of (8.6 ± 2.5) × 10−4 ph cm−2 s−1. All scientific achievements from the flight are predicated on calibrations performed before launch. These calibration procedures were repeated in advance of an intended 2020 balloon flight. Analyzing calibration data validates instrument performance and informs studies of detector effects, including charge sharing and charge trapping, that complicate the measurement process.

The next generation of COSI as a NASA Small Explorer satellite is slated for launch in 2027. It is anticipated to strengthen the spectroscopic measurement of Al-26 in the balloon flight and yield the most detailed images of Al-26 to date. Extensive testing of the imaging algorithm in COSI’s new analysis toolkit is presented as a first step towards producing these images. The desired culmination of these efforts is an enhanced understanding of Galactic Al-26 by way of thorough calibrations, novel insight into undesirable detector effects, and advanced analyses of high-resolution data from the COSI balloon and satellite instruments.

High-energy transient events can be used to probe the extreme physics that power them, as well as the properties of matter in violent astrophysical environments such as in the vicinity of compact objects or the birthplaces of black holes. X-ray binary outbursts provide important insights into the physics of accretion and the nature of black holes and neutron stars. Meanwhile, gamma-ray bursts (GRBs) carry signatures of the powerful progenitors that produce them, yet the origins of their prompt emission and their jet structure remain unclear. Spectral and timing analyses have been effective investigative tools for these extreme settings, especially as telescopes advance and models increase in predictive power. Pairing them with linear polarization analyses of high-energy emissions can add even more information about the emission mechanisms and source geometries of accreting black holes and GRBs.

The Compton Spectrometer and Imager (COSI) is a soft gamma-ray (0.2-5 MeV) telescope designed to study astrophysical sources including accreting black holes and GRBs. It has significant heritage as a balloon-borne telescope, and was selected as a NASA Small Explorer (SMEX) slated to launch on a satellite in 2027. COSI employs a compact Compton telescope designed to conduct high-resolution spectroscopy, imaging over a wide field-of-view, polarization studies, and effective suppression of background events. Compton telescopes detect multiple interactions from individual incoming photons, allowing for polarization information to be captured through measurements of the distribution of azimuthal angles. While the standard method relies on binning the photons to produce and fit an azimuthal scattering angle distribution, improved polarization sensitivity is obtained by using additional information to more accurately weigh each event’s contribution to the likelihood statistics. In this work, we report validations of COSI’s capabilities as a polarimeter. Furthermore, we develop tools that enable future spectral and polarimetric analyses of COSI GRB observations.

We also present the first observed outburst from the transient X-ray binary source MAXI J0637-430, based on observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT). We study the source's transition from a soft state dominated by disk-blackbody emission to a hard state dominated by a power-law or thermal Comptonization component, with NuSTAR providing the first coverage of MAXI J0637-430 above 10 keV. These broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test alternative excess emission models such as blackbody emission from the plunging region, a reflection component with a Comptonization continuum, and a reflection component of a blackbody illuminating the disk. The study demonstrates the importance of broadband spectral analyses of accreting compact objects. We include a discussion on how joint spectral and polarization analyses could be conducted for long-duration transient sources with COSI in the future.

High-mass X-ray binaries (HMXBs) consist of a black hole or neutron star accreting material from a high-mass stellar companion. Although these systems are very rare, with only about 100 having been discovered in the Milky Way Galaxy, they provide crucial insights into the evolution of high-mass stars and may have played an important role in the early Universe, heating the gas in the intergalactic medium and facilitating its reionization by the ultraviolet light produced by the first stars and galaxies. The advent of gravitational wave astronomy further motivates a more thorough understanding of HMXB populations, since HMXBs are the likely progenitors of many of the double compact binaries whose mergers will be detected by gravitational wave observatories. This dissertation presents three studies of HMXB populations, addressing open questions about the faint end of the HMXB luminosity function, the metallicity dependence of HMXB evolution, and the nature of compact objects in non-pulsating HMXBs.

In order to identify low-luminosity HMXBs and study their properties and Galactic number density, we surveyed a square-degree region in the direction of the Norma spiral arm with the Chandra and NuSTAR X-ray telescopes. We discovered three low-luminosity HMXB candidates, all of which have main-sequence Be/B-type counterparts and X-ray luminosities equal to 0.1-1 solar luminosities. The Chandra and NuSTAR surveys of the Norma region also provided the opportunity to study other low-luminosity X-ray populations in the Galaxy. We found that the majority of sources detected at energies above 2 keV are cataclysmic variables (CVs), which likely dominate the hard X-ray component of the Galactic Ridge X-ray Emission. The CV candidates in the Norma region have plasma temperatures of kT = 10-20 keV, whereas the CVs found in similar surveys of the Galactic Center region have temperatures of kT = 20-50 keV. The lower temperatures of Norma CVs may result from a significant number of them being nonmagnetic CVs, polars, or symbiotic binaries whereas the Galactic Center CVs are likely dominated by intermediate polars.

The second part of this dissertation discusses the X-ray emission of star-forming galaxies at redshifts between z=1.4 and z=2.6. Simulated models of HMXB populations predict that luminous HMXBs should be more numerous in low-metallicity environments. Studies of nearby galaxies have found an excess of luminous HMXBs in very metal-poor blue compact dwarf galaxies, and it has been suggested that the observed increase of the X-ray luminosity per star formation rate (SFR) of star-forming galaxies at higher redshifts is a result of the metallicity dependence of HMXBs. In order to test this hypothesis, we divided a sample of high-redshift galaxies from the MOSDEF survey into different metallicity bins, and stacked the X-ray data from deep Chandra extragalactic surveys to measure the average X-ray luminosity of the galaxies in each bin. Our preliminary results confirm the increase of the X-ray luminosity per SFR with redshift but do not find a significant correlation between the X-ray luminosity per SFR and the metallicity of galaxies.

The third part of this dissertation investigates the nature of the compact object in a non-pulsating HMXB. X-ray pulsations provide strong evidence that an HMXB hosts a neutron star (NS), but the absence of pulsations does not rule out the possibility that an HMXB hosts a NS. Using XMM-Newton and NuSTAR observations of IGR J18214-1318, we study the timing and spectral properties of this supergiant HMXB in detail. Our analysis rules out the presence of pulsations with periods shorter than an hour and detects an exponential cutoff to the power-law spectrum of the source with e-folding energy lower than 25 keV. This low cutoff energy indicates that the compact object in this HMXB is most likely a NS. This study exemplifies the powerful diagnostics provided by the combination of XMM-Newton and NuSTAR observations for the identification of compact objects in HMXBs.