The Lunar Occultation Explorer (LOX)
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Ex Luna, Scientia: The Lunar Occultation eXplorer (LOX) An Activity, Project, and Statement of the Profession (APC) White Paper Submitted to: Decadal Survey on Astronomy and Astrophysics (Astro2020) Board on Physics and Astronomy & Space Studies Board National Academy of Sciences Washington, DC 20001 Submitted by: Dr. Richard S. Miller The Johns Hopkins University Applied Physics Laboratory Mail Stop 200-W230 11101 Johns Hopkins Road Laurel, MD 20723-6099 240-592-1576 [email protected] On behalf of the LOX Collaboration: M. Ajello1, J.F. Beacom2, P.F. Bloser3, A. Burrows4, C.L. Fryer3, J.O. Goldsten5, D.H. Hartmann1, P. Hoeflich6, A. Hungerford3, D.J. Lawrence5, M.D. Leising1, P. Milne7, P.N. Peplowski5, F. Shirazi1, T. Sukhbold2, L.-S. The1, Z. Yokley5, C.A. Young8 1Clemson University 2The Ohio State University 3Los Alamos National Laboratory 4Princeton University 5The Johns Hopkins University Applied Physics Laboratory 6Florida State University 7University of Arizona - Steward Observatory 8National Aeronautics and Space Administration ASTRO2020 (APC) - LOX - R.S. MILLER (PI) 1. Introduction LOX is a lunar-orbiting nuclear astrophysics mission that will probe the Cosmos at MeV energies (Ex. 1). It is guided by open questions regarding thermonuclear, or Type-Ia, supernovae (SNeIa) and will resolve the enigma of these inherently radioactive objects by enabling a systematic survey of SNeIa at gamma-ray energies for the first time. Astronomical investigations from lunar orbit afford new opportunities to advance our understanding of the cosmos. The foundation of LOX is the lunar occultation technique (LOT), an observational approach well suited to the all-sky monitoring demands of supernova investigations and time-domain astronomy. Its inherently wide-field-of-view and continuous all- sky monitoring provides an innovative way of addressing Exhibit 1. Artist’s impression decadal survey questions at MeV energies (0.1–10 MeV). of the Lunar Occultation eXplorer (LOX). The LOX approach achieves high sensitivity with a simple instrument design that eliminates the need for complex, position- sensitive detectors, kinematic event reconstruction, masks, or other insensitive detector mass, while also mitigating technology development, implementation complexity, and their associated costs (e.g., [2, 4, 45]). LOX can be realized within existing programs, like Explorer. The tremendous potential of MeV gamma-ray astronomy in general, and supernova radioactivity measurements in particular, is currently unrealized for the simple reason that instrument sensitivity has been inadequate; in fact, from 1980 until today, sensitivity has improved by only a factor of ten. This contrasts markedly with advances in soft X-ray, hard X- ray, GeV gamma-ray, and TeV gamma-ray astrophysics. By employing a paradigm changing observing methodology, LOX will utilize the Moon as a platform for astrophysics and change this status quo. LOX will address high-value SNeIa science by characterizing the spectral evolution of their primary nuclear LOX Addresses High- gamma-ray emissions to probe fundamental processes and Priority, High-Value Science diversity, and expose new connections between matter–energy life cycles within galaxies. The anticipated opportunity for an • LOX enables systematic studies of SNeIa and their progenitors Explorer-class mission is timely since it coincides with operation of the Large Synoptic Survey Telescope (LSST, • LOX will quantify SNeIa population systematics www.lsst.org) and NASA’s Wide-Field Infrared Survey Telescope (WFIRST, wfirst.gsfc.nasa.gov) in the mid-2020s to • LOX will reveal mass-energy lifecycles within galaxies provide enhanced SNeIa-related context measurements and science return. • LOX will enable near-continuous all-sky monitoring supporting 2. Key Science Goals and Objectives multiple topics in MeV astrophysics Thermonuclear supernovae are deeply connected to topics • LOX offers new capabilities in time-domain astronomy and throughout astrophysics and cosmology. They have synthesized surveys ASTRO2020 (APC) - LOX - R.S. MILLER (PI) !1 the majority of iron in the universe, supplied much of the energy input to the interstellar medium (ISM), and may have also revealed to us the fate of the universe. They are, however, an enigma. What are their progenitor systems? Why do they explode? How diverse is their population? The answers to these fundamental questions will have far-reaching impact, and they have motivated several unresolved Astro2010 Decadal Survey findings (Ex. 2) and multiple Astro2020 Decadal Survey science white papers [1, 6, 26, 34, 44, 50, 52, 56, 64, 66]. The clues that reveal the identity of the SNeIa progenitor systems, and the Astro2010 Top-Level Top-Level physical conditions that govern their Decadal Survey Science Science Findings Goals Objectives explosions, remain elusive. The consensus is that SNeIa result from degenerate Characterize the Parameterization of What are the progenitors Spectral Evolution of Type-Ia Gamma-Ray carbon-oxygen (CO) white dwarfs (WD) of SNeIa and how do they Type-Ia Supernovae Light Curves explode? (SSE2) undergoing thermonuclear runaway [22, Population Studies of 23]. But a standard model picture is Why is the Universe Quantify the Type-Ia Supernovae accelerating? (CFP2) Diversity of Spectral Evolution, difficult to produce from ultraviolet, Type-Ia Supernovae Including Identification How do stars form? of Subclasses optical, and infrared (UVOIR) based (PSF1) trends, even for a single progenitor Perform Census of What controls the mass– Type-Ia Supernovae channel [43]. Information loss is a natural energy–chemical cycles Progenitor Subclasses within galaxies? (GAN2) Probe the consequence of the nuclear radiation Thermonuclear Perform Census of reprocessing responsible for the UVOIR “Areas of Unusual Physics & Type-Ia Supernovae Discovery Potential”: Standardization of Environments signatures [8]; complex interstellar and Type-Ia Supernovae Time-Domain Connection to Type-Ia circumstellar environments also affect this Astronomy & Surveys Supernovae UVOIR secondary emission. Therefore, despite Diagnostics considerable theoretical modeling efforts Exhibit 2. Summary of LOX science goals & (e.g. [5, 14, 15, 41, 60, 61]), the final objectives. Also shown are related Astro2010 Decadal Survey findings. verdict on SNeIa progenitors and their explosion mechanisms has not yet been reached. It is possible that multiple evolutionary pathways contribute to the observed SNeIa population. Gamma rays have been understood to be an excellent diagnostic of SNeIa for nearly five decades (e.g., [9, 13]). In contrast to UVOIR emission, the escaping nuclear radiation is a direct consequence of the nuclear physics that governs the structure and dynamics of these objects and is encoded with critical fundamental evidence that can travel great distances unscathed. The energy and time dependence of emergent gamma-rays provides unique and powerful insights into SNeIa and are the obvious choice as a probe to resolve the SNeIa enigma. Only measurements of this radioactivity—the ashes of nuclear burning—will reveal fundamental details of thermonuclear supernovae. Exhibits 3 and 4 are an illustrative example of the information content and diagnostic capabilities of nuclear gamma-ray measurements. These figures shows the evolution of nuclear gamma-ray emission for two viable SNeIa progenitor models: a Chandrasekhar mass detonation [53] and the merger of two white dwarfs [42]. At early epochs following the explosion, the magnitude of low-energy emission is significantly different for the two models. This is an ASTRO2020 (APC) - LOX - R.S. MILLER (PI) !2 Exhibit 3. Spectral evolution encodes information about SNeIa progenitor systems. (left to right) Simulated emission spectra at 25, 50, and 100 days post-explosion for two representative SNeIa progenitor models [42, 53]. Line widths are due to Doppler broadening. Horizontal red lines denote preliminary definitions for continuum energy bands that, in addition to individual gamma-ray lines, contain information about elemental origin and transport processes. indicator of radioactive elements buried beneath different amounts of material and a consequence of their distinct internal structures. At late epochs, when the ejecta are optically thin and gamma rays can escape freely, the spectra are dominated by the nuclear lines and are similar in intensity, reflecting the identical amount of 56Ni (and hence 56Co) produced in these two particular models. Gamma-ray energy bands (i.e., continuum energy bands) also contain information about elemental origin and transport processes (e.g., [54, 55]) relevant for SNeIa studies. This contrasts with the approach requiring high-spectral resolution that seeks to characterize the discrete energy of nuclear decay lines (e.g., [24, 49]). The benefits of this approach include an increase in sensitivity due to additional gamma-ray flux, while also preserving the information of interest. Spectral evolution and intensity changes are therefore also be summarized in energy-band light curves, which characterize the spectral evolution of SNeIa. One notional set of definitions for SNeIa energy bands is given in Ex. 3. Diagnostics supporting SNeIa progenitor classification and the extraction of physical parameters can be obtained directly from the gamma-ray spectra and their corresponding temporal evolution in the form of light curves (e.g., [38, 54, 55]). The rapid expansion of the ejecta Flux [arbitrary units] exposes successively