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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Response of ZrC to swift heavy ion irradiation

Zirconium carbide (ZrC) is commonly used for energy sector research, as well as a surrogate for the proposed advanced nuclear fuel candidate uranium carbide. Here this study investigates structural modifications to nanocrystalline and microcrystalline ZrC resulting from dense electronic excitations induced by swift heavy ion exposure. Samples were irradiated with 946 MeV Au ions to various fluences up to 6 × 10 13 ions cm –2 and characterized using synchrotron-based x-ray diffraction. The evolution of the unit-cell parameter and heterogeneous microstrain were evaluated as a function of fluence and compared with those of nanocrystalline and microcrystalline CeO 2 (a surrogate for UO 2 fuel) irradiated under identical conditions. Distinct differences were observed in the radiation responses of the carbide and oxide across both grain sizes. Most notably, microcrystalline ZrC exhibits swelling characterized by two distinct regimes, which does not result in saturation at the ion fluences achieved. This contrasts with CeO 2 , which exhibits the well-documented direct-impact defect accumulation mechanism, reaching a steady-state saturation of swelling at higher fluences. Nanocrystalline CeO 2 undergoes more pronounced swelling compared with microcrystalline CeO 2 , in contrast to nanocrystalline ZrC, which exhibits only minimal unit-cell changes. These results demonstrate that swift heavy ion-induced structural changes can be quite different in carbides and oxides, which must be considered when extrapolating fission-fragment type damage in current fuels to advanced fuels.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1981

Directionality effects in the transfer of X-rays from a magnetized atmosphere: Beam pulse shape

A formalism is presented for radiation transfer in two normal polarization modes in finite and semiinfinite plane parallel uniform atmospheres with a magnetic field perpendicular to the surface and arbitrary propagation angles. This method is based on the coupled integral equations of transfer, including emission, absorption, and scattering. Calculations are performed for atmosphere parameters typical of X-ray pulsars. The directionality of the escaping radiation is investigated for several cases, varying the input distributions. Theoretical pencil beam profiles and X-ray pulse shapes are obtained assuming the radiation is emitted from the polar caps of spinning neutron stars. Implications for realistic models of accreting magnetized X-ray sources are briefly discussed.

Meszaros, P.↗
DOE OSTI2024

Limits on non-relativistic matter during Big-bang nucleosynthesis

Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures ~10 MeV to ~100 keV. Here, we consider the effect of a cosmic matter-like species that is non-relativistic and pressureless during BBN. Such a component must decay; doing so during BBN can alter the baryon-to-photon ratio, η, and the effective number of neutrino species. We use light element abundances and the cosmic microwave background (CMB) constraints on η and N ν to place constraints on such a matter component. We find that electromagnetic decays heat the photons relative to neutrinos, and thus dilute the effective number of relativistic species to N eff < 3 for the case of three Standard Model neutrino species. Intriguingly, likelihood results based on Planck CMB data alone find N ν = 2.800 ± 0.294, and when combined with standard BBN and the observations of D and 4 He give N ν = 2.898 ± 0.141. While both results are consistent with the Standard Model, we find that a nonzero abundance of electromagnetically decaying matter gives a better fit to these results. Our best-fit results are for a matter species that decays entirely electromagnetically with a lifetime τ X = 0.89 sec and pre-decay density that is a fraction ξ = (ρ X /ρ rad |10 MeV = 0.0026 of the radiation energy density at 10 MeV; similarly good fits are found over a range where ξτ X 1/2 is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the (τ X ,ξ) plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. ξ = 0) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Molybdenum and silver photoemission survey spectra from hard x-rays

Electron emission from thick polished samples of polycrystalline molybdenum (Mo) and single crystalline $\langle$111$\rangle$ silver (Ag) was measured with hard x-ray photoemission spectroscopy. Six different excitation x-ray energies were used, nominally 8.0, 11.0, 13.0, 15.0, 18.0, and 21.5 keV. Survey spectra were recorded with each excitation to a kinetic energy of at most 15 keV, often capturing the entire emission range. The Mo 1s core peak was measured. Detailed LMM Auger spectra of Mo show marked increases in intensity and altered shape when x-ray energy exceeds the Mo 1s binding energy. The Mo and Ag L-shell photoelectron peaks are measured at four x-ray energies up to 18 keV showing the transition from 2p3/2 to 2s photoionization dominance.

47 OTHER INSTRUMENTATION↗
NASA NTRS1997

Learning about jet physics from gamma-ray blazars

The spectral properties of the MeV radiation-dominated blazars are used to place constraints on the physical parameters of relativistic jets in quasars. The luminosities and positions of high energy and low energy spectral components are used to derive constraints on the jet speeds, magnetic fields and the distances at which most of the nonthermal radiation is produced. By comparing the theoretically predicted bulk-Compton radiation with the observed soft X-ray luminosities, upper limits on the optical thickness and lower limits on the distance where the relativistic jet is formed and collimated, are identified. The results show that these jets should be Thomson optically thin and, in the case of gamma ray production dominated by the external radiation Compton process, favor proton-electron jets. Weaker constraints on the pair production are provided if the gamma ray production is dominated by the synchrotron self Compton (SSC) process. The values of the jet Lorentz factors predicted by the SSC models are smaller than those observed in quasars.

Sikora, M.↗
NASA NTRS2002

A Chandra X-ray Study of Cygnus A: The Nucleus - 2

We report Chandra Advanced CCD Imaging Spectrometer and quasi-simultaneous Rossi X-Ray Timing Explorer (RXTE) observations of the nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the properties of the active nucleus. In the Chandra observation, the hard (less than a few keV) X-ray emission is spatially unresolved with a size is approximately 1" (1.5 kpc, H(sub 0) = 50 km/s/Mpc) and coincides with the radio and near-infrared nuclei. In contrast, the soft (less than 2 keV) emission exhibits a bipolar nebulosity that aligns with the optical bipolar continuum and emission-line structures and approximately with the radio jet. In particular, the soft X-ray emission corresponds very well with the [O III] (lambda)5007 and H(alpha) + [N II] lambda(lambda)6548, 6583 nebulosity imaged with Hubble Space Telescope. At the location of the nucleus, there is only weak soft X-ray emission, an effect that may be intrinsic or result from a dust lane that crosses the nucleus perpendicular to the source axis. The spectra of the various X-ray components have been obtained by simultaneous fits to the six detectors. The compact nucleus is detected to 100 keV and is well described by a heavily absorbed power-law spectrum with Gamma(sub h) = 1.52(sup + 0.12, sub -0.12) (similar to other 0.12 narrow-line radio galaxies) and equivalent hydrogen column N(sub H)(nuc) = 2.0(sup +0.1, sub -0.1) x 10(exp 23)/sq cm. This 0.2 column is compatible with the dust obscuration to the near-infrared source for a normal gas-to-dust ratio. The soft (less than 2 keV) emission from the nucleus may be described by a power-law spectrum with the same index (i.e., Gamma(sub l) = Gamma(sub h), although direct fits suggest a slightly larger value for Gamma(sub l). Narrow emission lines from highly ionized neon and silicon, as well as a "neutral" Fe K(alpha) line, are detected in the nucleus and its vicinity (r approximately less than 2 kpc). The equivalent width (EW) of the Fe K(alpha) line (182(sup +40, sub -54) eV) is in good agreement with theoretical predictions for the EW versus N(sub H)(nuc) relationship in various geometries. An Fe K edge is also seen. The RXTE observations indicate a temperature of kT = 6.9(sup +0., sub -1.0) keV for the cluster gas (discussed in Paper III of this series) and cluster emission lines of Fe K(alpha) and Fe K(beta) and/or Ni K(alpha). We consider the possibility that the extended soft X-ray emission is electron-scattered nuclear radiation. Given that 1% of the unabsorbed 2 - 10 keV nuclear radiation would have to be scattered, the necessary gas column [N(sub H)(Scattering) approx. = 3.5 x 10(exp 22)/sq cm] would absorb the X-rays rather than scatter them if the gas is cold. Thus, the scattering plasma must be highly ionized. If this ionization is achieved through photoionization by the nucleus, the ionization parameter zeta greater than 1 ergs cm/s and the electron density n(sub e) approx. = 6 cc given the observed distance of the soft X-ray emission from the nucleus. The electron column density inferred from the X-ray observations is much too low to account for the extended optical scattered light, strongly suggesting that the polarized optical light is scattered by dust. The presence of highly ionized Ne lines in the soft X-ray spectrum requires 20 ergs cm/s approximately less than zeta approximately less than 300 ergs cm/s these lines may originate closer to the nucleus than the extended soft continuum or in a lower density gas. A collisionally ionized thermal model of the extended soft X-rays cannot be ruled out but is unattractive in view of the low metal abundance required (Z = 0.03 Z(mass)). The hard X-ray to far-infrared ratio for the nucleus of Cygnus A is similar to that seen in Seyfert 1 and unobscured radio galaxies. By means of the correlation between hard X-ray luminosity and nuclear optical absolute magnitude for these classes of object, we estimate M(sub B) = -22.4 for Cygnus A, near the .borderline between Seyfert galaxies and QSOs.

Young, Andrew J.↗
DOE OSTI2021

Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90 GPa

Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information in situ under high pressures in a diamond-anvil cell. Here, the dynamic structure factor of hydrogen was measured over a large energy range of 45 eV. The electronic band gap was found to decrease linearly from 10.9 to 6.57 eV, with an 8.6 times densification (ρ/ρ 0 ~ 8.6 ) from zero pressure up to 90 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Band-edge Engineering to Eliminate Radiation-Induced Defect States in Perovskite Scintillators

Under radiative environments such as extended hard X- or γ-rays, degradation of scintillation performance is often due to irradiation-induced defects. To overcome the effect of deleterious defects, novel design mitigation strategies are needed to identify and design more resilient materials. The potential for band-edge engineering to eliminate the effect of radiation-induced defect states in rare-earth-doped perovskite scintillators is explored, taking Ce 3+ -doped LuAlO 3 as a model material system, using density functional theory (DFT)-based DFT + U and hybrid Heyd–Scuseria–Ernzerhof (HSE) calculations. Furthermore, from spin-polarized hybrid HSE calculations, the Ce 3+ activator ground-state 4f position is determined to be 2.81 eV above the valence band maximum in LuAlO 3 . Except for the oxygen vacancies which have a deep level inside the band gap, all other radiation-induced defects in LuAlO 3 have shallow defect states or are outside the band gap, that is, relatively far away from either the 5d 1 or the 4f Ce 3+ levels. Finally, we examine the role of Ga doping at the Al site and found that LuGaO 3 has a band gap that is more than 2 eV smaller than that of LuAlO 3 . Specifically, the lowered conduction band edge envelopes the defect gap states, eliminating their potential impact on scintillation performance and providing direct theoretical evidence for how band-edge engineering could be applied to rare-earth-doped perovskite scintillators.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Correlated hard X-ray and ultraviolet variability in NGC 5548

Contemporaneous X-ray (from the Ginga satellite) and ultraviolet (IUE) observations of NGC 5548 in 1989-1990 show that the 1350 A and 2-10 keV flux both varied by a factor of 2.6 in a highly correlated fashion. Adding an additional simultaneous IUE-Exosat observation from 1984 significantly weakens the correlation, since the UV flux was 3.5 times larger then than its average 1990 value, while the X-ray flux was only 70 percent higher. In 1990 the UV and X-ray flux variations appear to be simultaneous to within no more than six days. These results, together with the simultaneity of the variations at 5000 and 1350 A, are incompatible with the standard geometrically thin accretion model. They may be explained, however, if part of the UV and optical radiation originates from reprocessing of X-rays emitted above the disk. Thus essentially all the UV flux in 1990 would be due to reprocessing. But the large UV outburst in 1984 probably represents a genuine increase in the disk throughput due to instability in the disk itself. The similar relationship between UV and X-ray emission in NGC 4151 suggests that this scheme may be applicable to Seyfert galaxies as a class.

Clavel, J.↗
DOE OSTI2020

Speckle correlation as a monitor of X-ray free-electron laser induced crystal lattice deformation

X-ray free-electron lasers (X-FELs) present new opportunities to study ultrafast lattice dynamics in complex materials. While the unprecedented source brilliance enables high fidelity measurement of structural dynamics, it also raises experimental challenges related to the understanding and control of beam-induced irreversible structural changes in samples that can ultimately impact the interpretation of experimental results. This is also important for designing reliable high performance X-ray optical components. In this work, X-FEL beam-induced lattice alterations are investigated by measuring the shot-to-shot evolution of near-Bragg coherent scattering from a single crystalline germanium sample. It is shown that X-ray photon correlation analysis of sequential speckle patterns measurements can be used to monitor the nature and extent of lattice rearrangements. Additionally, abrupt, irreversible changes are observed following intermittent high-fluence monochromatic X-ray pulses, thus revealing the existence of a threshold response to X-FEL pulse intensity.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1995

New upper limits on the local metagalactic ionizing radiation density

We have obtained H-alpha observations with the Maryland-Caltech Fabry-Perot Spectrometer attached to the Cassegrain focus of the 1.5 m telescope at Palomer Observatory in order to set limits on the number of ionizing photons from the local metagalactic radiation field. We have observed the SW component of the Haynes-Giovanelli cloud H I 1225+01, an intergalactic cloud which should be optimum for measuring the metagalactic flux because it is nearly opaque to ionizing photons, it does not appear to be significantly shielded from the metagalactic radiation field, and the limits on embedded or nearby ionizing sources are unusually low. For the area of the cloud with an H I column density greater than 10(exp 19)/sq cm we set a 2 sigma limit of 1.1 x 10(exp -19) ergs/sq cm/s/sq arcsec (20 mR) for the surface brightness of diffuse H-alpha. This implies a 2 sigma upper limit on the incident one-sided ionizing flux of Phi(sub ex) is less than 3 x 10(exp 4)/sq cm/s. For a radiation field of the form J(sub nu) is approximately nu(exp -1.4), this yields a firm 2 sigma upper limit on the local metagalactic photoionization rate of Gamma is less than 2 x 10(exp -13)/s, and an upper limit for the radiation field J(sub nu) at the Lyman limit of J(sub nu0) is less than 8 x 10(exp -23) ergs/sq cm/Hz/sr. We discuss previous efforts to constrain the metagalactic ionizing flux using H-alpha surface brightness observations and also other methods, and conclude that our result places the firmest upper limit on this flux. We also observed the 7 min diameter region centered on 3C 273 in which H-alpha emission at a velocity of approximately 1700 km/s was initially reported by Williams and Schommer. In agreement with T. B. Williams (private communication) we find the initial detection was spurious. We obtain a 2 sigma upper limit of 1.8 x 10(exp -19) ergs/sq cm/s/sq arcsec (32 mR) for the mean surface brightness of diffuse H-alpha, about a factor of 6 below the published value.

Vogel, Stuart N.↗
DOE OSTI2024

Sustainable Valorization of CO2 through Nuclear Power-to-X Pathways

Some of the issues concerning energy security and climate change can be addressed by employing nuclear power (NP) to supply the energy required for the conversion of carbon dioxide (CO2) into chemicals, products, and materials. Nuclear energy represents a neutral carbon source that can be generated sustainably, reliably, and consistently. Nuclear power plants (NPPs) could supply energy in the form of heat, electricity, and ionizing radiation to drive CO2 chemical reactions underpinning NP-to-X type of pathways. CO2 conversion processes are either commercially available or emerging technologies at different developmental maturity stages. This work reviews the published literature (articles and patents) that reports R&D results and the understanding and development of chemical reactions and processes, as well as the efforts in integrating NPPs and chemical processes (CPs). As will be made evident, a new industrial era for the manufacturing of decarbonized chemicals, products, and materials will be possible by developing and implementing new (more energy- and carbon-efficient) processes responding to the NP-to-X pathways. This new decarbonizing platform not only contributes to achieving net zero goals but also broadens the NPP product beyond electricity.

08 HYDROGEN↗
DOE OSTI2022

Stabilization of Cu 2 O through Site-Selective Formation of a Co 1 Cu Hybrid Single-Atom Catalyst

Single-atom catalysts (SACs) consist of a low coverage of isolated metal atoms dispersed on a metal substrate, called single-atom alloys (SAAs), or alternatively single metal atoms coordinated to oxygen atoms on an oxide support. We present the synthesis of a new type of Co 1 Cu SAC centers on a Cu 2 O(111) support by means of a site-selective atomic layer deposition technique. Isolated metallic Co atoms selectively coordinate to the native oxygen vacancy sites (Cu sites) of the reconstructed Cu 2 O(111) surface, forming a Co 1 Cu SAA with no direct Co–O x bonds. The centers, here referred to as Co 1 Cu hybrid SACs, are found to stabilize the active Cu + sites of the low-cost Cu 2 O catalyst that otherwise is prone to deactivation under reaction conditions. The stability of the Cu 2 O(111) surface was investigated by synchrotron radiation-based ambient-pressure X-ray photoelectron spectroscopy under reducing CO environment. The structure and reduction reaction are modeled by density functional theory calculations, in good agreement with experimental results.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

Developing High-Throughput Organ-on-a-Chip Models to Investigate the Effects of Ionizing Radiation on the Central Nervous System

One of the main health risks in human space exploration is central nervous system (CNS) damage by ionizing radiation due to exposure to the galactic cosmic rays (GCRs). In animal models, irradiation with simulated GCRs or their components has been shown to cause neuronal damage and neuroinflammation associated with cognitive and behavioral dysfunction. In general, the extent of CNS damage is partially regulated by the blood-brain barrier (BBB), which enables immune cells to enter the CNS. The main cellular regulators of BBB permeability are astrocytes, which also modulate neuronal death, immune responses and oxidative stress, and thus could serve as a robust CNS-specific target for countermeasure development. However, studies on BBB permeability and astrocyte functions in regulating CNS responses to ionizing radiation have been limited, especially in human tissue/organ analogs. Therefore, we established a high-throughput 3D organ-on-a-chip system to study human CNS and BBB impairments in response to ionizing radiation, based on commercially available OrganoPlates (Mimetas, Inc.) seeded with primary or induced pluripotent stem cell-derived human cells. We investigated both immediate and delayed CNS responses to major GCR components: 0.15-0.5 Gy 250MeV/n 4-He, and 0.3-0.8 Gy 600 MeV/n 56-Fe; as well as to 0.5-1 Gy X-rays. We observed ionizing radiation-mediated increases in BBB permeability that was exacerbated by astrocyte presence and accompanied by morphological changes in endothelial cells and tight junctions, altered cytokine profile including TNFa upregulation, and increased oxidative stress. We also quantified irradiation-mediated changes in astrocyte activation and neuronal functions, revealing major astrocyte damage mediated by 600MeV/n 56-Fe particles. Thus, we demonstrate that deep space radiation may contribute to CNS damage by disrupting both astrocyte and endothelial cell components of the blood-brain barrier. Our next steps include mapping and validating the transcriptomic changes induced by simulated GCRs and their components in human CNS models. Ultimately, we aim to uncover potential novel targets for countermeasure developments to mitigate CNS damage in long duration spaceflight.

Radiation↗
NASA NTRS2016

A Unified Model for GRB Prompt Emission from Optical to Gamma-Rays; Exploring GRBs as Standard Candles

The origin of prompt emission from gamma-ray bursts (GRBs) remains to be an open question. Correlated prompt optical and gamma-ray emission observed in a handful of GRBs strongly suggests a common emission region, but failure to adequately fit the broadband GRB spectrum prompted the hypothesis of different emission mechanisms for the low- and high-energy radiations. We demonstrate that our multi-component model for GRB -ray prompt emission provides an excellent fit to GRB 110205A from optical to gamma-ray energies. Our results show that the optical and highest gamma-ray emissions have the same spatial and spectral origin, which is different from the bulk of the X- and softest gamma-ray radiation. Finally, our accurate redshift estimate for GRB 110205A demonstrates promise for using GRBs as cosmological standard candles.

acceleration of particles – black hole physics â↗
DOE OSTI2024

Efficient boundary-guided scanning for high-resolution X-ray ptychography

In the realm of X-ray ptychography experiments, a considerable amount of ptychography scans are typically performed within a field of view encompassing the target sample. While it is crucial to obtain overlapping scans in small increments over the region of interest for achieving high-resolution sample reconstruction, a significant number of these scans often redundantly measure the empty background within the wide field of view. To address this inefficiency, an innovative algorithm is proposed that introduces automatic guidance for data acquisition. The algorithm first directs the scan point to actively search for the object of interest within the field of view. Subsequently, it intelligently scans along the perimeter of the sample, strategically acquiring measurements exclusively within the boundary of the region of interest. By employing this approach, a reduction in the number of measurements required to obtain high-resolution reconstruction images is demonstrated, as compared with conventional raster scanning methods. Furthermore, the automatic guidance provided by the algorithm offers the added advantage of saving valuable time during the reconstruction process. Through practical implementation on real experiments, these findings showcase the efficacy of the proposed algorithm in enhancing the efficiency and accuracy of X-ray ptychography experiments. This novel approach holds immense potential for advancing sample analysis and imaging techniques in various scientific disciplines.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1991

Radiative-hydrodynamical simulations of accretion disk coronae

An algorithm has been developed that will carry out detailed, two-dimensional, fully self-consistent radiative-hydrodynamical simulations of accretion disk coronae in X-ray-luminous compact sources. The calculation reported here, for an accreting neutron star radiating at 0.5 times the Eddington luminosity, reveals several striking features. (1) The corona is comprised of two main regions - an inner highly dynamic portion whose vertical structure varies cyclically on a dynamical time scale, and an outer, more stable zone in which the evaporated plasma rises to form a 'sheath' that gradually merges into a wind at large radii. (2) The coronal structure shows a density inversion that contrasts sharply with the Gaussian profiles of earlier hydrostatic models. (3) Interestingly, flow velocities as high as a few billion cm/s are not uncommon in portions of the corona. The relevance of this point to the large measured width of the emission features seen in low-mass X-ray binaries and the Galactic black hole candidates is discussed.

Melia, Fulvio↗
DOE OSTI2021

A Lyα Transit Left Undetected: the Environment and Atmospheric Behavior of K2-25b

K2-25b is a Neptune-sized exoplanet (3.45R {sub ⊕}) that orbits its M4.5 host with a period of 3.48 days. Due to its membership in the Hyades Cluster, the system has a known age (727 ± 75 Myr). K2-25b’s youth and its similarities with Gl 436b suggested that K2-25b could be undergoing strong atmospheric escape. We observed two transits of K2-25b at Lyα using HST/STIS in order to search for escaping neutral hydrogen. We were unable to detect an exospheric signature, but placed an upper limit of (R {sub p}/R {sub ⋆})∣{sub Ly α} < 0.56 at 95% confidence by fitting the light curve of the Lyα red wing, or < 1.20 in the blue wing. We reconstructed the intrinsic Lyα profile of K2-25 to determine its Lyα flux, and analyzed XMM-Newton observations to determined its X-ray flux. Based on the total X-ray and extreme ultraviolet irradiation of the planet (8763 ± 1049 erg s{sup −1} cm{sup −2}), we estimated the maximum energy-limited mass-loss rate of K2-25b to be 10.6{sub −6.13}{sup +15.2}×10{sup 10} g s{sup −1} (0.56M {sub ⊕} per 1 Gyr), five times larger than the similarly estimated mass-loss rate of Gl 436b (2.2 × 10{sup 10} g s{sup −1}). The photoionization time is about 3 hr, significantly shorter than Gl 436b’s 14 hr. A nondetection of a Lyα transit could suggest K2-25b is not significantly losing its atmosphere, or factors of the system are resulting in the mass loss being unobservable (e.g., atmosphere composition or the system’s large high-energy flux). Further observations could provide more stringent constraints.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗