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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

FINDING THE FIRST COSMIC EXPLOSIONS. I. PAIR-INSTABILITY SUPERNOVAE

The first stars are the key to the formation of primitive galaxies, early cosmological reionization and chemical enrichment, and the origin of supermassive black holes. Unfortunately, in spite of their extreme luminosities, individual Population III (Pop III) stars will likely remain beyond the reach of direct observation for decades to come. However, their properties could be revealed by their supernova explosions, which may soon be detected by a new generation of near-IR (NIR) observatories such as JWST and WFIRST. We present light curves and spectra for Pop III pair-instability supernovae calculated with the Los Alamos radiation hydrodynamics code RAGE. Our numerical simulations account for the interaction of the blast with realistic circumstellar envelopes, the opacity of the envelope, and Lyman absorption by the neutral intergalactic medium at high redshift, all of which are crucial to computing the NIR signatures of the first cosmic explosions. We find that JWST will detect pair-instability supernovae out to z ∼> 30, WFIRST will detect them in all-sky surveys out to z ∼ 15-20, and LSST and Pan-STARRS will find them at z ∼< 7-8. The discovery of these ancient explosions will probe the first stellar populations and reveal the existence of primitive galaxies that might not otherwise have been detected.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

The supercritical pile gamma-ray burst model: The GRB afterglow steep decline and plateau phase

We present a process that accounts for the steep decline and plateau phase of the Swift X-Ray Telescope (XRT) light curves, vexing features of gamma-ray burst (GRB) phenomenology. This process is an integral part of the 'supercritical pile' GRB model, proposed a few years ago to account for the conversion of the GRB kinetic energy into radiation with a spectral peak at E {sub pk} ∼ m{sub e}c {sup 2}. We compute the evolution of the relativistic blast wave (RBW) Lorentz factor Γ to show that the radiation-reaction force due to the GRB emission can produce an abrupt, small (∼25%) decrease in Γ at a radius that is smaller (depending on conditions) than the deceleration radius R{sub D} . Because of this reduction, the kinematic criticality criterion of the 'supercritical pile' is no longer fulfilled. Transfer of the proton energy into electrons ceases and the GRB enters abruptly the afterglow phase at a luminosity smaller by ∼m{sub p} /m{sub e} than that of the prompt emission. If the radius at which this slow-down occurs is significantly smaller than R{sub D} , the RBW internal energy continues to drive the RBW expansion at a constant (new) Γ and its X-ray luminosity remains constant until R{sub D} is reached, at which point it resumes its more conventional decay, thereby completing the 'unexpected' XRT light curve phase. If this transition occurs at R ≅ R{sub D} , the steep decline is followed by a flux decrease instead of a 'plateau,' consistent with the conventional afterglow declines. Besides providing an account of these peculiarities, the model suggests that the afterglow phase may in fact begin before the RBW reaches R ≅ R{sub D} , thus providing novel insights into GRB phenomenology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A multi-wavelength look at the young plerionic supernova remnant 0540-69.3

We present a study of the plerionic supernova remnant 0540-69.3 in the LMC in X-ray, radio, optical, and infrared. We find that the shell of 0540-69.3 is characterized in the X-ray by thermal nonequilibrium plasma with depleted Mg and Si abundances and a temperature of kT ∼ 0.7 keV. This thermal emission is superimposed with synchrotron emission in several regions. Based on X-ray spectra and on morphological considerations in all surveyed wavebands, we conclude that the shell is expanding into a clumpy and highly inhomogeneous medium. In one region of the shell we find an overabundance of Ne, suggesting the presence of ejecta near the edge of the remnant. We also see evidence for reheating of material via a reverse shock originating from the interaction of the supernova blast wave with a particularly dense cloud in the surrounding medium. Finally, we perform the first detailed study of the 'halo' region extending 1.2-2.2 pc from the central pulsar. We detect the presence of thermal and nonthermal spectral components but do not find evidence for mixing or ejecta. We conclude that the thermal component is not a counterpart to similar optical and infrared halos and that it is most likely due to the projection of shell material along the line of sight.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

On the non-existence of a sharp cooling break in gamma-ray burst afterglow spectra

Although the widely used analytical afterglow model of gamma-ray bursts (GRBs) predicts a sharp cooling break ν {sub c} in its afterglow spectrum, the GRB observations so far rarely show clear evidence for a cooling break in their spectra or a corresponding temporal break in their light curves. Employing a Lagrangian description of the blast wave, we conduct a sophisticated calculation of the afterglow emission. We precisely follow the cooling history of non-thermal electrons accelerated into each Lagrangian shell. We show that a detailed calculation of afterglow spectra does not in fact give rise to a sharp cooling break at ν {sub c}. Instead, it displays a very mild and smooth transition, which occurs gradually over a few orders of magnitude in energy or frequency. The main source of this slow transition is that different mini shells have different evolutionary histories of the comoving magnetic field strength B, so that deriving the current value of ν {sub c} of each mini shell requires an integration of its cooling rate over the time elapsed since its creation. We present the time evolution of optical and X-ray spectral indices to demonstrate the slow transition of spectral regimes and discuss the implications of our result in interpreting GRB afterglow data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A comparison of two- and three-dimensional neutrino-hydrodynamics simulations of core-collapse supernovae

We present numerical results on two- (2D) and three-dimensional (3D) hydrodynamic core-collapse simulations of an 11.2 M {sub ☉} star. By changing numerical resolutions and seed perturbations systematically, we study how the postbounce dynamics are different in 2D and 3D. The calculations were performed with an energy-dependent treatment of the neutrino transport based on the isotropic diffusion source approximation scheme, which we have updated to achieve a very high computational efficiency. All of the computed models in this work, including nine 3D models and fifteen 2D models, exhibit the revival of the stalled bounce shock, leading to the possibility of explosion. All of them are driven by the neutrino-heating mechanism, which is fostered by neutrino-driven convection and the standing-accretion-shock instability. Reflecting the stochastic nature of multi-dimensional (multi-D) neutrino-driven explosions, the blast morphology changes from model to model. However, we find that the final fate of the multi-D models, whether an explosion is obtained or not, is little affected by the explosion stochasticity. In agreement with some previous studies, higher numerical resolutions lead to slower onset of the shock revival in both 2D and 3D. Based on the self-consistent supernova models leading to the possibility of explosions, our results systematically show that the revived shock expands more energetically in 2D than in 3D.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Light-element nucleosynthesis in a molecular cloud interacting with a supernova remnant and the origin of beryllium-10 in the protosolar nebula

The presence of short-lived radionuclides (t {sub 1/2} < 10 Myr) in the early solar system provides important information about the astrophysical environment in which the solar system formed. The discovery of now extinct {sup 10}Be (t {sub 1/2} = 1.4 Myr) in calcium-aluminum-rich inclusions (CAIs) with Fractionation and Unidentified Nuclear isotope anomalies (FUN-CAIs) suggests that a baseline concentration of {sup 10}Be in the early solar system was inherited from the protosolar molecular cloud. In this paper, we investigate various astrophysical contexts for the nonthermal nucleosynthesis of {sup 10}Be by cosmic-ray-induced reactions. We first show that the {sup 10}Be recorded in FUN-CAIs cannot have been produced in situ by irradiation of the FUN-CAIs themselves. We then show that trapping of Galactic cosmic rays (GCRs) in the collapsing presolar cloud core induced a negligible {sup 10}Be contamination of the protosolar nebula, the inferred {sup 10}Be/{sup 9}Be ratio being at least 40 times lower than that recorded in FUN-CAIs ({sup 10}Be/{sup 9}Be ∼ 3 × 10{sup –4}). Irradiation of the presolar molecular cloud by background GCRs produced a steady-state {sup 10}Be/{sup 9}Be ratio ≲ 1.3 × 10{sup –4} at the time of the solar system formation, which suggests that the presolar cloud was irradiated by an additional source of CRs. Considering a detailed model for CR acceleration in a supernova remnant (SNR), we find that the {sup 10}Be abundance recorded in FUN-CAIs can be explained within two alternative scenarios: (1) the irradiation of a giant molecular cloud by CRs produced by ≳ 50 supernovae exploding in a superbubble of hot gas generated by a large star cluster of at least 20,000 members, and (2) the irradiation of the presolar molecular cloud by freshly accelerated CRs escaped from an isolated SNR at the end of the Sedov-Taylor phase. In the second picture, the SNR resulted from the explosion of a massive star that ran away from its parent OB association, expanded during most of its adiabatic phase in an intercloud medium of density of about 1 H-atom cm{sup –3}, and eventually interacted with the presolar molecular cloud only during the radiative stage. This model naturally provides an explanation for the injection of other short-lived radionuclides of stellar origin into the cold presolar molecular cloud ({sup 26}Al, {sup 41}Ca, and {sup 36}Cl) and is in agreement with the solar system originating from the collapse of a molecular cloud shocked by a supernova blast wave.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

GAMMA-RAY BURSTS ARE OBSERVED OFF-AXIS

We constrain the jet opening angle and, for the first time, the off-axis observer angle for gamma-ray bursts in the Swift-XRT catalog by using the ScaleFit package to fit afterglow light curves directly to hydrodynamic simulations. The ScaleFit model uses scaling relations in the hydrodynamic and radiation equations to compute synthetic light curves directly from a set of high-resolution two-dimensional relativistic blast wave simulations. The data sample consists of all Swift-XRT afterglows from 2005 to 2012 with sufficient coverage and a known redshift, 226 bursts in total. We find that the jet half-opening angle varies widely but is commonly less than 0.1 rad. The distribution of the electron spectral index is also broad, with a median at 2.30. We find the observer angle to have a median value of 0.57 of the jet opening angle over our sample, which has profound consequences for the predicted rate of observed jet breaks and affects the beaming-corrected total energies of gamma-ray bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE ENERGETICS OF A GLOBAL SHOCK WAVE IN THE LOW SOLAR CORONA

As the most energetic eruptions in the solar system, coronal mass ejections (CMEs) can produce shock waves at both their front and flanks as they erupt from the Sun into the heliosphere. However, the amount of energy produced in these eruptions, and the proportion of their energy required to produce the waves, is not well characterized. Here we use observations of a solar eruption from 2014 February 25 to estimate the energy budget of an erupting CME and the globally propagating ''EIT wave'' produced by the rapid expansion of the CME flanks in the low solar corona. The ''EIT wave'' is shown using a combination of radio spectra and extreme ultraviolet images to be a shock front with a Mach number greater than one. Its initial energy is then calculated using the Sedov-Taylor blast-wave approximation, which provides an approximation for a shock front propagating through a region of variable density. This approach provides an initial energy estimate of ≈2.8 × 10{sup 31} erg to produce the ''EIT wave'', which is approximately 10% the kinetic energy of the associated CME (shown to be ≈2.5 × 10{sup 32} erg). These results indicate that the energy of the ''EIT wave'' may be significant and must be considered when estimating the total energy budget of solar eruptions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ULTRAVIOLET SPECTROSCOPY OF TYPE IIB SUPERNOVAE: DIVERSITY AND THE IMPACT OF CIRCUMSTELLAR MATERIAL

We present new Hubble Space Telescope (HST) multi-epoch ultraviolet (UV) spectra of the bright Type IIb SN 2013df, and undertake a comprehensive analysis of the set of four SNe IIb for which HST UV spectra are available (SN 1993J, SN 2001ig, SN 2011dh, and SN 2013df). We find strong diversity in both continuum levels and line features among these objects. We use radiative-transfer models that fit the optical part of the spectrum well, and find that in three of these four events we see a UV continuum flux excess, apparently unaffected by line absorption. We hypothesize that this emission originates above the photosphere, and is related to interaction with circumstellar material (CSM) located in close proximity to the SN progenitor. In contrast, the spectra of SN 2001ig are well fit by single-temperature models, display weak continuum and strong reverse-fluorescence features, and are similar to spectra of radioactive {sup 56}Ni-dominated SNe Ia. A comparison of the early shock-cooling components in the observed light curves with the UV continuum levels which we assume trace the strength of CSM interaction suggests that events with slower cooling have stronger CSM emission. The radio emission from events having a prominent UV excess is perhaps consistent with slower blast-wave velocities, as expected if the explosion shock was slowed down by the CSM that is also responsible for the strong UV, but this connection is currently speculative as it is based on only a few events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Signature of a spin-up magnetar from multi-band afterglow rebrightening of GRB 100814A

In recent years, more and more gamma-ray bursts (GRBs) with late rebrightenings in their multi-band afterglows have revealed the late-time activity of their central engines. GRB 100814A is a special case among the well-sampled events, with complex temporal and spectral evolution. The single power-law shallow decay index of the optical light curve observed by GROND between 640 s and 10 ks is α{sub opt}=0.57±0.02, which apparently conflicts with expectations from the simple external shock model. In particular, there is remarkable rebrightening in the optical to near-infrared bands at late times, challenging the external shock model with synchrotron emission coming from the interaction of the blast wave with the surrounding interstellar medium. In this paper, we invoke a magnetar with spin evolution to explain the complex multi-band afterglow emission of GRB 100814A. The initial shallow decay phase in the optical bands and the plateau in the X-ray can be explained as being due to energy injection from a spin-down magnetar. At late times, with materials from the fall-back disk falling onto the central object of the burster, the angular momentum of the accreted materials is transferred to the magnetar, which leads to a spin up process. As a result, the magnetic dipole radiation luminosity will increase, resulting in significant rebrightening of the optical afterglow. We show that the model can well reproduce the observed multi-band afterglow emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Extending laser plasma accelerators into the mid-IR spectral domain with a next-generation ultra-fast CO 2 laser

Here we discuss how expanding the scope of relativistic plasma research to wavelengths longer than λ/≈0.8₋1.1μm covered by conventional mode-locked solid-state lasers would offer attractive opportunities due to the quadratic scaling of the ponderomotive electron energy and critical plasma density with λ. Answering this quest, a next-generation mid-IR laser project is being advanced at the BNL ATF as a part of the user facility upgrade. We discuss the technical approach to this conceptually new 100 TW, 100 fs, λ=9₋11 μm CO 2 laser BESTIA (Brookhaven Experimental Supra-Terawatt Infrared at ATF) that encompasses several innovations applied for the first time to molecular gas lasers. BESTIA will enable new regimes of laser plasma accelerators. One for example is shock-wave ion acceleration from gas jets. We review ongoing efforts to achieve stable, monoenergetic proton acceleration by dynamically shaping the plasma density profile from a hydrogen gas target with laser-produced blast waves. At its full power, 100 TW BESTIA promises to achieve proton beams at energy exceeding 200 MeV. In addition to ion acceleration in over-critical plasma, the ultra-intense mid-IR laser BESTIA will open new opportunities in driving wakefields in tenuous plasmas, expanding the landscape of Laser Wake Field Accelerator (LWFA) studies into unexplored long-wavelength spectral domain. Simple wavelength scaling suggests that a 100-TW CO2 laser beam will be capable to efficiently generate plasma “bubbles” thousand times bigger in volume compared to a near-IR solid state laser of an equivalent power. Combined with a femtosecond electron linac available at the ATF, this wavelength scaling will facilitate study of external seeding and staging of LWFA.

43 PARTICLE ACCELERATORS↗
DOE OSTI2024

Machine learning analysis of high-repetition-rate two-dimensional Thomson scattering spectra from laser-produced plasmas

With the emergence of high-repetition-rate two-dimensional Thomson scattering (TS) measurements, improving spectral data analysis is a key area of interest. Here, we present a new way to derive the electron temperature and density of laser-driven blast waves in plasmas from their TS spectra with machine learning (ML). This analysis occurs in both the non-collective (α < 1) and collective (α > 1) scattering regimes with the goal of autonomously and more accurately determining T c and n e both where spectral data has been collected and to give the ability to predict these attributes in regions where data has not been collected. We introduce three ML models, one trained only on experimental data, one only on synthetic data, and one using transfer learning, and compare their speed and accuracy with the conventional TS inversion algorithms in the open source PlasmaPy python package.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

XFEL imaging techniques for high energy density and inertial fusion energy research at HED-HiBEF

The imaging platform developed at the High Energy Density-Helmholtz International Beamline for Extreme Fields (HED-HiBEF) instrument at the European X-ray Free Electron Laser (XFEL) and its applications to HED and fusion related research are presented. The platform combines the XFEL beam with the high-intensity short-pulse laser ReLaX and the high-energy nanosecond-pulse laser DiPOLE-100X. The spatial resolution is better than 500 nm and the temporal resolution of the order of 50 fs. The influence of the XFEL source in the x-ray imaging method is discussed. Free-propagation x-ray phase contrast imaging and Talbot-Lau imaging setups are shown. We show examples of blast waves and converging cylindrical shocks in aluminum, resonant absorption measurements of specific charged states in copper with ReLaX and planar shocks in polystyrene material generated by DiPOLE-100X. For the first time, we show the application of Talbot-Lau interferometry to convergent cylindrical shocks as well as resonant absorption processes. We also discuss the possibilities introduced by combining this imaging platform with a kJ-class laser.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Energetic lanthanide complexes: coordination chemistry and explosives applications

Here, we report that metals are generally added to organic molecular explosives in a heterogeneous composite to improve overall heat and energy release. In order to avoid creating a mixture that can vary in homogeneity, energetic organic molecules can be directly bonded to high molecular weight metals, forming a single metal complex with Angstrom-scale separation between the metal and the explosive. To probe the relationship between the structural properties of metal complexes and explosive performance, a new series of energetic lanthanide complexes has been prepared using energetic ligands such as NTO (5-nitro-2,4-dihydro-1,2,4-triazole-3-one). These are the first examples of lanthanide NTO complexes where no water is coordinated to the metal, demonstrating novel control of the coordination environment. The complexes have been characterized by X-ray crystallography, NMR and IR spectroscopies, photoluminescence, and sensitivity testing. The structural and energetic properties are discussed in the context of enhanced blast effects and detection. Cheetah calculations have been performed to fine-tune physical properties, creating a systematic method for producing explosives with 'tailor made' characteristics. In conclusion, these new complexes will be benchmarks for further study in the field of metalized high explosives.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

CAN ULTRAHIGH-ENERGY COSMIC RAYS COME FROM GAMMA-RAY BURSTS? COSMIC RAYS BELOW THE ANKLE AND GALACTIC GAMMA-RAY BURSTS

The maximum cosmic-ray energy achievable by acceleration by a relativistic blast wave is derived. It is shown that forward shocks from long gamma-ray bursts (GRBs) in the interstellar medium accelerate protons to large enough energies, and have a sufficient energy budget, to produce the Galactic cosmic-ray component just below the ankle at 4 x 10{sup 18} eV, as per an earlier suggestion. It is further argued that, were extragalactic long GRBs responsible for the component above the ankle as well, the occasional Galactic GRB within the solar circle would contribute more than the observational limits on the outward flux from the solar circle, unless an avoidance scenario, such as intermittency and/or beaming, allows the present-day local flux to be less than 10{sup -3} of the average. Difficulties with these avoidance scenarios are noted.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MAGNETIC AND DENSITY SPIKES IN COSMIC-RAY SHOCK PRECURSORS

In shock precursors populated by accelerated cosmic rays (CRs), the CR return current instability is believed to significantly enhance the pre-shock perturbations of magnetic field. We have obtained fully nonlinear exact ideal MHD solutions supported by the CR return current. The solutions occur as localized spikes of circularly polarized Alfven envelopes (solitons or breathers). As the conventional (undriven) solitons, the obtained magnetic spikes propagate at a speed C proportional to their amplitude, C=C{sub A}B{sub max}/{radical}2B{sub 0}. The sufficiently strong solitons run thus ahead of the main shock and stand in the precursor, being supported by the return current. This property of the nonlinear solutions is strikingly different from the linear theory that predicts non-propagating (that is, convected downstream) circularly polarized waves. The nonlinear solutions may come either in isolated pulses (solitons) or in soliton-trains (cnoidal waves). The morphological similarity of such quasi-periodic soliton chains with recently observed X-ray stripes in the Tycho supernova remnant (SNR) is briefly discussed. The magnetic field amplification determined by the suggested saturation process is obtained as a function of decreasing SNR blast wave velocity during its evolution from the ejecta dominated to the Sedov-Taylor stage.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

SECONDARY FLARE RIBBONS OBSERVED BY THE SOLAR DYNAMICS OBSERVATORY

Using the observations from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we statistically investigate the flare ribbons (FRs) of 19 X-class flares of the 24th solar cycle from 2010 June to 2013 August. Of these 19 flares, the source regions of 16 can be observed by AIA and the FRs of each flare are well detected, and 11 of the 16 display multiple ribbons. Based on the ribbon brightness and the relationship between the ribbons and post-flare loops, we divide the multiple ribbons into two types: normal FRs, which are connected by post-flare loops and have been extensively investigated, and secondary flare ribbons (SFRs), which are weaker than the FRs, not connected by post-flare loops, and always have a short lifetime. Of the 11 SFRs, 10 appear simultaneously with the FRs, and none of them have post-flare loops. The last one, on the other hand, appears 80 minutes later than the FR, lasts almost two hours, and also has no post-flare loops detected. We suggest that the magnetic reconnection associated with this SFR is triggered by the blast wave that results from the main flare. These observations imply that in some flare processes, more than two sets of magnetic loops or more than twice the number of magnetic reconnections are involved.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

NEAR-IR STUDIES OF RECURRENT NOVA V745 SCORPII DURING ITS 2014 OUTBURST

The recurrent nova V745 Scorpii underwent its third known outburst on 2014 February 6. Infrared monitoring of the eruption on an almost daily basis, starting from 1.3 days after discovery, shows the emergence of a powerful blast wave generated by the high velocity nova ejecta exceeding 4000 km s{sup –1} plowing into its surrounding environment. The temperature of the shocked gas is raised to a high value exceeding 10{sup 8} K immediately after outburst commencement. The energetics of the outburst clearly surpass those of similar symbiotic systems like RS Oph and V407 Cyg which have giant secondaries. The shock does not show a free-expansion stage but rather shows a decelerative Sedov-Taylor phase from the beginning. Such strong shock fronts are known to be sites for γ-ray generation. V745 Sco is the latest nova, apart from five other known novae, to show γ-ray emission. It may be an important testbed to resolve the crucial question of whether or not all novae are generically γ-ray emitters by virtue of having a circumbinary reservoir of material that is shocked by the ejecta rather than γ-ray generation being restricted to only symbiotic systems with a shocked red giant (RG) wind. The lack of a free-expansion stage favors V745 Sco to have a density enhancement around the white dwarf (WD), above that contributed by a RG wind. Our analysis also suggests that the WD in V745 Sco is very massive and a potential progenitor for a future SN Ia explosion.

79 ASTRONOMY AND ASTROPHYSICS↗