arXiv subjects

Chris L. Fryer

Publications and source records attributed to Chris L. Fryer.

At least 73 records · Page 4Linked to original sources

arXiv2016

JINA-NuGrid Galactic Chemical Evolution Pipeline

Galactic chemical evolution is a topic that involves nuclear physics, stellar evolution, galaxy evolution, observation, and cosmology. Continuous communication and feedback between these fields is a key component in improving our understanding of how galaxies form and how elements are created and recycled in galaxies and intergalactic space. In this proceedings, we present the current state of the JINA-NuGrid chemical evolution pipeline. It is designed to probe the impact of nuclear astrophysics uncertainties on galactic chemical evolution, to improve our knowledges regarding the origin of the elements in a cosmological context, and to create the required interdisciplinary connections.

astro-ph.GA↗

arXiv2016

Explaining TeV Cosmic-Ray Anisotropies with Non-Diffusive Cosmic-Ray Propagation

Constraining the behavior of cosmic ray data observed at Earth requires a precise understanding of how the cosmic rays propagate in the interstellar medium. The interstellar medium is not homogeneous; although turbulent magnetic fields dominate over large scales, small coherent regions of magnetic field exist on scales relevant to particle propagation in the nearby Galaxy. Guided propagation through a coherent field is significantly different from random particle diffusion and could be the explanation of spatial anisotropies in the observed cosmic rays. We present a Monte Carlo code to propagate cosmic particle through realistic magnetic field structures. We discuss the details of the model as well as some preliminary studies which indicate that coherent magnetic structures are important effects in local cosmic-ray propagation, increasing the flux of cosmic rays by over two orders of magnitude at anisotropic locations on the sky. The features induced by coherent magnetic structure could be the cause of the observed TeV cosmic-ray anisotropy.

astro-ph.HE↗

arXiv2015

A Spatially Resolved Study of the Synchrotron Emission and Titanium in Tycho's Supernova Remnant with NuSTAR

We report results from deep observations (~750 ks) of Tycho's supernova remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over several energy bands to identify the regions producing the hardest X-rays and to search for radioactive decay line emission from 44Ti. We find that the hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where recent Chandra observations have revealed high emissivity "stripes" associated with particles accelerated to the knee of the cosmic-ray spectrum. We do not find evidence of 44Ti, and we set limits on its presence and distribution within the SNR. These limits correspond to a upper-limit 44Ti mass of M44 < 2.4x10^-4 M_sun for a distance of 2.3 kpc. We perform spatially resolved spectroscopic analysis of sixty-six regions across Tycho. We map the best-fit rolloff frequency of the hard X-ray spectra, and we compare these results to measurements of the shock expansion and ambient density. We find that the highest energy electrons are accelerated at the lowest densities and in the fastest shocks, with a steep dependence of the roll-off frequency with shock velocity. Such a dependence is predicted by models where the maximum energy of accelerated electrons is limited by the age of the SNR rather than by synchrotron losses, but this scenario requires far lower magnetic field strengths than those derived from observations in Tycho. One way to reconcile these discrepant findings is through shock obliquity effects, and future observational work is necessary to explore the role of obliquity in the particle acceleration process.

astro-ph.HE↗

arXiv2015

The First Gamma-Ray Bursts in the Universe

Gamma-ray bursts (GRBs) are the ultimate cosmic lighthouses, capable of illuminating the universe at its earliest epochs. Could such events probe the properties of the first stars at z $\sim$ 20, the end of the cosmic Dark Ages? Previous studies of Population III GRBs only considered explosions in the diffuse relic H II regions of their progenitors, or bursts that are far more more energetic than those observed to date. But the processes that produce GRBs at the highest redshifts likely reset their local environments, creating much more complicated structures than those in which relativistic jets have been modeled so far. These structures can greatly affect the luminosity of the afterglow, and hence the redshift at which it can be detected. We have now simulated Population III GRB afterglows in H II regions, winds, and dense shells ejected by the star during the processes that produce the burst. Our model, which has been used in previous work, has been extended to include contributions from reverse shocks, inverse Compton cooling and the effects of sphericity and beaming in the blast wave, and is valid in a variety of circumjet density profiles. We find that GRBs with E$_{\mathrm{iso},γ} =$ 10$^{ 51}$ - 10$^{53}$ erg will be visible at z $\gtrsim$ 20 to the next generation of near infrared and radio observatories. In many cases, the environment of the burst, and hence progenitor type, can be inferred from the afterglow light curve. Although some Population III GRBs are visible to Swift and the Very Large Array now, the optimal strategy for their detection will be future missions like EXIST and JANUS, which have large survey areas and onboard X-ray and infrared telescopes that can track their near infrared flux from the moment of the burst, thereby identifying its redshift.

astro-ph.HE↗

arXiv2015

Population III Hypernovae

Population III supernovae have been of growing interest of late for their potential to directly probe the properties of the first stars, particularly the most energetic events that are visible near the edge of the observable universe. But until now, hypernovae, the unusually energetic Type Ib/c supernovae that are sometimes associated with gamma-ray bursts, have been overlooked as cosmic beacons at the highest redshifts. In this, the latest of a series of studies on Population III supernovae, we present numerical simulations of 25 - 50 M$_{\odot}$ hypernovae and their light curves done with the Los Alamos RAGE and SPECTRUM codes. We find that they will be visible at z = 10 - 15 to the James Webb Space Telescope (JWST) and z = 4 - 5 to the Wide-Field Infrared Survey Telescope (WFIRST), tracing star formation rates in the first galaxies and at the end of cosmological reionization. If, however, the hypernova crashes into a dense shell ejected by its progenitor, it is expected that a superluminous event will occur that may be seen at z ~ 20, in the first generation of stars.

astro-ph.CO↗

arXiv2015

The Fate of the Compact Remnant in Neutron Star Mergers

Neutron star (binary neutron star and neutron star - black hole) mergers are believed to produce short-duration gamma-ray bursts. They are also believed to be the dominant source of gravitational waves to be detected by the advanced LIGO and the dominant source of the heavy r-process elements in the universe. Whether or not these mergers produce short-duration GRBs depends sensitively on the fate of the core of the remnant (whether, and how quickly, it forms a black hole). In this paper, we combine the results of merger calculations and equation of state studies to determine the fate of the cores of neutron star mergers. Using population studies, we can determine the distribution of these fates to compare to observations. We find that black hole cores form quickly only for equations of state that predict maximum non-rotating neutron star masses below 2.3-2.4 solar masses. If quick black hole formation is essential in producing gamma-ray bursts, LIGO observed rates compared to GRB rates could be used to constrain the equation of state for dense nuclear matter.

astro-ph.HE↗

arXiv2015

Assessing the Observability of Hypernovae and Pair-Instability Supernovae in the Early Universe

The era of the universe's first (Population III) stars is essentially unconstrained by observation. Ultra-luminous and massive stars from this time altered the chemistry of the cosmos, provided the radiative scaffolding to support the formation of the first protogalaxies, and facilitated the creation and growth of now-supermassive black holes. Unfortunately, because these stars lie literally at the edge of the observable universe, they will remain beyond the reach of even the next generation of telescopes such as the James Webb Space Telescope and the Thirty-Meter Telescope. In this paper, we provide a primer to supernovae modeling and the first stars to make our discussion accessible to those new to or outside our field. We review recent work of the Los Alamos Supernova Light Curve Project and Brigham Young University to explore the possibility of probing this era through observations of the spectacular deaths of the first stars. We find that many such brilliant supernova explosions will be observable as far back as $\sim 99$% of the universe's current age, tracing primordial star formation rates and the locations of their protogalaxies on the sky. The observation of Population III supernovae will be among the most spectacular discoveries in observational astronomy in the coming decade.

astro-ph.HE↗

arXiv2015

The Effects on Supernova Shock Breakout and Swift Light Curves Due to the Mass of the Hydrogen-Rich Envelope

Mass loss remains one of the primary uncertainties in stellar evolution. In the most massive stars, mass loss dictates the circumstellar medium and can significantly alter the fate of the star. Mass loss is caused by a variety of wind mechanisms and also through binary interactions. Supernovae are excellent probes of this mass loss, both the circumstellar material and the reduced mass of the hydrogen-rich envelope. In this paper, we focus on the effects of reducing the hydrogen-envelope mass on the supernova light curve, studying both the shock breakout and peak light curve emission for a wide variety of mass loss scenarios. Even though the trends of this mass loss will be masked somewhat by variations caused by different progenitors, explosion energies, and circumstellar media, these trends have significant effects on the supernova light-curves that should be seen in supernova surveys. We conclude with a comparison of our results to a few key observations.

astro-ph.HE↗

arXiv2015

Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuSTAR

We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of the Crab nebula and pulsar. We show that while the phase-averaged and spatially integrated nebula + pulsar spectrum is a power-law in this energy band, spatially resolved spectroscopy of the nebula finds a break at $\sim$9 keV in the spectral photon index of the torus structure with a steepening characterized by $ΔΓ\sim0.25$. We also confirm a previously reported steepening in the pulsed spectrum, and quantify it with a broken power-law with break energy at $\sim$12 keV and $ΔΓ\sim0.27$. We present spectral maps of the inner 100\as\ of the remnant and measure the size of the nebula as a function of energy in seven bands. These results find that the rate of shrinkage with energy of the torus size can be fitted by a power-law with an index of $γ= 0.094\pm 0.018$, consistent with the predictions of Kennel and Coroniti (1984). The change in size is more rapid in the NW direction, coinciding with the counter-jet where we find the index to be a factor of two larger. NuSTAR observed the Crab during the latter part of a $γ$-ray flare, but found no increase in flux in the 3 - 78 keV energy band.

astro-ph.HE↗

arXiv2015

Locating the most energetic electrons in Cassiopeia A

We present deep ($>$2.4 Ms) observations of the Cassiopeia A supernova remnant with {\it NuSTAR}, which operates in the 3--79 keV bandpass and is the first instrument capable of spatially resolving the remnant above 15 keV. We find that the emission is not entirely dominated by the forward shock nor by a smooth "bright ring" at the reverse shock. Instead we find that the $>$15 keV emission is dominated by knots near the center of the remnant and dimmer filaments near the remnant's outer rim. These regions are fit with unbroken power-laws in the 15--50 keV bandpass, though the central knots have a steeper ($Γ\sim -3.35$) spectrum than the outer filaments ($Γ\sim -3.06$). We argue this difference implies that the central knots are located in the 3-D interior of the remnant rather than at the outer rim of the remnant and seen in the center due to projection effects. The morphology of $>$15 keV emission does not follow that of the radio emission nor that of the low energy ($<$12 keV) X-rays, leaving the origin of the $>$15 keV emission as an open mystery. Even at the forward shock front we find less steepening of the spectrum than expected from an exponentially cut off electron distribution with a single cutoff energy. Finally, we find that the GeV emission is not associated with the bright features in the {\it NuSTAR} band while the TeV emission may be, suggesting that both hadronic and leptonic emission mechanisms may be at work.

astro-ph.HE↗

arXiv2014

Pair-Instability Supernovae in the Local Universe

The discovery of 150 - 300 M$_{\odot}$ stars in the Local Group and pair-instability supernova candidates at low redshifts has excited interest in this exotic explosion mechanism. Realistic light curves for pair-instability supernovae at near-solar metallicities are key to identifying and properly interpreting these events as more are found. We have modeled pair-instability supernovae of 150 - 500 M$_{\odot}$ Z $\sim$ 0.1 - 0.4 Z$_{\odot}$ stars. These stars lose up to 80% of their mass to strong line-driven winds and explode as bare He cores. We find that their light curves and spectra are quite different from those of Population III pair-instability explosions, which therefore cannot be used as templates for low-redshift events. Although non-zero metallicity pair-instability supernovae are generally dimmer than their Population III counterparts, in some cases they will be bright enough to be detected at the earliest epochs at which they can occur, the formation of the first galaxies at $z \sim$ 10 - 15. Others can masquerade as dim, short duration supernovae that are only visible in the local universe and that under the right conditions could be hidden in a wide variety of supernova classes. We also report for the first time that some pair-instability explosions can create black holes with masses of $\sim$ 100 M$_{\odot}$.

astro-ph.HE↗

arXiv2014

The Hard X-Ray View of the Young Supernova Remnant G1.9+0.3

NuSTAR observed G1.9+0.3, the youngest known supernova remnant in the Milky Way, for 350 ks and detected emission up to $\sim$30 keV. The remnant's X-ray morphology does not change significantly across the energy range from 3 to 20 keV. A combined fit between NuSTAR and CHANDRA shows that the spectrum steepens with energy. The spectral shape can be well fitted with synchrotron emission from a power-law electron energy distribution with an exponential cutoff with no additional features. It can also be described by a purely phenomenological model such as a broken power-law or a power-law with an exponential cutoff, though these descriptions lack physical motivation. Using a fixed radio flux at 1 GHz of 1.17 Jy for the synchrotron model, we get a column density of N$_{\rm H}$ = $(7.23\pm0.07) \times 10^{22}$ cm$^{-2}$, a spectral index of $α=0.633\pm0.003$, and a roll-off frequency of $ν_{\rm rolloff}=(3.07\pm0.18) \times 10^{17}$ Hz. This can be explained by particle acceleration, to a maximum energy set by the finite remnant age, in a magnetic field of about 10 $μ$G, for which our roll-off implies a maximum energy of about 100 TeV for both electrons and ions. Much higher magnetic-field strengths would produce an electron spectrum that was cut off by radiative losses, giving a much higher roll-off frequency that is independent of magnetic-field strength. In this case, ions could be accelerated to much higher energies. A search for $^{44}$Ti emission in the 67.9 keV line results in an upper limit of $1.5 \times 10^{-5}$ $\,\mathrm{ph}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ assuming a line width of 4.0 keV (1 sigma).

astro-ph.HE↗

arXiv2014

Hypercritical Accretion, Induced Gravitational Collapse, and Binary-Driven Hypernovae

The induced gravitational collapse (IGC) paradigm has been successfully applied to the explanation of the concomitance of gamma-ray bursts (GRBs) with supernovae (SNe) Ic. The progenitor is a tight binary system composed by a carbon-oxygen (CO) core and a neutron star (NS) companion. The explosion of the SN leads to hypercritical accretion onto the NS companion which reaches the critical mass, hence inducing its gravitational collapse to a black hole (BH) with consequent emission of the GRB. The first estimates of this process were based on a simplified model of the binary parameters and the Bondi-Hoyle-Lyttleton accretion rate. We present here the first full numerical simulations of the IGC phenomenon. We simulate the core-collapse and SN explosion of CO stars to obtain the density and ejection velocity of the SN ejecta. We follow the hydrodynamic evolution of the accreting material falling into the Bondi-Hoyle surface of the NS all the way up to its incorporation to the NS surface. The simulations go up to BH formation when the NS reaches the critical mass. For appropriate binary parameters the IGC occurs in short timescales (100-1000s) owing to the combined effective action of the photon trapping and the neutrino cooling near the NS surface. We also show that the IGC scenario leads to a natural explanation for why GRBs are associated only to SN Ic with totally absent or very little helium.

astro-ph.HE↗

arXiv2014

High-Energy X-ray Imaging of the Pulsar Wind Nebula MSH~15-52: Constraints on Particle Acceleration and Transport

We present the first images of the pulsar wind nebula (PWN) MSH 15-52 in the hard X-ray band (>8 keV), as measured with the Nuclear Spectroscopic Telescope Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the 3-7 keV band is similar to that seen in Chandra high-resolution imaging. However, the spatial extent decreases with energy, which we attribute to synchrotron energy losses as the particles move away from the shock. The hard-band maps show a relative deficit of counts in the northern region towards the RCW 89 thermal remnant, with significant asymmetry. We find that the integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a spectral break at 6 keV which may be explained by a break in the synchrotron-emitting electron distribution at ~200 TeV and/or imperfect cross calibration. We also measure spatially resolved spectra, showing that the spectrum of the PWN softens away from the central pulsar B1509-58, and that there exists a roughly sinusoidal variation of spectral hardness in the azimuthal direction. We discuss the results using particle flow models. We find non-monotonic structure in the variation with distance of spectral hardness within 50" of the pulsar moving in the jet direction, which may imply particle and magnetic-field compression by magnetic hoop stress as previously suggested for this source. We also present 2-D maps of spectral parameters and find an interesting shell-like structure in the NH map. We discuss possible origins of the shell-like structure and their implications.

astro-ph.HE↗

arXiv2014

The Formation and Gravitational-Wave Detection of Massive Stellar Black-Hole Binaries

If binaries consisting of two 100 Msun black holes exist they would serve as extraordinarily powerful gravitational-wave sources, detectable to redshifts of z=2 with the advanced LIGO/Virgo ground-based detectors. Large uncertainties about the evolution of massive stars preclude definitive rate predictions for mergers of these massive black holes. We show that rates as high as hundreds of detections per year, or as low as no detections whatsoever, are both possible. It was thought that the only way to produce these massive binaries was via dynamical interactions in dense stellar systems. This view has been challenged by the recent discovery of several stars with mass above 150 Msun in the R136 region of the Large Magellanic Cloud. Current models predict that when stars of this mass leave the main sequence, their expansion is insufficient to allow common envelope evolution to efficiently reduce the orbital separation. The resulting black-hole--black-hole binary remains too wide to be able to coalesce within a Hubble time. If this assessment is correct, isolated very massive binaries do not evolve to be gravitational-wave sources. However, other formation channels exist. For example, the high multiplicity of massive stars, and their common formation in relatively dense stellar associations, opens up dynamical channels for massive black hole mergers (e.g., via Kozai cycles or repeated binary-single interactions). We identify key physical factors that shape the population of very massive black-hole--black-hole binaries. Advanced gravitational-wave detectors will provide important constraints on the formation and evolution of very massive stars.

astro-ph.HE↗

arXiv2014

X-ray Spectral Components in the Afterglow of GRB 130925A

We have identified spectral features in the late-time X-ray afterglow of the unusually long, slow-decaying GRB 130925A using NuSTAR, Swift-XRT, and Chandra. A spectral component in addition to an absorbed power-law is required at $>4σ$ significance, and its spectral shape varies between two observation epochs at $2\times10^5$ and $10^6$ seconds after the burst. Several models can fit this additional component, each with very different physical implications. A broad, resolved Gaussian absorption feature of several keV width improves the fit, but it is poorly constrained in the second epoch. An additive black body or second power-law component provide better fits. Both are challenging to interpret: the blackbody radius is near the scale of a compact remnant ($10^8$ cm), while the second powerlaw component requires an unobserved high-energy cutoff in order to be consistent with the non-detection by Fermi-LAT.

astro-ph.HE↗

arXiv2014

Comparison of LIGO/Virgo upper limits with predicted compact binary merger rates

We compare the current LIGO/Virgo upper limits on double compact object volumetric merger rates with our theoretical predictions. Our optimistic models are a factor of 3 below the existing upper limits for massive BH-BH systems with total mass 50-70 Msun, suggesting that a small increase in observational sensitivity may bring the first detections. The LIGO/Virgo gravitational wave observatories are currently being upgraded to advanced design sensitivity. If a sizeable population of BH-BH binaries is detected, the maximum total binary mass of this population will discriminate between two general families of common envelope models. If no binaries are detected, the new upper limits will provide astrophysically useful information about the environment and physical processes (e.g., metallicity of host galaxies or BH natal kicks) crucial to the formation of binaries containing black holes. For NS-NS systems, our predicted rates are 3 orders of magnitude below the current upper limits; even if advanced instruments reach their design sensitivities (factors of 10 times in distance, and 1,000 times in volumetric rate) the detection of NS-NS systems is not assured. However, we note that although our predicted NS-NS merger rates are consistent with estimates derived from Galactic NS-NS binaries and short GRBs, they are on the low side of these empirical estimates.

astro-ph.HE↗

arXiv2013

Finding the First Cosmic Explosions. III. Pulsational Pair-Instability Supernovae

Population III supernovae have been the focus of growing attention because of their potential to directly probe the properties of the first stars, particularly the most energetic events that can be seen at the edge of the observable universe. But until now pair-pulsation supernovae, in which explosive thermonuclear burning in massive stars fails to unbind them but can eject their outer layers into space, have been overlooked as cosmic beacons at the earliest redshifts. These shells can later collide and, like Type IIn supernovae, produce superluminous events in the UV at high redshifts that could be detected in the near infrared today. We present numerical simulations of a 110 M$_{\odot}$ pair-pulsation explosion done with the Los Alamos radiation hydrodynamics code RAGE. We find that collisions between consecutive pair pulsations are visible in the near infrared out to z $\sim$ 15 - 20 and can probe the earliest stellar populations at cosmic dawn.

astro-ph.CO↗