Wesley Even

Swift and NuSTAR observations of GW170817: Detection of a blue kilonova

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Ultraviolet and x-ray observations of a binary neutron star merger show a hot kilonova but no jet along the line of sight. With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and x-ray observations by Swift and the Nuclear Spectroscopic Telescope Array of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading UV emission indicates a high mass (≈0.03 solar masses) wind-driven outflow with moderate electron fraction (Ye ≈ 0.27). Combined with the x-ray limits, we favor an observer viewing angle of ≈30° away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultrarelativistic, highly collimated ejecta (a γ-ray burst afterglow).

SN 2010jl: OPTICAL TO HARD X-RAY OBSERVATIONS REVEAL AN EXPLOSION EMBEDDED IN A TEN SOLAR MASS COCOON

Some supernovae (SNe) may be powered by the interaction of the SN ejecta with a large amount of circumstellar matter (CSM). However, quantitative estimates of the CSM mass around such SNe are missing when the CSM material is optically thick. Specifically, current estimators are sensitive to uncertainties regarding the CSM density profile and the ejecta velocity. Here we outline a method to measure the mass of the optically thick CSM around such SNe. We present new visible-light and X-ray observations of SN 2010jl (PTF 10aaxf), including the first detection of an SN in the hard X-ray band using NuSTAR. The total radiated luminosity of SN 2010jl is extreme—at least 9 × 10^50 erg. By modeling the visible-light data, we robustly show that the mass of the circumstellar material within ~10^16 cm of the progenitor of SN 2010jl was in excess of 10 M_☉. This mass was likely ejected tens of years prior to the SN explosion. Our modeling suggests that the shock velocity during shock breakout was ~6000 km s^–1, decelerating to ~2600 km s^–1 about 2 yr after maximum light. Furthermore, our late-time NuSTAR and XMM spectra of the SN presumably provide the first direct measurement of SN shock velocity 2 yr after the SN maximum light—measured to be in the range of 2000-4500 km s^–1 if the ions and electrons are in equilibrium, and ≳ 2000 km s^–1 if they are not in equilibrium. This measurement is in agreement with the shock velocity predicted by our modeling of the visible-light data. Our observations also show that the average radial density distribution of the CSM roughly follows an r^–2 law. A possible explanation for the ≳ 10 M_☉ of CSM and the wind-like profile is that they are the result of multiple pulsational pair instability events prior to the SN explosion, separated from each other by years.

Seeing the First Supernovae at the Edge of the Universe with JWST

The first stars ended the cosmic dark ages and created the first heavy elements necessary for the formation of planets and life. The properties of these stars remain uncertain, and it may be decades before individual Population III (Pop III) stars are directly observed. Their masses, however, can be inferred from their supernova explosions, which may soon be found in both deep-field surveys by the James Webb Space Telescope (JWST) and in all-sky surveys by the Wide Field Infrared Survey Telescope (WFIRST). We have performed radiation hydrodynamical simulations of the near-infrared signals of Pop III pair-instability supernovae in realistic circumstellar environments with Lyman absorption by the neutral intergalactic medium. We find that JWST and WFIRST will detect these explosions out to z {approx} 30 and 20, respectively, unveiling the first generation of stars in the universe.

SUPERMASSIVE POPULATION III SUPERNOVAE AND THE BIRTH OF THE FIRST QUASARS

The existence of supermassive black holes as early as z ~ 7 is one of the great, unsolved problems in cosmological structure formation. One leading theory argues that they are born during catastrophic baryon collapse in z ~?15 protogalaxies that form in strong Lyman-Werner UV backgrounds. Atomic line cooling in such galaxies fragments baryons into massive clumps that are thought to directly collapse to 104-105 M ? black holes. We have now discovered that some of these fragments can instead become supermassive stars that eventually explode as thermonuclear supernovae (SNe) with energies of ~1055 erg, the most energetic explosions in the universe. We have calculated light curves and spectra for supermassive Pop III SNe with the Los Alamos RAGE and SPECTRUM codes. We find that they will be visible in near-infrared all-sky surveys by Euclid out to z ~ 10-15 and by WFIRST and WISH out to z ~ 15-20, perhaps revealing the birthplaces of the first quasars.

Illuminating the Primeval Universe with Type IIn Supernovae

The detection of Population III (Pop III) supernovae (SNe) could directly probe the primordial initial mass function for the first time, unveiling the properties of the first galaxies, early chemical enrichment and reionization, and the seeds of supermassive black holes. Growing evidence that some Pop III stars were less massive than 100 M ☉ may complicate prospects for their detection, because even though they would have been more plentiful, they would have died as core-collapse SNe, with far less luminosity than pair-instability explosions. This picture greatly improves if the SN shock collides with a dense circumstellar shell ejected during a prior violent luminous blue variable type eruption. Such collisions can turn even dim SNe into extremely bright ones whose luminosities can rival those of pair-instability SNe. We present simulations of Pop III Type IIn SN light curves and spectra performed with the Los Alamos RAGE and SPECTRUM codes. Taking into account Lyα absorption in the early universe and cosmological redshifting, we find that 40 M ☉ Pop III Type IIn SNe will be visible out to z ~ 20 with the James Webb Space Telescope and out to z ~ 7 with WFIRST. Thus, even low mass Pop III SNe can be used to probe the primeval universe.

THE BIGGEST EXPLOSIONS IN THE UNIVERSE

Supermassive primordial stars are expected to form in a small fraction of massive protogalaxies in the early universe, and are generally conceived of as the progenitors of the seeds of supermassive black holes (BHs). Supermassive stars with masses of ∼ 55,000M⊙, however, have been found to explode and completely disrupt in a supernova (SN) with an energy of up to ∼ 10 55 erg instead of collapsing to a BH. Such events, ∼ 10,000 times more energetic than typical SNe today, would be among the biggest explosions in the history of the universe. Here we present a simulation of such a SN in two stages. Using the RAGE radiation hydrodynamics code we first evolve the explosion from an early stage through the breakout of the shock from the surface of the star until the blast wave has propagated out to several parsecs from the explosion site, which lies deep within an atomic cooling dark matter (DM) halo at z ≃ 15. Then, using the GADGET cosmological hydrodynamics code we evolve the explosion out to several kiloparsecs from the explosion site, far into the low-density intergalactic medium. The host DM halo, with a total mass of 4 × 10 7 M⊙, much more massive than typical primordial star-forming halos, is completely evacuated of high density gas after . 10Myr, although dense metal-enriched gas recollapses into the halo, where it will likely form second-generation stars with metallicities of ≃ 0.05Z⊙ after & 70Myr. The chemical signature of supermassive star explosions may be found in such long-lived second-generation stars today. Subject headings: Cosmology: theory — early universe — supernovae: general

THE SUPERNOVA THAT DESTROYED A PROTOGALAXY: PROMPT CHEMICAL ENRICHMENT AND SUPERMASSIVE BLACK HOLE GROWTH

The first primitive galaxies formed from accretion and mergers by z ~ 15, and were primarily responsible for cosmological reionization and the chemical enrichment of the early cosmos. But a few of these galaxies may have formed in the presence of strong Lyman-Werner UV fluxes that sterilized them of H2, preventing them from forming stars or expelling heavy elements into the intergalactic medium prior to assembly. At masses of 108 M ? and virial temperatures of 104?K, these halos began to rapidly cool by atomic lines, perhaps forming 104-106 M ? Pop?III stars and, later, the seeds of supermassive black holes. We have modeled the explosion of a supermassive Pop?III star in the dense core of a line-cooled protogalaxy with the ZEUS-MP code. We find that the supernova (SN) expands to a radius of ~1?kpc, briefly engulfing the entire galaxy, but then collapses back into the potential well of the dark matter. Fallback fully mixes the interior of the protogalaxy with metals, igniting a violent starburst and fueling the rapid growth of a massive black hole at its center. The starburst would populate the protogalaxy with stars in greater numbers and at higher metallicities than in more slowly evolving, nearby halos. The SN remnant becomes a strong synchrotron source that can be observed with eVLA and eMERLIN and has a unique signature that easily distinguishes it from less energetic SN remnants. Such explosions, and their attendant starbursts, may well have marked the birthplaces of supermassive black holes on the sky.

The Los Alamos Supernova Light-curve Project: Computational Methods

We have entered the era of explosive transient astronomy, in which current and upcoming real-time surveys such as the Large Synoptic Survey Telescope, the Palomar Transient Factory, and the Panoramic Survey Telescope and Rapid Response System will detect supernovae in unprecedented numbers. Future telescopes such as the James Webb Space Telescope may discover supernovae from the earliest stars in the universe and reveal their masses. The observational signatures of these astrophysical transients are the key to unveiling their central engines, the environments in which they occur, and to what precision they will pinpoint cosmic acceleration and the nature of dark energy. We present a new method for modeling supernova light curves and spectra with the radiation hydrodynamics code RAGE coupled with detailed monochromatic opacities in the SPECTRUM code. We include a suite of tests that demonstrate how the improved physics and opacities are indispensable to modeling shock breakout and light curves when radiation and matter are tightly coupled.

Fast evolving pair-instability supernova models: evolution, explosion, light curves

With an increasing number of superluminous supernovae (SLSNe) discovered the question of their origin remains open and causes heated debates in the supernova community. Currently, there are three proposed mechanisms for SLSNe: (1) pair-instability supernovae (PISN), (2) magnetar-driven supernovae, and (3) models in which the supernova ejecta interacts with a circumstellar material ejected before the explosion. Based on current observations of SLSNe, the PISN origin has been disfavoured for a number of reasons. Many PISN models provide overly broad light curves and too reddened spectra, because of massive ejecta and a high amount of nickel. In the current study we re-examine PISN properties using progenitor models computed with the GENEC code. We calculate supernova explosions with FLASH and light curve evolution with the radiation hydrodynamics code STELLA. We find that high-mass models (200 and 250 solar masses) at relatively high metallicity (Z=0.001) do not retain hydrogen in the outer layers and produce relatively fast evolving PISNe Type I and might be suitable to explain some SLSNe. We also investigate uncertainties in light curve modelling due to codes, opacities, the nickel-bubble effect and progenitor structure and composition.

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 pulsational pair-instability 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 ☉ pulsational pair-instability explosion done with the Los Alamos radiation hydrodynamics code Radiation Adaptive Grid Eulerian. We find that collisions between consecutive pulsations are visible in the near infrared out to z ~ 15-20 and can probe the earliest stellar populations at cosmic dawn.