Anthony L. Piro

Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave source

Photons from a gravitational wave event Two neutron stars merging together generate a gravitational wave signal and have also been predicted to emit electromagnetic radiation. When the gravitational wave event GW170817 was detected, astronomers rushed to search for the source using conventional telescopes (see the Introduction by Smith). Coulter et al. describe how the One-Meter Two-Hemispheres (1M2H) collaboration was the first to locate the electromagnetic source. Drout et al. present the 1M2H measurements of its optical and infrared brightness, and Shappee et al. report their spectroscopy of the event, which is unlike previously detected astronomical transient sources. Kilpatrick et al. show how these observations can be explained by an explosion known as a kilonova, which produces large quantities of heavy elements in nuclear reactions. Science, this issue p. 1556, p. 1570, p. 1574, p. 1583; see also p. 1554 A rapid astronomical search located the optical counterpart of the neutron star merger GW170817. On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.

Time-dependent models of accretion discs formed from compact object mergers

We present time-dependent models of the remnant accretion discs created during compact object mergers, focusing on the energy available from accretion at late times and the composition of the disc and its outflows. We calculate the dynamics near the outer edge of the disc, which contains the majority of the discs mass and determines the accretion rate on to the central black hole. This treatment allows us to follow the evolution over much longer time-scales (100s or longer) than current hydrodynamic simulations. At late times the disc becomes advective and its properties asymptote to self-similar solutions with an accretion rate M d ∞ t -4/3 (neglecting outflows). This late-time accretion can in principle provide sufficient energy to power the late-time activity observed by Swift from some short-duration gamma-ray bursts. However, because outflows during the advective phase unbind the majority of the remaining mass, it is difficult for the remnant disc alone to produce significant accretion power well beyond the onset of the advective phase. Unless the viscosity is quite low (α ≤ 10 -3 ), this occurs before the start of observed flaring at ∼30 s; continued mass inflow at late times thus appears required to explain the late-time activity from short-duration gamma-ray bursts. We show that the composition of the disc freezes-out when the disc is relatively neutron rich (electron fraction Y e ≃ 0.3). Roughly 10 -2 M ⊙ of this neutron-rich material is ejected by winds at late times. During earlier, neutrino-cooled phases of accretion, neutrino irradiation of the disc produces a wind with Y e ≃ 0.5, which synthesizes at most ∼10 -3 M ⊙ of 56 Ni. We highlight what conditions are favorable for 56 Ni production and predict, in the best cases, optical and infrared transients peaking ∼0.5-2d after the burst, with fluxes a factor of ∼10 below the current observational limits.

Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars

The coalescence of binary neutron stars (NSs) may in some cases produce a stable massive NS remnant rather than a black hole. Due to the substantial angular momentum from the binary, such a remnant is born rapidly rotating and likely acquires a strong magnetic field (a ‘millisecond magnetar’). Magnetic spin-down deposits a large fraction of the rotational energy from the magnetar behind the small quantity of mass ejected during the merger. If the magnetar outflow is indeed trapped behind the ejecta (instead of placing most of its energy into a collimated jet), this has the potential for creating a bright transient that could be useful for determining whether an NS or black hole was formed in the merger. We investigate the expected signature of such an event, including for the first time the important impact of e^± pairs injected by the millisecond magnetar into the surrounding nebula. These pairs cool via synchrotron and inverse Compton emission, producing a pair cascade and hard X-ray spectrum. A fraction of these X-rays are absorbed by the ejecta walls and re-emitted as thermal radiation, leading to an optical/UV transient peaking at a luminosity of ∼10^(43)–10^(44) erg s^(−1) on a time-scale of several hours to days. This is dimmer than predicted by simpler analytic models because the large optical depth of e^± pairs across the nebula suppresses the efficiency with which the magnetar spin-down luminosity is thermalized. Nevertheless, the optical/UV emission is more than two orders of magnitude brighter than a radioactively powered ‘kilonova’. In some cases, nebular X-rays are sufficiently luminous to re-ionize the ejecta, in which case non-thermal X-rays escape the ejecta unattenuated with a similar peak luminosity and time-scale as the optical radiation. We discuss the implications of our results for the temporally extended X-ray emission that is observed to follow some short gamma-ray bursts (GRBs), including the kilonova candidates GRB 080503 and GRB 130603B.

Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis

Photons from a gravitational wave event Two neutron stars merging together generate a gravitational wave signal and have also been predicted to emit electromagnetic radiation. When the gravitational wave event GW170817 was detected, astronomers rushed to search for the source using conventional telescopes (see the Introduction by Smith). Coulter et al. describe how the One-Meter Two-Hemispheres (1M2H) collaboration was the first to locate the electromagnetic source. Drout et al. present the 1M2H measurements of its optical and infrared brightness, and Shappee et al. report their spectroscopy of the event, which is unlike previously detected astronomical transient sources. Kilpatrick et al. show how these observations can be explained by an explosion known as a kilonova, which produces large quantities of heavy elements in nuclear reactions. Science, this issue p. 1556, p. 1570, p. 1574, p. 1583; see also p. 1554 Photometric observations of a neutron star merger show that it produced heavy elements through r-process nucleosynthesis. On 17 August 2017, gravitational waves (GWs) were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB 170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days postmerger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in rapid neutron capture (r-process) nucleosynthesis in the universe.

SHOCK BREAKOUT FROM TYPE Ia SUPERNOVA

The mode of explosive burning in Type Ia supernovae (SNe Ia) remains an outstanding problem. It is generally thought to begin as a subsonic deflagration, but this may transition into a supersonic detonation (the delayed detonation transition, DDT). We argue that this transition leads to a breakout shock, which would provide the first unambiguous evidence that DDTs occur. Its main features are a hard X-ray flash (~20 keV) lasting ~10–2 s with a total radiated energy of ~1040 erg, followed by a cooling tail. This creates a distinct feature in the visual light curve, which is separate from the nickel decay. This cooling tail has a maximum absolute visual magnitude of MV ≈ –9 to –10 at ≈1 day, which depends most sensitively on the white dwarf radius at the time of the DDT. As the thermal diffusion wave moves in, the composition of these surface layers may be imprinted as spectral features, which would help to discern between SN Ia progenitor models. Since this feature should accompany every SNe Ia, future deep surveys (e.g., m = 24) will see it out to a distance of ≈80 Mpc, giving a maximum rate of ~60 yr-1. Archival data sets can also be used to study the early rise dictated by the shock heating (at ≈20 days before maximum B-band light). A similar and slightly brighter event may also accompany core bounce during the accretion-induced collapse to a neutron star, but with a lower occurrence rate.

MAGNETOROTATIONAL CORE-COLLAPSE SUPERNOVAE IN THREE DIMENSIONS

We present results of new three-dimensional (3D) general-relativistic magnetohydrodynamic simulations of rapidly rotating strongly magnetized core collapse. These simulations are the first of their kind and include a microphysical finite-temperature equation of state and a leakage scheme that captures the overall energetics and lepton number exchange due to postbounce neutrino emission. Our results show that the 3D dynamics of magnetorotational core-collapse supernovae are fundamentally different from what was anticipated on the basis of previous simulations in axisymmetry (2D). A strong bipolar jet that develops in a simulation constrained to 2D is crippled by a spiral instability and fizzles in full 3D. While multiple (magneto-)hydrodynamic instabilities may be present, our analysis suggests that the jet is disrupted by an m = 1 kink instability of the ultra-strong toroidal field near the rotation axis. Instead of an axially symmetric jet, a completely new, previously unreported flow structure develops. Highly magnetized spiral plasma funnels expelled from the core push out the shock in polar regions, creating wide secularly expanding lobes. We observe no runaway explosion by the end of the full 3D simulation 185 ms after bounce. At this time, the lobes have reached maximum radii of ~900 km.

Neutron‐rich freeze‐out in viscously spreading accretion discs formed from compact object mergers

Accretion discs with masses ∼10 −3 -0.1 Mare believed to form during the merger of a neu- tron star (NS) with another NS and the merger of a NS with a black hole (BH). Soon after their formation, such hyperaccreting discs cool efficiently by neutrino emission and their composi- tion is driven neutron-rich by pair captures under degenerate conditions. However, as the disc viscously spreads and its temperature drops, neutrino cooling is no longer able to offset vis- cous heating and the disc becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the discs remaining mass. We calculate the thermal evolution and nuclear composition of viscously spreading accretion discs formed from compact object mergers using one-dimensional height-integrated simulations. We show that freeze-out from weak equilibrium necessarily accompanies the discs late-time transition to an advective state. As a result, hyperaccreting discs generically freeze-out neutron-rich (with electron fraction Y e ∼ 0.2-0.4), and their late-time outflows ro- bustly synthesize rare neutron-rich isotopes. Using the measured abundances of these isotopes in our Solar system, we constrain the compact object merger rate in the Milky Way to be 10 −5 (Md,0/0.1 M� ) −1 yr −1 , where Md,0 is the average initial mass of the accretion disc. Thus, either the NS-NS merger rate is at the low end of current estimates or the average disc mass produced during a typical merger is � 0.1 M� . Based on the results of current general relativistic merger simulations, the latter constraint suggests that prompt collapse to a BH is a more common outcome of NS-NS mergers than the formation of a transient hypermassive NS. We also show that if most short-duration gamma-ray bursts (GRBs) are produced by compact object mergers, their beaming fraction must exceed f b ≈ 0.13(Md,0/0.1 M� ), corresponding to a jet half-opening angle 30 ◦ (Md,0/0.1 M� ) 1/2 . This is consistent with other evidence that

The diversity of Type II supernova versus the similarity in their progenitors

The authors acknowledge the ASASSN, La Silla Quest, and LOSS surveys for discovering new SNe that made this study possible. This material is based upon work supported by the National Science Foundation (NSF) under Grant No. 1313484. MDS gratefully acknowledges generous support provided by the Danish Agency for Science and Technology and Innovation realized through a Sapere Aude Level 2 grant. MF is supported by the European Union FP7 programme through ERC grant number 320360. SJS acknowledges funding from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement No. [291222] and STFC grants ST/I001123/1 and ST/L000709/1. AVFs group at UC Berkeley is grateful for financial assistance from NSF grant AST-1211916, the TABASGO Foundation, Gary and Cynthia Bengier, and the Christopher R. Redlich Fund. This work was supported by the NSF under grants PHY-1125915 and AST-1109174. M.S. acknowledges support from EU/FP7-ERC grant no [615929]. This paper is based on observations made with the Swift, LCOGT, Gemini, and Keck Observatories; we thank their respective staffs for excellent assistance. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. Based on observations collected at the European Organization for Astronomical Research in the Southern hemisphere, Chile as part of PESSTO, (the Public ESO Spectroscopic Survey for Transient Objects Survey) ESO program ID 188.D-3003.

THE INTERNAL SHEAR OF TYPE Ia SUPERNOVA PROGENITORS DURING ACCRETION AND SIMMERING

A white dwarf (WD) gains substantial angular momentum during the accretion process that grows it toward a Chandrasekhar mass. It is therefore expected to be rotating quickly when it ignites as a Type Ia supernova. The thermal and shearing profiles are important for subsequent flame propagation. We highlight processes that could affect the WD shear during accretion, as well as during the ~1000 yr of pre-explosive simmering. Baroclinic instabilities and/or the shear growth of small magnetic fields provide sufficient torque to bring the WD very close to solid-body rotation during accretion. The lack of significant shear makes it difficult to grow a WD substantially past the typical Chandrasekhar mass. Once carbon ignites, a convective region spreads from the center of the WD. This phase occurs regardless of progenitor scenario, and therefore it is of great interest for understanding how the WD interior is prepared before the explosive burning begins. We summarize some of the key properties of the convective region, including a demonstration that the mass enclosed by convection at any given time depends most sensitively on a single parameter that can be expressed as either the ratio of temperatures or densities at the top and bottom of the convection zone. At low Rossby numbers, the redistribution of angular momentum by convection may result in significant shearing at the convective/nonconvective boundary.

The rising light curves of Type Ia supernovae

We present an analysis of the early, rising light curves of 18 Type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory and the La Silla-QUEST variability survey. We fit these early data flux using a simple power law (f(t) = α × t^n) to determine the time of first light (t_0), and hence the rise time (t_(rise)) from first light to peak luminosity, and the exponent of the power-law rise (n). We find a mean uncorrected rise time of 18.98 ± 0.54 d, with individual supernova (SN) rise times ranging from 15.98 to 24.7 d. The exponent n shows significant departures from the simple ‘fireball model’ of n = 2 (or f(t) ∝ t^2) usually assumed in the literature. With a mean value of n = 2.44 ± 0.13, our data also show significant diversity from event to event. This deviation has implications for the distribution of ^(56)Ni throughout the SN ejecta, with a higher index suggesting a lesser degree of ^(56)Ni mixing. The range of n found also confirms that the ^(56)Ni distribution is not standard throughout the population of SNe Ia, in agreement with earlier work measuring such abundances through spectral modelling. We also show that the duration of the very early light curve, before the luminosity has reached half of its maximal value, does not correlate with the light-curve shape or stretch used to standardize SNe Ia in cosmological applications. This has implications for the cosmological fitting of SN Ia light curves.