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 disc's 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 (100 s 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 (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 Ye≃ 0.3). Roughly 10−2M⊙ 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 Ye≃ 0.5, which synthesizes at most ∼10−3M⊙ of 56Ni. We highlight what conditions are favorable for 56Ni production and predict, in the best cases, optical and infrared transients peaking ∼0.5–2 d after the burst, with fluxes a factor of ∼10 below the current observational limits.
High-quality collections of Type II supernova (SN) light curves are scarce because they evolve for hundreds of days, making follow-up observations time consuming and often extending over multiple ...observing seasons. In light of these difficulties, the diversity of SNe II is not fully understood. Here we present ultraviolet and optical photometry of 12 SNe II monitored by the Las Cumbres Observatory Global Telescope Network during 2013 to 2014, and compare them with previously studied SNe having well-sampled light curves. We explore SN II diversity by searching for correlations between the slope of the linear light-curve decay after maximum light (historically used to divide SNe II into IIL and IIP) and other measured physical properties. While SNe IIL are found to be on average more luminous than SNe IIP, SNe IIL do not appear to synthesize more 56Ni than SNe IIP. Finally, optical nebular spectra obtained for several SNe in our sample are found to be consistent with models of red supergiant progenitors in the 12–16 M⊙ range. Consequently, SNe IIL appear not to account for the deficit of massive red supergiants as SN II progenitors.
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.
The Thermodynamics of Advanced Fuels – International Database (TAF-ID) was developed using the Calphad method to provide a computational tool to perform thermodynamic calculations on nuclear fuel ...materials under normal and off-normal conditions. Different kinds of fuels are considered: oxide, metallic, carbide and nitride fuels. Many fission products are introduced as well as structural materials (e.g., zirconium, steel, concrete, SiC) and absorbers (e.g., B4C), in order to investigate the thermochemistry of irradiated fuels and to predict their chemical interaction with the surrounding materials. The approach to develop the database and the models implemented in the database are described. Examples of models for key chemical systems are presented. Finally, a few examples of application calculations on severe accidents with UO2 fuels, irradiated fuel chemistry of MOX and metallic fuels and metallic fuel/cladding interaction show how this tool can be used. To validate the database, the calculations are compared to the available experimental data. A good agreement is obtained which gives confidence in the maturity degree and quality of the TAF-ID database. The working version is only accessible to the participants of the TAF-ID project (Canada, France, Japan, the Netherlands, Republic of Korea, United Kingdom, USA). A public version is accessible by all the NEA countries. The current version contains models on the Am–Fe, Am–Np, Am-O-Pu, Am–U, Am–Zr, C–O–U-Pu, Cr–U, Np–U, Np–Zr, O–U–Zr, Re–U, Ru–U, Si–U, Ti–U, U-Pu-Zr, U–W systems. It is progressively extended with our published assessments. Information on how to join the project is available on the website: https://www.oecd-nea.org/science/taf-id/.
•The TAF-ID international thermodynamic database is a computational tool for nuclear fuel applications.•Key systems for nuclear fuels are modelled using the Calphad method.•Fission product chemistry of irradiated fuels is predicted.•Chemical interaction with structural materials can be predicted under normal and severe accident conditions.•Coupling with kinetics, phase-field, Fuel Performance Code is being performed.
Accretion discs with masses ∼10−3–0.1 M⊙ are believed to form during the merger of a neutron 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 composition 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 viscous heating and the disc becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the disc's 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 disc's late-time transition to an advective state. As a result, hyperaccreting discs generically freeze-out neutron-rich (with electron fraction Ye∼ 0.2–0.4), and their late-time outflows robustly 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 fb≈ 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 short-duration GRB outflows are less collimated than those produced in long-duration GRBs.
Even though SN 2012cg is one of the best-studied Type Ia supernovae to date, the nature of its progenitor system has been debated in numerous studies. Specifically, it is difficult to reconcile ...recent claims of the detection of a ∼6 MS companion with recent deep, late-time flux limits. In this study we add three new constraints. (1) We analyze a new high-signal-to-noise, nebular-phase, Large Binocular Telescope/MODS spectrum of SN 2012cg and place an upper limit on the amount of low-velocity, solar-abundance material removed from a possible companion of . (2) We use Swift X-ray observations to constrain the pre-explosion mass-loss rate to be for . (3) We carefully reanalyze a prediscovery MASTER image, and with published light curves of SN 2012cg we estimate the time of first light and conservatively constrain the radius of a Roche-lobe overflowing companion to be . These observations disagree with a large nearby companion, and when considered with other studies of SN 2012cg's progenitor system, essentially rule out a non-degenerate companion.
ABSTRACT
We present observations of ASASSN-19dj, a nearby tidal disruption event (TDE) discovered in the post-starburst galaxy KUG 0810+227 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at ...a distance of d ≃ 98 Mpc. We observed ASASSN-19dj from −21 to 392 d relative to peak ultraviolet (UV)/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From the ASAS-SN g-band data, we determine that the TDE began to brighten on 2019 February 6.8 and for the first 16 d the rise was consistent with a flux ∝t2 power law. ASASSN-19dj peaked in the UV/optical on 2019 March 6.5 (MJD = 58548.5) at a bolometric luminosity of L = (6.2 ± 0.2) × 1044 erg s−1. Initially remaining roughly constant in X-rays and slowly fading in the UV/optical, the X-ray flux increased by over an order of magnitude ∼225 d after peak, resulting from the expansion of the X-ray emitting region. The late-time X-ray emission is well fitted by a blackbody with an effective radius of ∼1 × 1012 cm and a temperature of ∼6 × 105 K. The X-ray hardness ratio becomes softer after brightening and then returns to a harder state as the X-rays fade. Analysis of Catalina Real-Time Transient Survey images reveals a nuclear outburst roughly 14.5 yr earlier with a smooth decline and a luminosity of LV ≥ 1.4 × 1043 erg s−1, although the nature of the flare is unknown. ASASSN-19dj occurred in the most extreme post-starburst galaxy yet to host a TDE, with Lick HδA = 7.67 ± 0.17 Å.
Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their ...explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44Ti.
Recent observations of giant flares from soft gamma-ray repeaters have exhibited multiple 625-150 Hz oscillations. Frequencies in this range are expected for toroidal shear waves in a neutron star ...(NS) crust, lending support to Duncan's proposal that such modes may be excited in these events. This motivates a reassessment of how these waves reflect the NS structure and what role the magnetic field plays in setting their frequencies. We calculate the eigenfrequencies and eigenfunctions of toroidal oscillations for a realistic NS crust, including a vertical magnetic field at magnetar strengths (B 6 10 super(14)-10 super(15) G). The lowest radial-order mode has a redshifted frequency of -28 Hzl(l + 1)/6 super(1/2), with the prefactor depending on the NS's mass and radius and its crust's depth and composition. This mode is independent of the magnetic field for B < 4 x 10 super(15) G, a limit much greater than the inferred dipole magnetic fields for these objects. Although this is a good fit to the observed oscillations, only rather loose constraints can be made for the NSs' properties because all that can be fit is this prefactor (a single parameter). Modes with shorter radial wavelengths are more sensitive to the magnetic field starting at B 6 2 x 10 super(14) G and have higher frequencies (6600-2000 Hz). The discovery of these modes, coupled with the oscillations observed thus far, would provide a powerful probe to the NS crustal structure.
The merging neutron star gravitational-wave event GW170817 has been observed throughout the entire electromagnetic spectrum from radio waves to γ-rays. The resulting energetics, variability, and ...light curves are shown to be consistent with GW170817 originating from the merger of two neutron stars, in all likelihood followed by the prompt gravitational collapse of the massive remnant. The available γ-ray, X-ray, and radio data provide a clear probe for the nature of the relativistic ejecta and the non-thermal processes occurring within, while the ultraviolet, optical, and infrared emission are shown to probe material torn during the merger and subsequently heated by the decay of freshly synthesized r-process material. The simplest hypothesis, that the non-thermal emission is due to a low-luminosity short γ-ray burst (sGRB), seems to agree with the present data. While low-luminosity sGRBs might be common, we show here that the collective prompt and multi-wavelength observations are also consistent with a typical, powerful sGRB seen off-axis. Detailed follow-up observations are thus essential before we can place stringent constraints on the nature of the relativistic ejecta in GW170817.
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