The origin of the cosmic gamma-ray background in the MeV range

There has been much debate about the origin of the diffuse $\gamma$--ray background in the MeV range. At lower energies, AGNs and Seyfert galaxies can explain the background, but not above $\simeq$0.3 MeV. Beyond $\sim$10 MeV blazars appear to account for the flux observed. That leaves an unexplained gap for which different candidates have been proposed, including annihilations of WIMPS. One candidate are Type Ia supernovae (SNe Ia). Early studies concluded that they were able to account for the $\gamma$--ray background in the gap, while later work attributed a significantly lower contribution to them. All those estimates were based on SN Ia explosion models which did not reflect the full 3D hydrodynamics of SNe Ia explosions. In addition, new measurements obtained since 2010 have provided new, direct estimates of high-z SNe Ia rates beyond $z\sim$2. We take into account these new advances to see the predicted contribution to the gamma--ray background. We use here a wide variety of explosion models and a plethora of new measurements of SNe Ia rates. SNe Ia still fall short of the observed background. Only for a fit, which would imply $\sim$150\% systematic error in detecting SNe Ia events, do the theoretical predictions approach the observed fluxes. This fit is, however, at odds at the highest redshifts with recent SN Ia rates estimates. Other astrophysical sources such as FSRQs do match the observed flux levels in the MeV regime, while SNe Ia make up to 30--50\% of the observed flux.Comment: 40 pages, 13 Figures, accepted to be published in Ap

Hydrodynamical moving-mesh simulations of the tidal disruption of stars by supermassive black holes

When a star approaches a black hole closely, it may be pulled apart by gravitational forces in a tidal disruption event (TDE). The flares produced by TDEs are unique tracers of otherwise quiescent supermassive black holes (SMBHs) located at the centre of most galaxies. In particular, the appearance of such flares and the subsequent decay of the light curve are both sensitive to whether the star is partially or totally destroyed by the tidal field. However, the physics of the disruption and the fall-back of the debris are still poorly understood. We are here modelling the hydrodynamical evolution of realistic stars as they approach a SMBH on parabolic orbits, using for the first time the moving-mesh code AREPO, which is particularly well adapted to the problem through its combination of quasi-Lagrangian behaviour, low advection errors, and high accuracy typical of mesh-based techniques. We examine a suite of simulations with different impact parameters, allowing us to determine the critical distance at which the star is totally disrupted, the energy distribution and the fallback rate of the debris, as well as the hydrodynamical evolution of the stellar remnant in the case of a partial disruption. Interestingly, we find that the internal evolution of the remnant's core is strongly influenced by persistent vortices excited in the tidal interaction. These should be sites of strong magnetic field amplification, and the associated mixing may profoundly alter the subsequent evolution of the tidally pruned star.Comment: 13 pages, 9 figures. Accepted for publication by MNRA

Type Ia supernovae from exploding oxygen-neon white dwarfs

The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs (WDs), but for many of the explosion scenarios, particularly those involving the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch WD), there is also a possibility of having an oxygen-neon (ONe) WD as progenitor. We simulate detonations of ONe WDs and calculate synthetic observables from these models. The results are compared with detonations in CO WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic explosion simulations of detonations in initially hydrostatic ONe WDs for a range of masses below the Chandrasekhar mass (M Ch), followed by detailed nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The results are used to calculate synthetic spectra and light curves, which are then compared with observations of SNe Ia. We also perform binary evolution calculations to determine the number of SNe Ia involving ONe WDs relative to the number of other promising progenitor channels. The ejecta structures of our simulated detonations in sub-M Ch ONe WDs are similar to those from CO WDs. There are, however, small systematic deviations in the mass fractions and the ejecta velocities. These lead to spectral features that are systematically less blueshifted. Nevertheless, the synthetic observables of our ONe WD explosions are similar to those obtained from CO models. Our binary evolution calculations show that a significant fraction (3-10%) of potential progenitor systems should contain an ONe WD. The comparison of our ONe models with our CO models of comparable mass (1.2 Msun) shows that the less blueshifted spectral features fit the observations better, although they are too bright for normal SNe Ia.Comment: 6 pages, 5 figure

Collisions of red giants in galactic nuclei

In stellar-dense environments, stars can collide with each other. For collisions close to a supermassive black hole (SMBH), the collisional kinetic energy can be so large that the colliding stars can be completely destroyed, potentially releasing an amount of energy comparable to that of a supernova. These violent events have been examined mostly analytically, with the non-linear hydrodynamical effects being left largely unstudied. Using the moving-mesh hydrodynamics code {\small AREPO}, we investigate high-velocity ($>10^{3}$ km/s) collisions between 1M$_{\odot}$ giants with varying radii, impact parameters, and initial approaching velocities, and estimate their observables. Very strong shocks across the collision surface efficiently convert $\gtrsim10\%$ of the initial kinetic energy into radiation energy. The outcome is a gas cloud expanding supersonically, homologously, and quasi-spherically, generating a flare with a peak luminosity $\simeq 10^{41}-10^{44}$ erg/s in the extreme UV band ($\simeq 10$ eV). The luminosity decreases approximately following a power-law $t^{-0.7}$ initially, then $t^{-0.4}$ after $t\simeq$10 days at which point it would be bright in the optical band ($\lesssim 1$eV). Subsequent, and possibly even brighter, emission would be generated due to the accretion of the gas cloud onto the nearby SMBH, possibly lasting up to multi-year timescales. This inevitable BH-collision product interaction can contribute to the growth of BHs at all mass scales, in particular, seed BHs at high redshifts. Furthermore, the proximity of the events to the central BH makes them a potential tool for probing the existence of dormant BHs, even very massive ones which cannot be probed by tidal disruption events.Comment: 16 pages, 14 figures, 2 tables, submitted to MNRAS, comments welcome, movies here: https://www.youtube.com/playlist?list=PLxLK3qI02cQd9lyIo6DIqm1tQnx_-G3U

Optimizing Distributed Tensor Contractions using Node-Aware Processor Grids

We propose an algorithm that aims at minimizing the inter-node communication volume for distributed and memory-efficient tensor contraction schemes on modern multi-core compute nodes. The key idea is to define processor grids that optimize intra-/inter-node communication volume in the employed contraction algorithms. We present an implementation of the proposed node-aware communication algorithm into the Cyclops Tensor Framework (CTF). We demonstrate that this implementation achieves a significantly improved performance for matrix-matrix-multiplication and tensor-contractions on up to several hundreds modern compute nodes compared to conventional implementations without using node-aware processor grids. Our implementation shows good performance when compared with existing state-of-the-art parallel matrix multiplication libraries (COSMA and ScaLAPACK). In addition to the discussion of the performance for matrix-matrix-multiplication, we also investigate the performance of our node-aware communication algorithm for tensor contractions as they occur in quantum chemical coupled-cluster methods. To this end we employ a modified version of CTF in combination with a coupled-cluster code (Cc4s). Our findings show that the node-aware communication algorithm is also able to improve the performance of coupled-cluster theory calculations for real-world problems running on tens to hundreds of compute nodes.Comment: 15 pages, 4 figure

Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha

Stellar evolution models predict the existence of hybrid white dwarfs (WDs) with a carbon-oxygen core surrounded by an oxygen-neon mantle. Being born with masses ~1.1 Msun, hybrid WDs in a binary system may easily approach the Chandrasekhar mass (MCh) by accretion and give rise to a thermonuclear explosion. Here, we investigate an off-centre deflagration in a near-MCh hybrid WD under the assumption that nuclear burning only occurs in carbon-rich material. Performing hydrodynamics simulations of the explosion and detailed nucleosynthesis post-processing calculations, we find that only 0.014 Msun of material is ejected while the remainder of the mass stays bound. The ejecta consist predominantly of iron-group elements, O, C, Si and S. We also calculate synthetic observables for our model and find reasonable agreement with the faint Type Iax SN 2008ha. This shows for the first time that deflagrations in near-MCh WDs can in principle explain the observed diversity of Type Iax supernovae. Leaving behind a near-MCh bound remnant opens the possibility for recurrent explosions or a subsequent accretion-induced collapse in faint Type Iax SNe, if further accretion episodes occur. From binary population synthesis calculations, we find the rate of hybrid WDs approaching MCh to be on the order of 1 percent of the Galactic SN Ia rate.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in MNRA