arXiv subjects

Alexandra Kozyreva

Publications and source records attributed to Alexandra Kozyreva.

31 records · Page 2Linked to original sources

arXiv2020

Shock breakouts from red supergiants: analytical and numerical predictions

Shock breakout (SBO) signal is the first signature of the supernova explosion apart from gravitational waves and neutrinos. Observational properties of SBO, such as bolometric luminosity and colour temperature, connect to the supernova progenitor and explosion parameters. Detecting SBO or SBO-cooling will constrain the progenitor and explosion models of collapsing stars. In the light of recently launched eROSITA telescope, the rate for detection of SBO is a few events during a year. In the current study, we examine the analytic formulae derived by Shussman et al. (2016). We use four red supergiant models from their study, while running explosions with the radiation hydrodynamics code STELLA. We conclude that there is a good agreement between analytic and numerical approaches for bolometric luminosity and colour temperature during SBO. The analytic formulae for the SBO signal based on the global supernova parameters can be used instead of running time-consuming numerical simulations. We define spectral range where analytic formulae for the SBO spectra are valid. We provide improved analytical expression for the SBO spectral energy distribution. We confirm dependence of colour temperature on radius derived by analytical studies and suggest to use early time observations to confine the progenitor radius. Additionally we show the prediction for the SBO signal from red supergiants as seen by eROSITA instrument.

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arXiv2018

The role of radioactive nickel in shaping the plateau phase of Type II supernovae

In the present study, we systematically explore the effect of the radioactive 56Ni and its mixing properties in the ejecta on the plateau of Type IIP supernovae (SNe). We evaluate the importance of 56Ni in shaping light curves of SNe IIP by simulating light curves for two red supergiant models using different amounts of 56Ni and with different types of mixing: uniform distribution of 56Ni out to different fractions of the envelope and "boxcar" distribution of 56Ni. We find, similarly to previous studies, that 56Ni extends duration of the plateau. We find a formula to estimate the extension based on the observed bolometric light curves and show that for most SNe IIP 56Ni extends the plateau by about 20%. Another effect of 56Ni consists in reduction of the plateau decline rate, i.e. 56Ni presented in the ejecta flattens the plateau. Our simulations suggest that for typical SNe IIP it can reduce the decline rate by about 1 mag/100 day. We find that for the contribution of 56Ni seen in most SNe our simulated bolometric light curves resemble observed ones for various types of 56Ni mixing. We thereby cannot determine the level of 56Ni mixing in these SNe based on the light curve alone. However, for SN2009ib we find that only a model where 56Ni is mixed significantly throughout most of the hydrogen envelope is consistent with the observed light curve. Our light curves are available via link https://wwwmpa.mpa-garching.mpg.de/ccsnarchive/data/Kozyreva2018/.

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arXiv2018

OGLE14-073 - a promising pair-instability supernova candidate

The recently discovered bright type II supernova OGLE14-073 evolved very slowly. The light curve rose to maximum for 90 days from discovery and then declined at a rate compatible with the radioactive decay of 56Co. In this study, we show that a pair-instability supernova is a plausible mechanism for this event. We calculate explosion models and light curves with the radiation hydrodynamics code STELLA starting from two M(ZAMS) = 150 solar masses, Z=0.001 progenitors. We obtain satisfactory fits to OGLE14-073 broadband light curves by including additional 56Ni in the centre of the models and mixing hydrogen down into the inner layers of the ejecta to a radial mass coordinate of 10 solar masses. The extra 56Ni required points to a slightly more massive progenitor star. The mixing of hydrogen could be due to large scale mixing during the explosion. We also present synthetic spectra for our models simulated with the Monte Carlo radiative transfer code ARTIS. The synthetic spectra reproduce the main features of the observed spectra of OGLE14-073. We conclude that OGLE14-073 is one of the most promising candidates for a pair-instability explosion.

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arXiv2017

Pair-Instability Supernova Simulations: Progenitor Evolution, Explosion, and Light Curves

In recent years, the viability of the pair-instability supernova (PISN) scenario for explaining superluminous supernovae has all but disappeared except for a few slowly-evolving examples. However, PISN are not predicted to be superluminous throughout the bulk of their mass range. In fact, it is more likely that the first PISN we see (if we have not seen one already) will not be superluminous. Here, we present hydrodynamic simulations of PISNe for four stellar models with unique envelope properties spanning the PISN mass range. In addition, we compute synthetic light curves for comparison with current and future observations. We also investigate, in the context of our most massive model, the prospect of mixing in the supernova ejecta alleviating discrepancies between current PISN models and the remaining superluminous candidate events. To this end, we present the first published 3D hydrodynamic simulations of PISNe. After achieving convergence between 1D, 2D, and 3D simulations we examine mixing in the supernova ejecta and its affect on the bolometric light curve. We observe slight deviations from spherical symmetry which increase with the number of dimensions. We find no significant effects on the bolometric light curve, however we conclude that mixing between the silicon and oxygen rich layers caused by the Rayleigh-Taylor instability may affect spectra.

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arXiv2017

Ultraviolet light curves of Gaia16apd in superluminous supernova models

Observations of Gaia16apd revealed extremely luminous ultraviolet emission among superluminous supernovae (SLSNe). Using radiation hydrodynamics simulations we perform a comparison of UV light curves, color temperatures and photospheric velocities between the most popular SLSN models: pair-instability supernova, magnetar and interaction with circumstellar medium. We find that the interaction model is the most promising to explain the extreme UV luminosity of Gaia16apd. The differences in late-time UV emission and in color evolution found between the models can be used to link an observed SLSN event to the most appropriate model. Observations at UV wavelengths can be used to clarify the nature of SLSNe and more attention should be paid to them in future follow-up observations.

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arXiv2016

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.

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arXiv2016

Effects of Dimensionality on Pair-Instability Supernova Explosions

Since the emergence of the new class of extremely bright transients, super-luminous supernovae (SLSNe), three main mechanisms to power their light curves (LCs) have been discussed. They are the spin-down of a magnetar, interaction with circumstellar material, and the decay of large amounts of radioactive nickel in pair-instability supernovae (PISNe). Given the high degree of diversity seen within the class, it is possible that all three mechanisms are at work. PISN models can be self- consistently simulated from the main sequence phase of very massive stars (VMS) through to their explosion. These models robustly predict large amounts of radioactive nickel and thus very luminous SN events. However, PISN model LCs evolve more slowly than even the slowest evolving SLSNe. Multidimensional effects on the ejecta structure, specifically the mixing of radioactive nickel out to large radii, could alleviate this discrepancy with observation. Here we explore the multidimensional effects on the LC evolution by simulating the explosion phase in 1D, 2D, and 3D. We find that the ejecta from the multidimensional simulations have slightly shallower abundance gradients due to mixing at shell boundaries. We compute synthetic LCs whose shapes show no discernible differences due to the multidimensional effects.

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arXiv2016

How much radioactive nickel does ASASSN-15lh require?

The discovery of the most luminous supernova ASASSN-15lh triggered a shock-wave in the supernova community. The three possible mechanisms proposed for the majority of other superluminous supernovae do not produce a realistic physical model for this particular supernova. In the present study we show the limiting luminosity available from a nickel-powered pair-instability supernova. We computed a few exotic nickel-powered explosions with a total mass of nickel up to 1500 solar masses. We used the hydrostatic configurations prepared with the GENEVA and MESA codes, and the STELLA radiative-transfer code for following the explosion of these models. We show that 1500 solar masses of radioactive nickel is needed to power a luminosity of 2x10^45 erg/s. The resulting light curve is very broad and incompatible with the shorter ASASSN-15lh time-scale. This rules out a nickel-powered origin of ASASSN-15lh. In addition, we derive a simple peak luminosity - nickel mass relation from our data, which may serve to estimate of nickel mass from observed peak luminosities.

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arXiv2015

Can pair-instability supernova models match the observations of superluminous supernovae?

An increasing number of so-called superluminous supernovae (SLSNe) are discovered. It is believed that at least some of them with slowly fading light curves originate in stellar explosions induced by the pair instability mechanism. Recent stellar evolution models naturally predict pair instability supernovae (PISNe) from very massive stars at wide range of metallicities (up to Z=0.006, Yusof et al. 2013). In the scope of this study we analyse whether PISN models can match the observational properties of SLSNe with various light curve shapes. Specifically, we explore the influence of different degrees of macroscopic chemical mixing in PISN explosive products on the resulting observational properties. We artificially apply mixing to the 250 Msun PISN evolutionary model from Kozyreva et al. (2014) and explore its supernova evolution with the one-dimensional radiation hydrodynamics code STELLA. The greatest success in matching SLSN observations is achieved in the case of an extreme macroscopic mixing, where all radioactive material is ejected into the hydrogen-helium outer layer. Such an extreme macroscopic redistribution of chemicals produces events with faster light curves with high photospheric temperatures and high photospheric velocities. These properties fit a wider range of SLSNe than non-mixed PISN model. Our mixed models match the light curves, colour temperature and photospheric velocity evolution of two well-observed SLSNe PTF12dam and LSQ12dlf. However, these models' extreme chemical redistribution may be hard to realise in massive PISNe. Therefore, alternative models such as the magnetar mechanism or wind-interaction may still to be favourable to interpret rapidly rising SLSNe.

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arXiv2015

Can very massive Population III stars produce a super-collapsar?

A fraction of the first generation of stars in the early Universe may be very massive ($\gtrsim 300~\mathrm{M_\odot}$) as they form in metal-free environments. Formation of black holes from these stars can be accompanied by supermassive collapsars to produce long gamma-ray bursts of a unique type having a very high total energy ($\sim 10^{54}~\mathrm{erg}$) as recently suggested by several authors. We present new stellar evolution models of very massive Population III stars including the effect of rotation to provide theoretical constraints on super-collapsar progenitors. We find that the angular momentum condition for super-collapsar can be fulfilled if magnetic torques are ignored, in which case Eddington-Sweet circulations play the dominant role for the transport of angular momentum. We further find that the initial mass range for super-collapsar progenitors would be limited to $300~\mathrm{M_\odot} \lesssim M \lesssim 700~\mathrm{M_\odot}$. However, all of our very massive star models of this mass range end their lives as red supergiants rather than blue supergiants, in good agreement with most of the previous studies. The predicted final fate of these stars is either a jet-powered type IIP supernova or an ultra-long, relatively faint gamma-ray transient, depending on the initial amount of angular momentum.

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arXiv2014

Finding the First Cosmic Explosions. IV. 90 - 140 M$_{\odot}$ Pair-Instability Supernovae

Population III stars that die as pair-instability supernovae are usually thought to fall in the mass range of 140 - 260 M$_{\odot}$. But several lines of work have now shown that rotation can build up the He cores needed to encounter the pair instability at stellar masses as low as 90 $_{\odot}$. Depending on the slope of the initial mass function of Population III stars, there could be 4 - 5 times as many stars from 90 - 140 $_{\odot}$ in the primordial universe than in the usually accepted range. We present numerical simulations of the pair-instability explosions of such stars performed with the MESA, FLASH and RAGE codes. We find that they will be visible to supernova factories such as Pan-STARRS and LSST in the optical out to z $\sim$ 1 - 2 and to JWST and the 30 m-class telescopes in the NIR out to $z \sim$ 7 - 10. Such explosions will thus probe the stellar populations of the first galaxies and cosmic star formation rates in the era of cosmological reionization. These supernovae are also easily distinguished from more massive pair-instability explosions, underscoring the fact that there is far greater variety to the light curves of these events than previously understood.

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arXiv2014

Explosion and nucleosynthesis of low redshift pair instability supernovae

Both recent observations and stellar evolution models suggest that pair-instability supernovae (PISNe) could occur in the local Universe, at metallicities below Z_Sun/3. Previous PISN models were mostly produced at very low metallicities in the context of the early Universe. We present new PISNe models at a metallicity of Z=0.001, which are relevant for the local Universe. We take the self-consistent stellar evolutionary models of pair-instability progenitors with initial masses of 150 and 250 solar masses at metallicity of Z=0.001 by Langer et al. (2007) and follow the evolution of these models through the supernova explosions, using a hydrodynamics stellar evolution code with an extensive nuclear network including 200 isotopes. Both models explode as PISNe without leaving a compact stellar remnant. Our models produce a nucleosynthetic pattern that is generally similar to that of Population III PISN models, which is mainly characterized by the production of large amounts of alpha-elements and a strong deficiency of the odd-charged elements. However, the odd-even effect in our models is significantly weaker than that found in Population III models. The comparison with the nucleosynthetic yields from core-collapse supernovae at a similar metallicity (Z=0.002) indicates that PISNe could have strongly influenced the chemical evolution below Z=0.002, assuming a standard initial mass function. The odd-even effect is predicted to be most prominent for the intermediate mass elements between silicon and calcium. With future observations of chemical abundances in Population II stars, our result can be used to constrain the number of PISNe that occurred during the past evolution of our Galaxy.

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arXiv2005

New soft gamma-ray bursts in the BATSE records and spectral properties of X-ray rich bursts

A population of X-ray dominated gamma-ray bursts (GRBs) observed by Ginga, BeppoSax and Hete-2 should be represented in the BATSE data as presumably soft bursts. We have performed a search for soft GRBs in the BATSE records in the 25--100 keV energy band. A softness of a burst spectrum can be a reason why it has been missed by the on-board procedure and previous searches for untriggered GRBs tuned to 50--300 keV range. We have found a surprisingly small number (~20/yr down to 0.1 ph cm$^{-2}$ s$^{-1}$) of soft GRBs where the count rate is dominated by 25--50 keV energy channel. This fact as well as the analysis of HETE-2 and common BeppoSAX/BATSE GRBs indicates that the majority of GRBs with a low Epeak has a relatively hard tail with the high-energy power-law photon index >-3. An exponential cutoff in GRB spectra below 20 keV may be a distinguishing feature separating non-GRB events.

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