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Aims. We study a thermonuclear supernova (SN), emphasizing very late phases.

We present seven epochs between October 1999 and November 2007 of high resolution VLT/UVES echelle spectra of the ejecta-ring collision of SN 1987A.
The fluxes of most of the narrow lines from the unshocked gas decreased by a factor of $2{-}3$during this period, consistent with the decay from the initial ionization by the shock break-out. However, [O III] in particular shows an increase up to day ~6800. This agrees with radiative shock models where the pre-shocked gas is heated by the soft X-rays from the shock. The evolution of the [O III] line ratio shows a decreasing temperature of the unshocked ring gas, consistent with a transition from a hot, low density component which was heated by the initial flash ionization to the lower temperature in the pre-ionized gas ahead of the shocks.
The line emission from the shocked gas increases rapidly as the shock sweeps up more gas. We find that the neutral and high ionization lines follow the evolution of the Balmer lines roughly, while the intermediate ionization lines evolve less rapidly. Up to day ~6800, the optical light curves have a similar evolution to that of the soft X-rays. The break between day 6500 and day 7000 for [O III] and [Ne III] is likely due to recombination to lower ionization levels. Nevertheless, the evolution of the [Fe XIV] line, as well as the lines from the lowest ionization stages, continue to follow that of the soft X-rays, as expected.
There is a clear difference in the line profiles between the low and intermediate ionization lines, and those from the coronal lines at the earlier epochs. This shows that these lines arise from regions with different physical conditions, with at least a fraction of the coronal lines coming from adiabatic shocks. At later epochs the line widths of the low ionization lines, however, increase and approach those of the high ionization lines of [ Fe X-XIV] . The Hαline profile can be traced up to ~500 km s-1at the latest epoch. This is consistent with the cooling time of shocks propagating into a density of (1-4) $\times$104cm-3. This means that these shocks are among the highest velocity radiative shocks observed.

Aims. The aim of the work presented in this paper is to test and optimise supernova detection methods based on the optimal image subtractiontechnique. The main focus is on applying the detection methods to wide field supernova imaging surveys and in particular to the Stockholm VIMOS Supernova Survey (SVISS).

We discuss high resolution VLT/UVES observations (FWHM~ 6 km s-1) from October 2002 (day ~5700 past explosion) of the shock interaction of SN 1987A and its circumstellar ring. A large number of narrow emission lines from the unshocked ring, with ion stages from neutral up to Ne V and Fe VII, have been identified. A nebular analysis of the narrow lines from the unshocked gas indicates gas densities of (~1.5-5.0)$\times$103cm-3and temperatures of ~6.5$\times$103-2.4$\times$104K. This is consistent with the thermal widths of the lines. From the shocked component we observe a large range of ionization stages from neutral lines to [Fe XIV]. From a nebular analysis we find that the density in the low ionization region is 4$\times$106-107cm-3. There is a clear difference in the high velocity extension of the low ionization lines and that of lines from [ Fe X-XIV] , with the latter extending up to ~-390 km s-1in the blue wing for [Fe XIV], while the low ionization lines extend to typically ~-260 km s-1. For Hαa faint extension up to ~-450 km s-1can be seen probably arising from a small fraction of shocked high density clumps. We discuss these observations in the context of radiative shock models, which are qualitatively consistent with the observations. A fraction of the high ionization lines may originate in gas which has yet not had time to cool, explaining the difference in width between the low and high ionization lines. The maximum shock velocities seen in the optical lines are ~510 km s-1. We expect the maximum width of especially the low ionization lines to increase with time.

Context.SN 1987A in the Large Magellanic Cloud is close enough for a study of the very late time evolution of a supernova and its transition to a supernova remnant. Nearly two decades after explosion we are witnessing the supernova shock wave engaging the inner circumstellar ring, which had been fluorescing since being ionised by the soft X-ray flash from shock breakout.

High resolution spectra with UVES/VLT of SN 1987A from December 2000 until November 2005 show a number of high ionization lines from gas with velocities of ~ $\pm 350~\rm km\,s^{-1}$, emerging from the shocked gas formed by the ejecta–ring collision. These include coronal lines from [Fe X], [Fe XI] and [Fe XIV] which have increased by a factor of ~ 20 during the observed period. The evolution of the lines is similar to that of the soft X-rays, indicating that they arise in the same component. The line ratios are consistent with those expected from radiative shocks with velocity $310{-}390~\rm km\,s^{-1}$, corresponding to a shock temperature of $(1.6{-}2.5)\times 10^{6}$K. A fraction of the coronal emission may, however, originate in higher velocity adiabatic shocks.

The X-ray emission from the circumstellar interaction in type II supernovae with a dense circumstellar medium is calculated. In type IIL and type IIn supernovae, mass loss rates are generally high enough for the region behind the reverse shock to be radiative, producing strong radiation, particularly in X-rays. We present a model for the emission from the cooling region in the case of a radiative reverse shock. Under the assumption of a stationary flow, a hydrodynamic model is combined with a time-dependent ionization balance and multilevel calculations. The applicability of the steady state approximation is discussed for various values of the ejecta density gradient and different sets of chemical composition. We show how the emerging spectrum strongly depends on the reverse shock velocity and the composition of the shocked gas. We discuss differences between a spectrum produced by this model and a single-temperature spectrum. Large differences are found especially for the line emission, which can seriously affect abundance estimates. We also illustrate the effects of absorption in the cool shocked ejecta. The applicability of our model to various types of supernovae is discussed.

We present early time high-resolution (VLT/UVES) and late time low-resolution (VLT/FORS) optical spectra of the normal type Ia supernova, SN 2001el. The high-resolution spectra were obtained 9 and 2 days before (B-band) maximum light. This was in order to allow the detection of narrow hydrogen and/or helium emission lines from the circumstellar medium of the supernova. No such lines were detected in our data. We therefore use these spectra together with photoionisation models to derive upper limits of $9\times10^{-6} ~{M}_\odot~{\rm yr}^{-1}$and $5\times10^{-5} ~{M}_\odot~{\rm yr}^{-1}$for the mass loss rate from the progenitor system of SN 2001el assuming velocities of 10  km s-1and 50  km s-1, respectively, for a wind extending to outside at least a few $\times 10^{15}$cm away from the supernova explosion site. So far, these are the best Hαbased upper limits obtained for a type Ia supernova, and exclude a symbiotic star in the upper mass loss rate regime (so called Mira type stars) from being the progenitor of SN 2001el. The low-resolution spectrum was obtained in the nebular phase of the supernova, ~400 days after the maximum light, to search for any hydrogen rich gas originating from the supernova progenitor system. However, we see no signs of Balmer lines in our spectrum. Therefore, we model the late time spectra to derive an upper limit of ~0.03 $M_{\odot}$for solar abundance material present at velocities lower than 1000  km s-1within the supernova explosion site. According to numerical simulations of Marietta et al. (2000) this is less than the expected mass lost by a subgiant, red giant or a main-sequence secondary star at a small binary separation as a result of the SN explosion. Our data therefore exclude these scenarios as the progenitor of SN 2001el. Finally, we discuss the origin of high velocity Ca II lines previously observed in a few type Ia supernovae before the maximum light. We see both the Ca II IR triplet and the H&K lines in our earliest (-9 days) spectrum at a very high velocity of up to ~34 000 km s-1. The spectrum also shows a flat-bottomed Si II “6150 Å” feature similar to the one previously observed in SN 1990N (Leibundgut et al. 1991, ApJ, 371, L23) at 14 days before maximum light. We compare these spectral features in SN 2001el to those observed in SN 1984A and SN 1990N at even higher velocities.

Late time synthetic spectra of type Ia supernovae, based on three-dimensional deflagration models, are presented. We mainly focus on one model, “c3_3d_256_10s”, for which the hydrodynamics (Röpke 2005, A&A, 432, 969) and nucleosynthesis (Travaglio et al. 2004, A&A, 425, 1029) was calculated up to the homologous phase of the explosion. Other models with different ignition conditions and different resolution are also briefly discussed. The synthetic spectra are compared to observed late time spectra. We find that while the model spectra after 300 to 500 days show a good agreement with the observed Fe II-III features, they also show too strong O I and C I lines compared to the observed late time spectra. The oxygen and carbon emission originates from the low-velocity unburned material in the central regions of these models. To get agreement between the models and observations we find that only a small mass of unburned material may be left in the center after the explosion. This may be a problem for pure deflagration models, although improved initial conditions, as well as higher resolution decrease the discrepancy. The relative intensity from the different ionization stages of iron is sensitive to the density of the emitting iron-rich material. We find that clumping, with the presence of low density regions, is needed to reproduce the observed iron emission, especially in the range between 4000 and 6000 Å. Both temperature and ionization depend sensitively on density, abundances and radioactive content. This work therefore illustrates the importance of including the inhomogeneous nature of realistic three-dimensional explosion models. We briefly discuss the implications of the spectral modeling for the nature of the explosion.

We have conducted a systematic and comprehensive monitoring programme of the type Ia supernova 2000cx at late phases using the VLT and HST. The VLT observations cover phases 360 to 480 days past maximum brightness and include photometry in the BVRIJHbands, together with a single epoch in each of Uand Ks. While the optical bands decay by about 1.4 mag per 100 days, we find that the near-IR magnitudes stay virtually constant during the observed period. This means that the importance of the near-IR to the bolometric light curve increases with time. The finding is also in agreement with our detailed modeling of a type Ia supernova in the nebular phase. In these models, the increased importance of the near-IR is a temperature effect. We note that this complicates late-time studies where often only the Vband is well monitored. In particular, it is not correct to assume that any optical band follows the bolometric light curve at these phases, and any conclusions based on such assumptions, e.g., regarding positron-escape, must be regarded as premature. A very simple model where all positrons are trapped can reasonably well account for the observations. The nickel mass deduced from the positron tail of this light curve is lower than found from the peak brightness, providing an estimate of the fraction of late-time emission that is outside of the observed wavelength range. Our detailed models show the signature of an infrared catastrophe at these epochs, which is not supported by the observations.

Infrared and optical spectra of SN 1998bu at an age of one year after explosion are presented. The data show evidence for the radioactive decay of 56Co to 56Fe, long assumed to be the powering source for the supernova light curve past maximum light. The spectra provide direct evidence for at least 0.4 solar masses of iron being present in the ejecta of the supernova. The fits to the data also show that the widths of the emission lines increase with time. Photometric measurements in the H-band show that the supernova is not fading during the observation period. This is consistent with theoretical expectations.

The probable association with GRB 980425immediately put SN 1998bwat the forefront of supernova research. Here, we present revised late-time BVRIlight curves of the supernova, based on template images taken at the VLT. To follow the supernova to the very last observable phases we have used HST/STIS. Deep images taken in June and November 2000 are compared to images taken in August 2001. The identification of the supernova is firmly established. This allows us to measure the light curve to ~1000 days past explosion. The main features are a rapid decline up to more than 500 days after explosion, with no sign of complete positron trapping from the $\element[][56]{Co}$decay. Thereafter, the light curve flattens out significantly. One possible explanation is powering by more long lived radioactive isotopes, if they are abundantly formed in this energetic supernova.

ISO/SWS observations of SN 1987A on day 3425 show no emission in [Fe I] 24.05 $\mu{\rm m}$and [Fe II] 25.99 $\mu{\rm m}$down to the limits ~0.39 Jy and ~0.64 Jy, respectively. Assuming a homogeneous distribution of 44Ti inside $2000 \rm km s^{-1}$and negligible dust cooling, we have made time dependent theoretical models to estimate an upper limit on the mass of ejected 44Ti. Assessing various uncertainties of the model, and checking the late optical emission it predicts, we obtain an upper limit of $\simeq$$1.1\times 10^{-4} {M}_\odot$. This is lower than in our previous estimate using other ISO data, and we compare our new result with other models for the late emission, as well as with expected yields from explosion models. We also show that steady-state models for the optical emission are likely to overestimate the mass of ejected 44Ti. The low limit we find for the mass of ejected 44Ti could be higher if dust cooling is important. A direct check on this is provided by the gamma-ray emission at 1.157 Mev as a result of the radioactive decay of 44Ti.

Aims. We investigate the effects of radiative shocks on the observed X-ray emission from the type?II supernova?SN?1993J. To this end, the X-ray emission is modeled as a result of the interaction between the supernova ejecta and a dense circumstellar medium at an age of 8?years.Methods. The circumstances under which the reverse shock is radiative are discussed and the observed X-ray emission is analyzed using the numerical code described in Nymark et?al. (2006, A&A, 449, 171). We argue that the original analysis of the X-ray observations suffered from the lack of self-consistent models for cooling shocks with high density and velocity, leading to questionable conclusions about the temperatures and elemental abundances. We reanalyze the spectra with our numerical model, and discuss the expected spectra for different explosion models for the progenitors.Results. We find that the spectra of SN?1993J are compatible with a CNO-enriched composition and that the X-ray flux is dominated by the reverse shock.

Supernova surveys can be used to study a variety of subjects such as: (i) cosmology through type Ia supernovae (SNe), (ii) star-formation rates through core-collapse SNe, and (iii) supernova properties and their connection to host galaxy characteristics. The Stockholm VIMOS Supernova Survey (SVISS) is a multi-band imaging survey aiming to detect supernovae at redshift  ~0.5 and derive thermonuclear and core-collapse supernova rates at high redshift. In this paper we present the supernovae discovered in the survey along with light curves and a photometric classification into thermonuclear and core-collapse types. To detect the supernovae in the VLT/VIMOS multi-epoch images, we used difference imaging and a combination of automatic and manual source detection to minimise the number of spurious detections. Photometry for the found variable sources was obtained and careful simulations were made to estimate correct errors. The light curves were typed using a Bayesian probability method and Monte Carlo simulations were used to study misclassification. We detected 16 supernovae, nine of which had a core-collapse origin and seven had a thermonuclear origin. The estimated misclassification errors are quite small, in the order of 5%, but vary with both redshift and type. The mean redshift of the supernovae is 0.58. Additionally, we found a variable source with a very extended light curve that could possibly be a pair instability supernova.

SN 1987A provides a unique opportunity to study the evolution of a supernova from explosion into very late phases. Owing to the rich chemical structure, the multitude of physical processes involved and extensive radiative transfer effects, detailed modeling is needed to interpret the emission from this and other supernovae. In this paper, we analyze the late-time (about eight years) Hubble Space Telescope spectrum of the SN 1987A ejecta, where 44Ti is the dominant power source. Based on an explosion model for a 19 M⊙progenitor, we compute a model spectrum by calculating the degradation of positrons and gamma-rays from the radioactive decays, solving the equations governing temperature, ionization balance and NLTE level populations, and treating the radiative transfer with a Monte Carlo technique. We obtain a UV/optical/NIR model spectrum that reproduces most of the lines in the observed spectrum with good accuracy. We find non-local radiative transfer in atomic lines to be an important process also at this late stage of the supernova, with  ~30% of the emerging flux in the optical and NIR coming from scattering/fluorescence. We investigate the question of where the positrons deposit their energy, and favor the scenario where they are locally trapped in the Fe/He clumps by a magnetic field. Energy deposition into these largely neutral Fe/He clumps makes Fe I lines prominent in the emerging spectrum. With the best available estimates for the dust extinction, we determine the amount of 44Ti produced in the explosion to be \hbox{$1.5_{-0.5}^{+0.5}\e{-4}\Mo$}.