SN 1998bw at late phases

We present observations of the peculiar supernova SN 1998bw, which was probably associated with GRB 980425. The photometric and spectroscopic evolution is monitored up to 500 days past explosion. We also present modeling based on spherically symmetric, massive progenitor models and very energetic explosions. The models allow line identification and clearly show the importance of mixing. From the late light curves we estimate that about 0.3-0.9 solar masses of ejected Nickel-56 is required to power the supernova.Comment: With 3 figures Accepted for ApJ Letter

A spectropolarimetric view on the nature of the peculiar Type I SN 2005hk

We report two spectropolarimetric observations of SN 2005hk, which is a close copy of the "very peculiar" SN 2002cx, showing low peak luminosity, slow decline, high ionization near peak and an unusually low expansion velocity of only about 7,000 km s^-1. Further to the data presented by Chornock et al., (2006), at -4 days before maximum, we present data of this object taken on 9 November 2005 (near maximum) and 23 November (+ two weeks) that show the continuum and most of the spectral lines to be polarized at levels of about 0.2-0.3%. At both epochs the data corresponds to the Spectropolarimetric Type D1. The general low level of line polarization suggests that the line forming regions for most species observed in the spectrum have a similar shape to that of the photosphere, which deviates from a spherical symmetry by <10%. In comparison with spectropolarimetry of Type Ia and Core-collapse SNe at similar epochs, we find that the properties of SN 2005hk are most similar to those of Type Ia SNe. In particular, we find the low levels of continuum and line polarization to indicate that the explosion mechanism is approximately spherical, with homogeneous ejecta (unlike the chemically segregated ejecta of CCSNe). We discuss the possibility that SN 2005hk was the result of the pure deflagration of a white dwarf and note the issues concerning this interpretation.Comment: ApJ accepted, uses emulateapj, 16 pages, 10 figures, figures 3 and 4 update

Spectropolarimetry of the Type Ia SN 2007sr Two Months After Maximum Light

We present late time spectropolarimetric observations of SN 2007sr, obtained with the VLT telescope at ESO Paranal Observatory when the object was 63 days after maximum light. The late time spectrum displays strong line polarization in the CaII absorption features. SN 2007sr adds to the case of some normal Type Ia SNe that show high line polarization or repolarization at late times, a fact that might be connected with the presence of high velocity features at early times

Supernova 1996L: evidence of a strong wind episode before the explosion

Observations of the type II SN 1996L reveal the presence of a slowly expanding (V~700$ km/s) shell at ~ 10^(16) cm from the exploding star. Narrow emission features are visible in the early spectra superposed on the normal SN spectrum. Within about two months these features develop narrow symmetric P-Cygni profiles. About 100 days after the explosion the light curve suddenly flattens, the spectral lines broaden and the Halpha flux becomes larger than what is expected from a purely radioactive model. These events are interpreted as signatures of the onset of the interaction between the fast moving ejecta and a slowly moving outer shell of matter ejected before the SN explosion. At about 300 days the narrow lines disappear and the flux drops until the SN fades away, suggesting that the interaction phase is over and that the shell has been swept away. Simple calculations show that the superwind episode started 9 yr before the SN explosion and lasted 6 yr, with an average dM/dt=10^(-3) M_solar/yr. Even at very late epochs (up to day 335) the typical forbidden lines of [OI], CaII], [FeII] remain undetected or very weak. Spectra after day 270 show relatively strong emission lines of HeI. These lines are narrower than other emission lines coming from the SN ejecta, but broader than those from the CSM. These high excitation lines are probably the result of non-thermal excitation and ionization caused by the deposition of the gamma-rays emitted in the decay of radioactive material mixed in the He layer.Comment: 8 pages, 6 figures, Latex, To appear in M.N.R.A.

Diversity of Decline-Rate-Corrected Type Ia Supernova Rise Times: One Mode or Two?

B-band light-curve rise times for eight unusually well-observed nearby Type Ia supernovae (SNe) are fitted by a newly developed template-building algorithm, using light-curve functions that are smooth, flexible, and free of potential bias from externally derived templates and other prior assumptions. From the available literature, photometric BVRI data collected over many months, including the earliest points, are reconciled, combined, and fitted to a unique time of explosion for each SN. On average, after they are corrected for light-curve decline rate, three SNe rise in 18.81 +- 0.36 days, while five SNe rise in 16.64 +- 0.21 days. If all eight SNe are sampled from a single parent population (a hypothesis not favored by statistical tests), the rms intrinsic scatter of the decline-rate-corrected SN rise time is 0.96 +0.52 -0.25 days -- a first measurement of this dispersion. The corresponding global mean rise time is 17.44 +- 0.39 days, where the uncertainty is dominated by intrinsic variance. This value is ~2 days shorter than two published averages that nominally are twice as precise, though also based on small samples. When comparing high-z to low-z SN luminosities for determining cosmological parameters, bias can be introduced by use of a light-curve template with an unrealistic rise time. If the period over which light curves are sampled depends on z in a manner typical of current search and measurement strategies, a two-day discrepancy in template rise time can bias the luminosity comparison by ~0.03 magnitudes.Comment: As accepted by The Astrophysical Journal; 15 pages, 6 figures, 2 tables. Explanatory material rearranged and enhanced; Fig. 4 reformatte

SN 1993J VLBI (IV): A Geometric Determination of the Distance to M81 with the Expanding Shock Front Method

We compare the angular expansion velocities, determined with VLBI, with the linear expansion velocities measured from optical spectra for supernova 1993J in the galaxy M81, over the period from 7 d to ~9 yr after shock breakout. We estimate the distance to SN 1993J using the Expanding Shock Front Method (ESM). We find the best distance estimate is obtained by fitting the angular velocity of a point halfway between the contact surface and outer shock front to the maximum observed hydrogen gas velocity. We obtain a direct, geometric, distance estimate for M81 of D=3.96+-0.05+-0.29 Mpc with statistical and systematic error contributions, respectively, corresponding to a total standard error of $+-0.29 Mpc. The upper limit of 4.25 Mpc corresponds to the hydrogen gas with the highest observed velocity reaching no farther out than the contact surface a few days after shock breakout. The lower limit of 3.67 Mpc corresponds to this hydrogen gas reaching as far out as the forward shock for the whole period, which would mean that Rayleigh-Taylor fingers have grown to the forward shock already a few days after shock breakout. Our distance estimate is 9+-13 % larger than that of 3.63+-0.34 Mpc from the HST Key Project, which is near our lower limit but within the errors.Comment: 25 pages, 11 figures, accepted for publication in Ap

Properties of extragalactic dust inferred from linear polarimetry of Type Ia Supernovae

Aims: The aim of this paper is twofold: 1) to investigate the properties of extragalactic dust and compare them to what is seen in the Galaxy; 2) to address in an independent way the problem of the anomalous extinction curves reported for reddened Type Ia Supernovae (SN) in connection to the environments in which they explode. Methods: The properties of the dust are derived from the wavelength dependence of the continuum polarization observed in four reddened Type Ia SN: 1986G, 2006X, 2008fp, and 2014J. [...] Results: All four objects are characterized by exceptionally low total-to-selective absorption ratios (R_V) and display an anomalous interstellar polarization law, characterized by very blue polarization peaks. In all cases the polarization position angle is well aligned with the local spiral structure. While SN~1986G is compatible with the most extreme cases of interstellar polarization known in the Galaxy, SN2006X, 2008fp, and 2014J show unprecedented behaviours. The observed deviations do not appear to be connected to selection effects related to the relatively large amounts of reddening characterizing the objects in the sample. Conclusions: The dust responsible for the polarization of these four SN is most likely of interstellar nature. The polarization properties can be interpreted in terms of a significantly enhanced abundance of small grains. The anomalous behaviour is apparently associated with the properties of the galactic environment in which the SN explode, rather than with the progenitor system from which they originate. For the extreme case of SN2014J, we cannot exclude the contribution of light scattered by local material; however, the observed polarization properties require an ad hoc geometrical dust distribution.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

VLT Spectropolarimetry of the fast expanding Type Ia SN2006X

Using VLT-FORS1 we performed optical spectropolarimetric observations of the Type Ia SN2006X on 7 pre-maximum epochs (day -10 to day -1) and one post-maximum epoch (+39 days). The SN shows strong continuum interstellar polarization reaching about 8% at 4000A, characterized by a wavelength dependency that is substantially different from that of the Milky-Way dust mixture. Several SN features, like SiII 6355A and the CaII IR triplet, present a marked evolution. The CaII near-IR triplet shows a pronounced polarization (~1.4%) already on day -10 in correspondence with a strong high-velocity feature (HVF). The SiII polarization peaks on day -6 at about 1.1% and decreases to 0.8% on day -1. By day +39 no polarization signal is detected for the SiII line, while the CaII IR triplet shows a marked re-polarization at the level of 1.2%. As in the case of another strongly polarized SN (2004dt), no polarization was detected across the OI 7774A absorption. The fast-expanding SN2006X lies on the upper edge of the relation between peak polarization and decline rate, and it confirms previous speculations about a correlation between degree of polarization, expansion velocity, and HVF strength. The polarization of CaII detected in our last epoch, the most advanced ever obtained for a Type Ia SN, coincides in velocity with the outer boundary of the Ca synthesized during the explosion (15,000-17,000 km/s) in delayed-detonation models. This suggests a large scale chemical inhomogeneity as produced by off-center detonations, a rather small amount of mixing, or a combination of both effects. In contrast, the absence of polarization at the inner edge of the Ca-rich layer (8000-10,000 km/s) implies a substantial amount of mixing in these deeper regions.Comment: 28 pages, 39 figures, accepted for publication in Astronomy and Astrophysic