576 research outputs found
Pre-Maximum Spectropolarimetry of the Type Ia SN 2004dt
We report observations of SN 2004dt obtained with the Very Large Telescope of
the European Southern Observatory on August 13.30, 2004 when the supernova was
more than a week before optical maximum. SN 2004dt showed strong lines of
\ion{O}{1}, \ion{Mg}{2}, \ion{Si}{2}, and \ion{Ca}{2} with typical velocities
of absorption minimum around 17,000 \kms. The line profiles show material
moving at velocities as high as 25,000 \kms in these lines. The observations
also reveal absorption lines from \ion{S}{2} and \ion{Si}{3} with a velocity of
only 11,000 \kms. The highest velocity in the \ion{S}{2} features can be traced
no higher than 15,000 \kms, much lower than those of O, Mg, Si, and Ca. SN
2004dt has a polarization spectrum unlike any previously observed. The
variation of the polarization across some \ion{Si}{2} lines approaches 2%,
making SN 2004dt the most highly polarized SN Ia ever observed. In contrast,
the strong line of O I at 777.4 nm shows little or no polarization signature.
The degree of polarization points to a richly-structured partially burned
silicon layer with substantial departure from spherical symmetry. A geometry
that would account for the observations is one in which the distribution of
oxygen is essentially spherically symmetric, but with bubbles of
intermediate-mass elements with significant opacity within the oxygen
substrate.Comment: Submitted to Ap
VLT Spectropolarimetry of the optical transient in NGC300. Evidence for asymmetry in the circumstellar dust
AIMS: The main goal of this work is to study possible signs of asymmetry in
the bright optical transient in NGC300, with the aim of getting independent
information on the explosion mechanism, the progenitor star and its
circumstellar environment.
METHODS: Using VLT-FORS1 we have obtained low-resolution optical linear
spectropolarimetry of NGC300 OT2008-1 on two epochs, 48 and 55 days after the
discovery, covering the spectral range 3600--9330A.
RESULTS: The data show a continuum polarization at a very significant level.
At least two separate components are identified. The first is characterized by
a strong wavelength dependency and a constant position angle (68.6+/-0.3
degrees), which is parallel to the local spiral arm of the host galaxy. The
second shows a completely different position angle (151.3+/-0.4) and displays a
mild but statistically significant evolution between the two epochs. While the
former is identified as arising in the interstellar dust associated with
NGC300, the latter is most likely due to continuum polarization by dust
scattering in the circumstellar environment. No line depolarization is detected
in correspondence of the most intense emission lines, disfavoring electron
scattering as the source of intrinsic polarization. This implies a very small
deviation from symmetry in the continuum-forming region. Given the observed
level of intrinsic polarization, the transient must be surrounded by a
significant amount of dust (>4x10^-5 Msun), asymmetrically distributed within a
few thousand AU. This most likely implies that one or more asymmetric outflow
episodes took place during the past history of the progenitor.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 16
figure
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.
Supernova 2002ic: the collapse of a stripped-envelope, massive star in a dense medium ?
We revisit the case of SN2002ic that recently revived the debate about the
progenitors of SNeIa after the claim of the unprecedented presence of hydrogen
lines over a diluted SNIa spectrum. As an alternative to the previous
interpretation, we suggest that SN2002ic actually was a type Ic SN, the core
collapse of a massive star which lost its hydrogen and helium envelope. In this
scenario the observed interaction with a dense circumstellar material (CSM) is
the predictable consequence of the intense mass-loss of the progenitor and/or
of the presence of a gas rich environment. With this view we establish a link
between energetic SNeIc and highly interacting SNeIIn and add some credits to
the proposed association of some SNeIIn to GRBs.Comment: Accepted for publication on ApJ
Supernova type Ia luminosities, their dependence on second parameters, and the value of H_0
A sample of 35 SNe Ia with good to excellent photometry in B and V, minimum
internal absorption, and 1200 < v < \approx 30000 km/s is compiled from the
literature. As far as their spectra are known they are all Branch-normal. For
29 of the SNe Ia also peak magnitudes in I are known. The SNe Ia have uniform
colors at maximum, i.e. =-0.012 mag (sigma=0.051) and =-0.276 mag
(sigma=0.078). In the Hubble diagram they define a Hubble line with a scatter
of =0.21-0.16 mag, decreasing with wavelength. The scatter is further
reduced if the SNe Ia are corrected for differences in decline rate Delta_m_15
or color (B-V). A combined correction reduces the scatter to sigma<=0.13 mag.
After the correction no significant dependence remains on Hubble type or
galactocentric distance. The Hubble line suggests some curvature which can be
differently interpreted. A consistent solution is obtained for a cosmological
model with Omega_M=0.3, Omega_Lambda=0.7, which is indicated also by much more
distant SNe Ia. Absolute magnitudes are available for eight equally blue
(Branch-normal) SNe Ia in spirals, whose Cepheid distances are known. If their
well defined mean values of M_B, M_V, and M_I are used to fit the Hubble line
to the above sample of SNe Ia one obtains H_0=58.3 km/s/Mpc, or, after
adjusting all SNe Ia to the average values of Delta_m_15 and (B-V), H_0=60.9
km/s/Mpc. Various systematic errors are discussed whose elimination tends to
decrease H_0. The finally adopted value at the 90-percent level, including
random and systematic errors, is H_0=58.5 +/- 6.3 km/s/Mpc. Several higher
values of H_0 from SNe Ia, as suggested in the literature, are found to depend
on large corrections for variations of the light curve parameter and/or on an
unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.Comment: 42 pages, including 9 figures; submitted to Ap
Detection of a light echo from SN1998bu
About 500d after explosion the light curve of the Type Ia SN1998bu suddenly
flattened and at the same time the spectrum changed from the typical nebular
emission to a blue continuum with broad absorption and emission features
reminiscent of the SN spectrum at early phases. We show that in analogy to
SN1991Tbu (Schmidt et al. 1994), this can be explained by the emergence of a
light echo from a foreground dust cloud. Based on a simple model we argue that
the amount of dust required can consistently explain the extinction which has
been estimated by completely independent methods. Because of the similar echo
luminosity but much higher optical depth of the dust in SN1998bu compared with
SN1991T, we expect that the echo ring size of SN1998bu grows faster than in
SN1991T. HST observations have indeed confirmed this prediction.Comment: 5 pages (including 3 figures) - Accepted for pubblication in ApJ
Letter
Positron Escape from Type Ia Supernovae
We generate bolometric light curves for a variety of type Ia supernova models
at late times, simulating gamma-ray and positron transport for various
assumptions about the magnetic field and ionization of the ejecta. These
calculated light curve shapes are compared with light curves of specific
supernovae for which there have been adequate late observations. %The selection
of models is generally not based upon the %ability to fit the late
observations, but rather because the %model has been demonstrated by other
authors to approximate the spectra %and early light curves of that specific SN.
From these comparisons we draw two conclusions: whether a suggested model is
an acceptable approximation of a particular event, and, given that it is, the
magnetic field characteristics and degree of ionization that are most
consistent with the observed light curve shape. For the ten SNe included in
this study, five strongly suggest Co positron escape as would be
permitted by a weak or radially-combed magnetic field. Of the remaining five
SNe, none clearly show the upturned light curve expected for positron trapping
in a strong, tangled magnetic field. Chandrasekhar mass models can explain
normally, sub-, and super- luminous supernova light curves; sub-Chandrasekhar
mass models have difficulties with sub- (and potentially normally) luminous
SNe. An estimate of the galactic positron production rate from type Ia SNe is
compared with gamma-ray observations of Galactic 511 keV annihilation
radiation. Additionally, we emphasize the importance of correctly treating the
positron transport for calculations of spectra, or any properties, of type Ia
SNe at late epochs ( 200 d).Comment: 82 pages including 25 figure
Deep infrared observations of the puzzling central X-ray source in RCW103
1E 161348-5055 (1E 1613) is a point-like, soft X-ray source originally
identified as a radio-quiet, isolated neutron star, shining at the center of
the 2000 yr old supernova remnant RCW103. 1E 1613 features a puzzling 6.67 hour
periodicity as well as a dramatic variability over a time scale of few years.
Such a temporal behavior, coupled to the young age and to the lack of an
obvious optical counterpart, makes 1E 1613 a unique source among all compact
objects associated to SNRs. It could either be the first low-mass X-ray binary
system discovered inside a SNR, or a peculiar isolated magnetar with an
extremely slow spin period. Analysis of archival IR observations, performed in
2001 with the VLT/ISAAC instrument, and in 2002 with the NICMOS camera onboard
HST unveils a very crowded field. A few sources are positionally consistent
with the refined X-ray error region that we derived from the analysis of 13
Chandra observations. To shed light on the nature of 1E 1613, we have performed
deep IR observations of the field with the NACO instrument at the ESO/VLT,
searching for variability. We find no compelling reasons to associate any of
the candidates to 1E 1613. On one side, within the frame of the binary system
model for the X-ray source, it is very unlikely that one of the candidates be a
low-mass companion star to 1E 1613. On the other side, if the X-ray source is
an isolated magnetar surrounded by a fallback disc, we cannot exclude that the
IR counterpart be hidden among the candidates. If none of the potential
counterparts is linked to the X-ray source, 1E 1613 would remain undetected in
the IR down to Ks>22.1. Such an upper limit is consistent only with an
extremely low-mass star (an M6-M8 dwarf) at the position of 1E 1613, and makes
rather problematic the interpretation of 1E 1613 as an accreting binary system.Comment: 26 pages, 5 figures. Accepted for publication in Ap
- âŠ