15 research outputs found
Astrophysics in 2006
The fastest pulsar and the slowest nova; the oldest galaxies and the youngest
stars; the weirdest life forms and the commonest dwarfs; the highest energy
particles and the lowest energy photons. These were some of the extremes of
Astrophysics 2006. We attempt also to bring you updates on things of which
there is currently only one (habitable planets, the Sun, and the universe) and
others of which there are always many, like meteors and molecules, black holes
and binaries.Comment: 244 pages, no figure
Time evolution of the line emission from the inner circumstellar ring of SN 1987A and its hot spots
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 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 H-alpha line
profile can be traced up to ~500 km/s at the latest epoch. This is consistent
with the cooling time of shocks propagating into a density of (1-4)x10^4 cm-3.Comment: 25 Pages, 12 Figures, accepted for publication in A&
Spectral evolution and polarization of variable structures in the pulsar wind nebula of PSR B0540-69.3
We present high spatial resolution optical imaging and polarization
observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula
(PWN) performed with HST, and compare them with X-ray data obtained with the
Chandra X-ray Observatory. We have studied the bright region southwest of the
pulsar where a bright "blob" is seen in 1999. We show that it may be a result
of local energy deposition around 1999, and that the emission from this then
faded away. Polarization data from 2007 show that the polarization properties
show dramatic spatial variations at the 1999 blob position arguing for a local
process. Several other positions along the pulsar-"blob" orientation show
similar changes in polarization, indicating previous recent local energy
depositions. In X-rays, the spectrum steepens away from the "blob" position,
faster orthogonal to the pulsar-"blob" direction than along this axis of
orientation. This could indicate that the pulsar-"blob" orientation is an axis
along where energy in the PWN is mainly injected, and that this is then
mediated to the filaments in the PWN by shocks. We highlight this by
constructing an [S II]-to-[O III]-ratio map. We argue, through modeling, that
the high [S II]/[O III] ratio is not due to time-dependent photoionization
caused by possible rapid Xray emission variations in the "blob" region. We have
also created a multiwavelength energy spectrum for the "blob" position showing
that one can, to within 2sigma, connect the optical and X-ray emission by a
single power law. We obtain best power-law fits for the X-ray spectrum if we
include "extra" oxygen, in addition to the oxygen column density in the
interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen
is most naturally explained by the oxygen-rich ejecta of the supernova remnant.
The oxygen needed likely places the progenitor mass in the 20 - 25 Msun range.Comment: Accepted by MNRAS on December 6th 2010, 18 pages, 15 figures. The
article with full resolution figures is available here
ftp://ftp.astro.su.se/pub/peter/papers/pwn0540_2010_corrected.pd
Coronal emission from the shocked circumstellar ring of SN 1987A
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 roughly 350 km/s, 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 about 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 km/s, corresponding to a
shock temperature of (1.6-2.5) x 10^6 K. A fraction of the coronal emission
may, however, originate in higher velocity adiabatic shocks.Comment: 11 pages, 10 figures, accepted for publication in A&
Modeling the X-ray emission of SN 1993J
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. 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). 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. 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.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication in A&
The outer rings of SN 1987A
Aims. We investigate the physical properties and structure of the outer rings of SN 1987A to understand their formation and evolution.
Methods. We used low resolution spectroscopy from VLT/FORS1 and high resolution spectra from VLT/UVES to estimate the physical conditions in the outer rings, using nebular analysis for emission lines such as [O II], [O III], [N II], and [S II]. We also measured the velocity at two positions of the outer rings to test a geometrical model for the rings. Additionally, we used data from the HST science archives to check the evolution of the outer rings of SN 1987A for a period that covers almost 11 years.
Results. We measured the flux in four different regions, two for each outer ring. We chose regions away from the two bright neighbouring stars and as far as possible from the inner ring and created light curves for the emission lines of [O III], Hα, and [N II]. The light curves display a declining behaviour, which is consistent with the initial supernova-flash powering of the outer rings. The electron density of the emitting gas in the outer rings, as estimated by nebular analysis from the [O II] and [S II] lines, is ≲ 3 × 103 cm-3, has not changed over the last ~15 years, and the [N II] temperature remains also fairly constant at ~1.2 × 104 K. We find no obvious difference in density and temperature for the two outer rings. The highest density, as estimated from the decay of Hα, could be ~5 × 103 cm-3 however, and because the decay is somewhat faster in the southern outer ring than it is in the northern, the highest density in the outer rings may be found in the southern outer ring. For an assumed distance of 50 kpc to the supernova, the distance between the supernova and the closest parts of the outer rings could be as short as ~1.7 × 1018 cm. Interaction between the supernova ejecta and the outer rings could therefore start in less than ~20 years. We do not expect the outer rings to show the same optical display as the equatorial ring when this happens. Instead soft X-rays should provide a better way of observing the ejecta - outer rings interaction
High resolution spectroscopy of the inner ring of SN 1987A
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)103 cm-3 and temperatures of
~6.5103-2.4104 K. 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 4106-107 cm-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-1 in the
blue wing for [Fe XIV], while the low ionization lines extend to typically
~-260 km s-1. For Hα a faint extension up to ~-450 km s-1 can 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