3,436 research outputs found
Can Light Echoes Account for the Slow Decay of Type IIn Supernovae?
The spectra of type IIn supernovae indicate the presence of apre-existing
slow, dense circumstellar wind (CSW). If the CSW extends sufficiently far from
the progenitor star, then dust formation should occur in the wind. The light
from the supernova explosion will scatter off this dust and produce a light
echo. Continuum emission seen after the peak will have contributions from both
this echo as well as from the shock of the ejecta colliding with the CSW, with
a fundamental question of which source dominates the continuum. We calculate
the brightness of the light echo as a function of time for a range of dust
shell geometries, and use our calculations to fit to the light curves of SN
1988Z and SN 1997ab, the two slowest declining IIn supernovae on record. We
find that the light curves of both objects can be reproduced by the echo model.
However, their rate of decay from peak, color at peak and their observed peak
absolute magnitudes when considered together are inconsistent with the echo
model. Furthermore, when the observed values of M are corrected for the
effects of dust scattering, the values obtained imply that these supernovae
have unrealistically high luminosities. We conclude that light echoes cannot
properly account for the slow decline seen in some IIn's, and that the shock
interaction is likely to dominate the continuum emission.Comment: 15 pages, 9 figure
Possible Recovery of SN 1961V In Hubble Space Telescope Archival Images
SN 1961V in NGC 1058 was originally classified by Fritz Zwicky as a ``Type
V'' supernova. However, it has been argued that SN 1961V was not a genuine
supernova, but instead the superoutburst of an eta Carinae-like luminous blue
variable star. In particular, Filippenko et al. (1995, AJ, 110, 2261) used
pre-refurbishment HST WFPC images and the known radio position of SN 1961V to
conclude that the star survived the eruption and is likely coincident with a V
\~ 25.6 mag, V-I ~ 1.9 mag object. Recently, Stockdale et al. (2001, AJ, 122,
283) recovered the fading SN 1961V at radio wavelengths and argue that its
behavior is similar that of some Type II supernovae. We have analyzed
post-refurbishment archival HST WFPC2 data and find that the new radio position
is still consistent with the Filippenko et al. object, which has not changed in
brightness or color, but is also consistent with an adjacent, fainter (I ~ 24.3
mag) and very red (V-I > 1.0 mag) object. We suggest that this fainter object
could be the survivor of SN 1961V. Forthcoming HST observations may settle this
issue.Comment: 8 pages, 6 figures, to appear in the PASP (2002 July issue
A Chandra Observation of the Diffuse Emission in the Face-on Spiral NGC 6946
This paper describes the {\it Chandra} observation of the diffuse emission in
the face-on spiral NGC 6946. Overlaid on optical and H images, the
diffuse emission follows the spiral structure of the galaxy. An overlay on a 6
cm polarized radio intensity map confirms the phase offset of the polarized
emission. We then extract and fit the spectrum of the unresolved emission with
several spectral models. All model fits show a consistent continuum thermal
temperature with a mean value of 0.250.03 keV. Additional degrees of
freedom are required to obtain a good fit and any of several models satisfy
that need; one model uses a second continuum component with a temperature of
0.700.10 keV. An abundance measure of 3 for Si differs
from the solar value at the 90% confidence level; the net diffuse spectrum
shows the line lies above the instrumental Si feature. For Fe, the abundance
measure of 0.670.13 is significant at 99%. Multiple gaussians also provide
a good fit. Two of the fitted gaussians capture the O VII and O VIII emission;
the fitted emission is consistent with an {\it XMM-Newton} RGS spectrum of
diffuse gas in M81. The ratio of the two lines is 0.6-0.7 and suggests the
possibility of non-equilibrium ionization conditions exist in the ISM of NGC
6946. An extrapolation of the point source luminosity distribution shows the
diffuse component is not the sum of unresolved point sources; their
contribution is at most 25%.Comment: accepted for ApJ; 16 pages; 12 figs; to meet Archive size limits,
most converted to jpe
A Deep ROSAT HRI Observation of NGC 1313
We describe a series of observations of NGC 1313 using the ROSAT HRI with a
combined exposure time of 183.5 ksec. The observations span an interval between
1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray
flux of SN1978K, one of several luminous sources in the galaxy. No diffuse
emission is detected in the galaxy to a level of ~1-2x10^37 ergs/s/arcmin^-2. A
total of eight sources are detected in the summed image within the D_25
diameter of the galaxy. The luminosities of five of the eight range from
\~6x10^37 to ~6x10^38 erg/s; these sources are most likely accreting X-ray
binaries, similar to sources obseved in M31 and M33. The remaining three
sources all emit above 10^39 erg/s. We present light curves of the five
brightest sources. Variability is detected at the 99.9% level from four of
these. We identify one of the sources as an NGC 1313 counterpart of a Galactic
X-ray source. The light curve, though crudely sampled, most closely resembles
that of a Galactic black hole candidate such as GX339-4, but with considerably
higher peak X-ray luminosity. An additional seven sources lie outside of the
D_25 diameter and are either foreground stars or background AGN.Comment: 18 pages, 9 figures; accepted AJ, scheduled for November 200
Anharmonic transitions in nearly dry L-cysteine I
Two special dynamical transitions of universal character have been recently
observed in macromolecules at K and K.
Despite their relevance, a complete understanding of the nature of these
transitions and their consequences for the bio-activity of the macromolecule is
still lacking. Our results and analysis concerning the temperature dependence
of structural, vibrational and thermodynamical properties of the orthorhombic
polymorph of the amino acid L-cysteine (at a hydration level of 3.5%) indicated
that the two referred temperatures define the triggering of very simple and
specific events that govern all the biochemical interactions of the
biomolecule: activation of rigid rotors (), phonon-phonon interactions
with phonons of water dimer (), and water rotational barriers
surpassing ().Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Early Chandra X-ray Observations of Eta Carinae
Sub-arcsecond resolution Chandra observations of Eta Carinae reveal a 40
arcsec X 70 arcsec ring or partial shell of X-ray emission surrounding an
unresolved, bright, central source. The spectrum of the central source is
strongly absorbed and can be fit with a high-temperature thermal continuum and
emission lines. The surrounding shell is well outside the optical/IR bipolar
nebula and is coincident with the Outer Shell of Eta Carinae. The X-ray
spectrum of the Shell is much softer than that of the central source. The X-ray
Shell is irregular and only correlates well with optical features where a
bright X-ray knot coincides with a bright feature of the Outer Shell.
Implications for the binary model of the central source are discussed.Comment: 6 pages, 6 figures, accepted by Ap
Transversity and Transverse Spin in Nucleon Structure through SIDIS at Jefferson Lab
The JLab 12 GeV upgrade with a proposed solenoid detector and the CLAS12
detector can provide the granularity and three-dimensional kinematic coverage
in longitudinal and transverse momentum, , with to precisely measure the leading twist
chiral-odd and -odd quark distribution and fragmentation functions in SIDIS.
The large experimental reach of these detectors with a 12 GeV CEBAF at JLab
makes it {\em ideal} to obtain precise data on the {\em valence-dominated}
transversity distribution function and to access the tensor charge.Comment: 7 Pages, 2 figures. Summary of the working group on Transversity and
Transverse Spin Physics, from the workshop, "Inclusive and Semi-Inclusive
Spin Physics with High Luminosity and LargeAcceptance at 11 GeV", Thomas
Jefferson National Accelerator Facility (JLAB), December 13-14, 2006,
Jefferson Lab, Newport News, VA USA. Serves as input for the Nuclear Physics
Long Range Plan on QCD and Hadron Physic
Supernova 1996cr: SN 1987A's Wild Cousin?
We report on new VLT optical spectroscopic and multi-wavelength archival
observations of SN1996cr, a previously identified ULX known as Circinus Galaxy
X-2. Our optical spectrum confirms SN1996cr as a bona fide type IIn SN, while
archival imaging isolates its explosion date to between 1995-02-28 and
1996-03-16. SN1996cr is one of the closest SNe (~3.8 Mpc) in the last several
decades and in terms of flux ranks among the brightest radio and X-ray SNe ever
detected. The wealth of optical, X-ray, and radio observations that exist for
this source provide relatively detailed constraints on its post-explosion
expansion and progenitor history, including an preliminary angular size
constaint from VLBI. The archival X-ray and radio data imply that the
progenitor of SN1996cr evacuated a large cavity just prior to exploding: the
blast wave likely expanded for ~1-2 yrs before eventually striking the dense
circumstellar material which surrounds SN1996cr. The X-ray and radio emission,
which trace the progenitor mass-loss rate, have respectively risen by a factor
of ~2 and remained roughly constant over the past 7 yr. This behavior is
reminiscent of the late rise of SN1987A, but 1000 times more luminous and much
more rapid to onset. Complex Oxygen line emission in the optical spectrum
further hints at a possible concentric shell or ring-like structure. The
discovery of SN1996cr suggests that a substantial fraction of the closest SNe
observed in the last several decades have occurred in wind-blown bubbles. An
Interplanetary Network position allows us to reject a tentative GRB association
with BATSE 4B960202. [Abridged]Comment: 25 pages with tables, 12 figures (color), accepted to ApJ, comments
welcome; v2 - updated to reflect the subsequent rejection of our tentative
GRB association based on a revised error region from the Interplanetary
Network (thanks to Kevin Hurley) and include a few additional references; v3
- corrected some errors in Tables 7 and
- …