10 research outputs found
Is it Round? Spectropolarimetry of the Type II-P Supernova 1999em
We present the first multi-epoch spectropolarimetry of a type II plateau
supernova (SN II-P), with optical observations of SN 1999em on days 7, 40, 49,
159, and 163 after discovery. These data are used to probe the geometry of the
electron-scattering atmosphere before, during, and after the plateau phase,
which ended roughly 90 days after discovery. Weak continuum polarization with
an unchanging polarization angle (theta ~ 160 deg) is detected at all epochs,
with p ~ 0.2% on day 7, p ~ 0.3% on days 40 and 49, and p ~ 0.5% in the final
observations. Distinct polarization modulations across strong line features are
present on days 40, 49, 159, and 163. Uncorrected for interstellar polarization
(which is believed to be quite small), polarization peaks are associated with
strong P Cygni absorption troughs and nearly complete depolarization is seen
across the H-alpha emission profile. The temporal evolution of the continuum
polarization and sharp changes across lines indicate polarization intrinsic to
SN 1999em. When modeled in terms of the oblate, electron-scattering atmospheres
of Hoeflich, the observed polarization implies anasphericity of at least 7%
during the period studied. The temporal polarization increase may indicate
greater asphericity deeper into the ejecta. We discuss the implications of
asphericity on the use of type II-P supernovae as primary extragalactic
distance indicators through the expanding photosphere method (EPM). If
asphericity produces directionally dependant flux and peculiar galaxy motions
are characterized by sigma_v_rec = 300 km/s, it is shown that the agreement
between previous EPM measurements of SNe II and distances to the host galaxies
predicted by a linear Hubble law restrict mean SN II asphericity to values less
than 30% (3-sigma) during the photospheric phase.Comment: 65 pages (29 Figures, 4 Tables), Accepted for publication in the June
1, 2001 edition of ApJ. Revised statistical analysis of scatter in Hubble
diagram of previous EPM distances and the implications for mean SN II
asphericit
Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei III. Kinematics of Parsec-Scale Radio Jets
We report the results of a 15 GHz (2 cm) multi-epoch VLBA program, begun in
1994 to study the outflow in radio jets ejected from quasars and active
galaxies. The observed flow of 208 distinct features measured in 110 quasars,
active galaxies, and BL Lac objects shows highly collimated relativistic motion
with apparent transverse velocities typically between zero and about 15c, with
a tail extending up to about 34c. Within individual jets, different features
appear to move with a similar characteristic velocity which may represent an
underlying continuous jet flow, but we also see some stationary and even
apparently inward moving features which co-exist with the main features.
Comparison of our observations with published data at other wavelengths
suggests that there is a systematic decrease in apparent velocity with
increasing wavelength, probably because the observations at different
wavelengths sample different parts of the jet structure. The observed
distribution of linear velocities is not consistent with any simple ballistic
model. Either there is a rather broad range of Lorentz factors, a significant
difference between the velocity of the bulk relativistic flow and the pattern
speed of underlying shocks, or a combination of these options. Assuming a
ballistic flow, comparison of observed apparent velocities and Doppler factors
computed from the time scale of flux density variations is consistent with a
steep power law distribution of intrinsic Lorentz factors, an isotropic
distribution of orientations of the parent population, and intrinsic brightness
temperatures about an order of magnitude below the canonical inverse Compton
limit. It appears that the parent population of radio jets is not dominated by
highly relativistic flows (abridged).Comment: 25 pages, 11 figures, 3 tables, accepted for publication in the
Astrophysical Journa