210 research outputs found
Case Notes
For decades, optical time-domain searches have been tuned to find ordinary supernovae, which rise and fall in brightness over a period of weeks. Recently, supernova searches have improved their cadences and a handful of fast-evolving luminous transients have been identified(1-5). These have peak luminosities comparable to type Ia supernovae, but rise to maximum in less than ten days and fade from view in less than one month. Here we present the most extreme example of this class of object thus far: KSN 2015K, with a rise time of only 2.2 days and a time above half-maximum of only 6.8 days. We show that, unlike type Ia supernovae, the light curve of KSN 2015K was not powered by the decay of radioactive elements. We further argue that it is unlikely that it was powered by continuing energy deposition from a central remnant (a magnetar or black hole). Using numerical radiation hydrodynamical models, we show that the light curve of KSN 2015K is well fitted by a model where the supernova runs into external material presumably expelled in a pre-supernova mass-loss episode. The rapid rise of KSN 2015K therefore probes the venting of photons when a hypersonic shock wave breaks out of a dense extended medium.NASA
NNH15ZDA001N
NNX17AI64G
Australian Research Council Centre of Excellence for All-sky Astrophysics
CE11000102
WFPC2 Images of the Central Regions of Early-Type Galaxies - I. The Data
We present high resolution R-band images of the central regions of 67
early-type galaxies obtained with the Wide Field and Planetary Camera 2 (WFPC2)
aboard the Hubble Space Telescope (HST). Our sample strikingly confirms the
complex morphologies of the central regions of early-type galaxies. In
particular, we detect dust in 43 percent of all galaxies, and evidence for
embedded stellar disks in a remarkably large fraction of 51 percent. In 14 of
those galaxies the disk-like structures are misaligned with the main galaxy,
suggesting that they correspond to stellar bars in S0 galaxies. We analyze the
luminosity profiles of the galaxies in our sample, and classify galaxies
according to their central cusp slope. To a large extent we confirm the clear
dichotomy found in previous HST surveys: bright, boxy ellipticals with shallow
inner cusps (`core' galaxies) on one hand and faint, disky ellipticals with
steep central cusps (`power-law' galaxies) on the other hand. The advantages
and shortcomings of classification schemes utilizing the extrapolated central
cusp slope are discussed, and it is shown that this cusp slope might be an
inadequate representation for galaxies whose luminosity profile slope changes
smoothly with radius rather than resembling a broken power-law. In fact, we
find evidence for an `intermediate' class of galaxies, that cannot
unambiguously be classified as either core or power-law galaxies, and which
have central cusp slopes and absolute magnitudes intermediate between those of
core and power-law galaxies.Comment: 44 pages, 7 Postscript figures. Accepted for publication in the
Astronomical Journal. The associated Appendix with figures of luminosity
profiles, contour plots and isophotal parameters for all galaxies is
available at http://www.astro.washington.edu/rest/centralpro
The Transitional Stripped-Envelope SN 2008ax: Spectral Evolution and Evidence for Large Asphericity
Supernova (SN) 2008ax in NGC 4490 was discovered within hours after shock
breakout, presenting the rare opportunity to study a core-collapse SN beginning
with the initial envelope-cooling phase immediately following shock breakout.
We present an extensive sequence of optical and near-infrared spectra, as well
as three epochs of optical spectropolarimetry. Our initial spectra, taken two
days after shock breakout, are dominated by hydrogen Balmer lines at high
velocity. However, by maximum light, He I lines dominated the optical and
near-infrared spectra, which closely resembled those of normal Type Ib
supernovae (SNe Ib) such as SN 1999ex. This spectroscopic transition defines
Type IIb supernovae, but the strong similarity of SN 2008ax to normal SNe Ib
beginning near maximum light, including an absorption feature near 6270A due to
H-alpha at high velocities, suggests that many objects classified as SNe Ib in
the literature may have ejected similar amounts of hydrogen as SN 2008ax,
roughly a few x 0.01 M_sun. Early-time spectropolarimetry (6 and 9 days after
shock breakout) revealed strong line polarization modulations of 3.4% across
H-alpha, indicating the presence of large asphericities in the outer ejecta.
The continuum shares a common polarization angle with the hydrogen, helium, and
oxygen lines, while the calcium and iron absorptions are oriented at different
angles. This is clear evidence of deviations from axisymmetry even in the outer
ejecta. Intrinsic continuum polarization of 0.64% only nine days after shock
breakout shows that the outer layers of the ejecta were quite aspherical. A
single epoch of late-time spectropolarimetry, as well as the shapes of the
nebular line profiles, demonstrate that asphericities extended from the
outermost layers all the way down to the center of this SN. [Abridged]Comment: 24 pages, 21 figures, 4 tables, appendix, minor revisions to match
version accepted by Ap
Constraining Cosmic Evolution of Type Ia Supernovae
We present the first large-scale effort of creating composite spectra of
high-redshift type Ia supernovae (SNe Ia) and comparing them to low-redshift
counterparts. Through the ESSENCE project, we have obtained 107 spectra of 88
high-redshift SNe Ia with excellent light-curve information. In addition, we
have obtained 397 spectra of low-redshift SNe through a multiple-decade effort
at Lick and Keck Observatories, and we have used 45 UV spectra obtained by
HST/IUE. The low-redshift spectra act as a control sample when comparing to the
ESSENCE spectra. In all instances, the ESSENCE and Lick composite spectra
appear very similar. The addition of galaxy light to the Lick composite spectra
allows a nearly perfect match of the overall spectral-energy distribution with
the ESSENCE composite spectra, indicating that the high-redshift SNe are more
contaminated with host-galaxy light than their low-redshift counterparts. This
is caused by observing objects at all redshifts with the same slit width, which
corresponds to different projected distances. After correcting for the
galaxy-light contamination, subtle differences in the spectra remain. We have
estimated the systematic errors when using current spectral templates for
K-corrections to be ~0.02 mag. The variance in the composite spectra give an
estimate of the intrinsic variance in low-redshift maximum-light SN spectra of
~3% in the optical and growing toward the UV. The difference between the
maximum light low and high-redshift spectra constrain SN evolution between our
samples to be < 10% in the rest-frame optical.Comment: 22 pages, 22 figures, submitted to ApJ. Composite spectra can be
downloaded from http://astro.berkeley.edu/~rfoley/composite
Two Rare Magnetic Cataclysmic Variables with Extreme Cyclotron Features Identified in the Sloan Digital Sky Survey
Two newly identified magnetic cataclysmic variables discovered in the Sloan
Digital Sky Survey (SDSS), SDSSJ155331.12+551614.5 and SDSSJ132411.57+032050.5,
have spectra showing highly prominent, narrow, strongly polarized cyclotron
humps with amplitudes that vary on orbital periods of 4.39 and 2.6 hrs,
respectively. In the former, the spacing of the humps indicates the 3rd and 4th
harmonics in a magnetic field of ~60 MG. The narrowness of the cyclotron
features and the lack of strong emission lines imply very low temperature
plasmas and very low accretion rates, so that the accreting area is heated by
particle collisions rather than accretion shocks. The detection of rare systems
like these exemplifies the ability of the SDSS to find the lowest accretion
rate close binaries.Comment: Accepted for publication in the Astrophysical Journal, vol. 583,
February 1, 2003; slight revisions and additions in response to referee's
comments; 17 pages, 6 figures, AASTeX v4.
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