7 research outputs found
The correlations between optical variability and physical parameters of quasars in SDSS Stripe 82
We investigate the optical variability of 7658 quasars from SDSS Stripe 82.
Taking advantage of a larger sample and relatively more data points for each
quasar, we estimate variability amplitudes and divide the sample into small
bins of redshift, rest-frame wavelength, black hole mass, Eddington ratio and
bolometric luminosity respectively, to investigate the relationships between
variability and these parameters. An anti-correlation between variability and
rest-frame wavelength is found. The variability amplitude of radio-quiet
quasars shows almost no cosmological evolution, but that of radio-loud ones may
weakly anti-correlate with redshift. In addition, variability increases as
either luminosity or Eddington ratio decreases. However, the relationship
between variability and black hole mass is uncertain; it is negative when the
influence of Eddington ratio is excluded, but positive when the influence of
luminosity is excluded. The intrinsic distribution of variability amplitudes
for radio-loud and radio-quiet quasars are different. Both radio-loud and
radio-quiet quasars exhibit a bluer-when-brighter chromatism. Assuming that
quasar variability is caused by variations of accretion rate, the
Shakura-Sunyaev disk model can reproduce the tendencies of observed
correlations between variability and rest-frame wavelength, luminosity as well
as Eddington ratio, supporting that changes of accretion rate plays an
important role in producing the observed optical variability. However, the
predicted positive correlation between variability and black hole mass seems to
be inconsistent with the observed negative correlation between them in small
bins of Eddington ratio, which suggests that other physical mechanisms may
still need to be considered in modifying the simple accretion disk model.Comment: 51 pages, 28 figures, 2 tables, ApJ accepte
Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82
We present a new approach to analysing the dependence of quasar variability
on rest-frame wavelengths. We exploited the spectral archive of the Sloan
Digital Sky Survey (SDSS) to create a sample of more than 9000 quasars in the
Stripe 82. The quasar catalogue was matched with the Light Motion Curve
Catalogue for SDSS Stripe 82 and individual first-order structure functions
were computed. The structure functions are used to create a variability
indicator that is related to the same intrinsic timescales for all quasars (1
to 2 yr in the rest frame). We study the variability ratios for adjacent SDSS
filter bands as a function of redshift. While variability is almost always
stronger in the bluer passband compared to the redder, the variability ratio
depends on whether strong emission lines contribute to either one band or the
other. The variability ratio-redshift relations resemble the corresponding
colour index-redshift relations. From the comparison with Monte Carlo
simulations of variable quasar spectra we find that the observed variability
ratio-redshift relations are closely fitted assuming that (a) the r.m.s.
fluctuation of the quasar continuum follows a power law-dependence on the
intrinsic wavelength with an exponent -2 (i.e., bluer when brighter) and (b)
the variability of the emission line flux is only about 10% of that of the
underlying continuum. These results, based upon the photometry of more than
8000 quasars, confirm the previous findings by Wilhite et al. (2005) from 315
quasars with repeated SDSS spectroscopy. Finally, we find that quasars with
unusual spectra and weak emission lines tend to have less variability than
conventional quasars. This trend is opposite to what is expected from the
dilution effect of variability due to line emission and may be indicative of
high Eddington ratios in these unconventinal quasars.Comment: Accepted for publication in Astronomy and Astrophysic
Hypernovae and Other Black-Hole-Forming Supernovae
During the last few years, a number of exceptional core-collapse supernovae
(SNe) have been discovered. Their kinetic energy of the explosions are larger
by more than an order of magnitude than the typical values for this type of
SNe, so that these SNe have been called `Hypernovae'. We first describe how the
basic properties of hypernovae can be derived from observations and modeling.
These hypernovae seem to come from rather massive stars, thus forming black
holes. On the other hand, there are some examples of massive SNe with only a
small kinetic energy. We suggest that stars with non-rotating black holes are
likely to collapse "quietly" ejecting a small amount of heavy elements (Faint
supernovae). In contrast, stars with rotating black holes are likely to give
rise to very energetic supernovae (Hypernovae). We present distinct
nucleosynthesis features of these two types of "black-hole-forming" supernovae.
Hypernova nucleosynthesis is characterized by larger abundance ratios
(Zn,Co,V,Ti)/Fe and smaller (Mn,Cr)/Fe. Nucleosynthesis in Faint supernovae is
characterized by a large amount of fall-back. We show that the abundance
pattern of the most Fe deficient star, HE0107-5240, and other extremely
metal-poor carbon-rich stars are in good accord with those of
black-hole-forming supernovae, but not pair-instability supernovae. This
suggests that black-hole-forming supernovae made important contributions to the
early Galactic (and cosmic) chemical evolution.Comment: 49 pages, to be published in "Stellar Collapse" (Astrophysics and
Space Science; Kluwer) ed. C. L. Fryer (2003
On the Origin of S0 Galaxies
I will review the basic properties of S0 galaxies in the local Universe in
relation to both elliptical and spiral galaxies, their neighbours on the Hubble
sequence, and also in relation to dwarf spheroidal (dSph) galaxies. This will
include colours, luminosities, spectral features, information about the age and
metallicity composition of their stellar populations and globular clusters,
about their ISM content, as well as kinematic signatures and their implications
for central black hole masses and past interaction events, and the number
ratios of S0s to other galaxy types in relation to environmental galaxy
density. I will point out some caveats as to their morphological discrimination
against other classes of galaxies, discuss the role of dust and the wavelength
dependence of bulge/disk light ratios. These effects are of importance for
investigations into the redshift evolution of S0 galaxies -- both as individual
objects and as a population. The various formation and transformation scenarios
for S0 and dSph galaxies will be presented and confronted with the available
observations.Comment: Invited Review, 18 pages, ``BARS 2004'' Conference, South Africa,
June 2004, eds.: K. C. Freeman, D. L. Block, I. Puerari, R. Groess, Kluwer,
in pres