2,412 research outputs found
Sequences of Tibetan Macaque (Macaca thibetana) and tourist behaviors at Mt. Huangshan, China
Strongly Variable z=1.48 FeII and MgII Absorption in the Spectra of z=4.05 GRB 060206
We report on the discovery of strongly variable FeII and MgII absorption
lines seen at z=1.48 in the spectra of the z=4.05 GRB 060206 obtained between
4.13 to 7.63 hours (observer frame) after the burst. In particular, the FeII
line equivalent width (EW) decayed rapidly from 1.72+-0.25 AA to 0.28+-0.21 AA,
only to increase to 0.96+-0.21 AA in a later date spectrum. The MgII doublet
shows even more complicated evolution: the weaker line of the doublet drops
from 2.05+-0.25 AA to 0.92+-0.32 AA, but then more than doubles to 2.47+-0.41
AA in later data. The ratio of the EWs for the MgII doublet is also variable,
being closer to 1:1 (saturated regime) when the lines are stronger and becoming
closer to 2:1 (unsaturated regime) when the lines are weaker, consistent with
expectations based on atomic physics. We have investigated and rejected the
possibility of any instrumental or atmospheric effects causing the observed
strong variations. Our discovery of clearly variable intervening FeII and MgII
lines lends very strong support to their scenario, in which the characteristic
size of intervening patches of MgII ``clouds'' is comparable to the GRB beam
size, i.e, about 10^16 cm. We discuss various implications of this discovery,
including the nature of the MgII absorbers, the physics of GRBs, and
measurements of chemical abundances from GRB and quasar absorption lines.Comment: 14 pages, 3 figures, 1 table; ApJ Letters, accepte
Cyclotron effective mass of 2D electron layer at GaAs/AlGaAs heterojunction subject to in-plane magnetic fields
We have found that Fermi contours of a two-dimensional electron gas at
\rmGaAs/Al_xGa_{1-x}As interface deviate from a standard circular shape under
the combined influence of an approximately triangular confining potential and
the strong in-plane magnetic field. The distortion of a Fermi contour manifests
itself through an increase of the electron effective cyclotron mass which has
been measured by the cyclotron resonance in the far-infrared transmission
spectra and by the thermal damping of Shubnikov-de Haas oscillations in tilted
magnetic fields with an in-plane component up to 5 T. The observed increase of
the cyclotron effective mass reaches almost 5 \% of its zero field value which
is in good agreement with results of a self-consistent calculation.Comment: 4 pages, Revtex, figures can be obtained on request from
[email protected]; to appear in Phys. Rev. B (in press). No changes, the
corrupted submission replace
Spectroscopy of High-Redshift Supernovae from the ESSENCE Project: The First Four Years
We present the results of spectroscopic observations from the ESSENCE
high-redshift supernova (SN) survey during its first four years of operation.
This sample includes spectra of all SNe Ia whose light curves were presented by
Miknaitis et al. (2007) and used in the cosmological analyses of Davis et al.
(2007) and Wood-Vasey et al. (2007). The sample represents 273 hours of
spectroscopic observations with 6.5 - 10-m-class telescopes of objects detected
and selected for spectroscopy by the ESSENCE team. We present 174 spectra of
156 objects. Combining this sample with that of Matheson et al. (2005), we have
a total sample of 329 spectra of 274 objects. From this, we are able to
spectroscopically classify 118 Type Ia SNe. As the survey has matured, the
efficiency of classifying SNe Ia has remained constant while we have observed
both higher-redshift SNe Ia and SNe Ia farther from maximum brightness.
Examining the subsample of SNe Ia with host-galaxy redshifts shows that
redshifts derived from only the SN Ia spectra are consistent with redshifts
found from host-galaxy spectra. Moreover, the phases derived from only the SN
Ia spectra are consistent with those derived from light-curve fits. By
comparing our spectra to local templates, we find that the rate of objects
similar to the overluminous SN 1991T and the underluminous SN 1991bg in our
sample are consistent with that of the local sample. We do note, however, that
we detect no object spectroscopically or photometrically similar to SN 1991bg.
Although systematic effects could reduce the high-redshift rate we expect based
on the low-redshift surveys, it is possible that SN 1991bg-like SNe Ia are less
prevalent at high redshift.Comment: 21 pages, 17 figures, accepted to A
Exploring the Structure of Distant Galaxies with Adaptive Optics on the Keck-II Telescope
We report on the first observation of cosmologically distant field galaxies
with an high order Adaptive Optics (AO) system on an 8-10 meter class
telescope. Two galaxies were observed at 1.6 microns at an angular resolution
as high as 50 milliarcsec using the AO system on the Keck-II telescope. Radial
profiles of both objects are consistent with those of local spiral galaxies and
are decomposed into a classic exponential disk and a central bulge. A
star-forming cluster or companion galaxy as well as a compact core are detected
in one of the galaxies at a redshift of 0.37+/-0.05. We discuss possible
explanations for the core including a small bulge, a nuclear starburst, or an
active nucleus. The same galaxy shows a peak disk surface brightness that is
brighter than local disks of comparable size. These observations demonstrate
the power of AO to reveal details of the morphology of distant faint galaxies
and to explore galaxy evolution.Comment: 5 pages, Latex, 3 figures. Accepted for publication in P.A.S.
The Type Ic Hypernova SN 2003dh/GRB 030329
The spectra of SN 2003dh, identified in the afterglow of GRB030329, are
modeled using radiation transport codes. It is shown that SN 2003dh had a high
explosion kinetic energy ( erg in spherical symmetry),
making it one of the most powerful hypernovae observed so far, and supporting
the case for association between hypernovae and Gamma Ray Bursts. However, the
light curve derived from fitting the spectra suggests that SN 2003dh was not as
bright as SN 1998bw, ejecting only \sim 0.35\Msun of \Nifs. The spectra of SN
2003dh resemble those of SN 1998bw around maximum, but later they look more
like those of the less energetic hypernova SN 1997ef. The spectra and the
inferred light curve can be modeled adopting a density distribution similar to
that used for SN 1998bw at \kms but more like that of SN 1997ef at
lower velocities. The mass of the ejecta is \sim 8\Msun, somewhat less than
in the other two hypernovae. The progenitor must have been a massive star (M
\sim 35-40\Msun), as for other hypernovae. The need to combine different
one-dimensional explosion models strongly indicates that SN 2003dh was an
asymmetric explosion.Comment: 11 pages, 1 table and 5 figures. To appear in the Astrophysical
Journal (Letters). Revised version taking referee's comments into account,
minor change
Forgetting memory skepticism
Memory skepticism denies our memory beliefs could have any notable epistemic good. One route to memory skepticism is to challenge memory’s epistemic trustworthiness, that is, its functioning in a way necessary for it to provide epistemic justification. In this paper we develop and respond to this challenge. It could threaten memory in such a way that we altogether lack doxastic attitudes. If it threatens memory in this way, then the challenge is importantly self-defeating. If it does not threaten memory in this way, then the challenge leaves a foundation for an inference to the best explanation response, one we articulate and support
Spectral Identification of an Ancient Supernova using Light Echoes in the LMC
We report the successful identification of the type of the supernova
responsible for the supernova remnant SNR 0509-675 in the Large Magellanic
Cloud (LMC) using Gemini spectra of surrounding light echoes. The ability to
classify outbursts associated with centuries-old remnants provides a new window
into several aspects of supernova research and is likely to be successful in
providing new constraints on additional LMC supernovae as well as their
historical counterparts in the Milky Way Galaxy (MWG). The combined spectrum of
echo light from SNR 0509-675 shows broad emission and absorption lines
consistent with a supernova (SN) spectrum. We create a spectral library
consisting of 26 SNe Ia and 6 SN Ib/c that are time-integrated, dust-scattered
by LMC dust, and reddened by the LMC and MWG. We fit these SN templates to the
observed light echo spectrum using minimization as well as correlation
techniques, and we find that overluminous 91T-like SNe Ia with \dm15<0.9
match the observed spectrum best.Comment: 12 pages, 18 Figures, to be published in Ap
Exceptionally Bright Type Ib Supernova 1991D
Photometric and spectroscopic observations of the peculiar Type Ib Supernova
1991D are presented. SN 1991D was exceptionally bright for a Type Ib supernova.
The He I lines were rather weak and the velocity at the photosphere as a
function of time was unusually low. Comparison of the observed and synthetic
spectra indicates that either hydrogen was ejected with a minimum velocity of
12,000 km/s or the spectrum contained features caused by lines of Ne I. Light
curve modelling suggests that the progenitor probably had a very large radius
(~10^14 cm) and that a considerable amount of 56Ni was synthesized during the
explosion (~0.7 Msolar). We suggest a progenitor model of SN 1991D that
involves a binary system.Comment: 7 pages, 9 figures, MNRAS in pres
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