2,244 research outputs found
Intracluster supernovae in the Multi-epoch Nearby Cluster Survey
The Multi-Epoch Nearby Cluster Survey (MENeaCS) has discovered twenty-three
cluster Type Ia supernovae (SNe) in the 58 X-ray selected galaxy clusters (0.05
< z < 0.15) surveyed. Four of our SN Ia events have no host galaxy on close
inspection, and are likely intracluster SNe. Deep image stacks at the location
of the candidate intracluster SNe put upper limits on the luminosities of faint
hosts, with M_{r} > -13.0 mag and M_{g} > -12.5 mag in all cases. For such
limits, the fraction of the cluster luminosity in faint dwarfs below our
detection limit is <0.1%, assuming a standard cluster luminosity function. All
four events occurred within ~600 kpc of the cluster center (projected), as
defined by the position of the brightest cluster galaxy, and are more centrally
concentrated than the cluster SN Ia population as a whole. After accounting for
several observational biases that make intracluster SNe easier to discover and
spectroscopically confirm, we calculate an intracluster stellar mass fraction
of 0.16^{+0.13}_{-0.09} (68% CL) for all objects within R_{200}. If we assume
that the intracluster stellar population is exclusively old, and the cluster
galaxies themselves have a mix of stellar ages, we derive an upper limit on the
intracluster stellar mass fraction of <0.47 (84% one-sided CL). When combined
with the intragroup SNe results of McGee & Balogh, we confirm the declining
intracluster stellar mass fraction as a function of halo mass reported by
Gonzalez and collaborators. (Abridged)Comment: 24 pages, 8 figures, ApJ publishe
SDSS J124602.54+011318.8: A Highly Variable AGN, Not an Orphan GRB Afterglow
The optically variable source SDSS J124602.54+011318.8 first appears in Sloan
Digital Sky Survey (SDSS) data as a bright point source with nonstellar colors.
Subsequent SDSS imaging and spectroscopy showed that the point source declined
or disappeared, revealing an underlying host galaxy at redshift 0.385. Based on
these properties, the source was suggested to be a candidate ``orphan
afterglow'': a moderately beamed optical transient, associated with a gamma-ray
burst (GRB) whose highly beamed radiation cone does not include our line of
sight. We present new imaging and spectroscopic observations of this source.
When combined with a careful re-analysis of archival optical and radio data,
the observations prove that SDSS J124602.54+011318.8 is in fact an unusual
radio-loud AGN, probably in the BL Lac class. The object displays strong
photometric variability on time scales of weeks to years, including several
bright flares, similar to the one initially reported. The SDSS observations are
therefore almost certainly not related to a GRB. The optical spectrum of this
object dramatically changes in correlation with its optical brightness. At the
bright phase, weak, narrow oxygen emission lines and probably a broader
H line are superposed on a blue continuum. As the flux decreases, the
spectrum becomes dominated by the host galaxy light, with emerging stellar
absorption lines, while both the narrow and broad emission lines have larger
equivalent widths. We briefly discuss the implications of this discovery on the
study of AGNs and other optically variable or transient phenomena.Comment: 14 pages, 5 figures, AASTEX 5.0.2, slight modifications following
referee's report, PASP, in pres
Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II-P Supernovae Have Low Mass?
The stars that end their lives as supernovae (SNe) have been directly
observed in only a handful of cases, due mainly to the extreme difficulty in
identifying them in images obtained prior to the SN explosions. Here we report
the identification of the progenitor for the recent Type II-plateau
(core-collapse) SN 2005cs in pre-explosion archival images of the Whirlpool
Galaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera for
Surveys (ACS). From high-quality ground-based images of the SN from the
Canada-France-Hawaii Telescope, we precisely determine the position of the SN
and are able to isolate the SN progenitor to within 0".04 in the HST/ACS
optical images. We further pinpoint the SN location to within 0".005 from
HST/ACS ultraviolet images of the SN, confirming our progenitor identification.
From photometry of the SN progenitor obtained with the pre-SN ACS images, and
also limits to its brightness in pre-SN HST/NICMOS images, we infer that the
progenitor is a red supergiant star of spectral type K0--M3, with initial mass
7--9 Msun. We also discuss the implications of the SN 2005cs progenitor
identification and its mass estimate. There is an emerging trend that the most
common Type II-plateau SNe originate from low-mass supergiants 8--15 Msun.Comment: Submitted to ApJ. A high resolution version can be found at
http://astron.berkeley.edu/~weidong/sn05cs.p
A fourfold coordinated point defect in silicon
Due to their technological importance, point defects in silicon are among the
best studied physical systems. The experimental examination of point defects
buried in bulk is difficult and evidence for the various defects usually
indirect. Simulations of defects in silicon have been performed at various
levels of sophistication ranging from fast force fields to accurate density
functional calculations. The generally accepted viewpoint from all these
studies is that vacancies and self interstitials are the basic point defects in
silicon. We challenge this point of view by presenting density functional
calculations that show that there is a new fourfold coordinated point defect in
silicon that is lower in energy
The progenitor and early evolution of the Type IIb SN 2016gkg
We report initial observations and analysis on the Type IIb SN~2016gkg in the
nearby galaxy NGC~613. SN~2016gkg exhibited a clear double-peaked light curve
during its early evolution, as evidenced by our intensive photometric follow-up
campaign. SN~2016gkg shows strong similarities with other Type IIb SNe, in
particular with respect to the \he~emission features observed in both the
optical and near infrared. SN~2016gkg evolved faster than the prototypical
Type~IIb SN~1993J, with a decline similar to that of SN~2011dh after the first
peak. The analysis of archival {\it Hubble Space Telescope} images indicate a
pre-explosion source at SN~2016gkg's position, suggesting a progenitor star
with a mid F spectral type and initial mass \msun, depending on
the distance modulus adopted for NGC~613. Modeling the temperature evolution
within of explosion, we obtain a progenitor radius of
\rsun, smaller than that obtained from the analysis of the
pre-explosion images (\rsun).Comment: 7 pages, 5 figures. Submitted to ApJ Letter
The type IIb SN 2008ax: the nature of the progenitor
A source coincident with the position of the type IIb supernova (SN) 2008ax
is identified in pre-explosion Hubble Space Telescope (HST) Wide Field
Planetary Camera 2 observations in three optical filters. We identify and
constrain two possible progenitor systems: (i) a single massive star that lost
most of its hydrogen envelope through radiatively driven mass loss processes,
prior to exploding as a helium-rich Wolf-Rayet star with a residual hydrogen
envelope, and (ii) an interacting binary in a low mass cluster producing a
stripped progenitor. Late time, high resolution observations along with
detailed modelling of the SN will be required to reveal the true nature of this
progenitor star.Comment: 5 pages, 2 figures, resolution of figure 1 reduced, figure 2 revised,
some revision following referee's comments, accepted for publication in MNRAS
letter
Efficient Behavior of Small-World Networks
We introduce the concept of efficiency of a network, measuring how
efficiently it exchanges information. By using this simple measure small-world
networks are seen as systems that are both globally and locally efficient. This
allows to give a clear physical meaning to the concept of small-world, and also
to perform a precise quantitative a nalysis of both weighted and unweighted
networks. We study neural networks and man-made communication and
transportation systems and we show that the underlying general principle of
their construction is in fact a small-world principle of high efficiency.Comment: 1 figure, 2 tables. Revised version. Accepted for publication in
Phys. Rev. Let
The Caltech Core-Collapse Project (CCCP)
The cosmological utility of type Ia Supernovae prompted numerous studies of these events, and they are now well characterized observationally, both as individual objects and as a population. In contrast, all other types of supernovae (i.e. core-collapse events) are not as well observationally characterized. While some individual events have been studied in great detail (e.g. SN 1987A or SN 1998bw), the global properties of the core-collapse SN population are little known. However, in recent years, major drivers for change have emerged, among them the verification of the connection between core-collapse supernovae and long-duration Gamma-Ray Bursts (GRBs), the possible utility of some core-collapse supernovae (type II-P) as independent cosmological probes, and studies of core-collapse supernovae as high redshift targets for missions like the Supernova Acceleration Probe and the James Webb Space Telescope. The Caltech Core-Collapse Project is a large observational program using the Hale 200 inch and the robotic 60 inch telescopes at Palomar observatory to obtain optical photometry, spectroscopy and IR photometry of ~50 nearby core-collapse supernovae. The program is designed to provide a complete sample of core-collapse events, with well-defined selection criteria and uniform, high-quality optical/IR observations, as well as radio and X-ray light curves for some events. We will use this sample to characterize the little-studied properties of core-collapse supernovae as a population. The sample will be used as a comparison set for studies of supernovae associated with Gamma-Ray Bursts, to promote and calibrate the use of supernovae II-P for cosmography, and to set the stage for investigations of supernovae at high-z using coming space missions such as the Supernova Acceleration Probe and the James Webb Space Telescope
An automated archival VLA transients survey
In this paper we present the results of a survey for radio transients using
data obtained from the Very Large Array archive. We have reduced, using a
pipeline procedure, 5037 observations of the most common pointings - i.e. the
calibrator fields. These fields typically contain a relatively bright point
source and are used to calibrate `target' observations: they are therefore
rarely imaged themselves. The observations used span a time range ~ 1984 - 2008
and consist of eight different pointings, three different frequencies (8.4, 4.8
and 1.4 GHz) and have a total observing time of 435 hours. We have searched for
transient and variable radio sources within these observations using components
from the prototype LOFAR transient detection system. In this paper we present
the methodology for reducing large volumes of Very Large Array data; and we
also present a brief overview of the prototype LOFAR transient detection
algorithms. No radio transients were detected in this survey, therefore we
place an upper limit on the snapshot rate of GHz frequency transients > 8.0 mJy
to rho less than or equal to 0.032 deg^-2 that have typical timescales 4.3 to
45.3 days. We compare and contrast our upper limit with the snapshot rates -
derived from either detections or non-detections of transient and variable
radio sources - reported in the literature. When compared with the current Log
N - Log S distribution formed from previous surveys, we show that our upper
limit is consistent with the observed population. Current and future radio
transient surveys will hopefully further constrain these statistics, and
potentially discover dominant transient source populations. In this paper we
also briefly explore the current transient commissioning observations with
LOFAR, and the impact they will make on the field.Comment: Accepted for publication in MNRA
- âŠ