539 research outputs found
Search for Supermassive Black Hole Binaries in the Sloan Digital Sky Survey Spectroscopic Sample
Supermassive black hole (SMBH) binaries are expected in a Lambda CDM
cosmology given that most (if not all) massive galaxies contain a massive black
hole at their center. So far, however, direct evidence for such binaries has
been elusive. We use cross-correlation to search for temporal velocity shifts
in the MgII broad emission lines of 0.36 < z < 2 quasars with multiple
observations in the Sloan Digital Sky Survey. For ~ 10^9 Msun BHs in SMBH
binaries, we are sensitive to velocity drifts for binary separations of ~ 0.1
pc with orbital periods of ~100 years. We find seven candidate sub-pc--scale
binaries with velocity shifts > 3.4 sigma ~ 280 km/s, where sigma is our
systematic error. Comparing the detectability of SMBH binaries with the number
of candidates (N < 7), we can rule out that most 10^9 Msun BHs exist in ~
0.03-0.2 pc scale binaries, in a scenario where binaries stall at sub-pc scales
for a Hubble time. We further constrain that < one-third of quasars host SMBH
binaries after considering gas-assisted sub-pc evolution of SMBH binaries,
although this result is very sensitive to the assumed size of the broad line
region. We estimate the detectability of SMBH binaries with ongoing or
next-generation surveys (e.g., BOSS, Subaru Prime Focus Spectrograph), taking
into account the evolution of the sub-parsec binary in circumbinary gas disks.
These future observations will provide longer time baselines for searches
similar to ours and may in turn constrain the evolutionary scenarios of SMBH
binaries.Comment: Resubmitted to ApJ after referee's comments. 21 pages, 9 figure
Elemental Abundances in the Possible Type Ia Supernova Remnant G344.7-0.1
Recent studies on the Galactic supernova remnant (SNR) G344.7-0.1 have
commonly claimed its origin to be a core-collapse supernova (SN) explosion,
based on its highly asymmetric morphology and/or proximity to a star forming
region. In this paper, however, we present an X-ray spectroscopic study of this
SNR using Suzaku, which is supportive of a Type Ia origin. Strong K-shell
emission from lowly ionized Fe has clearly been detected, and its origin is
determined, for the first time, to be the Fe-rich SN ejecta. The abundance
pattern is highly consistent with that expected for a somewhat-evolved Type Ia
SNR. It is suggested, therefore, that the X-ray point-like source CXOU
J170357.8-414302 located at the SNR's geometrical center is not associated with
the SNR but is likely to be a foreground object. Our result further indicates
that G344.7-0.1 is the first possible Type Ia SNR categorized as a member of
the so-called "mixed-morphology" class. In addition, we have detected emission
from He-like Al at ~1.6 keV, the first clear detection of this element in the
spectrum of an extended X-ray source. The possible enhancement of the Al/Mg
abundance ratio from the solar value suggests that the ambient interstellar
medium has a relatively high metallicity (not less than 10% of the solar
value), if this SNR has indeed a Type Ia origin. We also report marginal
detection of Cr and Mn, although the measured fluxes have large statistical and
systematic uncertainties.Comment: ApJ in pres
The Origin of the Iron-Rich Knot in Tycho's Supernova Remnant
X-ray observations of supernova remnants (SNRs) allow us to investigate the
chemical inhomogeneity of ejecta, offering unique insight into the
nucleosynthesis in supernova explosions. Here we present detailed imaging and
spectroscopic studies of the "Fe knot" located along the eastern rim of the
Type Ia SNR Tycho (SN 1572) using Suzaku and Chandra long-exposure data.
Surprisingly, the Suzaku spectrum of this knot shows no emission from Cr, Mn,
or Ni, which is unusual for the Fe-rich regions in this SNR. Within the
framework of the canonical delayed-detonation models for SN Ia, the observed
mass ratios M_Cr/M_Fe < 0.023, M_Mn/M_Fe < 0.012, and M_Ni/M_Fe < 0.029 (at 90%
confidence) can only be achieved for a peak temperature of (5.3-5.7) x 10^9 K
and a neutron excess of < 2.0 x 10^-3. These constraints rule out the deep,
dense core of a Chandrasekhar-mass white dwarf as the origin of the Fe knot,
and favors either incomplete Si burning or the alpha-rich freeze-out regime,
probably close to their boundary. An explosive He burning regime is a possible
alternative, although this hypothesis is in conflict with the main properties
of this SNR.Comment: 13 pages, 13 figures, accepted for publication in Ap
The Absence of Ex-Companions in Type Ia Supernova Remnants
Type Ia supernovae (SNe Ia) play important roles in our study of the
expansion and acceleration of the Universe, but because we do not know the
exact nature or natures of the progenitors, there is a systematic uncertainty
that must be resolved if SNe Ia are to become more precise cosmic probes. No
progenitor system has ever been identified either in the pre- or post-explosion
images of a Ia event. There have been recent claims for and against the
detection of ex-companion stars in several SNe Ia remnants. These studies,
however, usually ignore the angular momentum gain of the progenitor WD, which
leads to a spin-up phase and a subsequent spin-down phase before explosion. For
spin-down timescales greater than 10^5 years, the donor star could be too dim
to detect by the time of explosion. Here we revisit the current limits on
ex-companion stars to SNR 0509-67.5, a 400 year old remnant in the Large
Magellanic Cloud (LMC). If the effects of possible angular momentum gain on the
WD are included, a wide range of single-degenerate progenitor models are
allowed for this remnant. We demonstrate that the current absence of evidence
for ex-companion stars in this remnant, as well as other SNe Ia remnants, does
not necessarily provide the evidence of absence for ex-companions. We discuss
potential ways to identify such ex-companion stars through deep imaging
observations.Comment: ApJ, accepted versio
Using Yellow Rocket as a Trap Crop for Diamondback Moth (Lepidoptera: Plutellidae)
Yellow rocket, Barbarea vulgaris (R. Br.) variety arcuata, was evaluated as a trap crop for diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), in cabbage, Brassica oleracea L. variety capitata, in 2003 and 2004. In 2003, the numbers of P. xylostella larvae found in field plots of cabbage alone were 5.2-11.3 times higher than those on cabbage plants in plots that included cabbage and several rows of yellow rocket. In an outdoor experiment in screenhouses, P. xylostella oviposition on cabbage was compared among six treatments that varied in the percentage of yellow rocket in relation to cabbage (0, 4, 8, 16, 24, and 32% of the plants were yellow rocket). Results indicated that the percentage of eggs laid on cabbage decreased as the percentage of yellow rocket in the treatment increased, but this decrease was not significant beyond 20% of the plants being yellow rocket. In 2004, the numbers of P. xylostella larvae in field plots of cabbage alone were 1.6-2.4 and 1.7-2.8 times higher than numbers in treatments with 10 and 20% trap crop, respectively. Sticky trap and sweep net captures of P. xylostella adults indicated that within-field dispersal was reduced by the presence of yellow rocket and aggregation occurred around yellow rocket plants. Our study suggests that using yellow rocket as a trap crop may reduce P. xylostella infestations in cabbage fields, and this possibility is discussed in the context of general crop and insect pest management practices in crucifer
Manipulating the Attractiveness and Suitability of Hosts for Diamondback Moth (Lepidoptera: Plutellidae)
Ovipositional preference and larval survival of the diamondback moth, Plutella xylostella (L.), were compared among cabbage, Brassica oleracea L. variety capitata; glossy collards, Brassica oleracea L. variety acephala; and yellow rocket, Barbarea vulgaris (R. Br.) variety arcuata in different treatments of planting density, host plant age, intercropping, and water stress in 2003 and 2004. P. xylostella laid nearly twice as many eggs per plant in the high planting densities of glossy collards and yellow rocket than in the standard planting densities. Ovipositional preference was positively correlated with plant age in cabbage, glossy collards, and yellow rocket. Larval survival on cabbage was 1.9 times higher on 6-wk than on 12-wk-old plants, whereas larval survival on collards was 12.1 times higher on the younger plants. No larvae survived on either 6- or 12-wk-old yellow rocket plants. Intercropping cabbage with either tomato, Lycopersicon esculentum Mill., or fava bean, Vicia fava L., did not reduce the number of eggs laid on cabbage. No significant differences in oviposition were found between water-stressed and well-irrigated host plants treatments. Yet, P. xylostella larval survival on water-stressed cabbage was 2.1 times lower than on well-irrigated cabbage plants. Based on our findings, the effectiveness of trap crops of glossy collards and yellow rocket could be enhanced by integrating the use of higher planting densities in the trap crop than in the main crop and seeding of the trap crop earlier than the main cro
Supernova Remnants as Clues to Their Progenitors
Supernovae shape the interstellar medium, chemically enrich their host
galaxies, and generate powerful interstellar shocks that drive future
generations of star formation. The shock produced by a supernova event acts as
a type of time machine, probing the mass loss history of the progenitor system
back to ages of 10 000 years before the explosion, whereas supernova
remnants probe a much earlier stage of stellar evolution, interacting with
material expelled during the progenitor's much earlier evolution. In this
chapter we will review how observations of supernova remnants allow us to infer
fundamental properties of the progenitor system. We will provide detailed
examples of how bulk characteristics of a remnant, such as its chemical
composition and dynamics, allow us to infer properties of the progenitor
evolution. In the latter half of this chapter, we will show how this exercise
may be extended from individual objects to SNR as classes of objects, and how
there are clear bifurcations in the dynamics and spectral characteristics of
core collapse and thermonuclear supernova remnants. We will finish the chapter
by touching on recent advances in the modeling of massive stars, and the
implications for observable properties of supernovae and their remnants.Comment: A chapter in "Handbook of Supernovae" edited by Athem W. Alsabti and
Paul Murdin (18 pages, 6 figures
A Chandrasekhar Mass Progenitor for the Type Ia Supernova Remnant 3C 397 from The Enhanced Abundances of Nickel and Manganese
Despite decades of intense efforts, many fundamental aspects of Type Ia
supernova (SNe Ia) remain elusive. One of the major open questions is whether
the mass of the exploding white dwarf (WD) is close to the Chandrasekhar limit.
Here we report the detection of strong K-shell emission from stable Fe-peak
elements in the Suzaku X-ray spectrum of the Type Ia supernova remnant (SNR) 3C
397. The high Ni/Fe and Mn/Fe mass ratios (0.11-0.24 and 0.018-0.033,
respectively) in the hot plasma component that dominates the K-shell emission
lines indicate a degree of neutronization in the SN ejecta which can only be
achieved by electron captures in the dense cores of exploding WDs with a
near-Chandrasekhar mass. This suggests a single-degenerate origin for 3C 397,
since Chandrasekhar mass progenitors are expected naturally if the WD accretes
mass slowly from a companion. Together with other results supporting the
double-degenerate scenario, our work adds to the mounting evidence that both
progenitor channels make a significant contribution to the SN Ia rate in
star-forming galaxies.Comment: Accepted by ApJL; 6 pages with 4 figures and 1 tabl
New Evidence for Efficient Collisionless Heating of Electrons at the Reverse Shock of a Young Supernova Remnant
Although collisionless shocks are ubiquitous in astrophysics, certain key
aspects of them are not well understood. In particular, the process known as
collisionless electron heating, whereby electrons are rapidly energized at the
shock front, is one of the main open issues in shock physics. Here we present
the first clear evidence for efficient collisionless electron heating at the
reverse shock of Tycho's supernova remnant (SNR), revealed by Fe-K diagnostics
using high-quality X-ray data obtained by the Suzaku satellite. We detect
K-beta (3p->1s) fluorescence emission from low-ionization Fe ejecta excited by
energetic thermal electrons at the reverse shock front, which peaks at a
smaller radius than Fe K-alpha (2p->1s) emission dominated by a relatively
highly-ionized component. Comparison with our hydrodynamical simulations
implies instantaneous electron heating to a temperature 1000 times higher than
expected from Coulomb collisions alone. The unique environment of the reverse
shock, which is propagating with a high Mach number into rarefied ejecta with a
low magnetic field strength, puts strong constraints on the physical mechanism
responsible for this heating, and favors a cross-shock potential created by
charge deflection at the shock front. Our sensitive observation also reveals
that the reverse shock radius of this SNR is about 10% smaller than the
previous measurement using the Fe K-alpha morphology from the Chandra
observations. Since strong Fe K-beta fluorescence is expected only from
low-ionization plasma where Fe ions still have many 3p electrons, this feature
is key to diagnosing the plasma state and distribution of the immediate
postshock ejecta in a young SNR.Comment: 7 pages, 9 figures, resubmitted to ApJ with minor changes following
the referee repor
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