7,640 research outputs found
Improving the sensitivity to gravitational-wave sources by modifying the input-output optics of advanced interferometers
We study frequency dependent (FD) input-output schemes for signal-recycling
interferometers, the baseline design of Advanced LIGO and the current
configuration of GEO 600. Complementary to a recent proposal by Harms et al. to
use FD input squeezing and ordinary homodyne detection, we explore a scheme
which uses ordinary squeezed vacuum, but FD readout. Both schemes, which are
sub-optimal among all possible input-output schemes, provide a global noise
suppression by the power squeeze factor, while being realizable by using
detuned Fabry-Perot cavities as input/output filters. At high frequencies, the
two schemes are shown to be equivalent, while at low frequencies our scheme
gives better performance than that of Harms et al., and is nearly fully
optimal. We then study the sensitivity improvement achievable by these schemes
in Advanced LIGO era (with 30-m filter cavities and current estimates of
filter-mirror losses and thermal noise), for neutron star binary inspirals, and
for narrowband GW sources such as low-mass X-ray binaries and known radio
pulsars. Optical losses are shown to be a major obstacle for the actual
implementation of these techniques in Advanced LIGO. On time scales of
third-generation interferometers, like EURO/LIGO-III (~2012), with
kilometer-scale filter cavities, a signal-recycling interferometer with the FD
readout scheme explored in this paper can have performances comparable to
existing proposals. [abridged]Comment: Figs. 9 and 12 corrected; Appendix added for narrowband data analysi
Generating and Analyzing Constrained Dark Energy Equations of State and Systematics Functions
Some functions entering cosmological analysis, such as the dark energy
equation of state or systematic uncertainties, are unknown functions of
redshift. To include them without assuming a particular form we derive an
efficient method for generating realizations of all possible functions subject
to certain bounds or physical conditions, e.g. w\in[-1,+1] as for quintessence.
The method is optimal in the sense that it is both pure and complete in filling
the allowed space of principal components. The technique is applied to
propagation of systematic uncertainties in supernova population drift and dust
corrections and calibration through to cosmology parameter estimation and bias
in the magnitude-redshift Hubble diagram. We identify specific ranges of
redshift and wavelength bands where the greatest improvements in supernova
systematics due to population evolution and dust correction can be achieved.Comment: 12 pages, 11 figures; v2 minor revisions, higher resolution figures,
matches PRD versio
Adaptive Optics Imaging of IRAS 18276-1431: a bipolar pre-planetary nebula with circumstellar "searchlight beams" and "arcs"
We present high-angular resolution images of the post-AGB nebula
IRAS18276-1431 (also known as OH17.7-2.0) obtained with the Keck II Adaptive
Optics (AO) system in its Natural Guide Star (NGS) mode in the Kp, Lp, and Ms
near-infrared bands. We also present supporting optical F606W and F814W HST
images as well as interferometric observations of the 12CO(J=1-0), 13CO(J=1-0),
and 2.6mm continuum emission with OVRO. The envelope of IRAS18276-1431 displays
a clear bipolar morphology in our optical and NIR images with two lobes
separated by a dark waist and surrounded by a faint 4.5"x3.4" halo. Our Kp-band
image reveals two pairs of radial ``searchlight beams'' emerging from the
nebula center and several intersecting, arc-like features. From our CO data we
derive a mass of M>0.38[D/3kpc]^2 Msun and an expansion velocity v_exp=17km/s
for the molecular envelope. The density in the halo follows a radial power-law
proportional to r^-3, which is consistent with a mass-loss rate increasing with
time. Analysis of the NIR colors indicates the presence of a compact central
source of ~300-500K dust illuminating the nebula in addition to the central
star. Modeling of the thermal IR suggests a two-shell structure in the dust
envelope: 1) an outer shell with inner and outer radius R_in~1.6E16cm and
R_out>~1.25E17cm, dust temperature T_d~105-50K, and a mean mass-loss rate of
Mdot~1E-3Msun/yr; and 2) an inner shell with R_in~6.3E14cm, T_dust~500-105K,
and Mdot~3E-5Msun/yr. An additional population of big dust grains (radius
a>~0.4mm) with T_dust=150-20K and mass M_dust=(0.16-1.6)E-3 [D/3kpc]^2 Msun can
account for the observed sub-mm and mm flux excess. The mass of the envelope
enclosed within R_out=1.25E17cm derived from SED modeling is ~1[D/3kpc]^2 Msun.Comment: 46 pages, 14 figures, 3 tables, accepted for publication in ApJ.
Figures 12 & 13 in low resolution. Full resolution versions are available
upon request to the first autho
The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
The size of a planet is an observable property directly connected to the
physics of its formation and evolution. We used precise radius measurements
from the California-Kepler Survey (CKS) to study the size distribution of 2025
planets in fine detail. We detect a factor of 2 deficit
in the occurrence rate distribution at 1.5-2.0 R. This gap splits
the population of close-in ( < 100 d) small planets into two size regimes:
R < 1.5 R and R = 2.0-3.0 R, with few planets in
between. Planets in these two regimes have nearly the same intrinsic frequency
based on occurrence measurements that account for planet detection
efficiencies. The paucity of planets between 1.5 and 2.0 R supports
the emerging picture that close-in planets smaller than Neptune are composed of
rocky cores measuring 1.5 R or smaller with varying amounts of
low-density gas that determine their total sizes.Comment: Paper III in the California-Kepler Survey series, accepted to the
Astronomical Journa
SN2008am: A Super-Luminous Type IIn Supernova
We present observations and interpretation of the Type IIn supernova SN
2008am discovered by the ROTSE Supernova Verification Project (RSVP). SN 2008am
peaked at approximately -22.3 mag at a redshift of z=0.2338, giving it a peak
luminosity of 3 x 10^{44}erg/s and making it one of the most luminous
supernovae ever observed. The total radiated energy is ~ 2 x 10^{51} erg.
Photometric observations in the ultraviolet, optical and infrared bands
(J,H,Ks) constrain the SED evolution. We obtained six optical spectra of the
supernova, five on the early decline from maximum light and a sixth nearly a
year later plus a very late-time spectrum (~2 yr) of the host galaxy. The
spectra of SN 2008am show strong Balmer-line and He I lambda 5876A emission
with intermediate widths (~25A) in the first ~40 days after optical maximum. We
examine a variety of models for the line wings and conclude that multiple
scattering is most likely, implying that our spectra contain no specific
information on the bulk flow velocity. We examine a variety of models for the
ROTSE light curve subject to the rise time and the nature of the spectra,
including radioactive decay, shocks in optically-thick and optically-thin
circumstellar media (CSM) and a magnetar. The most successful model is one for
which the CSM is optically-thick and in which diffusion of forward
shock-deposited luminosity gives rise to the observed light curve. Diffusion of
the shock-deposited energy from the forward shock is found to be important to
account for the rising part of the light curve. Although there are differences
in detail, SN 2008am appears to be closely related to other super-luminous Type
IIn supernovae, SN 2006gy, SN 2006tf and perhaps SN 2008iy, that may represent
the deaths of very massive LBV-type progenitors and for which the luminosity is
powered by the interaction of the ejecta with a dense circumstellar medium.Comment: 58 pages, 14 figure
SN 2006oz: rise of a super-luminous supernova observed by the SDSS-II SN Survey
We study SN 2006oz, a newly-recognized member of the class of H-poor,
super-luminous supernovae. We present multi-color light curves from the SDSS-II
SN Survey, that cover the rise time, as well as an optical spectrum that shows
that the explosion occurred at z~0.376. We fitted black body functions to
estimate the temperature and radius evolution of the photosphere and used the
parametrized code SYNOW to model the spectrum. We constructed a bolometric
light curve and compared it with explosion models. The very early light curves
show a dip in the g- and r-bands and a possible initial cooling phase in the
u-band before rising to maximum light. The bolometric light curve shows a
precursor plateau with a duration of 6-10 days in the rest-frame. A lower limit
of M_u < -21.5 can be placed on the absolute peak luminosity of the SN, while
the rise time is constrained to be at least 29 days. During our observations,
the emitting sphere doubled its radius to 2x10^15 cm, while the temperature
remained hot at 15000 K. As for other similar SNe, the spectrum is best modeled
with elements including O II and Mg II, while we tentatively suggest that Fe
III might be present. We suggest that the precursor plateau might be related to
a recombination wave in a circumstellar medium (CSM) and discuss whether this
is a common property of all similar explosions. The subsequent rise can be
equally well described by input from a magnetar or by ejecta-CSM interaction,
but the models are not well constrained owing to the lack of post-maximum
observations, and CSM interaction has difficulties accounting for the precursor
plateau self-consistently. Radioactive decay is less likely to be the mechanism
that powers the luminosity. The host galaxy, detected in deep imaging with the
10 m GTC, is a moderately young and star-forming, but not a starburst, galaxy.
It has an absolute magnitude of M_g = -16.9.Comment: Contains minor changes (of editorial nature) with respect to v1 in
order to match the published version. The abstract has been modified to fit
the arXiv space requirements. 11 pages, 8 figures, 3 table
Ultra-Low Power Wake Up Receiver For Medical Implant Communications Service Transceiver
This thesis explores the specific requirements and challenges for the design of a dedicated wake-up receiver for medical implant communication services equipped with a novel “uncertain-IF†architecture combined with a high – Q filtering MEMS resonator and a free running CMOS ring oscillator as the RF LO. The receiver prototype, implements an IBM 0.18μm mixed-signal 7ML RF CMOS technology and achieves a sensitivity of -62 dBm at 404MHz while consuming \u3c100 μW from a 1 V supply
Modeling the System Parameters of 2M1533+3759: A New Longer-Period Low-Mass Eclipsing sdB+dM Binary
We present new photometric and spectroscopic observations for 2M 1533+3759 (=
NSVS 07826147). It has an orbital period of 0.16177042 day, significantly
longer than the 2.3--3.0 hour periods of the other known eclipsing sdB+dM
systems. Spectroscopic analysis of the hot primary yields Teff = 29230 +/- 125
K, log g = 5.58 +/- 0.03 and log N(He)/N(H) = -2.37 +/- 0.05. The sdB velocity
amplitude is K1 = 71.1 +/- 1.0 km/s. The only detectable light contribution
from the secondary is due to the surprisingly strong reflection effect. Light
curve modeling produced several solutions corresponding to different values of
the system mass ratio, q(M2/M1), but only one is consistent with a core helium
burning star, q=0.301. The orbital inclination is 86.6 degree. The sdB primary
mass is M1 = 0.376 +/- 0.055 Msun and its radius is R1 = 0.166 +/- 0.007 Rsun.
2M1533+3759 joins PG0911+456 (and possibly also HS2333+3927) in having an
unusually low mass for an sdB star. SdB stars with masses significantly lower
than the canonical value of 0.48 Msun, down to as low as 0.30 Msun, were
theoretically predicted by Han et al. (2002, 2003), but observational evidence
has only recently begun to confirm the existence of such stars. The existence
of core helium burning stars with masses lower than 0.40--0.43 Msun implies
that at least some sdB progenitors have initial main sequence masses of
1.8--2.0 Msun or more, i.e. they are at least main sequence A stars. The
secondary is a main sequence M5 star.Comment: 47 pages, 7 figure
Morphology and Composition of the Helix Nebula
We present new narrow-band filter imagery in H-alpha and [N II] 6584 along
with UV and optical spectrophotometry measurements from 1200 to 9600 Angstroms
of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large
diameter and low surface brightness. Detailed models of the observable ionized
nebula support the recent claim that the Helix is actually a flattened disk
whose thickness is roughly one-third its diameter with an inner region
containing hot, highly ionized gas which is generally invisible in narrow-band
images. The outer visible ring structure is of lower ionization and temperature
and is brighter because of a thickening in the disk. We also confirm a central
star effective temperature and luminosity of 120,000K and 100L_sun, and we
estimate a lower limit to the nebular mass to be 0.30M_sun. Abundance
measurements indicate the following values: He/H=0.12 (+/-0.017),
O/H=4.60x10^-4 (+/-0.18), C/O=0.87 (+/-0.12), N/O=0.54 (+/-0.14), Ne/O=0.33
(+/-0.04), S/O=3.22x10^-3 (+/-0.26), and Ar/O=6.74x10^-3 (+/-0.76). Our carbon
abundance measurements represent the first of their kind for the Helix Nebula.
The S/O ratio which we derive is anomalously low; such values are found in only
a few other planetary nebulae. The central star properties, the super-solar
values of He/H and N/O, and a solar level of C/O are consistent with a 6.5M_sun
progenitor which underwent three phases of dredge-up and hot bottom burning
before forming the planetary nebula.Comment: 50-page manuscript plus 11 postscript figures. This revised version
corrects a typo in earlier submission. Nothing else has changed. Accepted for
publication in the Astrophysical Journa
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