400 research outputs found
Performance of the GLAS Space Lidar Receiver Through Its Seven-Year Space Mission
NASA s Ice, Cloud, and land Elevation Satellite (ICESat) mission [1,2] carrying the Geoscience Laser Altimeter System (GLAS) Instrument, was launched on January 12, 2003. The three lasers on ICESat have made a total of 1.98 billion laser shot measurements of the Earth s surface and atmosphere during its 17 science data collection campaigns over its seven year operating lifetime. ICESat completed its science mission after the last laser stopped operating in October 2009. The spacecraft was de-orbited on August 30, 2010. The GLAS instrument carried 3 diode-pumped Q-switched Nd:YAG lasers, which emitted 6-nsec wide pulses at 1064 and 532 nm at a 40-Hz rate. There are three lidar receiver channels, a 1064 nm surface altimetry channel, a 1064 nm cloud backscattering lidar channel, and a 532 nm cloud and aerosol backscattering lidar channel. The altimetry and cloud backscatter channels used Si avalanche photodiode (APD) operated in analog mode as in the Mars Global Surveyor s Mars Orbital Laser Altimeter [3,4]. GLAS also utilized a number of new technologies and techniques for space lidar, including passively Q-switched diode-pumped Nd:YAG lasers, a 1-m diameter telescope, a temperature tuned etalon optical bandpass filter, Si APD single photon counting detectors, 1 Gsample/sec waveform digitizers, ultra stable clock oscillators, and digital signal processing and detection algorithms [5]. A global position system (GPS) receiver was used to provide the spacecraft position and epoch times. The ICESat mission provided a unique opportunity to monitor the lidar component performance in the space environment over a multi-year time period. We performed a number of engineering tests periodically to monitor the lidar receiver performance, including receiver sensitivity, timing precision, detector dark noise, etc. A series of engineering tests were also performed after the end of the science mission to evaluate the performance of the spare detector, oscillator, waveform digitizer, and GPS receiver. An experiment was conducted which pointed GLAS to Venus to test the receiver sensitivity to star light and to verify GLAS bore sight with respect to the spacecraft coordinate system. These tests provided unique data to assess the degradation and the rate of change of these key lidar components due to space radiation and aging. They also helped to validate new techniques to operate and calibrate future space lidars
Spectral Decomposition of Broad-Line AGNs and Host Galaxies
Using an eigenspectrum decomposition technique, we separate the host galaxy
from the broad line active galactic nucleus (AGN) in a set of 4666 spectra from
the Sloan Digital Sky Survey (SDSS), from redshifts near zero up to about 0.75.
The decomposition technique uses separate sets of galaxy and quasar
eigenspectra to efficiently and reliably separate the AGN and host
spectroscopic components. The technique accurately reproduces the host galaxy
spectrum, its contributing fraction, and its classification. We show how the
accuracy of the decomposition depends upon S/N, host galaxy fraction, and the
galaxy class. Based on the eigencoefficients, the sample of SDSS broad-line AGN
host galaxies spans a wide range of spectral types, but the distribution
differs significantly from inactive galaxies. In particular, post-starburst
activity appears to be much more common among AGN host galaxies. The
luminosities of the hosts are much higher than expected for normal early-type
galaxies, and their colors become increasingly bluer than early-type galaxies
with increasing host luminosity. Most of the AGNs with detected hosts are
emitting at between 1% and 10% of their estimated Eddington luminosities, but
the sensitivity of the technique usually does not extend to the Eddington
limit. There are mild correlations among the AGN and host galaxy
eigencoefficients, possibly indicating a link between recent star formation and
the onset of AGN activity. The catalog of spectral reconstruction parameters is
available as an electronic table.Comment: 18 pages; accepted for publication in A
Dust Reddening in SDSS Quasars
We explore the form of extragalactic reddening toward quasars using a sample
of 9566 quasars with redshifts 0<z<2.2, and accurate optical colors from the
Sloan Digital Sky Survey (SDSS). We confirm that dust reddening is the primary
explanation for the red ``tail'' of the color distribution of SDSS quasars. Our
fitting to 5-band photometry normalized by the modal quasar color as a function
of redshift shows that this ``tail'' is well described by SMC-like reddening
but not by LMC-like, Galactic, or Gaskell et al. (2004) reddening. Extension to
longer wavelengths using a subset of 1886 SDSS-2MASS matches confirms these
results at high significance. We carry out Monte-Carlo simulations that match
the observed distribution of quasar spectral energy distributions using a
Lorentzian dust reddening distribution; 2% of quasars selected by the main SDSS
targeting algorithm (i.e., which are not extincted out of the sample) have
E_{B-V} > 0.1; less than 1% have E_{B-V} > 0.2, where the extinction is
relative to quasars with modal colors. Reddening is uncorrelated with the
presence of intervening narrow-line absorption systems, but reddened quasars
are much more likely to show narrow absorption at the redshift of the quasar
than are unreddened quasars. Thus the reddening towards quasars is dominated by
SMC-like dust at the quasar redshift.Comment: 29 pages including 8 figures. AJ, September 2004 issu
Constraints on the Compact Object Mass in the Eclipsing HMXB XMMU J013236.7+303228 in M33
We present optical spectroscopic measurements of the eclipsing High Mass
X-ray Binary XMMU J013236.7+303228 in M33. Based on spectra taken at multiple
epochs of the 1.73d binary orbital period we determine physical as well as
orbital parameters for the donor star. We find the donor to be a B1.5IV
sub-giant with effective temperature T=22,000-23,000 K. From the luminosity,
temperature and known distance to M33 we derive a radius of R = 8.9 \pm 0.5
R_sun. From the radial--velocity measurements, we determine a velocity
semi-amplitude of K_opt = 63 \pm 12 km/sec. Using the physical properties of
the B-star determined from the optical spectrum, we estimate the star's mass to
be M_opt = 11 \pm 1 M_sun. Based on the X-ray spectrum, the compact companion
is likely a neutron star, although no pulsations have yet been detected. Using
the spectroscopically derived B-star mass we find the neutron star companion
mass to be M_X = 2.0 \pm 0.4 M_sun, consistent with the neutron star mass in
the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_sun found for
many millisecond pulsars. We attempt to use as an additional constraint that
the B star radius inferred from temperature, flux, and distance, should equate
the Roche radius, since the system accretes by Roche lobe overflow. This leads
to substantially larger masses, but from trying to apply the technique to known
systems, we find that the masses are consistently overestimated. Attempting to
account for that in our uncertainties, we derive M_X = 2.2^{+0.8}_{-0.6} M_sun
and M_opt =13 \pm 4 M_sun. We conclude that precise constraints require
detailed modeling of the shape of the Roche surface.Comment: 11 pages emulateapj, 9 figures, ApJ accepte
Testing stellar population synthesis models with Sloan Digital Sky Survey colors of M31's globular clusters
Accurate stellar population synthesis models are vital in understanding the
properties and formation histories of galaxies. In order to calibrate and test
the reliability of these models, they are often compared with observations of
star clusters. However, relatively little work has compared these models in the
ugriz filters, despite the recent widespread use of this filter set. In this
paper, we compare the integrated colors of globular clusters in the Sloan
Digital Sky Survey (SDSS) with those predicted from commonly used simple
stellar population (SSP) models. The colors are based on SDSS observations of
M31's clusters and provide the largest population of star clusters with
accurate photometry available from the survey. As such, it is a unique sample
with which to compare SSP models with SDSS observations. From this work, we
identify a significant offset between the SSP models and the clusters' g-r
colors, with the models predicting colors which are too red by g-r\sim0.1. This
finding is consistent with previous observations of luminous red galaxies in
the SDSS, which show a similar discrepancy. The identification of this offset
in globular clusters suggests that it is very unlikely to be due to a minority
population of young stars. The recently updated SSP model of Maraston &
Stromback better represents the observed g-r colors. This model is based on the
empirical MILES stellar library, rather than theoretical libraries, suggesting
an explanation for the g-r discrepancy.Comment: 8 pages, 4 figures, accepted for publication in Ap
Vodcast as ideating medium in STEM lesson plan in teaching heat transfer
This study attempts to link the clay soil in the locality to the topic of heat transfer as a contextualization point. It attempted to slightly modify the seven-step STEM lesson by having the part of the prototyping be first tried by the teacher, thereby having a change-of-hat to anticipate the ‘what if’ questions of the students. The teacher-researchers experimentation provided critical information to scaffold the students in the prototyping part. The evaluation of experts shows that the modified STEM lesson can be an excellent tool and the vodcast has been found to be a very satisfactory component of the STEM lesson. It is described as a very useful material in teaching heat transfer and related thermodynamics concepts and is highly recommended for use in both distance learning and face-to-face modality. Further, the clay oven exploration has come up with a refined clay oven production process, wherein the clay oven prototype has the capacity for the contextualization of heat transfer. It is recommended that a formal implementation be conducted to refine and standardize the lesson delivery
Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio
Polarimetry and the High-Energy Emission Mechanisms in Quasar Jets
The emission mechanisms in extragalactic jets include synchrotron and various
inverse-Compton processes. At low (radio through infrared) energies, it is
widely agreed that synchrotron emission dominates in both low-power (FR I) and
high-power (FR II and quasar) jets, because of the power-law nature of the
spectra observed and high polarizations. However, at higher energies, the
emission mechanism for high-power jets at kpc scales is hotly debated. Two
mechanisms have been proposed: either inverse-Compton of cosmic microwave
background photons or synchrotron emission from a second, high-energy
population of electrons. Here we discuss optical polarimetry as a method for
diagnosing the mechanism for the high-energy emission in quasar jets, as well
as revealing the jet's three-dimensional energetic and magnetic field
structure. We then discuss high-energy emission mechanisms for powerful jets in
the light of the HST polarimetry of PKS 1136-135.Comment: 4 pages, 1 figure. To appear in proceedings of "The Monster's Fiery
Breath: Feedback in galaxies, groups, and clusters" meeting, June 1-5, 2009
held in Madison, WI, US
- …