844 research outputs found
SDSSJ14584479+3720215: A Benchmark JHK Blazar Light Curve from the 2MASS Calibration Scans
Active galactic nuclei (AGNs) are well-known to exhibit flux variability
across a wide range of wavelength regimes, but the precise origin of the
variability at different wavelengths remains unclear. To investigate the
relatively unexplored near-IR variability of the most luminous AGNs, we conduct
a search for variability using well sampled JHKs-band light curves from the
2MASS survey calibration fields. Our sample includes 27 known quasars with an
average of 924 epochs of observation over three years, as well as one
spectroscopically confirmed blazar (SDSSJ14584479+3720215) with 1972 epochs of
data. This is the best-sampled NIR photometric blazar light curve to date, and
it exhibits correlated, stochastic variability that we characterize with
continuous auto-regressive moving average (CARMA) models. None of the other 26
known quasars had detectable variability in the 2MASS bands above the
photometric uncertainty. A blind search of the 2MASS calibration field light
curves for AGN candidates based on fitting CARMA(1,0) models (damped-random
walk) uncovered only 7 candidates. All 7 were young stellar objects within the
{\rho} Ophiuchus star forming region, five with previous X-ray detections. A
significant {\gamma}-ray detection (5{\sigma}) for the known blazar using 4.5
years of Fermi photon data is also found. We suggest that strong NIR
variability of blazars, such as seen for SDSSJ14584479+3720215, can be used as
an efficient method of identifying previously-unidentified {\gamma}-ray
blazars, with low contamination from other AGN.Comment: 6 pages, 3 figures, ApJ Accepte
Ultraviolet-Selected Field and Pre-Main-Sequence Stars Towards Taurus and Upper Scorpius
We have carried out a Galaxy Evolution Explorer (GALEX) Cycle 1 guest
investigator program covering 56 square degrees near the Taurus T association
and 12 square degrees along the northern edge of the Upper Scorpius OB
association. We combined photometry in the GALEX FUV and NUV bands with data
from the Two Micron All Sky Survey to identify candidate young (<100 Myr old)
stars as those with an ultraviolet excess relative to older main sequence
stars. Follow-up spectroscopy of a partial sample of these candidates suggest 5
new members of Taurus, with 8-20 expected from additional observations, and 5
new members of Upper Scorpius, with 3-6 expected from additional observations.
These candidate new members appear to represent a distributed, non-clustered
population in either region, although our sample statistics are as of yet too
poor to constrain the nature or extent of this population. Rather, our study
demonstrates the ability of GALEX observations to identify young stellar
populations distributed over a wide area of the sky. We also highlight the
necessity of a better understanding of the Galactic ultraviolet source
population to support similar investigations. In particular, we report a large
population of stars with an ultraviolet excess but no optical indicators of
stellar activity or accretion, and briefly argue against several
interpretations of these sources.Comment: 46 pages, 16 figures, 13 tables; Accepted to the Astronomical Journa
Sub-systems in nearby solar-type wide binaries
We conducted a deep survey of resolved sub-systems among wide binaries with
solar-type components within 67 pc from the Sun. Images of 61 stars in the K
and H bands were obtained with the NICI adaptive-optics instrument on the 8-m
Gemini-South telescope. Our maximum detectable magnitude difference is about
5mag and 7.8mag at 0.15" and 0.9" separations, respectively. This enables a
complete census of sub-systems with stellar companions in the projected
separation range from 5 to 100 AU. Out of 7 such companions found in our
sample, only one was known previously. We determine that the fraction of
sub-systems with projected separations above 5 AU is 0.12 +- 0.04 and that the
distribution of their mass ratio is flat, with a power-law index 0.2 +- 0.5.
Comparing this with the properties of closer spectroscopic sub-systems
(separations below 1 AU), it appears that the mass-ratio distribution does not
depend on the separation. The frequency of sub-systems in the separation ranges
below 1 AU and between 5 and 100 AU is similar, about 0.15. Unbiased statistics
of multiplicity higher than two, advanced by this work, provide constraints on
star-formation theory.Comment: Accepted by Astronomical Journal. 13 pages, 5 figure
NEOWISE Reactivation Mission Year One: Preliminary Asteroid Diameters and Albedos
We present preliminary diameters and albedos for 7,959 asteroids detected in
the first year of the NEOWISE Reactivation mission. 201 are near-Earth
asteroids (NEAs). 7,758 are Main Belt or Mars-crossing asteroids. 17% of these
objects have not been previously characterized using WISE or NEOWISE thermal
measurements. Diameters are determined to an accuracy of ~20% or better. If
good-quality H magnitudes are available, albedos can be determined to within
~40% or better.Comment: 42 pages, 5 figure
Asteroid Diameters and Albedos from NEOWISE Reactivation Mission Years 4 and 5
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft has been conducting a two-band thermal infrared survey to detect and characterize asteroids and comets since its reactivation in 2013 December. Using the observations collected during the fourth and fifth years of the survey, our automated pipeline detected candidate moving objects that were verified and reported to the Minor Planet Center. Using these detections, we perform thermal modeling of each object from the near-Earth object (NEO) and Main Belt asteroid (MBA) populations to constrain their sizes. We present thermal model fits of asteroid diameters for 189 NEOs and 5831 MBAs detected during the fourth year of the survey, and 185 NEOs and 5776 MBAs from the fifth year. To date, the NEOWISE Reactivation survey has provided thermal model characterization for 957 unique NEOs. Including all phases of the original Wide-field Infrared Survey Explorer survey brings the total to 1473 unique NEOs that have been characterized between 2010 and the present
A Global Photometric Analysis of 2MASS Calibration Data
We present results from the application of a global photometric calibration
(GPC) procedure to calibration data from the first 2 years of The Two Micron
All Sky Survey (2MASS). The GPC algorithm uses photometry of both primary
standards and moderately bright `tracer' stars in 35 2MASS calibration fields.
During the first two years of the Survey, each standard was observed on
approximately 50 nights, with about 900 individual measurements. Based on the
photometry of primary standard stars and secondary tracer stars and under the
assumption that the nightly zeropoint drift is linear, GPC ties together all
calibration fields and all survey nights simultaneously, producing a globally
optimized solution. Calibration solutions for the Northern and Southern
hemisphere observatories are found separately, and are tested for global
consistency based on common fields near the celestial equator.
Several results from the GPC are presented, including establishing candidate
secondary standards, monitoring of near-infrared atmospheric extinction
coefficients, and verification of global validity of the standards. The
solution gives long-term averages of the atmospheric extinction coefficients,
A_J=0.096, A_H=0.026, A_{K_s}=0.066 (North) and A_J=0.092, A_H=0.031,
A_{K_s}=0.065 (South), with formal error of 0.001. The residuals show small
seasonal variations, most likely due to changing atmospheric content of water
vapor. Extension of the GPC to approximately 100 field stars in each of the 35
calibration fields yields a catalog of more than two thousand photometric
standards ranging from 10th to 14th magnitude, with photometry that is globally
consistent to .Comment: 19 pages, 10 figures; Submitted to AJ. The table of secondary
standards is available from ftp://nova.astro.umass.edu/pub/nikolaev/ or
ftp://anon-ftp.ipac.caltech.edu/pub/2mass/globalcal
NEOWISE Reactivation Mission Year Three: Asteroid Diameters and Albedos
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE)
reactivation mission has completed its third year of surveying the sky in the
thermal infrared for near-Earth asteroids and comets. NEOWISE collects
simultaneous observations at 3.4 um and 4.6 um of solar system objects passing
through its field of regard. These data allow for the determination of total
thermal emission from bodies in the inner solar system, and thus the sizes of
these objects. In this paper we present thermal model fits of asteroid
diameters for 170 NEOs and 6110 MBAs detected during the third year of the
survey, as well as the associated optical geometric albedos. We compare our
results with previous thermal model results from NEOWISE for overlapping sample
sets, as well as diameters determined through other independent methods, and
find that our diameter measurements for NEOs agree to within 26% (1-sigma) of
previously measured values. Diameters for the MBAs are within 17% (1-sigma).
This brings the total number of unique near-Earth objects characterized by the
NEOWISE survey to 541, surpassing the number observed during the fully
cryogenic mission in 2010.Comment: Accepted for publication in A
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