447 research outputs found
Photometric variability of candidate white dwarf binary systems from Palomar Transient Factory archival data
We present a sample of 59 periodic variables from the Palomar Transient
Factory, selected from published catalogues of white dwarf (WD) candidates. The
variability can likely be attributed to ellipsoidal variation of the tidally
distorted companion induced by the gravity of the primary (WD or hot subdwarf)
or to the reflection of hot emission by a cooler companion. We searched 11311
spectroscopically or photometrically selected WD candidates from three hot
star/WD catalogues, using the Lomb-Scargle periodogram to single out promising
sources. We present period estimates for the candidates, 45 of which were not
previously identified as periodic variables, and find that most have a period
shorter than a few days. Additionally, we discuss the eclipsing systems in our
sample and present spectroscopic data on selected sources
Statistical Searches for Microlensing Events in Large, Non-Uniformly Sampled Time-Domain Surveys: A Test Using Palomar Transient Factory Data
Many photometric time-domain surveys are driven by specific goals, such as
searches for supernovae or transiting exoplanets, which set the cadence with
which fields are re-imaged. In the case of the Palomar Transient Factory (PTF),
several sub-surveys are conducted in parallel, leading to non-uniform sampling
over its footprint. While the median PTF field has been imaged 40 times in \textit{R}-band,
have been observed 100 times. We use PTF data to
study the trade-off between searching for microlensing events in a survey whose
footprint is much larger than that of typical microlensing searches, but with
far-from-optimal time sampling. To examine the probability that microlensing
events can be recovered in these data, we test statistics used on uniformly
sampled data to identify variables and transients. We find that the von Neumann
ratio performs best for identifying simulated microlensing events in our data.
We develop a selection method using this statistic and apply it to data from
fields with 10 -band observations, light curves,
uncovering three candidate microlensing events. We lack simultaneous,
multi-color photometry to confirm these as microlensing events. However, their
number is consistent with predictions for the event rate in the PTF footprint
over the survey's three years of operations, as estimated from near-field
microlensing models. This work can help constrain all-sky event rate
predictions and tests microlensing signal recovery in large data sets, which
will be useful to future time-domain surveys, such as that planned with the
Large Synoptic Survey Telescope.Comment: 13 pages, 14 figures; accepted for publication in ApJ. fixed author
lis
More Flexibility in Representing Geometric Distortion in Astronomical Images
A number of popular software tools in the public domain are used by astronomers, professional and amateur alike, but some of the tools that have similar purposes cannot be easily interchanged, owing to the lack of a common standard. For the case of image distortion, SCAMP and SExtractor, available from Astromatic.net, perform astrometric calibration and source-object extraction on image data, and image-data geometric distortion is computed in celestial coordinates with polynomial coefficients stored in the FITS header with the PV i_j keywords. Another widely-used astrometric-calibration service, Astrometry.net, solves for distortion in pixel coordinates using the SIP convention that was introduced by the Spitzer Science Center. Up until now, due to the complexity of these distortion representations, it was very difficult to use the output of one of these packages as input to the other. New Python software, along with faster-computing C-language translations, have been developed at the Infrared Processing and Analysis Center (IPAC) to convert FITS-image headers from PV to SIP and vice versa. It is now possible to straightforwardly use Astrometry.net for astrometric calibration and then SExtractor for source-object extraction. The new software also enables astrometric calibration by SCAMP followed by image visualization with tools that support SIP distortion, but not PV . The software has been incorporated into the image-processing pipelines of the Palomar Transient Factory (PTF), which generate FITS images with headers containing both distortion representations. The software permits the conversion of archived images, such as from the Spitzer Heritage Archive and NASA/IPAC Infrared Science Archive, from SIP to PV or vice versa. This new capability renders unnecessary any new representation, such as the proposed TPV distortion convention
Disk-related Bursts and Fades in Young Stars
We present first results from a new, multiyear, time domain survey of young stars in the North America Nebula complex using the Palomar Transient Factory. Our survey is providing an unprecedented view of aperiodic variability in young stars on timescales of days to years. The analyzed sample covers R_(PTF) ā 13.5-18 and spans a range of mid-infrared color, with larger-amplitude optical variables (exceeding 0.4 mag root mean squared) more likely to have mid-infrared evidence for circumstellar material. This paper characterizes infrared excess stars with distinct bursts above or fades below a baseline of lower-level variability, identifying 41 examples. The light curves exhibit a remarkable diversity of amplitudes, timescales, and morphologies, with a continuum of behaviors that cannot be classified into distinct groups. Among the bursters, we identify three particularly promising sources that may represent theoretically predicted short-timescale accretion instabilities. Finally, we find that fading behavior is approximately twice as common as bursting behavior on timescales of days to years, although the bursting and fading duty cycle for individual objects often varies from year to year
Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory
We report the discovery of two new halo velocity groups (Cancer groups A and B) traced by 8 distant RR Lyrae stars and observed by the Palomar Transient Factory (PTF) survey at R.A.~129Ā°, Dec~20Ā° (l~205Ā°, b~32Ā°). Located at 92 kpc from the Galactic center (86 kpc from the Sun), these are some of the most distant substructures in the Galactic halo known to date. Follow-up spectroscopic observations with the Palomar Observatory 5.1-m Hale telescope and W. M. Keck Observatory 10-m Keck I telescope indicate that the two groups are moving away from the Galaxy at v_(gsr) = 78.0+-5.6 km s^(-1) (Cancer group A) and v_(gsr) = 16.3+-7.1 km s^(-1) (Cancer group B). The groups have velocity dispersions of Ļ_(v_)gsr))=12.4+-5.0 km s^(-1) and Ļ _(v_(gsr))=14.9+-6.2 km s^(-1), and are spatially extended (about several kpc) making it very unlikely that they are bound systems, and are more likely to be debris of tidally disrupted dwarf galaxies or globular clusters. Both groups are metal-poor (median metallicities of [Fe/H]^A = -1.6 dex and [Fe/H]^B =-2.1 dex), and have a somewhat uncertain (due to small sample size) metallicity dispersion of ~0.4 dex, suggesting dwarf galaxies as progenitors. Two additional RR Lyrae stars with velocities consistent with those of the Cancer groups have been observed ~25 Ā° east, suggesting possible extension of the groups in that direction
Corrected QT interval as a predictor of mortality in elderly patients with syncope
Background: Prolonged corrected QT interval (QTc) holds independent prognostic importance
in predicting mortality in patients with coronary artery disease, diabetes mellitus and
congestive heart failure. However, its association with all cause or cardiac mortality in the
general population remains unclear. We evaluated the relationship between prolonged QTc
and total mortality among patients with syncope.
Methods: This was a retrospective study of 348 patients presenting to the emergency department
with syncope of any etiology over a period of one year. All patients with atrial fibrillation,
left bundle branch block and cardiac devices (pacemaker/defibrillator) were excluded. Prolonged
QTc interval was defined as QTc interval ≥ 440 ms. The primary end point for this
study was total mortality in patients presenting with syncope.
Results: There were 58 (16%) deaths in this population during a mean follow-up of 30 months.
Patients with prolonged QTc interval had significantly higher mortality when compared to
those with normal QTc interval (22% vs 11%; p = 0.004). This significance was not retained
after adjustment for covariates in the Cox regression model, where we found that age ≥ 65 years
(hazard ratio [HR] 7.9; 95% confidence interval [CI] 1.9–32.9; p = 0.004) and QTc interval
≥ 500 ms (HR 3.5; 95% CI 1.56–8.12; p = 0.002) were predictors of increased mortality
among patients with syncope.
Conclusions: In elderly patients presenting to the emergency department with syncope, QTc
interval ≥ 500 ms helps identify patients at higher risk of adverse outcomes. (Cardiol J 2011;
18, 4: 395–400
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