312 research outputs found
The unblinking eye on the sky
From near-Earth asteroids to superluminous supernovae and counterparts to
gravitational wave sources, the Zwicky Transient Facility will soon scan the
night sky for transient phenomena.Comment: Author's version of "Mission Control" profile published in Nature
Astronomy. 3 pages, 1 figure. https://www.nature.com/articles/s41550-017-007
Volumetric Survey Speed: A Figure of Merit for Transient Surveys
Time-domain surveys can exchange sky coverage for revisit frequency, complicating the comparison of their relative capabilities. By using different revisit intervals, a specific camera may execute surveys optimized for discovery of different classes of transient objects. We propose a new figure of merit, the instantaneous volumetric survey speed, for evaluating transient surveys. This metric defines the trade between cadence interval and snapshot survey volume and so provides a natural means of comparing survey capability. The related metric of areal survey speed imposes a constraint on the range of possible revisit times: we show that many modern time-domain surveys are limited by the amount of fresh sky available each night. We introduce the concept of "spectroscopic accessibility" and discuss its importance for transient science goals requiring followup observing. We present an extension of the control time algorithm for cases where multiple consecutive detections are required. Finally, we explore how survey speed and choice of cadence interval determine the detection rate of transients in the peak absolute magnitude–decay timescale phase space
The Zwicky Transient Facility
The Zwicky Transient Facility (ZTF) is a next-generation optical synoptic survey that builds on the experience and infrastructure of the Palomar Transient Factory (PTF). Using a new 47 deg2 survey camera, ZTF will survey more than an order of magnitude faster than PTF to discover rare transients and variables. I describe the survey and the camera design. Searches for young supernovae, fast transients, counterparts to gravitational-wave detections, and rare variables will benefit from ZTF's high cadence, wide area survey
Volumetric Survey Speed: A Figure of Merit for Transient Surveys
Time-domain surveys can exchange sky coverage for revisit frequency, complicating the comparison of their relative capabilities. By using different revisit intervals, a specific camera may execute surveys optimized for discovery of different classes of transient objects. We propose a new figure of merit, the instantaneous volumetric survey speed, for evaluating transient surveys. This metric defines the trade between cadence interval and snapshot survey volume and so provides a natural means of comparing survey capability. The related metric of areal survey speed imposes a constraint on the range of possible revisit times: we show that many modern time-domain surveys are limited by the amount of fresh sky available each night. We introduce the concept of "spectroscopic accessibility" and discuss its importance for transient science goals requiring followup observing. We present an extension of the control time algorithm for cases where multiple consecutive detections are required. Finally, we explore how survey speed and choice of cadence interval determine the detection rate of transients in the peak absolute magnitude–decay timescale phase space
The Outer Halo of the Milky Way as Probed by RR Lyr Variables from the Palomar Transient Facility
RR Lyr stars are ideal massless tracers that can be used to study the total
mass and dark matter content of the outer halo of the Milky Way. This is
because they are easy to find in the light curve databases of large stellar
surveys and their distances can be determined with only knowledge of the light
curve. We present here a sample of 112 RR Lyr beyond 50 kpc in the outer halo
of the Milky Way, excluding the Sgr streams, for which we have obtained
moderate resolution spectra with Deimos on the Keck 2 Telescope. Four of these
have distances exceeding 100 kpc. These were selected from a much larger set of
447 candidate RR Lyr which were datamined using machine learning techniques
applied to the light curves of variable stars in the Palomar Transient Facility
database. The observed radial velocities taken at the phase of the variable
corresponding to the time of observation were converted to systemic radial
velocities in the Galactic standard of rest. From our sample of 112 RR Lyr we
determine the radial velocity dispersion in the outer halo of the Milky Way to
be ~90 km/s at 50 kpc falling to about 65 km/s near 100 kpc once a small number
of major outliers are removed. With reasonable estimates of the completeness of
our sample of 447 candidates and assuming a spherical halo, we find that the
stellar density in the outer halo declines as the -4 power of r.Comment: Accepted for publication in the Ap
Origins of the 1/4 keV Soft X-Ray Background
Snowden and coworkers have presented a model for the 1/4 keV soft X-ray
diffuse background in which the observed flux is dominated by a ~ 10^6 K
thermal plasma located in a 100-300 pc diameter bubble surrounding the Sun, but
has significant contributions from a very patchy Galactic halo. Halo emission
provides about 11% of the total observed flux and is responsible for half of
the H I anticorrelation. The remainder of the anticorrelation is presumably
produced by displacement of disk H I by the varying extent of the local hot
bubble (LHB). The ROSAT R1 and R2 bands used for this work had the unique
spatial resolution and statistical precision required for separating the halo
and local components, but provide little spectral information. Some consistency
checks had been made with older observations at lower X-ray energies, but we
have made a careful investigation of the extent to which the model is supported
by existing sounding rocket data in the Be (73-111 eV) and B bands (115-188 eV)
where the sensitivities to the model are qualitatively different from the ROSAT
bands. We conclude that the two-component model is well supported by the
low-energy data. We find that these combined observations of the local
component may be consistent with single-temperature thermal emission models in
collisional ionization equilibrium if depleted abundances are assumed. However,
different model implementations give significantly different results, offering
little support for the conclusion that the astrophysical situation is so
simple.Comment: 17 pages, 6 figures, accepted by the Astrophysical Journa
A novel method for transient detection in high-cadence optical surveys: Its application for a systematic search for novae in M31
[abridged] In large-scale time-domain surveys, the processing of data, from
procurement up to the detection of sources, is generally automated. One of the
main challenges is contamination by artifacts, especially in regions of strong
unresolved emission. We present a novel method for identifying candidates for
variables and transients from the outputs of such surveys' data pipelines. We
use the method to systematically search for novae in iPTF observations of the
bulge of M31. We demonstrate that most artifacts produced by the iPTF pipeline
form a locally uniform background of false detections approximately obeying
Poissonian statistics, whereas genuine variables and transients as well as
artifacts associated with bright stars result in clusters of detections, whose
spread is determined by the source localization accuracy. This makes the
problem analogous to source detection on images produced by X-ray telescopes,
enabling one to utilize tools developed in X-ray astronomy. In particular, we
use a wavelet-based source detection algorithm from the Chandra data analysis
package CIAO. Starting from ~2.5x10^5 raw detections made by the iPTF data
pipeline, we obtain ~4000 unique source candidates. Cross-matching these
candidates with the source-catalog of a deep reference image, we find
counterparts for ~90% of them. These are either artifacts due to imperfect PSF
matching or genuine variable sources. The remaining ~400 detections are
transient sources. We identify novae among these candidates by applying
selection cuts based on the expected properties of nova lightcurves. Thus, we
recovered all 12 known novae registered during the time span of the survey and
discovered three nova candidates. Our method is generic and can be applied for
mining any target out of the artifacts in optical time-domain data. As it is
fully automated, its incompleteness can be accurately computed and corrected
for.Comment: 16 pages, 8 figures, accepted to A&
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