Day 1293

Wednesday. 

1708.00003
Precise time delays from chromatically micro lensed Type Ia supernovae
Goldstein, Nugent, Kasen, Collett

Perform detailed population, microlensing, radiation transport, and light-curve simulations to quantify (a) the effect of microlensing on the strongly lensed Type Ia supernova (LSN Ia) yield of the LSST and (b) the effect of microlenising on the precision and accuracy of time delays that can be extracted from LSST LSNe Ia.  Microlensing has a negligible effect on the LSST LSN Ia yield, but it can be increased by a factor of ~2 to 930 systems (comparable to the expected yield of l lensed quasars) using a novel photometric identification technique based on spectral template fitting.  Crucially, the microlensing of LSNe Ia is achromatic until 3 rest-frame weeks after the explosion, making features in the early-time color curves precise time delay indicators.  By fitting simulated flux and color observations of microlensed LSNe Ia with their underlying, unlensed spectral templates, forecast the distribution of absolute time delay error due to mcirolensing for LSST, which is unbiased at the sub-percent level and peaked at 1% for color curve observations in the achromatic phase, while for light curve observations it is comparable to mass modeling uncertainties (4%).  About 70% of LSST LSN Ia images should be discovered during the achromatic phase, indicating the microlensing time delay uncertainties can be minimized if prompt multicolor follow-up observations are obtained.  Accounting for microlensing, the 1-2 day time delay on the recently discovered LSN Ia iPTF16geu can be measured to 40% precision, limiting is cosmological utility.  The relatively low precision of this time delay is due to (a) its remarkably short duration and (b) the fact that follow-up observations began long after peak brightness, during a period of significant chromatic uncertainty..


1708.00022
In the crosshair: astrometric exoplanet detection with WFIRST's diffraction spikes
Melchior, Spergel, Lanz

WFIRST will conduct a corona graphic program of characterizing the atmospheres of planets around bright nearly stars.  When observed with the WFIRST WFC, these stars will saturate the detector and produce very strong diffraction spikes.  In this paper, forecast the astrometric precision that WFIRST can achieve by centering on the diffraction spikes of highly saturated stars.  This measurement principle is strongly facilitated by the WFIRST H4RG detectors, which confine excess charges within the potential well of saturated pixels.  By adopting a simplified analytical model of the diffraction spike caused by a single support strut obscuring the telescope aperture, integrated over the WFIRST pixel size, predict the performance of this approach with the Fisher-matrix formalism.  Discuss the validity of the model and find that 10 mas astrometric precision is achievable with a single 100s exposure of a R=6 or a J=5 star.  Discuss observational limitations from the optical distortion correction and pixel-level artifacts, which need to be calibrated at the level of 10-20 mas so as to not dominate the error budget.  To suppress those systematics, suggest a series of short exposures, dithered by at least several hundred pixels, to reach an effective per-visit astrometric precision of better than 10 mas.  If this can be achieved, a dedicated WFIRST GO program will be able to detect Earth-mass exoplanets with orbital periods of 1 yr around stars within a few pc as well as Neptune-like planets with shorter periods or around more massive or distant stars.  Such a program will also enable mass  measurements of many anticipated direct-imaging exoplanet targets of the WFIRST coronagraph and a "star shade" occulter.