Exoplanet Gravitational Microlensing Observations with the Deep Impact Flyby Spacecraft

1. Exoplanet Gravitational Microlensing Observations • with the Deep Impact Flyby Spacecraft • R. K. Barry1*, M. Albrow2, D. Bennett3, J. Christiansen4, J. Yee5, • Becker6, D. Wellnitz7, K. Klaasen8 • 1. NASA/GSFC, 2. U. Canterbury, NZ, 3. U. Notre Dame, IN, 4. NASA/Ames, • 5. Harvard CfA, Sagan Fellow, 6. U. Washington, WA, 7. U. Maryland, MD, 8. NASA/JPL Abstract: Observations conducted using the High Resolution Instrument (HRI) aboard the Deep Impact Flyby spacecraft of ongoing exoplanet microlensing events in the Galactic Bulge, detected and monitored by ground observatories are described. A permanent HRI defocus anomaly, a strong PSF chromaticity, and significant spacecraft pointing jitter present a particularly challenging data set that are used to explore methodologies for crowded-field photometry. Time series photometry are first extracted of events in the highly blended field to demonstrate the efficacy of an approach based on a standard ground observation pipeline, Pysis3, using differencing photometry. A separate approach based on a modified drizzle algorithm to provide sub-pixel image and repeatable photometric aperture positioning is explored. Once robust photometric results are obtained, the gravitational microlensing parallax signature for individual events observed simultaneously with an approximate 1AU baseline will be extracted. Observations: We conducted observations of the Galactic Bulge from May 30, 2012 to August 10, 2012 with the Deep Impact HRI visible channel during the Demonstration Extension of the NASA/EPOXI Extended Mission – on cruise between principal cometary targets. Spacecraft targeting was uploaded in near real time, depending on up-link availability, as promising high-magnification targets were detected from a network of ground observatories (Fig. 1). Science images are in 128x128, 2 micro-radian pixel, sub-frame mode while 256x256 frame images are interleaved to facilitate field recognition and post-processing. All 14-bit data were obtained in visible light through the instrument’s CLEAR1 filter with a center wavelength of 650 nm and bandwidth exceeding 700 nm. Data were subsequently calibrated using the Deep Impact standard, reversible pipeline prior to the science team’s ongoing analysis. Pipeline processing includes stripe removal, frame-transfer smear removal, and radiance calibration. Data were not compressed on the spacecraft prior to downlink. Integration time for all images is between 30.5 and 180.5 seconds. The Point Spread Function: All HRI images have a well-documented permanent defocus anomaly due to GSE calibration errors in preflight cryogenic testing. The instrument PSF is approximately 9.5 pixels, FWHM, and is strongly chromatic with the central portion of the PSF being more or less ‘filled’ depending on the color of the observed object (Fig. 2). Post processing by the science team requires the creation of target color-specific PSFs. We construct these using a drizzle process that co-adds PSFs from isolated calibrator stars (Barry, et al., 2010). CR Identification and Removal: Due to the nature of these crowded-field observations and the needed photometric sensitivity, the science team conducted a nearly exhaustive exploration of schemes to flag cosmic ray strikes in images. We have tested the following algorithms: imgclean, crfind, di_crrej, Laplacian edge detection, and STSDAS crrej, among others. We find that Laplacian edge detection works well on these data and use it to flag and mask suspect CR strikes. We then stack 256x256 images that have been so processed and median them in the Z direction to obtain a reference image for flagging and masking of CRs in individual 128x128 science images (Fig. 3). Image Registration: The team has found that image registration can be quite challenging with these data. The FITS header files do not provide WCS information, and, due to the crowded and blended fields and well-documented spacecraft pointing jitter, individual images must be separately assessed and the target – or a nearby bright fiducial star - identified to facilitate registration. Subsequently, the images, after an intermediate CR smoothing step, are masked with a broad Gaussian to limit edge effects. The relative sub-pixel shift between images are then determined using a discrete Fourier transform up-sampling method. Full-frame images are subsequently drizzled together to form a reference image, while science images are registered to the reference for later photometric steps. Photometry: One approach to extracting time-series photometry from these data is through the use of new difference imaging approach that incorporates iterative correction of image-to-image offsets and rotations, and iterative self-determination of the flat field (Fig. 4). Another approach that is being explored by the team is to attempt target color-specific PSF deconvolution of the crowded-field images prior to registration and differencing photometry. Initial experiments using iterative, Richardson-Lucy minimized deconvolution have shown some promise that using this additional color ‘dimension’ as mapped to the shape of the PSF may help separate targets in these crowded, blended fields. Additionally, use of our drizzled, sub-sampled PSFs rather than a single, critically sampled image, can be used to restore images with sub-pixel resolution and may be a critical element in registering these images. The Untimely Demise of Deep Impact: NASA officially terminated the Deep Impact mission one month after the Jet Propulsion Laboratory lost contact with the spacecraft on August 8, 2013. Unfortunately, for the work outlined above, final observations of these fields were required to establish a brightness baseline for the source stars in the Galactic bulge. While it is no longer possible to complete the absolute calibration of our time-series photometry, the team will attempt to construct a new relative calibration of the DI data. This should permit us to retrieve the simultaneous microlensing parallax even in the absence of instrument-specific baseline brightness information from this exceedingly challenging data set. Figure 1. Ground-based image of target field. Gravitationally lensed Galactic Bulge source star is marked with cursor – OGLE-2012-BLG-0406. Extremely dense star fields present a difficult photometric problem even when the instrument PSF is well understood and modeled. Figure 3. HRI image of target field before (L) processing. The image on the right shows a cutout of the reference image of the same field. The large, chromatic HRI PSF poses interesting challenges to the analysis of this crowded Bulge field. Figure 4. Time series photometry of event OGLE-2012-BLG-0406. These results will be further calibrated and, when taken together with ground-based data, a simultaneous parallax signature will be extracted. Co-I Barry observing target OB0406 at Canopus Observatory, Tasmania, while the team conducts simultaneous observations from 1AU with the Deep Impact Flyby spacecraft. How cool is that!? * Richard.K.Barry@NASA.gov, 301-286-0664 Figure 2. The HRI PSF showing defocus anomaly and chromaticity. All images are of Canopus through differing filters and have been drizzled. Similar results are obtained of stars of varying color.