Galactic Bulge Time Domain Survey

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Roman Simulated Image

Roman Simulated Image

This Roman Simulated Image (1/140th Roman field of view) of center of our Galaxy Download Hi-res

[Credit] Matthew T. Penny (Ohio State University)

The Galactic Bulge Time Domain Survey will provide an unprecedented census of extrasolar planets detected by gravitational microlensing of background stars, which is among Roman’s top level mission objectives. It will also enable exoplanet detection by the transit method, studies of variable star physics, and insights into the structure of our Milky Way Galaxy. Some of these applications are explored in the STScI Roman fact sheet, "Stars by the Billions".

The observing program described below is an example of a possible survey design. The actual survey will be defined by a future community process.

The survey will consist of repeated Wide Field Instrument (WFI) images over a set of fields selected for high stellar density and low foreground dust obscuration within the Galactic bulge (GB) region, to obtain light curve measurements at varying exoplanet, source star and host star geometries over time. The notional microlensing survey concept cycles through seven WFI fields (~2 deg2 total area) every 15 minutes, almost continuously, throughout six 62-day “bulge seasons” when the Galactic Bulge is visible given Roman’s orbital and pointing constraints.

Planets are detected via light curve deviations that differ from the normal stellar lens light curves. Usually, the signal occurs when one of the two images due to lensing by the host star passes close to the location of the planet, but planets are also detected at very high magnification where the gravitational field of the planet destroys the symmetry of the Einstein ring.

Microlensing is sensitive to a wide range of planet-star separations and host star types. The host stars for planets detected by microlensing are a random sample of stars that happen to pass close to the line-of-sight to the source stars in the Galactic bulge, so all common types of stars are surveyed, including G, K, and M-dwarfs, as well as white dwarfs and brown dwarfs. Microlensing is most sensitive to planets at a separation of ~RE (usually 2-3 AU) due to the strong stellar lens magnification at this separation, but the sensitivity extends to arbitrarily large separations. It is only planets well inside RE that are missed because the stellar lens images that would be distorted by these inner planets have very low magnifications and a very small contribution to the total brightness.

Microlensing relies upon the high density of source and lens stars towards the Galactic bulge to generate the stellar alignments needed to generate microlensing events, but this high star density also means that the bulge main sequence source stars are not generally resolved in ground-based images. This means that the precise photometry needed to detect planets of less than a tenth of the Earth's mass is not possible from the ground unless the magnification due to the stellar lens is moderately high. This, in turn, implies that ground-based microlensing is only sensitive to terrestrial planets located close to the Einstein ring (at ~2-3 AU). The full sensitivity to terrestrial planets in all orbits from the outer habitable zone to ∞ comes only from a space-based survey.

Roman observes 7 fields in the Galactic bulge on a 15 minute cadence for six 62 day seasons, interrupted only by monthly lunar avoidance cutouts. The microlensing events are continuously monitored in a single wide band to measure the basic light curve parameters. The 7 fields are monitored in the bluest filter for one exposure every 12 hours in order to measure the color of the microlensing source stars. The first and last observing seasons are separated by more than 2 years to measure lens-source relative proper motion.

The geomtry of a microlensing planet search towards the Galactic bulge.

The geometry of a microlensing planet search towards the Galactic bulge. Main sequence stars in the bulge are monitored for magnification due to gravitational lensing by foreground stars and planets in the Galactic disk and bulge. Download Hi-res

mocd figure

Notional placement of the Galactic Bulge Time Domain Survey fields. The upper image shows the inner half of our Milky Way galaxy in false color, with the Galactic Bulge at the center. Dark patches are regions where the background starlight is obscured by interstellar dust. The lower image is a 6x6 degree region near the Galactic center, where the color-bar indicates the amount of dust extinction. The locations of seven Galactic Bulge Time Domain Survey WFI fields shown in the inset optimizes a combination of high stellar density and comparatively low dust extinction, to maximize the yield of microlensing events. Download Hi-res

Time Series Photometry

Schematic illustration of how the Roman Space Telescope discovers signals caused by planetary companions in primary microlensing events, and how planet parameters can be extracted from these signals. The left panel shows a simulated primary microlensing event, containing a planetary deviation from an Earth-mass companion to the primary lens. The offset of the deviation from the peak of the primary event, when combined with the primary event parameters, is related to the projected separation of the planet. The right panel shows an enlargement of the planetary perturbation. The width and precise shape of the planetary deviation yield the mass of the companion relative to that of the primary host lens. Download Hi-res

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