NIRSpec BOTS Observations of GJ 1214b

Example Science Program #32

This example science program describes a JWST NIRSpec Bright Object Time Series exoplanet transit observation of Gliese 1214b (GJ 1214b), including Exposure Time Calculation and  Astronomers Proposal Tool considerations.

The JWST NIRSpec bright object time-series (BOTS) mode is for observations of bright sources that require high throughput and stable time-resolved spectroscopy. This mode is optimized for the study of transiting exoplanets around their bright host stars; such observations are expected to be the primary use of the BOTS mode. BOTS mode can only be used with NIRSpec's S1600A1 aperture. This 1.6" square aperture specifically enables high precision time-series spectroscopy of bright objects. 

This science use case presents considerations for transit spectroscopy ETC calculations and APT observation definition for the Gliese 1214b exoplanet in two sub-articles:

Considerations for the GJ 1214b transit 

Gliese 1214b (often shortened to GJ 1214b) is a super earth exoplanet that orbits the star Gliese 1214, a very low mass M star (M 4.5 spectral type).  GJ 1214b has an estimated temperature of ~500K. Previous transit spectroscopy of GJ 1214b suggests a possible flat transmission spectral character with few deep or wide spectral absorption features. Observations at JWST sensitivities will improve upon past measurements, and higher spectral resolution observations may reveal narrow molecular features missed by prior low resolution spectroscopy. Many molecules of interest in exoplanet atmospheres (water, carbon monoxide, hydrogen cyanide, methane, and ammonia) are expected to show significant spectral features in the 1–5 μm NIRSpec BOTS wavelength range—depending on atmospheric pressure and temperature. The NIRSpec BOTS mode can acquire transit spectra in this spectral range using a single setting (R=100) prism mode, or in multiple settings with the medium or high resolution gratings. Because GJ 1214 has a J-mag (Vega) of 9.8—and the NIRSpec-Prism begins to saturate at a J-mag (Vega) of 10.2 (for Teff = 2,500 K).

Figure 1 shows an example white light signal and the observation timing for the GJ 1214b transit observation.  The duration of the exoplanet transit of GJ 1214b is one hour (T14=1hr).  As a result, we plan to observe GJ 1214b for a total time of 3 T14 + 1 hr = 4 hours real time for each visit. This allows for sufficient detector settling time (currently estimated to be ~15 minutes), the primary transit, and one transit duration before and after the primary transit to reduce photon noise. Padding the observation by a factor of 2 T14 + 1 hr also allows some margin in case the observation execution initiation window is defined to be large or if the primary transit occurs at a slightly different time than predicted. Timing windows of 1 hour or more of margin plus settling time will also avoid an extra timing overhead on tight observation execution windows (< 1 hr).

Figure 1. Timing of GJ 1214b Primary Transits

Transit timing to maximize science return from NIRSpec BOTS observations of GJ 1214b. The period indicated in the figure as "1 hr settling time and margin" includes the padding needed for 15 minute detector setup and 45 minutes of extra margin.



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