Spectroscopic time series performance of the Mid-Infrared Instrument on the JWST
Authors: Jeroen Bouwman (Max Planck Institute for Astronomy), Sarah Kendrew (Space Telescope Science Institute), Thomas P. Greene (NASA Ames Research Center), Taylor J. Bell (NASA Ames Research Center), Pierre-Olivier Lagage (CEA), Juergen Schreiber (Max Planck Institute for Astronomy), Daniel Dicken (UK Astronomy Technology Centre), G. C. Sloan (Space Telescope Science Institute), Nestor Espinoza (Space Telescope Science Institute), Silvia Scheithauer (Max Planck Institute for Astronomy), Alain Coulais (CEA), Ori D. Fox (Space Telescope Science Institute), Rene Gastaud (CEA), Adrian M. Glauser (ETH), Olivia C. Jones (UK Astronomy Technology Centre), Alvaro Labiano (Telespazio UK for the European Space Agency), Fred Lahuis (SRON), Jane E. Morrison (University of Arizona), Katherine Murray (Space Telescope Science Institute), Michael Mueller (University of Groningen), Omnarayani Nayak (Space Telescope Science Institute), Gillian S. Wright (UK Astronomy Technology Centre), Alistair Glasse (UK Astronomy Technology Centre), George Rieke (University of Arizona)
Abstract: We present here the first ever mid-infrared spectroscopic time series observation of the transiting exoplanet \object{L 168-9 b} with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope. The data were obtained as part of the MIRI commissioning activities, to characterize the performance of the Low Resolution Spectroscopy (LRS) mode for these challenging observations. To assess the MIRI LRS performance, we performed two independent analyses of the data. We find that with a single transit observation we reached a spectro-photometric precision of $\sim$50 ppm in the 7-8 \micron range at R=50, consistent with $\sim$25 ppm systematic noise. The derived band averaged transit depth is 524 $\pm$ 15 ppm and 547 $\pm$ 13 ppm for the two applied analysis methods, respectively, recovering the known transit depth to within 1 $\sigma$. The measured noise in the planet's transmission spectrum is approximately 15-20 \% higher than random noise simulations over wavelengths $6.8 \lesssim \lambda \lesssim 11$ $\mu$m. \added{We observed an larger excess noise at the shortest wavelengths of up to a factor of two, for which possible causes are discussed.} This performance was achieved with limited in-flight calibration data, demonstrating the future potential of MIRI for the characterization of exoplanet atmospheres.
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