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Register a new userTwo NIRCam channels are Better than One: How JWST Can Do More Science with NIRCams Short-Wavelength Dispersed Hartmann Sensor
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The James Webb Space Telescope (JWST) offers unprecedented sensitivity, stability, and wavelength coverage for transiting exoplanet studies, opening up new avenues for measuring atmospheric abundances, structure, and temperature profiles. Taking full advantage of JWST spectroscopy of planets from 0.6um to 28um, however, will require many observations with a combination of the NIRISS, NIRCam, NIRSpec, and MIRI instruments. In this white paper, we discuss a new NIRCam mode (not yet approved or implemented) that can reduce the number of necessary observations to cover the 1.0um to 5.0um wavelength range. Even though NIRCam was designed primarily as an imager, it also includes several grisms for phasing and aligning JWSTs 18 hexagonal mirror segments. NIRCams long-wavelength channel includes grisms that cover 2.4um to 5.0um with a resolving power of R = 1200 - 1550 using two separate configurations. The long-wavelength grisms have already been approved for science operations, including wide field and single object (time series) slitless spectroscopy. We propose a new mode that will simultaneously measure spectra for science targets in the 1.0um to 2.0um range using NIRCams short-wavelength channel. This mode, if approved, would take advantage of NIRCams Dispersed Hartmann Sensor (DHS), which produces 10 spatially separated spectra per source at R ~ 300. We discuss the added benefit of the DHS in constraining abundances in exoplanet atmospheres as well as its ability to observe the brightest systems. The DHS essentially comes for free (at no time cost) with any NIRCam long-wavelength grism observation, but the detector integration parameters have to be selected to ensure that the long-wavelength grism observations do not saturate and that JWST data volume downlink constraints are not violated.
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JWST transmission and emission spectra will provide invaluable glimpses of transiting exoplanet atmospheres, including possible biosignatures. This promising science from JWST, however, will require exquisite precision and understanding of systematic errors that can impact the time series of planets crossing in front of and behind their host stars. Here, we provide estimates of the random noise sources affecting JWST NIRCam time-series data on the integration-to-integration level. We find that 1/f noise can limit the precision of grism time series for 2 groups (230 ppm to 1000 ppm depending on the extraction method and extraction parameters), but will average down like the square root of N frames/reads. The current NIRCam grism time series mode is especially affected by 1/f noise because its GRISMR dispersion direction is parallel to the detector fast-read direction, but could be alleviated in the GRISMC direction. Care should be taken to include as many frames as possible per visit to reduce this 1/f noise source: thus, we recommend the smallest detector subarray sizes one can tolerate, 4 output channels and readout modes that minimize the number of skipped frames (RAPID or BRIGHT2). We also describe a covariance weighting scheme that can significantly lower the contributions from 1/f noise as compared to sum extraction. We evaluate the noise introduced by pre-amplifier offsets, random telegraph noise, and high dark current RC pixels and find that these are correctable below 10 ppm once background subtraction and pixel masking are performed. We explore systematic error sources in a companion paper.
JWST holds great promise in characterizing atmospheres of transiting exoplanets, potentially providing insights into Earth-sized planets within the habitable zones of M dwarf host stars if photon-limited performance can be achieved. Here, we discuss the systematic error sources that are expected to be present in grism time series observations with the NIRCam instrument. We find that pointing jitter and high gain antenna moves on top of the detectors subpixel crosshatch patterns will produce relatively small variations (less than 6 parts per million, ppm). The time-dependent aperture losses due to thermal instabilities in the optics can also be kept to below 2 ppm. To achieve these low noise sources, it is important to employ a sufficiently large (more than 1.1 arcseconds) extraction aperture. Persistence due to charge trapping will have a minor (less than 3 ppm) effect on time series 20 minutes into an exposure and is expected to play a much smaller role than it does for the HST WFC3 detectors. We expect temperature fluctuations to be less than 3 ppm. In total, our estimated noise floor from known systematic error sources is only 9 ppm per visit. We do however urge caution as unknown systematic error sources could be present in flight and will only be measurable on astrophysical sources like quiescent stars. We find that reciprocity failure may introduce a perennial instrument offset at the 40 ppm level, so corrections may be needed when stitching together a multi-instrument multi-observatory spectrum over wide wavelength ranges.
The original intensity interferometers were instruments built in the 1950s and 60s by Hanbury Brown and collaborators, achieving milli-arcsec resolutions in visible light without optical-quality mirrors. They exploited a then-novel physical effect, now known as HBT correlation after the experiments of Hanbury Brown and Twiss, and nowadays considered fundamental in quantum optics. Now a new generation of inten- sity interferometers is being designed, raising the possibility of measuring intensity correlations with three or more detectors. Quantum optics predicts some interesting features in higher-order HBT. One is that HBT correlation increases combinatorially with the number of detectors. Signal to noise considerations suggest, that many-detector HBT correlations would be mea- surable for bright masers, but very difficult for thermal sources. But the more modest three-detector HBT correlation seems measurable for bright stars, and would provide image information (namely the bispectrum) not present in standard HBT.
There is increasing evidence of a local population of short duration Gamma-ray Bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible Luminosity Functions (LF) for both neutron star binary mergers and giant flares from Soft Gamma Repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall BATSE sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best fit parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population.
With a peak luminosity of ~10^47 erg/s, the December 27th 2004 giant flare from SGR1806-20 would have been visible by BATSE (the Burst and Transient Source Experiment) out to ~50 Mpc. It is thus plausible that some fraction of the short duration Gamma-Ray Bursts (sGRBs) in the BATSE catalogue were due to extragalactic magnetar giant flares. According to the most widely accepted current models, the remaining BATSE sGRBs were most likely produced by compact object (neutron star-neutron star or neutron star-black hole) mergers with intrinsically higher luminosities. Previously, by examining correlations on the sky between BATSE sGRBs and galaxies within 155 Mpc, we placed limits on the proportion of nearby sGRBs. Here, we examine the redshift distribution of sGRBs produced by assuming both one and two populations of progenitor with separate Luminosity Functions (LFs). Using the local Galactic SGR giant flare rate and theoretical NS-NS merger rates evolved according to well-known Star Formation Rate parameterisations, we constrain the predicted distributions by BATSE sGRB overall number counts. We show that only a dual population consisting of both SGR giant flares and NS-NS mergers can reproduce the likely local distribution of sGRBs as well as the overall number counts. In addition, the best fit LF parameters of both sub-populations are in good agreement with observed luminosities.
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