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The magnetic fields of the solar system’s planets provide valuable insights into their interiors and can have dramatic consequences for the evolution of their atmospheres and interaction with the solar wind. However, we have little direct knowledge of magnetic fields in exoplanets. Here we present a method for detecting magnetic fields in the atmospheres of close-in exoplanets based on spectropolarimetric transit observations at the wavelength of the helium line at 1083 nm. This methodology has been successfully applied for exploring magnetic fields in solar coronal filaments. Strong absorption signatures (transit depths on the order of a few percent) in the 1083 nm line have recently been observed for several close-in exoplanets. We show that in the conditions in these escaping atmospheres, metastable helium atoms should be optically pumped by the starlight and, for field strengths more than a few × 10−4 G, should align with the magnetic field. This results in linearly polarized absorption at 1083 nm that traces the field direction (the Hanle effect), which we explore by both analytic computation and the Hazel numerical code. The linear polarization ranges from ∼10−3 in optimistic cases down to a few × 10−5 for particularly unfavorable cases, with very weak dependence on field strength. The line-of-sight component of the field results in a slight circular polarization (the Zeeman effect), also reaching . We discuss the detectability of these signals with current (SPIRou) and future (Extremely Large Telescope) high-resolution infrared spectropolarimeters, and we briefly comment on possible sources of astrophysical contamination. [ABSTRACT FROM AUTHOR]

Weak gravitational lensing studies aim to measure small distortions in the shapes of distant galaxies, thus placing very tight demands on the understanding of detector-induced systematic effects in astronomical images. The Wide-field Infrared Survey Telescope will carry out weak lensing measurements in the near-infrared using the new Teledyne H4RG-10 detector arrays, which makes the range of possible detector systematics very different from traditional weak lensing measurements using optical CCDs. One of the nonlinear detector effects observed in CCDs is the brighter-fatter effect (BFE), in which charge already accumulated in a pixel alters the electric field geometry and causes new charge to be deflected away from brighter pixels. Here we describe the formalism for measuring the BFE using flat-field correlation functions in infrared detector arrays. The autocorrelation of CCD flat fields is often used to measure the BFE, but because the infrared detector arrays are read out with the charge “in place,” the flat-field correlations are dominated by capacitive cross-talk between neighboring pixels (the interpixel capacitance, or IPC). Conversely, if the BFE is present and one does not account for it, it can bias correlation measurements of the IPC and photon transfer curve measurements of the gain. We show that one can compute numerous cross-correlation functions between different time slices of the same flat exposures, and that correlations due to IPC and BFE leave distinct imprints. We generate a suite of simulated flat fields and show that the underlying IPC and BFE parameters can be extracted, even when both are present in the simulation. There are some biases in the BFE coefficients up to 12%, which are likely caused by higher-order terms that are dropped from this analysis. The method is applied to laboratory data in the companion Paper II. [ABSTRACT FROM AUTHOR]

The Wide Field Infrared Survey Telescope (WFIRST) will investigate the origins of cosmic acceleration using weak gravitational lensing at near-infrared wavelengths. Lensing analyses place strict constraints on the precision of size and ellipticity measurements of the point-spread function. WFIRST will use infrared detector arrays, which must be fully characterized to inform data reduction and calibration procedures such that unbiased cosmological results can be achieved. Hirata & Choi introduces formalism to connect the cross-correlation signal of different flat field time samples to nonlinear detector behaviors such as the brighter fatter effect (BFE) and nonlinear inter-pixel capacitance (NL-IPC), and this paper applies that framework to a WFIRST development detector, SCA 18237. We find a residual correlation signal after accounting for classical nonlinearity. This residual correlation contains a combination of the BFE and NL-IPC; however, further tests suggest that the BFE is the dominant mechanism. If interpreted as a pure BFE, it suggests that the effective area of a pixel is increased by (2.87 ± 0.03) × 10−7 (stat.) for every electron in the 4 nearest neighbors, with a rapid ∼r−5.6±0.2 fall-off of the effect for more distant neighbors. We show that the IPC inferred from hot pixels contains the same large-scale spatial variations as the IPC inferred from auto-correlations, albeit with an overall offset of ∼0.06%. The NL-IPC inferred from hot pixels is too small to explain the cross-correlation measurement, further supporting the BFE hypothesis. This work presents the first evidence for the BFE in an H4RG-10 detector, demonstrates some of the useful insights that can be gleaned from flat field statistics, and represents a significant step toward calibration of WFIRST data. [ABSTRACT FROM AUTHOR]

The observed binary black hole (BBH) mergers indicate a large Galactic progenitor population continuously evolving from large orbital separations and low gravitational-wave (GW) frequencies to the final merger phase. We investigate the equilibrium distribution of BBHs in the Galaxy. Given the observed BBH merger rate, we contrast the expected number of systems radiating in the low-frequency 0.1–10 mHz GW band under two assumptions: (1) that all merging systems originate from near-circular orbits, as may be indicative of isolated binary evolution, and (2) that all merging systems originate at very high eccentricity, as predicted by models of dynamically formed BBHs and triple and quadruple systems undergoing Lidov–Kozai eccentricity oscillations. We show that the equilibrium number of systems expected at every frequency is higher in the eccentric case (2) than in the circular case (1) by a factor of ≃2–15. This follows from the fact that eccentric systems spend more time than circular systems radiating in the low-frequency GW bands. The GW emission comes in pulses at periastron separated by the orbital period, which may be days to years. For a LISA-like sensitivity curve, we show that if eccentric systems contribute significantly to the observed merger rate, then ≃10 eccentric systems should be seen in the Galaxy. [ABSTRACT FROM AUTHOR]

The diagnostic potential of the spectral signatures of Raman scattering, imprinted in planetary albedo spectra at short optical wavelengths, has been demonstrated in research on planets in the solar system, and has recently been proposed as a probe of exoplanet atmospheres, complementary to albedo studies at longer wavelengths. Spectral features caused by Raman scattering offer insight into the properties of planetary atmospheres, such as the atmospheric depth, composition, and temperature, as well as the possibility of detecting and spectroscopically identifying spectrally inactive species, such as H2 and N2, in the visible wavelength range. Raman albedo features, however, depend on both the properties of the atmosphere and the shape of the incident stellar spectrum. Identical planetary atmospheres can produce very different albedo spectra depending on the spectral properties of the host star. Here we present a set of geometric albedo spectra calculated for atmospheres with H2/He, N2, and CO2 composition, irradiated by different stellar types ranging from late A to late K stars. Prominent albedo features caused by Raman scattering appear at different wavelengths for different types of host stars. We investigate how absorption due to the alkali elements sodium and potassium may affect the intensity of Raman features, and we discuss the preferred strategies for detecting Raman features in future observations. [ABSTRACT FROM AUTHOR]

An important source of opacity in exoplanet atmospheres at short visible and near-UV wavelengths is Rayleigh scattering of light on molecules. It is accompanied by a related, albeit weaker process—Raman scattering. We analyze the signatures of Raman scattering imprinted in the reflected light and the geometric albedo of exoplanets, which could provide information about atmospheric properties. Raman scattering affects the geometric albedo spectra of planets in the following ways. First, it causes filling-in of strong absorption lines in the incident radiation, thus producing sharp peaks in the albedo. Second, it shifts the wavelengths of spectral features in the reflected light causing the so-called Raman ghost lines. Raman scattering can also cause a broadband reduction of the albedo due to wavelength shifting of a stellar spectrum with red spectral index. Observing the Raman peaks in the albedo could be used to measure the column density of gas, thus providing constraints on the presence of clouds in the atmosphere. Observing the Raman ghost lines could be used to spectroscopically identify the main scatterer in the atmosphere, even molecules like H2 or N2, which do not have prominent spectral signatures in the optical wavelength range. If detected, ghost lines could also provide information about the temperature of the atmosphere. In this paper, we investigate the effects of Raman scattering in hydrogen- and nitrogen-dominated atmospheres. We analyze the feasibility of detecting the signatures of Raman scattering with the existing and future observational facilities, and of using these signatures as probes of exoplanetary atmospheres. [ABSTRACT FROM AUTHOR]

Unlike optical CCDs, near-infrared detectors, which are based on CMOS hybrid readout technology, typically suffer from electrical crosstalk between the pixels. The interpixel capacitance (IPC) responsible for the crosstalk affects the point-spread function (PSF) of the telescope, increasing the size and modifying the shape of all objects in the images while correlating the Poisson noise. Upcoming weak lensing surveys that use these detectors, such as WFIRST, place stringent requirements on the PSF size and shape (and the level at which these are known), which in turn must be translated into requirements on IPC. To facilitate this process, we present a first study of the effect of IPC on WFIRST PSF sizes and shapes. Realistic PSFs are forward-simulated from physical principles for each WFIRST bandpass. We explore how the PSF size and shape depends on the range of IPC coupling with pixels that are connected along an edge or corner; for the expected level of IPC in WFIRST, IPC increases the PSF sizes by ∼5%. We present a linear fitting formula that describes the uncertainty in the PSF size or shape due to uncertainty in the IPC, which could arise for example due to unknown time evolution of IPC as the detectors age or due to spatial variation of IPC across the detector. We also study of the effect of a small anisotropy in the IPC, which further modifies the PSF shapes. Our results are a first, critical step in determining the hardware and characterization requirements for the detectors used in the WFIRST survey. [ABSTRACT FROM AUTHOR]

The sky-averaged, or global, background of redshifted 21 cm radiation is expected to be a rich source of information on cosmological reheating and reionization. However, measuring the signal is technically challenging: one must extract a small, frequency-dependent signal from under much brighter spectrally smooth foregrounds. Traditional approaches to study the global signal have used single antennas, which require one to calibrate out the frequency-dependent structure in the overall system gain (due to internal reflections, for example) as well as remove the noise bias from auto-correlating a single amplifier output. This has motivated proposals to measure the signal using cross-correlations in interferometric setups, where additional calibration techniques are available. In this paper we focus on the general principles driving the sensitivity of the interferometric setups to the global signal. We prove that this sensitivity is directly related to two characteristics of the setup: the cross-talk between readout channels (i.e., the signal picked up at one antenna when the other one is driven) and the correlated noise due to thermal fluctuations of lossy elements (e.g., absorbers or the ground) radiating into both channels. Thus in an interferometric setup, one cannot suppress cross-talk and correlated thermal noise without reducing sensitivity to the global signal by the same factor—instead, the challenge is to characterize these effects and their frequency dependence. We illustrate our general theorem by explicit calculations within toy setups consisting of two short-dipole antennas in free space and above a perfectly reflecting ground surface, as well as two well-separated identical lossless antennas arranged to achieve zero cross-talk. [ABSTRACT FROM AUTHOR]

Geosynchronous orbits are appealing for solar or astrophysical observatories because they permit continuous data downlink at high rates. The radiation environment in these orbits presents unique challenges, however. This paper describes both the characteristics of the radiation environment in geosynchronous orbit and the mechanisms by which this radiation generates backgrounds in photon detectors. Shielding considerations are described, and a preliminary shielding design for the proposed Wide-Field InfraRed Survey Telescope observatory is presented as a reference for future space telescope concept studies that consider a geosynchronous orbit. [ABSTRACT FROM AUTHOR]