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Polarimetry in the mid-infrared: new vistas for SOFIA, NASA's airborne observatory

An astronomical instrument promises to significantly extend the currently available range of useful wavelengths, resulting in greater accuracy and observing efficiency.
11 December 2007, SPIE Newsroom. DOI: 10.1117/2.1200711.0944

The Stratospheric Observatory for Infrared Astronomy (SOFIA), operated by the National Aeronautics and Space Administration (NASA), represents a significant step forward for IR to submillimeter astronomy, offering near-uninterrupted wavelength coverage from 0.3μm to 1.6mm, and affording the 2.5m telescope the status of a premier IR and submillimeter observatory.1 A suite of nine instruments is approaching completion, offering both imaging and spectroscopic capabilities. SOFIA holds a unique advantage, as it operates at a sufficiently high altitude to be above most of the obscuring water vapor that limits ground-based telescopes. Yet, it returns to Earth at the end of each observing run, clearly unlike satellite observatories. The maintainability, upgradability, and timeliness of SOFIA offer the chance to deploy novel or intrinsically more complex instruments than those currently available. Its high-altitude capability (at ∼12.5km) will enable observations in an otherwise opaque part of the atmosphere

Polarimetry has proved crucial to understanding astronomical objects as diverse as galaxies and active galactic nuclei (AGNs, such as quasars), star-forming regions, and planet-spawning debris disks around young stars. But it has rarely been deployed on space-based telescopes, with the notable exception of the Hubble Space Telescope. The main reason is that it requires additional mechanisms or telescope pointing, which means greater risk of failure and complexity. Such considerations are paramount in planning space-based instruments, and favor a more conservative approach to developing them. SOFIA, however, will return to the ground at the end of each observing night, thus mitigating fears that malfunctioning mechanisms could lead to lost science potential. Problems can be fixed during the day, or the instrument can be replaced with another until repairs can be carried out. At wavelengths from ∼0.3 to 5μm, so-called dual-beam polarimeters record orthogonally polarized beams through the inclusion of a Wollaston prism. We plan to extend the successful designs of both our Wollaston prism-based 7.5–13.5μm instrument (CanariCam) for the 10m Gran Telescopio Canarias telescope2 and a 1–5μm polarimeter for the 6.5m Multiple Mirror Telescope3 to that of a combined ∼5–40μm device for SOFIA.

To optimally address the main science drivers for such an instrument,4 both imaging and spectropolarimetry, covering as large a wavelength range as possible, are highly desirable. The minimization of temporal atmospheric effects is typically accomplished by modulating the polarimetric signature faster than the timescale of typical sky variations. This will be achieved using a half-wave retarder or meanderline components,5 located as far upstream in the instrument's optical path as possible to reduce polarization. If funding is available, the polarimeter will be dual-beam to further improve accuracy and observing efficiency. This design increases accuracy because it lessens the effect of sky variations in both transmission and emission, and yields twice the number of photons for compact objects.

Calibration of the instrument should be readily accomplished during a typical night's observations, and takes a modicum of time. It is done by introducing a wire-grid polarizer that produces a previously known high degree of polarization. We expect at least some of the instrumental throughput to be channeled along two separate and distinct optical paths, sharing commonality in the early section. Through the use of Si:As- and Si:Sb-based detectors, the blue and red arms will span ∼5–25μm and ∼25–40μm, respectively. Adopting the dual-arm approach will lessen demands on the optics, and we can set the plate scale to Nyquist sample the diffraction limit at the shortest wavelengths reached by both arms.

Given the expected wavelength range, a design employing off-axis parabolas to provide the optical power will likely be necessary, where possible, to avoid chromatic aberrations otherwise introduced by transmissive optics. For the spectral dispersive element we will investigate the advantages offered by immersion compared to reflective gratings. Immersion-grating technology at mid-IR wavelengths has progressed rapidly, and allows the instrument to be significantly smaller.6 The resulting spectral resolution will be low, likely in the low hundreds.

As a relevant application, here we discuss observations of the nuclear, dusty torus of an AGN. Varying our viewing angle to this torus and its nuclear `engine', commonly thought to be fueled by gas and dust spiraling into a supermassive black hole, enables unification of the numerous AGN classifications. Some of the earliest mid-IR polarimetric observations of AGNs were obtained7 for NGC 1068, using an on-source exposure time of seven hours with the 4m Anglo-Australian Telescope. We8 observed the same object using the large collecting area of the Gemini-North 8.1m telescope equipped with the Michelle polarimeter, and an on-source exposure time of 148s (see Figure 1). This setup revealed significant, previously unresolved structure in the polarization thought to be caused by three dominant mechanisms. North of the nucleus the polarization arises from aligned dust in the narrow-emission-line region. South, east, and west of the nucleus the mechanism is consistent with dust being channeled toward the central engine. A central minimum of polarization implies the presence of a compact (≤22pc) torus. The observations provide continuity between the torus and the host galaxy's nuclear environments, and represent the first published mid-IR polarimetry from an 8m telescope.

Figure 1. Ten-micron observations8 of NGC 1068 obtained with the Michelle polarimeter on the Gemini-North telescope. The total flux is displayed in color and contours, and the vectors indicate the degree and position angle of the polarization.
C.P. acknowledges helpful discussions with Mason, Young, and Ebizuka. We are grateful for the generous support of the UCF-UF Space Research Initiative.

Chris Packham
Department of Astronomy
University of Florida
Gainesville, FL
David Axon
Rochester Institute of Technology
Rochester, NY
James Hough
University of Hertfordshire
Hatfield, UK
Terry Jones
University of Minnesota
Minneapolis, MN
Patrick Roche
University of Oxford
Oxford, UK
Motohide Tamura
National Astronomical Observatory of Japan
Mitaka, Japan
Charles Telesco
University of Florida
Gainesville, FL