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D.C. Hines n Steward Observatory
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NICMOS cameras 1 and 2 each carry a set of three polarizing elements to provide high sensitivity observations of linearly polarized light. The polarizers are bandpass limited and provide diffraction-limited imaging in camera 1 at 0.8 - 1.3um, and in camera 2 at 1.9-2.1um. The NICMOS design specified the intra-camera primary axis angles of the polarizers to be differentially offset by 120 degree, and with identical polarizing efficiency and transmittance. While this ideal concept was not strictly achieved, accurate polarimetry in both cameras, over their full (11 and ~19.2 square) fields of view was enabled through ground and on-orbit calibration of the as-built and HST-integrated systems. The Cycle 7 & 7N calibration program enabled and demonstrated excellent imaging polarimetric performance with uncertainties in measured polarization fractions <=1%. After the installation of the NICMOS Cooling System (NCS), the polarimetric calibration was re-established in Cycle 11, resulting in systemic performance comparable to (or better than) Cycle 7 & 7N. The NCS era NICMOS performance inspired the development of an earlier conceived, but non-implemented, observing mode combining high contrast coronagraphic imaging and polarimetry in camera 2. We successfully executed a program to calibrate and commission the Coronagraphic Polarimetry mode in NICMOS in Cycle 13, and the mode was made available for GO use in Cycle 14. We discuss the data reduction and calibration of direct and coronagraphic NICMOS polarimetry. Importantly, NICMOS coronagraphic polarimetry provides unique access to polarized light near bright targets over a range of spatial scales intermediate between direct polarimetry and ground-based (coronagraphic) polarimetry using adaptive optics.
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The ability of NICMOS to perform high accuracy polarimetry is currently hampered by an uncalibrated residual instrumental polarization at a level of 1.2-1.5%. To better quantify and characterize this residual we obtained observations of three polarimetric standard stars at three separate space-craft roll angles. Combined with archival data, these observations were used to characterize the residual instrumental polarization to enable NICMOS to reach its full polarimetric potential. Using these data, we calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, dominate the accuracy of p and theta. However, the new coefficients now enable imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15 degrees.
The findings of a nine orbit calibration plan carried out during HST Cycle 15, to fully determine the NICMOS camera 2 (2.0 micron) polarization calibration to high accuracy, are reported. Recently Ueta et al. and Batcheldor et al. have suggested that NICMOS possesses a residual instrumental polarization at a level of 1.2-1.5%. This would completely inhibit the data reduction in a number of GO programs, and hamper the ability of the instrument to perform high accuracy polarimetry. We obtained polarimetric calibration observations of three polarimetric standards at three spacecraft roll angles separated by ~60deg. Combined with archival data, these observations were used to characterize the residual instrumental polarization in order for NICMOS to reach its full potential of accurate imaging polarimetry at p~1%. Using these data, we place an 0.6% upper limit on the instrumental polarization and calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, are seen to dominate the accuracy of p and theta. However, the updated coefficients do allow imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15deg. This work enables a new caliber of science with HST.
The nuclei of Seyfert 1 galaxies exhibit a range of optical polarization characteristics that can be understood in terms of two scattering regions producing orthogonal polarizations: an extended polar scattering region (PSR) and a compact equatorial scattering region (ESR), located within the circum-nuclear torus. Here we present NICMOS 2.0 micron imaging polarimetry of 6 polar scattered Seyfert 1 (S1) galaxies, in which the PSR dominates the optical polarization. The unresolved nucleus (<0.58 arcsec) is significantly polarized in only three objects, but 5 of the 6 exhibit polarization in a 0.58 to 1.5 arcsec circum-nuclear annulus. In Fairall 51 and ESO 323-G077, the polarization position angle at 2 microns (theta2m) is consistent with the average for the optical spectrum (thetav), implying that the nuclear polarization is dominated by polar scattering at both wavelengths. The same is probably true for NGC 3227. In both NGC 4593 and Mrk 766, there is a large difference between theta2m and thetav off nucleus, where polar scattering is expected to dominate. This may be due to contamination by interstellar polarization in NGC 4593, but there is no clear explanation in the case of the strongly polarized Mrk 766. Lastly, in Mrk 1239, a large change (~ 60deg) in theta2m between the nucleus and the annulus indicates that the unresolved nucleus and its immediate surroundings have different polarization states at 2 microns, which we attribute to the ESR and PSR, respectively. A further implication is that the source of the scattered 2 micron emission in the unresolved nucleus is the accretion disk, rather than torus hot dust emission.
In the first half year of operation the satellite borne POLAR instrument detected a total of 55 Gamma-Ray Bursts about 10 of which were bright enough to allow for detailed polarization studies, thereby forming the start of the first Gamma-Ray Burst polarization catalog. In this paper a brief overview of the previous GRB polarization studies will be presented followed by an overview of the POLAR detector along with the first result of the in-flight performance. The detected Gamma-Ray bursts will be presented and finally prospects for polarization measurements of these events will be discussed.
We provide an overview of the most important calibration aspects of the NICMOS instrument on board of HST. We describe the performance of the instrument after the installation of the NICMOS Cooling System, and show that the behavior of the instrument has become very stable and predictable. We detail the improvements made to the NICMOS pipeline and outline plans for future developments. The derivation of the absolute photometric zero-point calibration is described in detail. Finally, we describe and quantify a newly discovered count-rate dependent non-linearity in the NICMOS cameras. This new non-linearity is distinctly different from the total count dependent non-linearity that is well known for near-infrared detectors. We show that the non-linearity has a power law behavior, with pixels with high system, or vice versa, pixels with low count rate detecting slightly less than expected. The effect has a wavelength dependence with observations at the shortest wavelengths being the most affected (~0.05-0.1 mag per dex flux change at ~1 micron, 0.03 mag per dex at 1.6 micron).
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