High resolution imaging at Palomar

For the last two years we have embarked on a program of understanding the ultimate limits of ground-based optical imaging. We have designed and fabricated a camera specifically for high resolution imaging. This camera has now been pressed into service at the prime focus of the Hale 5 m telescope. We have concentrated on two techniques: the Non-Redundant Masking (NRM) and Weigelt's Fully Filled Aperture (FFA) method. The former is the optical analog of radio interferometry and the latter is a higher order extension of the Labeyrie autocorrelation method. As in radio Very Long Baseline Interferometry (VLBI), both these techniques essentially measure the closure phase and, hence, true image construction is possible. We have successfully imaged binary stars and asteroids with angular resolution approaching the diffraction limit of the telescope and image quality approaching that of a typical radio VLBI map. In addition, we have carried out analytical and simulation studies to determine the ultimate limits of ground-based optical imaging, the limits of space-based interferometric imaging, and investigated the details of imaging tradeoffs of beam combination in optical interferometers

Noise in optical synthesis images. I. Ideal Michelson interferometer

We study the distribution of noise in optical images produced by the aperture synthesis technique, in which the principal source of noise is the intrinsic shot noise of photoelectric detection. The results of our analysis are directly applicable to any space-based optical interferometer. We show that the signal-to-noise ratio of images synthesized by such an ideal interferometric array is essentially independent of the details of the beam-combination geometry, the degree of array redundancy, and whether zero-spatial-frequency components are included in image synthesis. However, the distribution of noise does depend on the beam-combination geometry. A highly desirable distribution, one of uniform noise across the entire image, is obtained only when the beams from the n primary apertures are subdivided and combined pairwise on n(n - 1)/2 detectors

The unblinking eye on the sky

From near-Earth asteroids to superluminous supernovae and counterparts to gravitational wave sources, the Zwicky Transient Facility will soon scan the night sky for transient phenomena.Comment: Author's version of "Mission Control" profile published in Nature Astronomy. 3 pages, 1 figure. https://www.nature.com/articles/s41550-017-007

Optical identification of binary pulsars: Implications for magnetic field decay in neutron stars

We report the discovery of the optical counterparts of two binary pulsar systems-0655+64 and 0820+02. In accordance with stellar evolution scenarios we find the optical counterparts to be white dwarfs. The existence of a cool and therefore old white dwarf in the 0655+64 system contradicts the standard hypothesis of exponential decay of magnetic fields in neutron stars. We resolve this contradiction by hypothesizing that the magnetic field in neutron stars consists of two components-an exponentially decaying field and a steady field. The former probably resides in the crust and the latter perhaps in the core: The consequences of this hypothesis for the lifetimes of millisecond pulsars, binary pulsars, and galactic bulge X-ray sources are discussed

Deep narrow band imagery of the diffuse ISM in M33

Very deep narrow band images were obtained for several fields in the local group spiral galaxy M33 using a wide field reimaging Charge Coupled Device (CCD) camera on the 1.5 m telescope at Palomar Observatory. The reimaging system uses a 306 mm collimator and a 58 mm camera lens to put a 16 minute by 16 minute field onto a Texas Instruments 800 x 800 pixel CCD at a resolution of 1.2 arcseconds pixel (-1). The overall system is f/1.65. Images were obtained in the light of H alpha (S II) lambda lambda 6717, 6731, (O III) lambda 5007, and line-free continuum bands 100A wide, centered at 6450A and 5100A. Assuming a distance of 600 kpc to M33 (Humphreys 1980, Ap. J., 241, 587), this corresponds to a linear scale of 3.5 pc pixel (-1), and a field size of 2.8 kpc x 2.8 kpc. Researchers discuss the H alpha imagery of a field centered approx. equal to 8 minutes NE of the nucleus, including the supergiant HII region complex NGC 604. Two 2000 second H alpha images and two 300 second red continuum images were obtained of two slightly offset fields. The fields were offset to allow for discrimination between real emission and possible artifacts in the images. All images were resampled to align them with one of the H alpha frames. The continuum images were normalized to the line images using the results of aperture photometry on a grid of stars in the field, then the rescaled continuum data were directly subtracted from the line data

M51: Molecular spiral arms, GMAs and superclouds

Researchers present an aperture synthesis image of M51 in the CO 1 to 0 line at 9 seconds x 7 seconds resolution made with the Owens Valley Millimeter Interferometer. The image is a mosaic of 30 one-arcminute fields. The image shows narrow spiral arms which are coincident with the optical dust lanes and non-thermal radio emission, but are offset from the ridges of H alpha emission. Many dense concentrations of CO emission, termed Giant Molecular Associations (GMAs), are seen both along and between the arms. The typical GMA mass is about 3 times 10(exp 7) solar mass. Most of the on-arm GMAs appear to be gravitationally bound. These GMAs consist of several spectral components (Molecular Superclouds) with typical mass 10(exp 7) solar mass, which also appear to be bound. The observed streaming motions in the GMAs are consistent with density wave theory. The interarm GMAs are not gravitationally bound, and are likely to be due to a secondary compression of the density wave

Noise in optical synthesis images. II. Sensitivity of an ^nC_2 interferometer with bispectrum imaging

We study the imaging sensitivity of a ground-based optical array of n apertures in which the beams are combined pairwise, as in radio-interferometric arrays, onto n(n - 1)/2 detectors, the so-called ^nC_2 interferometer. Groundbased operation forces the use of the fringe power and the bispectrum phasor as the primary observables rather than the simpler and superior observable, the Michelson fringe phasor. At high photon rates we find that bispectral imaging suffers no loss of sensitivity compared with an ideal array (space based) that directly uses the Michelson fringe phasor. In the opposite limit, when the number of photons per spatial coherence area per coherence time drops below unity, the sensitivity of the array drops rapidly relative to an ideal array. In this regime the sensitivity is independent of n, and hence it may be efficient to have many smaller arrays, each operating separately and simultaneously

The Highest L(sub X)/L(sub opt) Sources in the ROSAT All-Sky Survey

The purpose of our program was to identify new and interesting soft X-ray sources from the Bright Source Catalog of the ROSAT All-Sky Survey. Our intent was to use XMM to observe a sample of BSC objects that had been identified as less than 10% likely to be associated with any object in the USNO-A2.0 catalog (Rutledge et al. 2000). We requested a single 5-ks pointing for each of 32 sources in this category in order to make a systematic examination of the properties of these sources. Ultimately, we hoped to identify new isolated neutron stars from within this population. Our requested observations were scheduled for execution at the end of Cycle 2, for two of our targets only (selected by sky position). A two-target sample does not allow for the statistical investigation that we originally proposed; however, based on the identification of the BSC object in the XMM data, an improved position, and reevaluation of likely off-band counterparts, it does enable a source-by-source evaluation of whether the BSC object is an isolated neutron star (INS)

Asteroids in GALEX: Near-ultraviolet photometry of the major taxonomic groups

We present ultraviolet photometry (NUV band, 180--280 nm) of 405 asteroids observed serendipitously by the Galaxy Evolution Explorer (GALEX) from 2003--2012. All asteroids in this sample were detected by GALEX at least twice. Unambiguous visible-color-based taxonomic labels (C type versus S type) exist for 315 of these asteroids; of these, thermal-infrared-based diameters are available for 245. We derive NUV-V color using two independent models to predict the visual magnitude V at each NUV-detection epoch. Both V models produce NUV-V distributions in which the S types are redder than C types with more than 8-sigma confidence. This confirms that the S types' redder spectral slopes in the visible remain redder than the C types' into the NUV, this redness being consistent with absorption by silica-containing rocks. The GALEX asteroid data confirm earlier results from the International Ultraviolet Explorer, which two decades ago produced the only other sizeable set of UV asteroid photometry. The GALEX-derived NUV-V data also agree with previously published Hubble Space Telescope (HST) UV observations of asteroids 21 Lutetia and 1 Ceres. Both the HST and GALEX data indicate that NUV band is less useful than u band for distinguishing subgroups within the greater population of visible-color-defined C types (notably, M types and G types).Comment: 13 pages, 11 figures, accepted 2015-May-6 to The Astrophysical Journal. Includes one machine-readable table of NUV asteroid detections. Version 2 includes a corrected citation to Waszczak et al. (2015) arXiv abstrac

Star formation and the distribution of HI and infrared emission in M51

H I, infrared, CO, H alpha and beta band observations of M51, the prototypical grand-design spiral galaxy, are used to study the consequences of star formation for the distribution of H I and dust. Using the H I and CO data sets new tests of the idea that the H I is largely a dissociation product in star-forming regions were performed. It was confirmed that the H I spiral arms are generally coincident with the H II region arms, and offset downstream from the CO arms. The radial distributions of total gas, H alpha and H I surface density have a simple explanation in the dissociation picture. The distributions also demonstrate how the surface density of H I might be related to the star formation efficiency in molecule-rich galaxies. The large width of the H I regions along the arms compared to that of the H II regions can be understood in terms of a simple Stroemgren sphere calculation. The longer lifetime of the stars producing dissociating radiation vs. those producing ionizing radiation will also contribute to the greater width of the H I arms if stars are continuously forming on the arms. The lack of detailed coincidence of the H I and H II regions along the inner arms has a variety of possible explanations within the dissociation scenario. Two simple tests to probe the origin of the IRAS emission in M51 were performed