Showing papers by "Michael W. Werner published in 2012"

Contribution of the accretion disk, hot corona, and obscuring torus to the luminosity of Seyfert galaxies: INTEGRAL and Spitzer observations

Abstract: We estimate the relative contributions of the supermassive black hole (SMBH) accretion disk, corona, and obscuring torus to the bolometric luminosity of Seyfert galaxies, using Spitzer mid-infrared (MIR) observations of a complete sample of 68 nearby active galactic nuclei (AGNs) from the INTEGRAL all-sky hard X-ray (HX) survey. This is the first HX-selected (above 15 keV) sample of AGNs with complementary high angular resolution, high signal-to-noise, MIR data. Correcting for the host galaxy contribution, we find a correlation between HX and MIR luminosities: L{sub 15{mu}m}{proportional_to}L{sup 0.74{+-}0.06}{sub HX}. Assuming that the observed MIR emission is radiation from an accretion disk reprocessed in a surrounding dusty torus that subtends a solid angle decreasing with increasing luminosity (as inferred from the declining fraction of obscured AGNs), the intrinsic disk luminosity, L{sub Disk}, is approximately proportional to the luminosity of the corona in the 2-300 keV energy band, L{sub Corona}, with the L{sub Disk}/L{sub Corona} ratio varying by a factor of 2.1 around a mean value of 1.6. This ratio is a factor of {approx}2 smaller than for typical quasars producing the cosmic X-ray background. Therefore, over three orders of magnitude in luminosity, HX radiation carries a large, and roughly comparable, fraction of themore » bolometric output of AGNs. We estimate the cumulative bolometric luminosity density of local AGNs at {approx}(1-3) Multiplication-Sign 10{sup 40} erg s{sup -1} Mpc{sup -3}. Finally, the Compton temperature ranges between kT{sub c} Almost-Equal-To 2 and Almost-Equal-To 6 keV for nearby AGNs, compared to kT{sub c} Almost-Equal-To 2 keV for typical quasars, confirming that radiative heating of interstellar gas can play an important role in regulating SMBH growth.« less

Spitzer Observations of Hotspots in Radio Lobes

Abstract: We have carried out a systematic search with Spitzer Warm Mission and archival data for infrared emission from the hotspots in radio lobes that have been described by Hardcastle et al. (2004). These hotspots have been detected with both radio and X-ray observations, but an observation at an intermediate frequency in the infrared can be critical to distinguish between competing models for particle acceleration and radiation processes in these objects. Between the archival and warm mission data, we report detections of 18 hotspots; the archival data generally include detections at all four IRAC bands, the Warm Mission data only at 3.6 microns. Using a theoretical formalism adopted from Godfrey et al. (2009), we fit both archival and warm mission spectral energy distributions [SEDs] - including radio, X-ray, and optical data from Hardcastle as well as the Spitzer data - with a synchrotron self-Compton [SSC] model, in which the X-rays are produced by Compton scattering of the radio frequency photons by the energetic electrons which radiate them. With one exception, a SSC model requires that the magnetic field be less or much less than the equipartition value which minimizes total energy and has comparable amounts of energy in the magnetic field and in the energetic particles. This conclusion agrees with those of comparable recent studies of hotspots, and with the analysis presented by Hardcastle et al. (2004). We also show that the infrared data rule out the simplest synchrotron only models for the SEDs. We briefly discuss the implications of these results and of alternate interpretations of the data.

WISE Detections of Dust in the Habitable Zones of Planet-bearing Stars

Abstract: We use data from the Wide-field Infrared Survey Explorer (WISE) all-sky release to explore the incidence of warm dust in the habitable zones around exoplanet-host stars. Dust emission at 12 and/or 22 microns (T(sub dust) approx.300 and/or approx.150 K) traces events in the terrestrial planet zones; its existence implies replenishment by evaporation of comets or collisions of asteroids, possibly stirred by larger planets. Of the 591 planetary systems (728 extrasolar planets) in the Exoplanet Encyclopedia as of 2012 January 31, 350 are robustly detected by WISE at > or = 5(sigma) level. We perform detailed photosphere subtraction using tools developed for Spitzer data and visually inspect all the WISE images to confirm bona fide point sources. We find nine planet-bearing stars show dust excess emission at 12 and/or 22 microns at > or = 3(sigma) level around young, main-sequence, or evolved giant stars. Overall, our results yield an excess incidence of approx.2.6% for stars of all evolutionary stages, but approx.1% for planetary debris disks around main-sequence stars. Besides recovering previously known warm systems, we identify one new excess candidate around the young star UScoCTIO 108.

Spitzer observations of hotspots in radio lobes

Abstract: We have carried out a systematic search with Spitzer Warm Mission and archival data for infrared emission from the hotspots in radio lobes that have been described by Hardcastle et al. These hotspots have been detected with both radio and X-ray observations, but an observation at an intermediate frequency in the infrared can be critical to distinguish between competing models for particle acceleration and radiation processes in these objects. Between the archival and warm mission data, we report detections of 18 hotspots; the archival data generally include detections at all four IRAC bands, the Warm Mission data only at 3.6 μm. Using a theoretical formalism adopted from Godfrey et al., we fit both archival and warm mission spectral energy distributions (SEDs)—including radio, X-ray, and optical data from Hardcastle as well as the Spitzer data—with a synchrotron self-Compton (SSC) model, in which the X-rays are produced by Compton scattering of the radio frequency photons by the energetic electrons which radiate them. With one exception, an SSC model requires that the magnetic field be less or much less than the equipartition value which minimizes total energy and has comparable amounts of energy in the magnetic field and in the energetic particles. This conclusion agrees with those of comparable recent studies of hotspots, and with the analysis presented by Hardcastle et al. We also show that the infrared data rule out the simplest synchrotron-only models for the SEDs. We briefly discuss the implications of these results and of alternate interpretations of the data.

The Spitzer Space Telescope

Abstract: The Spitzer Space Telescope, which has operated very successfully since 2003 in its unique Earth-trailing solar orbit, is NASA's Great Observatory for infrared astronomy. We provide a quick overview of the optical characteristics of Spitzer and review the observatory design. The main emphasis is on two unique on-orbit activities used to optimize the scientific return from Spitzer: 1. an unusual approach to focusing the telescope that minimized the use of the cryogenic focus mechanism, and 2. a methodology for extending the cryogenic lifetime of Spitzer by actively controlling the telescope temperature.

Probing the mass and structure of the Ring Nebula in Lyra with SOFIA/GREAT observations of the [CII] 158 micron line

Abstract: We have obtained new velocity-resolved spectra of the [CII] 158 μm line towards the Ring Nebula in Lyra (NGC 6720), one of the best-studied planetary nebulae, in order to probe its controversial 3-dimensional structure and to estimate the mass of circumstellar material in this object. We used the terahertz receiver GREAT aboard the SOFIA airborne telescope to obtain the [CII] spectra at eight locations within and outside the bright optical ring of NGC 6720. Emission was detected at all positions except for the most distant position along the nebula’s minor axis, and generally covers a broad velocity range, ΔV ∼ 50 km s −1 (FWZI), except at a position along the major axis located just outside the optical ring, where it is significantly narrower (ΔV ∼ 25 km s −1 ). The one narrow spectrum appears to be probing circumstellar material lying outside the main nebular shell that has not been accelerated by past fast wind episodes from the central star, and therefore most likely comes from equatorial and/or low-latitude regions of this multipolar nebula. Along lines-of-sight passing within about 10 �� of the nebular center, the CII column density is a factor 46 higher than the CO column density. The total mass of gas associated with the [CII] emission inside a circular region of diameter 87. �� 5i s at least 0.11M� . A significant amount of [CII] flux arises from a photodissociation region immediately outside the bright optical ring, where we find a CII to CO ratio of >6.5, lower than that seen towards the central region. Comparing our data with lower-quality CI spectra, which indicate similarly large CI/CO ratios in NGC 6720, we conclude that the bulk of elemental carbon in NGC 6720 is divided roughly equally between CII and CI, and that the emissions from these species are far more robust tracers of circumstellar material than CO in this object and other evolved planetary nebulae.

Probing the Mass and Structure of the Ring Nebula in Lyra with SOFIA/GREAT Observations of the [CII] 158 micron line

Abstract: We have obtained new velocity-resolved spectra of the [CII] 158 micron line towards the Ring Nebula in Lyra (NGC 6720), one of the best-studied planetary nebulae, in order to probe its controversial 3-dimensional structure and to estimate the mass of circumstellar material in this object. We used the Terahertz receiver GREAT aboard the SOFIA airborne telescope to obtain the [CII] spectra at eight locations within and outside the bright optical ring of NGC 6720. Emission was detected at all positions except for the most distant position along the nebula's minor axis, and generally covers a broad velocity range, ~50 km/s (FWZI), except at a position along the major axis located just outside the optical ring, where it is significantly narrower (~25 km/s). The one narrow spectrum appears to be probing circumstellar material lying outside the main nebular shell that has not been accelerated by past fast wind episodes from the central star, and therefore most likely comes from equatorial and/or low-latitude regions of this bipolar nebula. Along lines-of-sight passing within about 10 arcsec of the nebular center, the CII column density is a factor 46 higher than the CO column density. The total mass of gas associated with the [CII] emission inside a circular region of diameter 87.5 arcsec is at least 0.11 solar masses. A significant amount of [CII] flux arises from a photodissociation region immediately outside the bright optical ring, where we find a CII to CO ratio of 6.5, lower than that seen towards the central region. Comparing our data with lower-quality CI spectra, which indicate similarly large CI/CO ratios in NGC 6720, we conclude that the bulk of elemental carbon in NGC 6720 is divided roughly equally between CII and CI, and that the emissions from these species are far more robust tracers of circumstellar material than CO in this object and other evolved planetary nebulae.

Contribution of the accretion disk, hot corona, and obscuring torus to the luminosity of Seyfert galaxies: INTEGRAL and Spitzer observations

Abstract: We estimate the relative contributions of the supermassive black hole (SMBH) accretion disk, corona, and obscuring torus to the bolometric luminosity of Seyfert galaxies, using Spizter mid-infrared (MIR) observations of a complete sample of 68 nearby active galactic nuclei from the INTEGRAL all-sky hard X-ray (HX) survey. This is the first HX-selected (above 15 keV) sample of AGNs with complementary high angular resolution, high signal to noise, MIR data. Correcting for the host galaxy contribution, we find a correlation between HX and MIR luminosities: L_MIR L_HX^(0.74+/-0.06). Assuming that the observed MIR emission is radiation from an accretion disk reprocessed in a surrounding dusty torus that subtends a solid angle decreasing with increasing luminosity (as inferred from the declining fraction of obscured AGNs), the intrinsic disk luminosity, L_D, is approximately proportional to the luminosity of the corona in the 2-300 keV energy band, L_C, with the L_D/L_C ratio varying by a factor of 2.1 around a mean value of 1.6. This ratio is a factor of ~2 smaller than for typical quasars producing the cosmic X-ray background (CXB). Therefore, over three orders of magnitude in luminosity, HX radiation carries a large, and roughly comparable, fraction of the bolometric output of AGNs. We estimate the cumulative bolometric luminosity density of local AGNs at ~(1-3)x10^40 erg/s/Mpc^3. Finally, the Compton temperature ranges between kT_c~2 and ~6 keV for nearby AGNs, compared to kT_c~2 keV for typical quasars, confirming that radiative heating of interstellar gas can play an important role in regulating SMBH growth.

Astronomical opportunities from the outer solar system

Abstract: How bright is the sky? For cosmologists, the answer depends on how galaxies formed and how luminous the first generations of stars might have been. While of fundamental importance in astronomy, measurements of extragalactic sky brightness have been frustrated by contamination from local sources, especially at optical and IR wavelengths.1 Even in space, away from terrestrial lights and Earth’s atmosphere, the sky brightness is dominated by zodiacal light, sunlight scattered by the interplanetary dust (IPD) in our solar system. Our team has been studying the possibility of carrying out observations during the underused ‘cruise phase’ of a planetary science mission to the outer planets, beyond the bulk of the IPD cloud where the sky is much darker than at Earth’s orbit, one astronomical unit (AU) from the Sun. As shown in Figure 1, we estimate that the sky brightness is 30 times fainter from the orbit of Jupiter, and 100 times fainter from the orbit of Saturn, than the brightness available from Earth’s orbit.2 This unique vantage point would enable us to measure the extragalactic background light (EBL), as well as the structure and properties of dust in the outer solar system,3 with unprecedented accuracy. We developed a concept for an optical to near-IR instrument, the Zodiacal dust, Extragalactic Background, and ReionizationApparatus (ZEBRA4). As shown in Figure 2, ZEBRA is a small instrument, specialized for absolute surface brightness photometry. We designed ZEBRA for minimal mass (16.4kg), power (12.4W), and data resources, with simple interfaces to the parent satellite, and incorporating technologies that are well developed and readily available. ZEBRA uses two small wide-field telescopes operating from 0.4–5 m, to measure the EBL and to map the IPD cloud in scattered sunlight. The optics are based on three mirror off-axis designs, with multiple field and aperture Figure 1. Left: Estimated brightness of zodiacal light from 1AU (astronomical unit) to 10AU (cyan curves) based on Pioneer 10 and 11 measurements out to 3AU, with shaded areas indicating the uncertainty in the extrapolation. Diffuse galactic light (DGL, green curve) arises from starlight scattered by interstellar dust and emission from the interstellar medium. The large reduction in zodiacal brightness enables a precise measurement of the extragalactic background light (EBL, a model is indicated by the red curve) and a deep search for photons from reionization, shown by the violet shaded region. Note that the reionization EBL is constrained to have the minimum level required to produce sufficient photons for reionization. Right top: For the darkest field observed by the Diffuse Infrared Background Experiment (DIRBE) from 1AU, we show contributions to the total observed intensity from zodiacal light, undetected stars, DGL, and the estimated EBL. Clearly, zodiacal light is the dominant factor in current EBL measurements from 1AU. Right bottom: The same field observed by the Zodiacal dust, Extragalactic Background, and Reionization Apparatus (ZEBRA) at 10AU with higher spatial resolution is dominated by the EBL, before any attempts are made to subtract these foregrounds. I : Measure of surface brightness.