Almus T. Kenter

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (76)86.34 Total impact

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    ABSTRACT: We introduce Miniature X-ray Optics to bring highly successful Wolter-I X-ray optics to planetary science within affordable mass, power, and cost constraints.
    02/2014;
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    ABSTRACT: Every object in the Solar system emits X-rays, although the nature and magnitude of this emission varies. In every case sensitive X-ray imaging and spectroscopic measurements would provide direct information about a wide range of atmospheric, magnetospheric, and geologic processes that cannot be gleaned from observations in other energy bands. We are developing a CMOS-based sensor that when combined with existing X-ray telescope technologies would provide a significant advance in capability to the planetary community. Advantages of this sensor technology over other Si-based imagers such as CCDs include near Fano-limited performance to 100 eV, extreme radiation hardness, high speed windowing, high frame rates to handle high fluxes in single counting mode, and low power mW). Our instrument could detect all the CHNOPS elements, and most importantly carbon, via X-ray fluorescence from rocky bodies in the Solar system. Additionally, our sensor could be used for plasma investigations in the magnetospheres of the gas giants in particle environments that would rapidly destroy other Si imaging technologies. In this poster we describe the unique planetary investigations that could be made with this instrument, and present the current status of our instrumentation development.
    10/2013;
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    ABSTRACT: More than 99.9999% of the volume of our Solar system lies beyond the orbit of Neptune and Pluto, and this region is almost totally unexplored. We are developing a mission concept, called Whipple, to study the small bodies of the outer Solar System using a blind stellar occultation survey. This space-based mission would consist of a nominal 0.77 m diameter optical telescope with a focal plan comprising 32 Teledyne H1RG hybrid CMOS sensors. We would simultaneously monitor the brightnesses of tens of thousands of stars at a cadence of up to 40 Hz to search for occultations by Kuiper Belt, inner Oort cloud, and outer Oort Cloud objects. We will detect tens of thousands of these objects over the course of the mission, which will allow us to characterize the spatial and size distribution of these outer Solar System bodies. These data will be used to constrain models of the early history and evolution of the Solar System. In this poster, we summarize the scientific goals for this mission and present the current status of instrumentation development.
    10/2013;
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    ABSTRACT: The Whipple mission, "Reaching into the Outer Solar System", was proposed to NASA's 2010 Discovery program and was awarded funding for technology development. Whipple will conduct the first direct, systematic study of the outer Solar system, the Kuiper belt, the Sedna region, and the Oort cloud, using a blind stellar occultation survey. The instrument will monitor photometrically 10,000 stars at a cadence of 40 Hz (or 20,000 at 20 Hz, ...) to search for stellar occultations by outer solar system objects to characterize the size and spatial distribution of these objects. The occultations typically last less than a second, so the photometer must be able to continuously monitor the light curves of a large number of stars at video rates. The focal plane will be composed of 32 Teledyne H1RG sensors, each with a dedicated SIDECAR ASIC and an FPGA to process the light curve data and identify candidate occultation events. The H1RG sensor will be operated in a windowing mode with between 700 and 3000 windows per sensor at rates up to 40 Hz. We are currently developing an end-to-end system at SAO to evaluate the focal plane concept. This system includes a stellar occultation simulator that stimulates the sensor with simulated light curves, a Teledyne sensor with custom readout software to operate the sensor and SIDECAR ASIC in this windowed mode, and an FPGA that will process the light curves and identify candidate events. In this presentation we will outline the scientific capabilities of the Whipple mission and discuss the current status of our laboratory efforts.
    10/2012;
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    ABSTRACT: The Whipple mission was proposed to the 2010 Discovery program and funded for technology development. Whipple will conduct the first direct study of the outer solar system using a blind occultation survey. We discuss the status of our project.
    LPI Contributions. 10/2012;
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    ABSTRACT: We are developing monolithic backside illuminated CMOS detectors as soft X-ray imaging spectrometers for high energy astrophysics missions. These devices represent a significant advance over CCD technology and have unique properties that would make them ideal sensors for various planetary mission concepts. The benefits of CMOS include higher levels of integration which provide maximum pixel gain and therefore very low noise, very fast parallel output signal chains for high frame rates. CMOS imaging detectors have zero or one charge transfer so that they can withstand many orders of magnitude more radiation than conventional CCDs before degradation. The capability of high read rates provides dynamic range and temporal resolution. Additionally, the rapid read rates minimize shot noise from thermal dark current and optical light. CMOS detectors can therefore run at warmer temperatures and with ultra-thin optical blocking filters. Thin OBFs provide near unity quantum efficiency below 1 keV, thus maximizing response at the C and O lines. Possible mission concepts for these sensors include X-ray fluorescence studies of rocky bodies, and investigation of the magnetospheres of the gas giants and their moons. In this presentation, we discuss the current state of our technology development and outline its scientific potential for planetary physics with particular emphasis on studies of the Jovian magnetosphere. We contrast the capabilities of our instrument with that which has been achieved by the current generation of Earth-orbiting X-ray observatories.
    LPI Contributions. 10/2012;
  • A. T. Kenter, R. Kraft, S. S. Murray
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    ABSTRACT: We present preliminary results from our ongoing program to develop CMOS detectors as single photon counting, soft X-ray imaging spectrometers. The Smithsonian Astrophysical Observatory in collaboration with SRI International/ Sarnoff has been developing monolithic CMOS detectors optimized for x-ray astronomy. Our latest detector consists of an array of 1k × 1k 16 μm pixels manufactured on 15μm epitaxial Si. These detectors are designed to be packaged and thinned for back illumination. The devices have on-chip CDS and are optimized to have high (~ 40 frame/sec) read-out rates. Such monolithic CMOS imaging sensors would be ideal candidate detectors for the focal planes of space-borne soft x-ray astronomy missions. The high through-put, low noise and excellent low energy response, provide high dynamic range and good time resolution; bright and time varying x-ray features could be temporally and spectrally resolved without saturation or photon pile-up.
    Proc SPIE 07/2012;
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    ABSTRACT: We present one of the most precise measurements to date of the spatial clustering of X-ray-selected active galactic nuclei (AGNs) using a sample derived from the Chandra X-ray Observatory survey in the Boötes field. The real-space two-point correlation function over a redshift interval from z = 0.17 to z ~ 3 is well described by the power law, ξ(r) = (r/r 0)–γ, for comoving separations r 20 h –1 Mpc. We find γ = 1.84 ± 0.12 and r 0 consistent with no redshift trend within the sample (varying between r 0 = 5.5 ± 0.6 h –1 Mpc for z = 0.37 and r 0 = 6.9 ± 1.0 h –1 Mpc for z = 1.28). Furthermore, we are able to measure the projections of the two-point correlation function both on the sky plane and in the line of sight. We use these measurements to show that the Chandra/Boötes AGNs are predominantly located at the centers of dark matter halos with circular velocity v max > 320 km s–1 or M 180 > 4.1 × 1012 h –1 M ☉, and tend to avoid satellite galaxies in halos of this or higher mass. The halo occupation properties inferred from the clustering properties of Chandra/Boötes AGNs—the mass scale of the parent dark matter halos, the lack of significant redshift evolution of the clustering length, and the low satellite fraction—are broadly consistent with the Hopkins et al. scenario of quasar activity triggered by mergers of similarly sized galaxies.
    The Astrophysical Journal 10/2011; 741(1):15. · 6.73 Impact Factor
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    ABSTRACT: The proposedWhipple mission is intended to detect Trans-Neptunian Objects (TNOs) via the "blind" occultation technique. The size, number and spatial distribution of these objects provides critical input to evolutionary models of our solar system. The Whipple project was proposed as a NASA Discovery class mission in 2010 and though not selected, it was funded to continue technology development. As part of the proposal preparation, a functional segment of the focal plane was instrumented in the laboratory. The purpose of this test segment was to verify basic detector parameters such as read noise and to detect simulated occultation events. We describe the operation of the detector and a simulator to test and verify the candidate focal plane for the proposed Whipple mission. This paper describes the design, construction and operation of the Whipple event simulator and operation of the laboratory detector.
    Proc SPIE 09/2011;
  • Chandra News. 01/2010;
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    ABSTRACT: We combine IR, optical, and X-ray data from the overlapping, 9.3 deg2 NOAO Deep Wide-Field Survey, AGN and Galaxy Evolution Survey (AGES), and XBoötes Survey to measure the X-ray evolution of 6146 normal galaxies as a function of absolute optical luminosity, redshift, and spectral type over the largely unexplored redshift range 0.1 z 0.5. Because only the closest or brightest of the galaxies are individually detected in X-rays, we use a stacking analysis to determine the mean properties of the sample. Our results suggest that X-ray emission from spectroscopically late-type galaxies is dominated by star formation, while that from early-type galaxies is dominated by a combination of hot gas and active galactic nucleus (AGN) emission. We find that the mean star formation and supermassive black hole accretion rate densities evolve like ~(1 + z)3±1, in agreement with the trends found for samples of bright, individually detectable starburst galaxies and AGN. Our work also corroborates the results of many previous stacking analyses of faint source populations, with improved statistics.
    The Astrophysical Journal 04/2009; 696(2):2206. · 6.73 Impact Factor
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    ABSTRACT: We explore the connection between different classes of active galactic nuclei (AGNs) and the evolution of their host galaxies, by deriving host galaxy properties, clustering, and Eddington ratios of AGNs selected in the radio, X-ray, and infrared (IR) wavebands. We study a sample of 585 AGNs at 0.25 < z < 0.8 using redshifts from the AGN and Galaxy Evolution Survey (AGES). We select AGNs with observations in the radio at 1.4 GHz from the Westerbork Synthesis Radio Telescope, X-rays from the Chandra XBoötes Survey, and mid-IR from the Spitzer IRAC Shallow Survey. The radio, X-ray, and IR AGN samples show only modest overlap, indicating that to the flux limits of the survey, they represent largely distinct classes of AGNs. We derive host galaxy colors and luminosities, as well as Eddington ratios, for obscured or optically faint AGNs. We also measure the two-point cross-correlation between AGNs and galaxies on scales of 0.3-10 h –1 Mpc, and derive typical dark matter halo masses. We find that: (1) radio AGNs are mainly found in luminous red sequence galaxies, are strongly clustered (with M halo ~ 3 × 1013 h –1 M ☉), and have very low Eddington ratios λ 10–3; (2) X-ray-selected AGNs are preferentially found in galaxies that lie in the "green valley" of color-magnitude space and are clustered similar to the typical AGES galaxies (M halo ~ 1013 h –1 M ☉), with 10–3 λ 1; (3) IR AGNs reside in slightly bluer, slightly less luminous galaxies than X-ray AGNs, are weakly clustered (M halo 1012 h –1 M ☉), and have λ>10–2. We interpret these results in terms of a simple model of AGN and galaxy evolution, whereby a "quasar" phase and the growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a critical mass between ~1012 and 1013 M ☉. After this event, star formation ceases and AGN accretion shifts from radiatively efficient (optical- and IR-bright) to radiatively inefficient (optically faint, radio-bright) modes.
    The Astrophysical Journal 04/2009; 696(1):891. · 6.73 Impact Factor
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    ABSTRACT: The low energy grating readout for the AXAF-I mission will be implemented with microchannel plates in conjunction with a novel semi-solid substrate strip charge detector. One axis of the charge detector consists of a conventional wire grid. The other axis consists of charge pickup strips formed on a ceramic substrate. This configuration allows the construction of a three segment detector with the approximate curvature of the Rowland circle. A prototype detector has been built and tested. Spatial resolution is commensurate with conventional crossgrid detectors (< 25 micrometers FWHM).
    Proc SPIE 01/2009;
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    ABSTRACT: We present first results from a Chandra X-Ray Observatory observation of the radio galaxy Centaurus A with the High-Resolution Camera. All previously reported major sources of X-ray emission including the bright nucleus, the jet, individual point sources, and diffuse emission are resolved or detected. The spatial resolution of this observation is better than 1'' in the center of the field of view and allows us to resolve X-ray features of this galaxy not previously seen. In particular, we resolve individual knots of emission in the inner jet and diffuse emission between the knots. All of the knots are diffuse at the 1'' level, and several exhibit complex spatial structure. We find the nucleus to be extended by a few tenths of an arcsecond. Our image also suggests the presence of an X-ray counterjet. Weak X-ray emission from the southwest radio lobe is also seen, and we detect 63 pointlike galactic sources (probably X-ray binaries and supernova remnants) above a luminosity limit of ~1.7 × 1037 ergs s-1.
    The Astrophysical Journal 12/2008; 531(1):L9. · 6.73 Impact Factor
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    A. T. Kenter, S. S. Murray
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    ABSTRACT: We present a technique for determining the number of X-ray sources per flux density interval, the log N-log S relationship, that is mathematically analogous to spectral fitting. This technique is ideal for X-ray source counts obtained using the Chandra X-ray Observatory since the telescope and the focal plane instruments have been well modeled. This technique is of general applicability. In this paper, we apply it to a wavelet source-detect analysis of a Chandra Advanced CCD Imaging Spectrometer (ACIS-I) mosaic image of a 1.35 deg2 survey of the Lockman Hole. We verify the technique via Monte Carlo simulations.
    The Astrophysical Journal 12/2008; 584(2):1016. · 6.73 Impact Factor
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    ABSTRACT: We present results from a Chandra survey of the 9 deg2 Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). This XBootes survey consists of 126 separate contiguous ACIS-I observations each of approximately 5000 s in duration. These unique Chandra observations allow us to search for large-scale structure and to calculate X-ray source statistics over a wide, contiguous field of view with arcsecond angular resolution and uniform coverage. Optical spectroscopic follow-up observations and the rich NDWFS data set will allow us to identify and classify these X-ray-selected sources. Using wavelet decomposition, we detect 4642 point sources with n ≥ 2 counts. In order to keep our detections ~99% reliable, we limit our list to sources with n ≥ 4 counts. For a 5000 s observation and assuming a canonical unabsorbed active galactic nucleus (AGN) type X-ray spectrum, a 4 count on-axis source corresponds to a flux of 4.7 × 10-15 ergs cm-2 s-1 in the soft (0.5-2 keV) band, 1.5 × 10-14 ergs cm-2 s-1 in the hard (2-7 keV) band, and 7.8 × 10-15 ergs cm-2 s-1 in the full (0.5-7 keV) band. The full 0.5-7 keV band n ≥ 4 count list has 3293 point sources. In addition to the point sources, 43 extended sources have been detected, consistent with the depth of these observations and the number counts of clusters. We present here the X-ray catalog for the XBootes survey, including source positions, X-ray fluxes, hardness ratios, and their uncertainties. We calculate and present the differential number of sources per flux density interval, N(S), for the point sources. In the soft (0.5-2 keV) band, N(S) is well fitted by a broken power law with slope of 2.60 at bright fluxes and 1.74 for faint fluxes. The hard source N(S) is well described by a single power law with an index of -2.93.
    The Astrophysical Journal Supplement Series 12/2008; 161(1):9. · 16.24 Impact Factor
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    ABSTRACT: We investigate the evolution of the hard X-ray luminosity of the red galaxy population using a large sample of 3316 red galaxies selected over a wide range in redshift (0.3 < z < 0.9) from a 1.4 deg2 region in the Boötes field of the NOAO Deep Wide-Field Survey (NDWFS). The red galaxies are early-type, bulge-dominated galaxies and are selected to have the same evolution-corrected, absolute R-band magnitude distribution as a function of redshift to ensure that we are tracing the evolution in the X-ray properties of a comparable optical population. Using a stacking analysis of 5 ks Chandra/ACIS observations within this field to study the X-ray emission from these red galaxies in three redshift bins, we find that the mean X-ray luminosity increases as a function of redshift. The large mean X-ray luminosity and the hardness of the mean X-ray spectrum suggest that the X-ray emission is largely dominated by active galactic nuclei (AGNs) rather than stellar sources. The hardness ratio can be reproduced by either an absorbed (NH ≈ 2 × 1022 cm-2) Γ = 1.7 power-law source, consistent with that of a population of moderately obscured Seyfert-like AGNs, or an unabsorbed Γ = 0.7 source, suggesting a radiatively inefficient accretion flow (e.g., an advection-dominated accretion flow). We also find that the emission from this sample of red galaxies constitutes at least 5% of the hard X-ray background. These results suggest a global decline in the mean AGN activity of normal early-type galaxies from z ~ 1 to the present, which indicates that we are witnessing the tailing off of the accretion activity onto supermassive black holes in early-type galaxies since the quasar epoch.
    The Astrophysical Journal 12/2008; 626(2):723. · 6.73 Impact Factor
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    ABSTRACT: The XBoötes Survey is a 5 ks Chandra survey of the Boötes Field of the NOAO Deep Wide-Field Survey (NDWFS). This survey is unique in that it is the largest (9.3 deg2) contiguous region imaged in X-ray with complementary deep optical and near-infrared (near-IR) observations. We present a catalog of the optical counterparts to the 3213 X-ray point sources detected in the XBoötes survey. Using a Bayesian identification scheme, we successfully identified optical counterparts for 98% of the X-ray point sources. The optical colors suggest that the optically detected galaxies are a combination of z < 1 massive early-type galaxies and bluer star-forming galaxies whose optical AGN emission is faint or obscured, whereas the majority of the optically detected point sources are likely quasars over a large redshift range. Our large-area, X-ray-bright, optically deep survey enables us to select a large subsample of sources (773) with high X-ray-to-optical flux ratios (fX/fo > 10). These objects are likely high-redshift and/or dust-obscured AGNs. These sources have generally harder X-ray spectra than sources with 0.1 < fX/fo < 10. Of the 73 X-ray sources with no optical counterpart in the NDWFS catalog, 47 are truly optically blank down to R ~ 25.5 (the average 50% completeness limit of the NDWFS R-band catalogs). These sources are also likely to be high-redshift and/or dust-obscured AGNs.
    The Astrophysical Journal 12/2008; 641(1):140. · 6.73 Impact Factor
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    ABSTRACT: We combine the results of the Spitzer IRAC Shallow Survey and the Chandra XBoötes Survey of the 8.5 deg2 Boötes field of the NOAO Deep Wide-Field Survey to produce the largest comparison of mid-IR and X-ray sources to date. The comparison is limited to sources with X-ray fluxes >8 × 10−15 ergs cm−2 s−1 in the 0.5-7.0 keV range and mid-IR sources with 3.6 μm fluxes brighter than 18.4 mag (12.3 μJy). In this most sensitive IRAC band, 85% of the 3086 X-ray sources have mid-IR counterparts at an 80% confidence level based on a Bayesian matching technique. Only 2.5% of the sample have no IRAC counterpart at all based on visual inspection. Even for a smaller but a significantly deeper Chandra survey in the same field, the IRAC Shallow Survey recovers most of the X-ray sources. A majority (65%) of the Chandra sources detected in all four IRAC bands occupy a well-defined region of IRAC [3.6] − [4.5] versus [5.8] − [8.0] color-color space. These X-ray sources are likely infrared-luminous, unobscured type I AGNs with little mid-infrared flux contributed by the AGN host galaxy. Of the remaining Chandra sources, most are lower luminosity type I and type II AGNs whose mid-IR emission is dominated by the host galaxy, while approximately 5% are either Galactic stars or very local galaxies.
    The Astrophysical Journal 12/2008; 679(2):1040. · 6.73 Impact Factor
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    ABSTRACT: The DEEP2 Survey (ref) fields are being observed with Chandra. Fields 2 and 4 were observed with a mosaic of 12 and 11 ACIS pointings with exposure times of 7 to 9 ksec per pointing, and Field (Extended Groth Strip; EGS) has been observed for about 200 ksec per pointing.Field 3 is scheduled to be observed in August 2008. For Fields 2 and 4 we have analyzed the data in a manner similar to that for the Xbootes Shallow Survey (Murray et al. 2005) and detect about 550 sources per field (each DEEP2 field is about 1/2 square degree in total area) to a flux limit of 3 counts/9 ksec which corresponds to about 3x10-15 erg/cm2/sec. We have matched the X-ray sources against the DEEP2 photometric catalog and also the redshift catalog. About 25% of the X-ray sources have a counterpart in the redshift catalog (most are galaxies with redshifts between 0.7 and 1.4, as expected) and about 67% of the X-ray sources have a\ counterpart in the photometric catalog. We examine the redshift distributions and the R magnitude distributions of the X-ray sources and compare with the XBootes Shallow Survey results. For the EGS, the exposure times are much longer and the corresponding flux limit is about 8 x 10-16 erg/cm2/sec. For EGS we use the IRAC and X-ray data to study the average X-ray properties (e.g., luminosity and spectrum) of selected sub-samples.
    03/2008;