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  • 1
    Electronic Resource
    Electronic Resource
    Old pulsars in the low-density globular clusters MI3 and M53 (1991)
    [s.l.] : Nature Publishing Group
    Nature 349 (1991), S. 47-49 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] We discovered the pulsars during a survey of globular clusters with the Arecibo 305-m radio telescope6'7. The observations were conducted at a centre frequency of 430 MHz and bandwidth of 10 MHz using the observatory's digital correlator in a manner identical to that described in ref. 8. The data ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/349047a0
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Discovery of two radio pulsars in the globular cluster M15 (1990)
    [s.l.] : Nature Publishing Group
    Nature 346 (1990), S. 42-44 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The initial observations were made using the 305-m Arecibo radio telescope at a central frequency of 430 MHz and a 10-MHz receiver bandwidth. The data were sampled at an effective rate of 1.974kHz using the Arecibo 40-MHz three-level correlation spectrometer in a manner identical to that used in ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/346042a0
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  • 3
    Electronic Resource
    Electronic Resource
    Gamma-ray and hard X-ray imaging of solar flares (1988)
    Springer
    Solar physics 118 (1988), S. 269-290 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We discuss the scientific and technical aspects of high-resolution γ-ray and X-ray imaging of solar flares. The scientific necessity for imaging observations of solar flares and the implications of future observations for the study of solar flare electrons and ions are considered. Performance parameters for a future hard X-ray and γ-ray imager are then summarized. We briefly survey techniques for high-energy photon imaging including direct collimation imaging, coded apertures, and modulation collimators. We then discuss in detail the technique of Fourier-transform imaging. The basic formalism is presented, followed by a discussion of several practical aspects of the technique. We conclude our discussion of imaging techniques with a description of the options for detectors and grid fabrication. Several planned future high-energy imagers are described including the Solar-A hard X-ray imager, the balloon-borne GRID γ-ray imager, and the Pinhole/Occulter Facility.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148596
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  • 4
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    Photometric variability of candidate white dwarf binary systems from Palomar Transient Factory archival data (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-15
    Description: We present a sample of 59 periodic variables from the Palomar Transient Factory, selected from published catalogues of white dwarf (WD) candidates. The variability can likely be attributed to ellipsoidal variation of the tidally distorted companion induced by the gravity of the primary (WD or hot subdwarf) or to the reflection of hot emission by a cooler companion. We searched 11 311 spectroscopically or photometrically selected WD candidates from three hot star/WD catalogues, using the Lomb–Scargle periodogram to single out promising sources. We present period estimates for the candidates, 45 of which were not previously identified as periodic variables, and find that most have a period shorter than a few days. Additionally, we discuss the eclipsing systems in our sample and present spectroscopic data on selected sources.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Five new outbursting AM CVn systems discovered by the Palomar Transient Factory (2013)
    Oxford University Press
    Details
    Publication Date: 2013-03-02
    Description: We present five new outbursting AM CVn systems and one candidate discovered as part of an ongoing search for such systems using the Palomar Transient Factory (PTF). This is the first large-area, systematic search for AM CVn systems using only large-amplitude photometric variability to select candidates. Three of the confirmed systems and the candidate system were discovered as part of the PTF transient search. Two systems were found as part of a search for outbursts through the PTF photometric data base. We discuss the observed characteristics of each of these systems, including the orbital periods of two systems. We also consider the position of these systems, selected in a colour-independent survey, in colour–colour space and compare to systems selected solely by their colours. We find that the colours of our newly discovered systems do not differ significantly from those of previously known systems, but significant errors preclude a definitive answer.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Direct detection of SDSS J0926+3624 orbital expansion with ARCONS (2014)
    Oxford University Press
    Details
    Publication Date: 2014-03-21
    Description: AM Canum Venaticorum (AM CVn) stars belong to a class of ultracompact, short-period binaries with spectra dominated largely by helium. SDSS J0926+3624 is of particular interest as it is the first observed eclipsing AM CVn system. We observed SDSS J0926+3624 with the Array Camera for Optical to Near-IR Spectrophotometry (ARCONS) at the Palomar 200'' telescope. ARCONS uses a relatively new type of energy-resolved photon counters called Microwave Kinetic Inductance Detectors. ARCONS, sensitive to radiation from 350 to 1100 nm, has a time resolution of several microseconds and can measure the energy of a photon to ~10 per cent. We present the light curves for these observations and examine changes in orbital period from prior observations. Using a quadratic ephemeris model, we measure a period rate of change P = (3.07 ± 0.56) x 10 –13 . In addition, we use the high timing resolution of ARCONS to examine the system's high-frequency variations and search for possible quasi-periodic oscillations (QPOs). Finally, we use the instrument's spectral resolution to examine the light curves in various wavelength bands. We do not find any high-frequency QPOs or significant spectral variability throughout an eclipse.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Long-term photometric behaviour of outbursting AM CVn systems (2014)
    Oxford University Press
    Details
    Publication Date: 2014-11-13
    Description: The AM CVn systems are a class of He-rich, post-period minimum, semidetached, ultracompact binaries. Their long-term light curves have been poorly understood due to the few systems known and the long (hundreds of days) recurrence times between outbursts. We present combined photometric light curves from the Lincoln Near Earth Asteroid Research, Catalina Real-Time Transient Survey, and Palomar Transient Factory synoptic surveys to study the photometric variability of these systems over an almost 10 yr period. These light curves provide a much clearer picture of the outburst phenomena that these systems undergo. We characterize the photometric behaviour of most known outbursting AM CVn systems and establish a relation between their outburst properties and the systems’ orbital periods. We also explore why some systems have only shown a single outburst so far and expand the previously accepted phenomenological states of AM CVn systems. We conclude that the outbursts of these systems show evolution with respect to the orbital period, which can likely be attributed to the decreasing mass transfer rate with increasing period. Finally, we consider the number of AM CVn systems that should be present in modelled synoptic surveys.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Connection between optical and {gamma}-ray variability in blazars (2014)
    Oxford University Press
    Details
    Publication Date: 2014-03-01
    Description: We use optical data from the Palomar Transient Factory (PTF) and the Catalina Real-Time Transient Survey (CRTS) to study the variability of -ray-detected and non-detected objects in a large population of active galactic nuclei selected from the Candidate Gamma-Ray Blazar Survey and Fermi Gamma-Ray Space Telescope catalogues. Our samples include 714 sources with PTF data and 1244 sources with CRTS data. We calculate the intrinsic modulation index to quantify the optical variability amplitude in these samples. We find the -ray-detected objects to be more variable than the non-detected ones. The flat spectrum radio quasars (FSRQs) are more variable than the BL Lac objects in our sample, but the significance of the difference depends on the sample used. When dividing the objects based on their synchrotron peak frequency, we find the low synchrotron peaked (LSP) objects to be significantly more variable than the high synchrotron peaked (HSP) ones, explaining the difference between the FSRQs and BL Lacs. This could be due to the LSPs being observed near their electron energy peak, while in the HSPs the emission is caused by lower energy electrons, which cool more slowly. We also find a significant correlation between the optical and -ray fluxes that is stronger in the HSP BL Lacs than in the FSRQs. The FSRQs in our sample are also more Compton dominated than the HSP BL Lacs. These findings are consistent with models where the -ray emission of HSP objects is produced by the synchrotron self-Compton mechanism, while the LSP objects need an additional external Compton component that increases the scatter in the flux–flux correlation.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Phase-resolved spectroscopy and Kepler photometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-20
    Description: Kepler satellite photometry and phase-resolved spectroscopy of the ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The average spectra reveal a variety of weak metal lines of different species, including silicon, sulphur and magnesium as well as many lines of nitrogen, beside the strong absorption lines of neutral helium. The phase-folded spectra and the Doppler tomograms reveal an S-wave in emission in the core of the He i 4471 Å absorption line at a period of P orb  = 1085.7 ± 2.8 s identifying this as the orbital period of the system. The Si ii , Mg ii and the core of some He i lines show an S-wave in absorption with a phase offset of 170° ± 15° compared to the S-wave in emission. The N ii , Si iii and some helium lines do not show any phase variability at all. The spectroscopic orbital period is in excellent agreement with a period at P orb  = 1085.108(9) s detected in the 3 yr Kepler light curve. A Fourier analysis of the Q6–Q17 short-cadence data obtained by Kepler revealed a large number of frequencies above the noise level where the majority shows a large variability in frequency and amplitude. In an Observed-minus-computed analysis, we measured a $\vert \dot{P}\vert \sim 1.0\,$ x 10 –8 s s –1 for some of the strongest variations and set a limit for the orbital period to be $\vert \dot{P}\vert 〈 10^{-10}$  s s –1 . The shape of the phase-folded light curve on the orbital period indicates the motion of the bright-spot. Models of the system were constructed to see whether the phases of the radial velocity curves and the light-curve variation can be combined to a coherent picture. However, from the measured phases neither the absorption nor the emission can be explained to originate in the bright-spot.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Characterization of 9380 contact binaries from the CRTS Variable Sources Catalogue (2017)
    Oxford University Press
    Details
    Publication Date: 2017-01-01
    Description: We construct a sample of 9380 contact binaries (W UMa systems) by using the Catalina Real-Time Transient Survey Variables Sources Catalogue. By measuring brightness change rates, light-curve statistics, and temperatures for this sample, we improve the understanding of contact binary light-curve characteristics, and luminosity variability on decadal time-scales. We show that binaries with convective outer envelopes have a different distribution of light-curve amplitudes and magnitude differences between eclipse minima than binaries with radiative outer envelopes. We find that more than 2000 binaries exhibit a linear change in mean brightness over the 8-yr timespan of observations with at least 3 significance. We note that 25.9 per cent of binaries with convective outer envelopes exhibit a significant change in brightness, while only 10.5 per cent of radiative binaries exhibit a significant change in brightness. In 205 binaries (2.2 per cent), we find that a sinusoid model better describes the luminosity trend within the 8-yr observation timespan. For these binaries, we report the amplitudes and periods (as estimated using observed half-periods) of this sinusoidal brightness variation and discuss possible mechanisms driving the variation.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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