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  • 1
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    Polar research: Six priorities for Antarctic science (2014)
    Nature Publishing Group (NPG)
    Details
    Publication Date: 2014-08-08
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kennicutt, Mahlon C 2nd -- Chown, Steven L -- Cassano, John J -- Liggett, Daniela -- Massom, Rob -- Peck, Lloyd S -- Rintoul, Steve R -- Storey, John W V -- Vaughan, David G -- Wilson, Terry J -- Sutherland, William J -- England -- Nature. 2014 Aug 7;512(7512):23-5. doi: 10.1038/512023a.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Texas A&M University, College Station, Texas, USA, and past-president of the Scientific Committee on Antarctic Research. ; Monash University, Victoria, Australia. ; Cooperative Institute for Research in Environmental Sciences, Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, USA. ; Gateway Antarctica, University of Canterbury, Christchurch, New Zealand. ; Australian Antarctic Division, and Antarctic Climate and Ecosystems Cooperative Research Centre, Hobart, Australia. ; British Antarctic Survey, Cambridge, UK. ; Antarctic Climate and Ecosystems Cooperative Research Centre, Hobart, Tasmania, Australia. ; School of Physics, University of New South Wales, Sydney, Australia. ; School of Earth Sciences, Ohio State University, Columbus, Ohio, USA. ; Conservation Science Group, Department of Zoology, University of Cambridge, Cambridge, UK.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25100467" target="_blank"〉PubMed〈/a〉
    Keywords: Antarctic Regions ; Astronomy ; Atmosphere/chemistry ; Biological Evolution ; Budgets ; *Climate Change ; Conservation of Natural Resources/methods ; Ecology ; Exobiology ; Ice Cover ; International Cooperation ; Oceans and Seas ; *Policy Making ; Research/economics/*trends
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Nature Publishing Group (NPG)
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  • 2
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    On the Dramatic Spin-up/Spin-Down Torque Reversals in Accreting Pulsars (1997)
    In:  Other Sources
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    Publication Date: 2011-08-23
    Description: Dramatic torque reversals between spin-up and spin-down have been observed in half of the persistent X-ray pulsars monitored by the Burst and Transient Space Experiment (BATSE) all-sky monitor on the Compton Gamma Ray Observatory. Theoretical models developed to explain early pulsar timing data can explain spin-down torques via a disk-magnetosphere interaction if the star nearly corotates with the inner accretion disk. To produce the observed BATSE torque reversals, however, these equilibrium models require the disk to alternate between two mass accretion rates, with M+/- producing accretion torques of similar magnitude but always of opposite sign. Moreover, in at least one pulsar (GX 1+4) undergoing secular spin-down, the neutron star spins down faster during brief (approximately 20 day) hard X-ray flares-this is opposite the correlation expected from standard theory, assuming that BATSE pulsed flux increases with mass accretion rate. The 10 day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or viscous timescale near the inner disk boundary and are more suggestive of a global disk phenomenon. We discuss possible explanations of the observed torque behavior. Despite the preferred sense of rotation defined by the binary orbit, the BATSE observations are surprisingly consistent with an earlier suggestion for GX 1+4: the disks in these systems somehow alternate between episodes of prograde and retrograde rotation. We are unaware of any mechanism that could produce a stable retrograde disk in a binary undergoing Roche lobe overflow, but such flip-flop behavior does occur in numerical simulations of wind-fed systems. One possibility is that the disks in some of these binaries are fed by an X-ray-excited wind.
    Keywords: Astronomy
    Type: Astrophysical Journal; Volume 488; L117-L120
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  • 3
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    Discovery of an Afterglow Extension of the Prompt Phase of Two Gamma Ray Bursts Observed by Swift (2005)
    In:  Other Sources
    Details
    Publication Date: 2018-06-12
    Description: Contemporaneous BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, show clearly a prompt component of the afterglow emission. The rapid slewing capability of the spacecraft enables X-ray observations immediately after the burst, typically approximately 100 s following the initiation of the prompt gamma-ray phase. By fitting a power law form to the gamma-ray spectrum, we extrapolate the time dependent fluxes measured by the BAT, in the energy band 15 - 350 keV, into the spectral regime observed by the XRT, 0.2 - 10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to approximately 300 s, followed by a flatter decay. The duration of the steep decay, approximately 100 s in the source frame after correcting for cosmological time dilation, agrees with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where T(sub e),(BAT)approximately equal to 24 plus or minus 2 s, and T(sub e)(XRT) approximately equal to 35 plus or minus 2 s. For GRB 050319 a power law decay -d lnf/d lnt = n, where n approximately equal to 3, provides a reasonable fit. The early time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission, and provide our first quantitative measure of the decay of the prompt gamma-ray emission over a large dynamic range. The initial steep decay is expected from the high latitude emission from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs lasts for hundreds of seconds longer than previously thought.
    Keywords: Astronomy
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  • 4
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    Discovery of an Afterglow Extension of the Prompt Phase of Two Gamma Ray Bursts Observed by Swift (2005)
    In:  CASI
    Details
    Publication Date: 2018-06-06
    Description: Contemporaneous BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, show clearly a prompt component joining the onset of the afterglow emission. The rapid slewing capability of the spacecraft enables X-ray observations immediately after the burst, typically ~ 100 s following the initiation of the prompt y-ray phase. By fitting a power law form to the y-ray spectrum, we extrapolate the time dependent fluxes measured by the BAT, in the energy band 15 - 350 keV, into the spectral regime observed by the XRT 0.2 - 10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to ~ 300 s, followed by a flatter decay. The duration of the steep decay, ~ 100 s in the source frame after correcting for cosmological time dilation, agrees roughly with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where one e-folding decay time Te (BAT)~ = 24 f 2 s, and Te,(XRT)~ = 35 f 2 s. For GRB 050319, a power law decay - d l n f / d l n t = n, where n approx. ~ = 3, provides a reasonable fit. The early time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission, and provide our first quantitative measure of the decay of the prompt y-ray emission over a large dynamic range in flux. The initial steep decay is expected due to the delayed high latitude photons from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs remains observable for hundreds of seconds longer than previously thought.
    Keywords: Astronomy
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  • 5
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    The Physical Interpretation of X-Ray Phase Lags and Coherence: RXTE Observations of Cygnus X-1 as a Case Study (1998)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: There have been a number of recent spectral models that have been successful in reproducing the observed X-ray spectra of galactic black hole candidates (GBHC). However, there still exists controversy over such issues as: what are the sources of hard radiation, what is the system's geometry, is the accretion efficient or inefficient, etc. A potentially powerful tool for distinguishing among these possibilities, made possible by the Rossi X-ray Timing Explorer (RXTE), is the variability data, especially the observed phase lags and variability coherence. These data, in conjunction with spectral modeling, have the potential of determining physical sizes of the system, as well as placing strong constraints on both Compton corona and advection models. As an example, we present RXTE variability data of Cygnus X-1.
    Keywords: Astronomy
    Type: Nuclear Physics B (Proceedings Supplements) (ISSN 0920-5632); 69; 3-Jan; 302-307
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  • 6
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    Discovery and Orbital Determination of the Transient X-Ray Pulsar GRO J1750-27 (1997)
    In:  Other Sources
    Details
    Publication Date: 2019-07-17
    Description: We report on the discovery and hard X-ray (20-70 keV) observations of the 4.45 second period transient X-ray pulsar GRO J1750-27 with the BATSE all-sky monitor on board CCRO. A relatively faint outburst (〈 30 mCrab peak) lasting at least 60 days was observed during which the spin-up rate peaked at 38 pHz/sec and was correlated with the pulsed intensity. An orbit with a period of 29.8 days was found. The large spin-up rate, spin period and orbital period together suggest that accretion is occurring from a disk and that the outburst is a 'giant' outburst typical of a Be/X-ray transient system. No optical counterpart has been reported yet.
    Keywords: Astronomy
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  • 7
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    Arakelian 564: An XMM-Newton View (2003)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: We report on two XMM-Newton observations of the bright narrow-line Seyfert 1 galaxy Ark 564 taken one year apart (2000 June and 2001 June). The 0.6-10 keV continuum is well described by a soft blackbody component (kTau approximately equal 140-150 eV) plus a steep power law (Tau approximately equal to 2.50-2.55). No significant spectral changes are observed between the two observations, although the X-ray flux in the second observation is approximately equal to 40-50 per cent lower. In both observations we detect a significant absorption edge at a rest-frame energy of approximately equal to 0.73 keV, corresponding to O VII. The presence of the absorption feature is confirmed by a simultaneous Chandra grating observation in 2000 June, although the best-fitting edge threshold is at a slightly lower energy in the Chandra data, possibly because of a different parameterization of the underlying X-ray continuum. We find tentative evidence for a broad iron emission line in the 2000 June observation. The results from an analysis of the power spectral density (PSD) function are also presented. The present XMM-Newton data support the idea that the PSD shown two breads, although the location of the high-frequency break requires further constraints.
    Keywords: Astronomy
    Type: astro-ph/0310278
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  • 8
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    The Discovery of an Evolving Dust Scattered X-ray Halo Around GRB 031203 (2003)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: We report the first detection of a time-dependent, dust-scattered X-ray halo around a gamma-ray burst. GRB3 031203 was observed by XMM-Newton starting six hours after the burst. The halo appeared as concentric ring-like structures centered on the GRB location. The radii of these structures increased with time as t(sup 1/2), consistent with small-angle X-ray scattering caused by a large column of dust along the line of sight to a cosmologically distant GRB. The rings are due to dust concentrated in two distinct slabs in the Galaxy located at distances of 880 and 1390 pc, consistent with known Galactic features. The halo brightness implies an initial soft X-ray pulse consistent with the observed GRB.
    Keywords: Astronomy
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  • 9
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    Observations of Accreting Pulsars (1997)
    In:  Other Sources
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    Publication Date: 2019-08-15
    Description: We summarize 5 years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered five new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A0535+26, GRO J2058+42, 4U 1145-619, and A1118-616), and also measured the accretion torque history during outbursts of six of those transients whose orbital param- eters were also known. We have also continuously measured the pulsed flux and spin frequency for eiaht persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic neutron stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long timescales, which blurs the con- ventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars but are uncorrelated, or even anti- correlated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA/Goddard Space Flight Center.
    Keywords: Astronomy
    Type: Astrophysical Journal Supplement Series; 113; 367-408
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  • 10
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    RXTE Observation of Cygnus X-1: Spectra and Timing (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: We present first results from the analysis of an RXTE observation of Cyg X-1 in its low state, taken about two months after the end of the high state. With Gamma approx. equal to 1.45 the spectrum is considerably harder than previous low-state measurements. The observed spectrum can be explained by a Comptonization spectrum as that emitted from a spherical corona surrounded by a cold accretion disk. The optical depth of the corona is between 2 and 2.5 and the temperature is between 60 and 80 keV. Temporal analysis shows a typical Root Mean Square (RMS) noise of approximately 25%. The Pulse Shape Discrimination (PSD) can be described as consisting of a flat component followed by an 1/f power-law, followed by an f(sup -1.6) power-law. The lag of the hard photons with respect to the soft photons is consistent with prior observations. The coherence function is remarkably close to unity from 0.01 Hz to 10 Hz.
    Keywords: Astronomy
    Type: NASA-CR-204635 , NAS 1.26:204635
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