364 research outputs found
The impact resistance of SiC and other mechanical properties of SiC and Si3N4
Studies focused on the impact and mechanical behavior of SiC and Si3N4 at high temperatures are summarized. Instrumented Charpy impact testing is analyzed by a compliance method and related to strength; slow crack growth is related to processing, and creep is discussed. The transient nature of flaw populations during oxidation under load is emphasized for both SiC and Si3N4
The effects of alewife (Alosa pseudoharengus) on zooplankton community structure in Depot Pond NH and a comparison of seven New Hampshire lakes
Physical, chemical and biological features of seven New Hampshire lakes were examined in September and October of 1997. Zooplankton communities exhibited evidence of âtop-downâ control in Milton Three Ponds (Depot, Norteast, and Townhouse Ponds), showing effects of a planktivorous fish, Alosa pseudoharengus: small mean body size, dominance of small grazers such as Bosmina, and absence of large grazers such as Daphnia. Phosphorus concentrations were positively correlated to fluorescence of all water fractions, chlorophyll a and a phytoplankton biotic pollution index (modified from Hillsenhoff, 1978), revealing a level of âbottom-upâ control
Discovery of Microsecond Soft Lags in the X-Ray Emission of the Atoll Source 4U1636-536
Exploiting the presence of kilohertz quasi-periodic oscillations (QPOs) in
the timing power spectrum, we find that the soft x-ray emission of the
neutron-star X-ray binary and atoll source 4U1636-536 modulated at the QPO
frequency lags behind that of the hard x-ray emission. Emission in the 3.8-6.4
keV band is delayed by 25.0 +/- 3.3 microseconds relative to the 9.3-69 keV
band. The delay increases in magnitude with increasing energy. Our results are
consistent with those of Vaughan et al. (1997), when the sign is corrected
(Vaughan et al. 1998), for the atoll source 4U1608-52. The soft lag could be
produced by Comptonization of hard photons injected into a cooler electron
cloud or by intrinsic spectral softening of the emission during each
oscillation cycle.Comment: Accepted to the Astrophysical Journal Letters, 4 page
On the Spin History of the X-ray Pulsar in Kes 73: Further Evidence For an Utramagnetized Neutron Star
In previous papers, we presented the discovery of a 12-s X-ray pulsar in the
supernova remnant Kes 73, providing the first direct evidence for an
ultramagnetized neutron star, a magnetar, with an equivalent dipole field of
nearly twenty times the quantum critical magnetic field. Our conclusions were
based on two epochs of measurement of the spin, along with an age estimate of
the host supernova remnant. Herein, we present a spin chronology of the pulsar
using additional GINGA, ASCA, XTE, & SAX datasets spanning over a decade.
Timing and spectral analysis confirms our initial results and severely limit an
accretion origin for the observed flux. Over the 10 year baseline, the pulsar
is found to undergo a rapid, constant spindown, while maintaining a steady flux
and an invariant pulse profile. Within the measurement uncertainties, no
systematic departures from a linear spin-down are found - departures as in the
case of glitches or simply stochastic fluctuations in the pulse
times-of-arrival (e.g. red timing noise). We suggest that this pulsar is akin
to the soft gamma-ray repeaters, however, it is remarkably stable and has yet
to display similar outbursts; future gamma-ray activity from this object is
likely.Comment: 6 pages with 3 embedded figures, LaTex, emulateapj.sty. Submitted to
the ApJ Letter
Discovery of 16.6 and 25.5 s Pulsations from the Small Magellanic Cloud
We report the serendipitous detection of two previously unreported pulsars
from the direction of the Small Magellanic Cloud, with periods of 16.6 and 25.5
seconds. The detections are based on archival PCA data from the Rossi X-ray
Timing Explorer (RXTE). The observation leading to these detections occurred in
September 2000 extending over 2.1 days with an exposure of 121 ks. A possible
identification of the 16.6 s pulsar with an X-ray source RX J0051.8-7310 seen
by both ROSAT and ASCA imaging X-ray satellites is presented.Comment: 9 pages with 3 figures. Submitted to ApJ Letter
Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288
We report evidence for an Fe K-alpha fluorescence line feature and disk
reflection in the Very High, High, and Low State X-ray spectra of the galactic
microquasar XTE J1748-288 during its June~1998 outburst. Spectral analyses are
made on data gathered throughout the outburst by the Rossi X-ray Timing
Explorer Proportional Counter Array. Gaussian line, relativistic disk emission
line, and ionized disk reflection models are fit to the data. In the Very High
State the line profile appears strongly redshifted, consistent with disk
emission from the innermost stable orbits around a maximally rotating Kerr
black hole. In the High State the line profile is less redshifted and
increasingly prominent. The Low State line profile is very strong (~0.5 keV
equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model
fits indicate that the inner edge of the disk fluctuates between ~20 and ~100
gravitational radii in this state. The disk reflection fraction is traced
through the outburst; reflection from an ionized disk is preferred in the VHS
and HS, and reflection from a relatively neutral disk is preferred in the LS.
We discuss the implications of our findings for the binary system dynamics and
accretion flow geometry in XTE J1748-288.Comment: 32 pages, 8 figures, 5 tables. ApJ accepted 8/16/0
Aquatic biosurvey of the Lovell River on UNH land
We assessed the physical, chemical and biological conditions at two sites along the Lovell River on University of New Hampshire (UNH) -owned conservation land. The discharge was 4.4 m3 s-1 at Site 1 and 5.7 m3 s -1 downstream at Site 2. Canopy coverage ranged from 8-25%. Canopy was dominated by Eastern Hemlock (79-84%). Much of the stream was strewn with large boulders and the substrate consisted of rocks of highly variable sizes ( 3-549 cm dia.). Specific conductivity (22.1-23.3 ”S), pH (6.4) and temperature (7.9-8.3 °C) varied little between sites. Macro-invertebrate bio-indices indicated either excellent water quality with no apparent organic pollution (3.0/10) or good water quality with possible slight organic pollution (4.4/10)
X-Ray Flares and Oscillations from the Black Hole Candidate X-Ray Transient XTE J1650-500 at Low Luminosity
We report on X-ray observations made with the Rossi X-ray Timing Explorer of
the black hole candidate (BHC) transient XTE J1650-500 at the end of its first,
and currently only, outburst. By monitoring the source at low luminosities over
several months, we found 6 bright ~100 second X-ray flares and long time scale
oscillations of the X-ray flux. The oscillations are aperiodic with a
characteristic time scale of 14.2 days and an order of magnitude variation in
the 2.8-20 keV flux. The oscillations may be related to optical
"mini-outbursts" that have been observed at the ends of outbursts for other
short orbital period BHC transients. The X-ray flares have durations between 62
and 215 seconds and peak fluxes that are 5-24 times higher than the persistent
flux. The flares have non-thermal energy spectra and occur when the persistent
luminosity is near 3E34 (d/4 kpc)^2 erg/s (2.8-20 keV). The rise time for the
brightest flare demonstrates that physical models for BHC systems must be able
to account for the situation where the X-ray flux increases by a factor of up
to 24 on a time scale of seconds. We discuss the flares in the context of
observations and theory of Galactic BHCs and compare the flares to those
detected from Sgr A*, the super-massive black hole at the Galactic center. We
also compare the flares to X-ray bursts that are seen in neutron star systems.
While some of the flare light curves are similar to those of neutron star
bursts, the flares have non-thermal energy spectra in contrast to the blackbody
spectra exhibited in bursts. This indicates that X-ray bursts should not be
taken as evidence that a given system contains a neutron star unless the
presence of a blackbody component in the burst spectrum can be demonstrated.Comment: 9 pages, accepted by Ap
Hard X-ray emission of the microquasar GRO J1655-40 during the rise of its 2005 outburst
We present the analysis of the high energy emission of the Galactic black
hole GRO J1655-40 at the beginning of its 2005 outburst. The data from 458 ks
of INTEGRAL observations, spread over 4 weeks, are analyzed, along with the
existing simultaneous RXTE and Swift data. The high energy data allow us to
detect the presence of a high energy cut-off and to study its evolution during
the outburst rise. This high energy feature is generally related to thermal
mechanisms in the framework of Comptonization models from which we can estimate
the plasma parameters. We found an electron temperature of about 30-40 keV and
an optical depth around 1.8-2.1. The high energy cut-off decreased along with
the radio flux, and disappeared as the jet turned off.Comment: 24 pages, 5 figures, accepted for publication in the Astrophsical
Journa
GRB060602B = Swift J1749.4-2807: an unusual transiently accreting neutron-star X-ray binary
We present an analysis of the Swift BAT and XRT data of GRB060602B, which is
most likely an accreting neutron star in a binary system and not a gamma-ray
burst. Our analysis shows that the BAT burst spectrum is consistent with a
thermonuclear flash (type-I X-ray burst) from the surface of an accreting
neutron star in a binary system. The X-ray binary nature is further confirmed
by the report of a detection of a faint point source at the position of the XRT
counterpart of the burst in archival XMM-Newton data approximately 6 years
before the burst and in more recent XMM-Newton data obtained at the end of
September 2006 (nearly 4 months after the burst). Since the source is very
likely not a gamma-ray burst, we rename the source Swift J1749.4-2807, based on
the Swift/BAT discovery coordinates. Using the BAT data of the type-I X-ray
burst we determined that the source is at most at a distance of 6.7+-1.3 kpc.
For a transiently accreting X-ray binary its soft X-ray behaviour is atypical:
its 2-10 keV X-ray luminosity (as measured using the Swift/XRT data) decreased
by nearly 3 orders of magnitude in about 1 day, much faster than what is
usually seen for X-ray transients. If the earlier phases of the outburst also
evolved this rapidly, then many similar systems might remain undiscovered
because the X-rays are difficult to detect and the type-I X-ray bursts might be
missed by all sky surveying instruments. This source might be part of a class
of very-fast transient low-mass X-ray binary systems of which there may be a
significant population in our Galaxy.Comment: Accepted for publication in MNRA
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