237 research outputs found
Production of defect-poor nanostructured ceramics of yttria-zirconia
For the production of nanostructured ceramics of yttria-zirconia four powders differing in agglomerate strength, agglomerate size and crystallite size are compared. An ultra-fine-grained ceramic with a final density of 98% and a grain size of 0.18 ÎŒm could be produced from a hydrothermally crystallized ethanol-washed powder. The remaining porosity is caused by some residual defects which are present due to the irregular shape of the agglomerates and which cause improper die filling. A commercially available powder was also investigated. This powder consists of homogeneous porous, spherical, weak agglomerates. The resulting ceramic has a high density (â„ 99%) but cannot be obtained with ultra-fine grain size (minimum grain size is 0.3 ÎŒm). The air-crystallized ethanol-washed powder resulted, after sintering, in larger porosities. In this case the powder consists of weak and some strong agglomerates and a few defect clusters are found in the sintered ceramic which limit the maximum attainable density to 92%. The air-crystallized water-washed powder consists of agglomerates which are too strong to be fractured during compaction. The sintered ceramics contain a large amount of porosity (20%) which is attributed to the presence of inter-agglomerate pores.\u
Constraints on the Compact Object Mass in the Eclipsing HMXB XMMU J013236.7+303228 in M33
We present optical spectroscopic measurements of the eclipsing High Mass
X-ray Binary XMMU J013236.7+303228 in M33. Based on spectra taken at multiple
epochs of the 1.73d binary orbital period we determine physical as well as
orbital parameters for the donor star. We find the donor to be a B1.5IV
sub-giant with effective temperature T=22,000-23,000 K. From the luminosity,
temperature and known distance to M33 we derive a radius of R = 8.9 \pm 0.5
R_sun. From the radial--velocity measurements, we determine a velocity
semi-amplitude of K_opt = 63 \pm 12 km/sec. Using the physical properties of
the B-star determined from the optical spectrum, we estimate the star's mass to
be M_opt = 11 \pm 1 M_sun. Based on the X-ray spectrum, the compact companion
is likely a neutron star, although no pulsations have yet been detected. Using
the spectroscopically derived B-star mass we find the neutron star companion
mass to be M_X = 2.0 \pm 0.4 M_sun, consistent with the neutron star mass in
the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_sun found for
many millisecond pulsars. We attempt to use as an additional constraint that
the B star radius inferred from temperature, flux, and distance, should equate
the Roche radius, since the system accretes by Roche lobe overflow. This leads
to substantially larger masses, but from trying to apply the technique to known
systems, we find that the masses are consistently overestimated. Attempting to
account for that in our uncertainties, we derive M_X = 2.2^{+0.8}_{-0.6} M_sun
and M_opt =13 \pm 4 M_sun. We conclude that precise constraints require
detailed modeling of the shape of the Roche surface.Comment: 11 pages emulateapj, 9 figures, ApJ accepte
Nebular spectroscopy of SN 2014J: Detection of stable nickel in near infrared spectra
We present near infrared (NIR) spectroscopy of the nearby supernova 2014J
obtained 450 d after explosion. We detect the [Ni II] 1.939 m line
in the spectra indicating the presence of stable Ni in the ejecta. The
stable nickel is not centrally concentrated but rather distributed as the iron.
The spectra are dominated by forbidden [Fe II] and [Co II] lines. We use lines,
in the NIR spectra, arising from the same upper energy levels to place
constraints on the extinction from host galaxy dust. We find that that our data
are in agreement with the high and low found in earlier studies
from data near maximum light. Using a Ni mass prior from near maximum
light -ray observations, we find 0.05 M of stable nickel
to be present in the ejecta. We find that the iron group features are
redshifted from the host galaxy rest frame by 600 km s.Comment: 6 pages, 4 figures, submitted to A&
PSR J1909-3744, a Binary Millisecond Pulsar with a Very Small Duty Cycle
We report the discovery of PSR J1909-3744, a 2.95 millisecond pulsar in a
nearly circular 1.53 day orbit. Its narrow pulse width of 43 microseconds
allows pulse arrival times to be determined with great accuracy. We have
spectroscopically identified the companion as a moderately hot (T = 8500 K)
white dwarf with strong absorption lines. Radial velocity measurements of the
companion will yield the mass ratio of the system. Our timing data suggest the
presence of Shapiro delay; we expect that further timing observations, combined
with the mass ratio, will allow the first accurate determination of a
millisecond pulsar mass. We have measured the timing parallax and proper motion
for this pulsar which indicate a transverse velocity of 140 (+80/-40) km/s.
This pulsar's stunningly narrow pulse profile makes it an excellent candidate
for precision timing experiments that attempt to detect low frequency
gravitational waves from coalescing supermassive black hole binaries.Comment: 12 pages, 4 figures. Accepted for publication in ApJ
Tkachenko waves, glitches and precession in neutron star
Here I discuss possible relations between free precession of neutron stars,
Tkachenko waves inside them and glitches. I note that the proposed precession
period of the isolated neutron star RX J0720.4-3125 (Haberl et al. 2006) is
consistent with the period of Tkachenko waves for the spin period 8.4s. Based
on a possible observation of a glitch in RX J0720.4-3125 (van Kerkwijk et al.
2007), I propose a simple model, in which long period precession is powered by
Tkachenko waves generated by a glitch. The period of free precession,
determined by a NS oblateness, should be equal to the standing Tkachenko wave
period for effective energy transfer from the standing wave to the precession
motion. A similar scenario can be applicable also in the case of the PSR
B1828-11.Comment: 6 pages, no figures, accepted to Ap&S
Optical Detection of Two Intermediate Mass Binary Pulsar Companions
We report the detection of probable optical counterparts for two Intermediate
Mass Binary Pulsar (IMBP) systems, PSR J1528-3146 and PSR J1757-5322. Recent
radio pulsar surveys have uncovered a handful of these systems with putative
massive white dwarf companions, thought to have an evolutionary history
different from that of the more numerous class of Low Mass Binary Pulsars
(LMBPs) with He white dwarf companions. The study of IMBP companions via
optical observations offers us several new diagnostics: the evolution of main
sequence stars near the white-dwarf-neutron star boundary, the physics of white
dwarfs close to the Chandrasekhar limit, and insights into the recycling
process by which old pulsars are spun up to high rotation frequencies. We were
unsuccessful in our attempt to detect optical counterparts of PSR J1141-6545,
PSR J1157-5112, PSR J1435-6100, and PSR J1454-5846.Comment: 9 pages, 2 figures, accepted for publication in ApJ
Intrinsic and Reprocessed Optical Emission from the Companion to PSR J2051_0827
Hubble Space Telescope observations of the companion to the eclipsing millisecond pulsar PSR J2051-0827 have revealed its "dark" side. The R magnitude at minimum is ~26, while the difference between the side heated by the impinging pulsar radiation and the unirradiated side is ~3.3 mag. For the first time, these data show that there is an asymmetry in the optical light curve of the companion. Furthermore, significant variability is seen in the companion brightness measured after optical maximum on successive orbits. The data are modeled by a gravitationally distorted low-mass secondary star that is irradiated by the pulsar wind. These model fits indicate that the system is only mildly inclined (i ~ 40°) and the unilluminated side of the companion has a temperature likely less than 3000 K
Neutron stars and strange stars in the chiral SU(3) quark mean field model
We investigate the equations of state for pure neutron matter and strange
hadronic matter in -equilibrium, including , and
hyperons. The masses and radii of pure neutron stars and strange hadronic stars
are obtained. For a pure neutron star, the maximum mass is about , while for a strange hadronic star, the maximum mass is
around . The typical radii of pure neutron stars and
strange hadronic stars are about 11.0-12.3 km and 10.7-11.7 km, respectively.Comment: 18 pages, 7 figure
The relativistic jet of Cygnus X-3 in gamma rays
High energy gamma-rays have been detected from Cygnus X-3, a system composed
of a Wolf-Rayet star and a black hole or neutron star. The gamma-ray emission
is linked to the radio emission from the jet launched in the system. The flux
is modulated with the 4.8 hr orbital period, as expected if high energy
electrons are upscattering photons emitted by the Wolf-Rayet star to gamma-ray
energies. This modulation is computed assuming that high energy electrons are
located at some distance along a relativistic jet of arbitrary orientation.
Modeling shows that the jet must be inclined and that the gamma ray emitting
electrons cannot be located within the system. This is consistent with the idea
that the electrons gain energy where the jet is recollimated by the stellar
wind pressure and forms a shock. Jet precession should strongly affect the
gamma-ray modulation shape at different epochs. The power in non-thermal
electrons represents a small fraction of the Eddington luminosity only if the
inclination is low i.e. if the compact object is a black hole.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
- âŠ