1,136 research outputs found
An X-Ray Dip in the X-Ray Transient 4U 1630-47
An x-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer
observation of the recurrent x-ray transient 4U 1630-47. During the dip, the
2-60 keV x-ray flux drops by a factor of about three, and, at the lowest point
of the dip, the x-ray spectrum is considerably softer than at non-dip times. We
find that the 4U 1630-47 dip is best explained by absorption of the inner part
of an accretion disk, while the outer part of the disk is unaffected. The
spectral evolution during the dip is adequately described by the variation of a
single parameter, the column density obscuring the inner disk.Comment: 13 pages, 4 figures, Accepted for publication in Ap
Magnetic anisotropy switching in (Ga,Mn)As with increasing hole concentration
We study a possible mechanism of the switching of the magnetic easy axis as a
function of hole concentration in (Ga,Mn)As epilayers. In-plane uniaxial
magnetic anisotropy along [110] is found to exceed intrinsic cubic
magnetocrystalline anisotropy above a hole concentration of p = 1.5 * 10^21
cm^-3 at 4 K. This anisotropy switching can also be realized by post-growth
annealing, and the temperature-dependent ac susceptibility is significantly
changed with increasing annealing time. On the basis of our recent scenario
[Phys. Rev. Lett. 94, 147203 (2005); Phys. Rev. B 73, 155204 (2006).], we
deduce that the growth of highly hole-concentrated cluster regions with [110]
uniaxial anisotropy is likely the predominant cause of the enhancement in [110]
uniaxial anisotropy at the high hole concentration regime. We can clearly rule
out anisotropic lattice strain as a possible origin of the switching of the
magnetic anisotropy.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
A Method of Mass Measurement in Black Hole Binaries Using Timing and High Resolution X-ray Spectroscopy
In X-ray binaries, several percent of the compact object luminosity is
intercepted by the surface of the normal companion and re-radiated through
Compton reflection and the K-fluorescence. This reflected emission follows the
variability of the compact object with a delay approximately equal to the
orbital radius divided by the speed of light. This provides the possibility of
measuring the orbital radius and thus substantially refining the compact object
mass determination compared to using optical data alone. We demonstrate that it
may be feasible to measure the time delay between the direct and reflected
emission using cross-correlation of the light curves observed near the Kalpha
line and above the K-edge of neutral iron. In the case of Cyg X-1, the time
delay measurement is feasible with a 300--1000 ksec observation by a telescope
with a 1000 cm^2 effective area near 6.4 keV and with a ~5eV energy resolution.
With longer exposures, it may be possible to obtain mass constraints even if an
X-ray source in the binary system lacks an optical counterpart.Comment: ApJ Letters, in press. 4 pages, 3 figures, uses emulateapj.st
Infra-red effects of Non-linear sigma model in de Sitter space
We extend our investigation on a possible de Sitter symmetry breaking
mechanism in non-linear sigma models. The scale invariance of the quantum
fluctuations could make the cosmological constant time dependent signaling the
de Sitter symmetry breaking. To understand such a symmetry breaking mechanism,
we investigate the energy-momentum tensor. We show that the leading infra-red
logarithms cancel to all orders in perturbation theory in a generic non-linear
sigma model. When the target space is an N sphere, the de Sitter symmetry is
preserved in the large N limit. For a less symmetric target space, the
infra-red logarithms appear at the three loop level. However there is a counter
term to precisely cancel it. The leading infra-red logarithms do not cancel for
higher derivative interactions. We investigate such a model in which the
infra-red logarithms first appear at the three loop level. A nonperturbative
investigation in the large N limit shows that they eventually grow as large as
the one loop effect.Comment: 39page
Dynamic relaxation of magnetic clusters in a ferromagnetic (Ga,Mn)As epilayer
A new scenario of the mechanism of intriguing ferromagnetic properties in
Mn-doped magnetic semiconductor (Ga,Mn)As is examined in detail. We find that
magnetic features seen in zero-field cooled and field cooled magnetizations are
not interpreted with a single domain model [Phys. Rev. Lett. 95, 217204
(2005)], and the magnetic relaxation, which is similar to that seen in magnetic
particles and granular systems, is becoming significant at temperatures above
the lower-temperature peak in the temperature dependence of ac susceptibility,
supporting the cluster/matrix model reported in our previous work [Phys. Rev.
Lett. 94, 147203 (2005)]. Cole-Cole analysis reveals that magnetic interactions
between such (Ga,Mn)As clusters are significant at temperatures below the
higher-temperature peak in the temperature dependent ac susceptibility. The
magnetizations of these films disappear above the temperature showing the
higher-temperature peak, which is generally referred to as the Curie
temperature. However, we suggest that these combined results are evidence that
the temperature is actually the blocking temperature of (Ga,Mn)As clusters with
a relatively high hole concentration compared to the (Ga,Mn)As matrix.Comment: 8 pages, 7 figures, to appear in Phys. Rev.
High Resolution Spectroscopy of the X-ray Photoionized Wind in Cygnus X-3 with the Chandra High Energy Transmission Grating Spectrometer
We present a preliminary analysis of the 1--10 keV spectrum of the massive
X-ray binary Cyg X-3, obtained with the High Energy Transmission Grating
Spectrometer on the Chandra X-ray Observatory. The source reveals a richly
detailed discrete emission spectrum, with clear signatures of
photoionization-driven excitation.
Among the spectroscopic novelties in the data are the first astrophysical
detections of a number of He-like 'triplets' (Si, S, Ar) with emission line
ratios characteristic of photoionization equilibrium, fully resolved narrow
radiative recombination continua of Mg, Si, and S, the presence of the H-like
Fe Balmer series, and a clear detection of a ~ 800 km/s large scale velocity
field, as well as a ~1500 km/s FWHM Doppler broadening in the source. We
briefly touch on the implications of these findings for the structure of the
Wolf-Rayet wind.Comment: 11 pages, 3 figures; Accepted for publication in ApJ Letter
New Measurements of Orbital Period Change in Cygnus X-3
A nonlinear nature of the binary ephemeris of Cygnus X-3 indicates either a
change in the orbital period or an apsidal motion of the orbit. We have made
extended observations of Cygnus X-3 with the Pointed Proportional Counters
(PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1999 July 3-13
and October 11-14. Using the data from these observations and the archival data
from ROSAT, ASCA, BeppoSAX and RXTE, we have extended the data base for this
source. Adding these new arrival time measurements to the published results, we
make a comparison between the various possibilities, (a) orbital decay due to
mass loss from the system, (b) mass transfer between the stars, and (c) apsidal
motion of the orbit due to gravitational interaction between the two
components. Orbital decay due to mass loss from the companion star seems to be
the most probable scenario.Comment: 7 pages, 4 figures, accepted for publication in A&
Evidence for Doppler-Shifted Iron Emission Lines in Black Hole Candidate 4U 1630-47
We report the first detection of a pair of correlated emission lines in the
X-ray spectrum of black hole candidate 4U 1630-47 during its 1996 outburst,
based on RXTE observations of the source. At the peak plateau of the outburst,
the emission lines are detected, centered mostly at 5.7 keV and 7.7
keV, respectively, while the line energies exhibit random variability 5%.
Interestingly, the lines move in a concerted manner to keep their separation
roughly constant. The lines also vary greatly in strength, but with the
lower-energy line always much stronger than the higher-energy one. The measured
equivalent width ranges from 50 eV to 270 eV for the former, and
from insignificant detection to 140 eV for the latter; the two are
reasonably correlated.
The correlation between the lines implies a causal connection --- perhaps
they share a common origin. Both lines may arise from a single
line of highly ionized iron that is Doppler-shifted either in a Keplerian
accretion disk or in a bi-polar outflow or even both. In both scenarios, a
change in the line energy might simply reflect a change in the ionization state
of line-emitting matter. We discuss the implication of the results and also
raise some questions about such interpretations.Comment: To appear in Ap
Inclination Effects and Beaming in Black Hole X-ray Binaries
We investigate the dependence of observational properties of black hole X-ray
binaries on the inclination angle i of their orbits. We find the following: (1)
Transient black hole binaries show no trend in their quiescent X-ray
luminosities as a function of i, suggesting that the radiation is not
significantly beamed. This is consistent with emission from an accretion disk.
If the X-rays are from a jet, then the Lorentz factor gamma of the jet is less
than 1.24 at the 90% confidence level. (2) The X-ray binary 4U1543-47 with i of
order 21 degrees has a surprisingly strong fluorescent iron line in the high
soft state. Quantifying an earlier argument by Park et al. (2004), we conclude
that if the continuum X-ray emission in this source is from a jet, then gamma <
1.04. (3) None of the known binaries has cos i 75 degrees. This
fact, plus the lack of eclipses among the 20 black hole binaries in our sample,
strongly suggests at the 99.5% confidence level that systems with large
inclination angles are hidden from view. The obscuration could be the result of
disk flaring, as suggested by Milgrom (1978) for neutron star X-ray binaries.
(4) Transient black hole binaries with i ~ 70-75 degrees have significantly
more complex X-ray light curves than systems with i < 65 degrees. This may be
the result of variable obscuration and/or variable height above the disk of the
radiating gas.Comment: 26 pages, to appear in The Astrophysical Journal, vol. 624, May 1,
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