739 research outputs found
Where is the Inner Edge of an Accretion Disk Around a Black Hole?
What is meant by the "inner edge" of an accretion disk around a black hole
depends on the property that defines the edge. We discuss four such definitions
using data from recent high-resolution numerical simulations. These are: the
"turbulence edge", where flux-freezing becomes more important than turbulence
in determining the magnetic field structure; the "stress edge", where plunging
matter loses dynamical contact with the outer accretion flow; the "reflection
edge", the smallest radius capable of producing significant X-ray reflection
features; and the "radiation edge", the innermost place from which significant
luminosity emerges. All these edges are dependent on the accretion rate and are
non-axisymmetric and time-variable. Although all are generally located in the
vicinity of the marginally stable orbit, significant displacements can occur,
and data interpretations placing the disk edge precisely at this point can be
misleading. If observations are to be used successfully as diagnostics of
accretion in strong gravity, the models used to interpret them must take
careful account of these distinctions.Comment: accepted by Ap.J., 26 p
Chandra deep X-ray observation on the Galactic plane
Using the Chandra ACIS-I instruments, we have carried out the deepest X-ray
observation on a typical Galactic plane region at l 28.5 deg, where no discrete
X-ray sources have been known previously. We have detected, as well as strong
diffuse emission, 275 new point X-ray sources (4 sigma confidence) within two
partially overlapping fields (~250 arcmin^2 in total) down to ~3 x 10^{-15} erg
s^{-1} cm^{-2} (2 -- 10 keV) or ~ 7 x 10^{-16} erg s^{-1} cm^{-2} (0.5 -- 2
keV). We have studied spectral distribution of these point sources, and found
that very soft sources detected only below ~ 3 keV are more numerous than hard
sources detected only above ~ 3 keV. Only small number of sources are detected
both in the soft and hard bands. Surface density of the hard sources is almost
consistent with that at high Galactic regions, thus most of the hard sources
are considered to be Active Galactic Nuclei seen through the milky way. On the
other hand, some of the bright hard X-ray sources which show extremely flat
spectra and iron line or edge features are considered to be Galactic,
presumably quiescent dwarf novae. The soft sources show thermal spectra and
small interstellar hydrogen column densities, and some of them exhibit X-ray
flares. Therefore, most of the soft sources are probably X-ray active nearby
late type stars.Comment: Contribution to the proceedings of the "New Visions of the X-Ray
Universe in the XMM-Newton and Chandra Era" symposium at ESTEC, The
Netherlands. 26-30 Nov. 200
Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities
Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we
have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near
the end of its 2001-2002 outburst after its transition to the low-hard state at
X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source
distance of 4 kpc). Our results include a characterization of the spectral and
timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra
observation to measure the power spectrum at low frequencies. For the 3 epochs
at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum
is consistent with a spectral model consisting of a power-law with interstellar
absorption. We detect evolution in the power-law photon index from 1.66 +/-
0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source
softens at low luminosities. The power spectra are characterized by strong
(20-35% fractional rms) band-limited noise, which we model as a zero-centered
Lorentzian. Including results from an RXTE study of XTE J1650-500 near the
transition to the low-hard state by Kalemci et al. (2003), the half-width of
the zero-centered Lorentzian (roughly where the band-limited noise cuts off)
drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at
L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the
spectral and timing parameters evolve with luminosity, it is notable that the
general shapes of the energy and power spectra remain the same, indicating that
the source stays in the low-hard state. This implies that the X-ray emitting
region of the system likely keeps the same overall structure, while the
luminosity changes by a factor of 470. We discuss how these results may
constrain theoretical black hole accretion models.Comment: 11 pages, accepted by ApJ after minor revision
The Pattern of Correlated X-ray Timing and Spectral Behavior in GRS 1915+105
From data obtained from the PCA in the 2-11 keV and 11-30.5 keV energy range,
GRS 1915+105 is seen during RXTE observations between 1996 May and October on
two separate branches in a hardness intensity diagram. On the hard branch, GRS
1915+105 exhibits narrow quasi-periodic oscillations ranging from 0.5 to 6 Hz
with . The QPOs are observed over intensities
ranging from about 6,000 to 20,000 counts s in the 2 - 12.5 keV energy
band, indicating a strong dependence on source intensity. Strong harmonics are
seen, especially, at lower frequencies. As the QPO frequency increases, the
harmonic feature weakens and disappears. On the soft branch, narrow QPOs are
absent and the low frequency component of the power density spectrum is
approximated by a power-law, with index for low count rates and
for high count rates (\gta 18000 cts/s). Occasionally, a broad
peaked feature in the 1-6 Hz frequency range is also observed on this branch.
The source was probably in the very high state similar to those of other black
hole candidates. Thermal-viscous instabilities in accretion disk models do not
predict the correlation of the narrow QPO frequency and luminosity unless the
fraction of luminosity from the disk decreases with the total luminosity.Comment: ApJ Lett accepte
A Chandra Study of the Dense Globular Cluster Terzan 5
We report a Chandra ACIS-I observation of the dense globular cluster Terzan
5. The previously known transient low-mass x-ray binary (LMXB) EXO 1745-248 in
the cluster entered a rare high state during our August 2000 observation,
complicating the analysis. Nevertheless nine additional sources clearly
associated with the cluster are also detected, ranging from L_X(0.5-2.5
keV)=5.6*10^{32} down to 8.6*10^{31} ergs/s. Their X-ray colors and
luminosities, and spectral fitting, indicate that five of them are probably
cataclysmic variables, and four are likely quiescent LMXBs containing neutron
stars. We estimate the total number of sources between L_X(0.5-2.5 keV)=10^{32}
and 10^{33} ergs/s as 11.4^{+4.7}_{-1.8} by the use of artificial point source
tests, and note that the numbers of X-ray sources are similar to those detected
in NGC 6440. The improved X-ray position allowed us to identify a plausible
infrared counterpart to EXO 1745-248 on our 1998 Hubble Space Telescope NICMOS
images. This blue star (F110W=18.48, F187W=17.30) lies within 0.2'' of the
boresighted LMXB position. Simultaneous Rossi X-ray Timing Explorer (RXTE)
spectra, combined with the Chandra spectrum, indicate that EXO 1745-248 is an
ultracompact binary system, and show a strong broad 6.55 keV iron line and an 8
keV smeared reflection edge.Comment: 18 pages, 8 figures, accepted to Ap
Temporal Properties of Cygnus X-1 During the Spectral Transitions
We report the results from our timing analysis of 15 RXTE observations of
Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be
divided into 3 distinct phases: (1) transition from the hard to soft state, (2)
soft state, and (3) transition from the soft state back to the hard state. The
observed X-ray properties in phases 1 and 3 are remarkably similar, suggesting
that the same physical processes are likely involved in triggering such
transitions. The power density spectrum (PDS) during the transition can be
characterized by a red noise component, followed by a white noise component
which extends to roughly 1-3 Hz where it is cut off, and a steeper power law at
higher frequencies. The X-ray flux also exhibits apparent quasi-periodic
oscillation (QPO) with the centroid frequency varying in the range of 4-12 Hz.
The QPO shows no correlation with the source flux, but becomes more prominent
at higher energies. This type of PDS bears resemblance to that of other black
hole candidates often observed in a so-called very high state, although the
origin of the observed QPO may be very different. The low-frequency red noise
has not been observed in the hard state, thus seems to be positively correlated
with the disk mass accretion rate which is presumably low in the hard state and
high in the soft state; in fact, it completely dominates the PDS in the soft
state. In the framework of thermalComptonization models, Cui et al. (see
astro-ph/9610071 and astro-ph/9610072) speculated that the difference in the
observed spectral and timing properties between the hard and soft states is due
to the presence of a ``fluctuating'' Comptonizing corona during the transition.
Here we present the measured hard X-ray time lags and coherence functions
between various energy bands, and show that the results strongly support such a
scenario.Comment: AASTex file. 29 pages including 11 figures. To appear in Ap
X-ray Nova XTE J1550-564: RXTE Spectral Observations
Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was
provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense
(6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray
source observed with RXTE. We present a spectral analysis utilizing 60
Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a
nearly continuous All Sky Monitor light curve. The spectra were fit to a model
including multicolor blackbody disk and power-law components. XTE J1550-564 is
observed in the very high, high/soft, and intermediate canonical outburst
states of Black Hole X-ray Novae.Comment: 14 pages including 1 table and 4 figures, Accepted to ApJ Letter
Hall Effect in the mixed state of moderately clean superconductors
The Hall conductivity in the mixed state of a clean () type-II
s-wave superconductor is determined from a microscopic calculation within a
quasiclassical approximation. We find that below the superconducting transition
the contribution to the transverse conductivity due to dynamical fluctuations
of the order parameter is compensated by the modification of the quasiparticle
contribution. In this regime the nonlinear behaviour of the Hall angle is
governed by the change in the effective quasiparticle scattering rate due to
the reduction in the density of states at the Fermi level. The connection with
experimental results is discussed
Geometrically Frustrated Crystals: Elastic Theory and Dislocations
Elastic theory of ring-(or cylinder-)shaped crystals is constructed and the
generation of edge dislocations due to geometrical frustration caused by the
bending is studied. The analogy to superconducting (or superfluid) vortex state
is pointed out and the phase diagram of the ring-crystal, which depends on
radius and thickness, is discussed.Comment: 4 pages, 3 figure
- …