676 research outputs found
Extracting black-hole rotational energy: The generalized Penrose process
In the case involving particles the necessary and sufficient condition for
the Penrose process to extract energy from a rotating black hole is absorption
of particles with negative energies and angular momenta. No torque at the
black-hole horizon occurs. In this article we consider the case of arbitrary
fields or matter described by an unspecified, general energy-momentum tensor
and show that the necessary and sufficient condition for
extraction of a black hole's rotational energy is analogous to that in the
mechanical Penrose process: absorption of negative energy and negative angular
momentum. We also show that a necessary condition for the Penrose process to
occur is for the Noether current (the conserved energy-momentum density vector)
to be spacelike or past directed (timelike or null) on some part of the
horizon. In the particle case, our general criterion for the occurrence of a
Penrose process reproduces the standard result. In the case of relativistic
jet-producing "magnetically arrested disks" we show that the negative energy
and angular-momentum absorption condition is obeyed when the Blandford-Znajek
mechanism is at work, and hence the high energy extraction efficiency up to
found in recent numerical simulations of such accretion flows
results from tapping the black hole's rotational energy through the Penrose
process. We show how black-hole rotational energy extraction works in this case
by describing the Penrose process in terms of the Noether current.Comment: 24 pages, 14 figures, version published in Phys. Rev.
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
XMM-Newton observations of two black hole X-ray transients in quiescence
We report on XMM-Newton observations of GRO J1655-40 and GRS 1009-45, which
are two black hole X-ray transients currently in their quiescent phase. GRO
J1655-40 was detected with a 0.5 - 10 keV luminosity of 5.9 10^{31} erg/s. This
luminosity is comparable to a previous Chandra measurement, but ten times lower
than the 1996 ASCA value, most likely obtained when the source was not yet in a
true quiescent state. Unfortunately, XMM-Newton failed to detect GRS 1009-45. A
stringent upper limit of 8.9 10^{30} erg/s was derived by combining data from
the EPIC-MOS and PN cameras.
The X-ray spectrum of GRO J1655-40 is very hard as it can be fitted with a
power law model of photon index ~ 1.3 +/- 0.4. Similarly hard spectra have been
observed from other systems; these rule out coronal emission from the secondary
or disk flares as the origin of the observed X-rays. On the other hand, our
observations are consistent with the predictions of the disc instability model
in the case that the accretion flow forms an advection dominated accretion flow
(ADAF) at distances less than a fraction ~ 0.1 - 0.3) of the circularization
radius. This distance corresponds to the greatest extent of the ADAF that is
thought to be possible.Comment: 6 pages, 4 figures. Submitted to Astronomy and Astrophysic
Subordinated Langevin Equations for Anomalous Diffusion in External Potentials - Biasing and Decoupled Forces
The role of external forces in systems exhibiting anomalous diffusion is
discussed on the basis of the describing Langevin equations. Since there exist
different possibilities to include the effect of an external field the concept
of {\it biasing} and {\it decoupled} external fields is introduced.
Complementary to the recently established Langevin equations for anomalous
diffusion in a time-dependent external force-field [{\it Magdziarz et al.,
Phys. Rev. Lett. {\bf 101}, 210601 (2008)}] the Langevin formulation of
anomalous diffusion in a decoupled time-dependent force-field is derived
The origin and fate of short-period low-mass black-hole binaries
We present results of a population synthesis study for semidetached short
orbital period binaries which contain low-mass(<1.5 Msun) donors and black hole
(>4 Msun) accretors. Evolution of these binaries is determined by nuclear
evolution of the donors and/or orbital angular momentum loss due to magnetic
braking by the stellar wind of the donors and gravitational wave radiation.
According to our model, the estimated total number of this type of black-hole
binaries in the Galaxy is about 10000. If the magnetic braking is described by
the Verbunt & Zwaan formula, the model predicts around 3000 transient systems
with periods >2 hours and around 300 luminous stable systems with periods
between 3 and 8 hours. Several dozens of these bright systems should be above
the RXTE ASM sensitivity limit. The absence of such systems implies that
angular momentum losses are reduced by a factor more than 2 with respect to the
Verbunt & Zwaan prescription. We show that it is unlikely that the transient
behaviour of black-hole short-period X-ray binaries is explained by the evolved
nature of the stellar companion. A substantial fraction of black-hole binaries
with periods >3 hours could be faint with truncated, stable cold accretion
discs as proposed by Menou et al. Most of the semidetached black-hole binaries
are expected to have periods shorter than ~2 hours. Properties of such, still
to be observed, very small mass-ratio (q<0.02) binaries are different from
those of their longer period cousins.Comment: 13 pages, 6 figures, accepted for publication in A&
Three Additional Quiescent Low-Mass X-ray Binary Candidates in 47 Tucanae
We identify through their X-ray spectra one certain (W37) and two probable
(W17 and X4) quiescent low-mass X-ray binaries (qLMXBs) containing neutron
stars in a long Chandra X-ray exposure of the globular cluster 47 Tucanae, in
addition to the two previously known qLMXBs. W37's spectrum is dominated by a
blackbody-like component consistent with radiation from the hydrogen atmosphere
of a 10 km neutron star. W37's lightcurve shows strong X-ray variability which
we attribute to variations in its absorbing column depth, and eclipses with a
probable 3.087 hour period. For most of our exposures, W37's blackbody-like
emission (assumed to be from the neutron star surface) is almost completely
obscured, yet some soft X-rays (of uncertain origin) remain. Two additional
candidates, W17 and X4, present X-ray spectra dominated by a harder component,
fit by a power-law of photon index ~1.6-3. An additional soft component is
required for both W17 and X4, which can be fit with a 10 km hydrogen-atmosphere
neutron star model. X4 shows significant variability, which may arise from
either its power-law or hydrogen-atmosphere spectral component. Both W17 and X4
show rather low X-ray luminosities, Lx(0.5-10 keV)~5*10^{31} ergs/s. All three
candidate qLMXBs would be difficult to identify in other globular clusters,
suggesting an additional reservoir of fainter qLMXBs in globular clusters that
may be of similar numbers as the group of previously identified objects. The
number of millisecond pulsars inferred to exist in 47 Tuc is less than 10 times
larger than the number of qLMXBs in 47 Tuc, indicating that for typical
inferred lifetimes of 10 and 1 Gyr respectively, their birthrates are
comparable.Comment: Accepted for publication in ApJ. 13 pages, 7 figures (2 color
A Population of Faint Non-Transient Low Mass Black Hole Binaries
We study the thermal and viscous stability of accretion flows in Low Mass
Black Hole Binaries (LMBHBs). We consider a model in which an inner
advection-dominated accretion flow (ADAF) is surrounded by a geometrically thin
accretion disk, the transition between the two zones occurring at a radius
R_tr. In all the known LMBHBs, R_tr appears to be such that the outer disks
could suffer from a global thermal-viscous instability. This instability is
likely to cause the transient behavior of these systems. However, in most
cases, if R_tr were slightly larger than the estimated values, the systems
would be globally stable. This suggests that a population of faint persistent
LMBHBs with globally stable outer disks could be present in the Galaxy. Such
LMBHBs would be hard to detect because they would lack large amplitude
outbursts, and because their ADAF zones would have very low radiative
efficiencies, making the systems very dim. We present model spectra of such
systems covering the optical and X-ray bands.Comment: LateX, 37 pages, 11 figures; Accepted for publication in The
Astrophysical Journa
Black Hole and Neutron Star Transients in Quiescence
We consider the X-ray luminosity difference between neutron star and black
hole soft X-ray transients (NS and BH SXTs) in quiescence. The current
observational data suggest that BH SXTs are significantly fainter than NS SXTs.
The luminosities of quiescent BH SXTs are consistent with the predictions of
binary evolution models for the mass transfer rate if (1) accretion occurs via
an ADAF in these systems and (2) the accreting compact objects have event
horizons. The luminosities of quiescent NS SXTs are not consistent with the
predictions of ADAF models when combined with binary evolution models, unless
most of the mass accreted in the ADAF is prevented from reaching the neutron
star surface. We consider the possibility that mass accretion is reduced in
quiescent NS SXTs because of an efficient propeller and develop a model of the
propeller effect that accounts for the observed luminosities. We argue that
modest winds from ADAFs are consistent with the observations while strong winds
are probably not.Comment: LateX, 37 pages, 7 figures; Accepted for publication in The
Astrophysical Journa
Resonances of the cusp family
We study a family of chaotic maps with limit cases the tent map and the cusp
map (the cusp family). We discuss the spectral properties of the corresponding
Frobenius--Perron operator in different function spaces including spaces of
analytic functions. A numerical study of the eigenvalues and eigenfunctions is
performed.Comment: 14 pages, 3 figures. Submitted to J.Phys.
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