10 research outputs found
Confronting the models of 3:2 quasiperiodic oscillations with the rapid spin of the microquasar GRS 1915+105
Spectral fitting of the spin a in the microquasar GRS 1915+105 estimate
values higher than a=0.98. However, there are certain doubts about this
(nearly) extremal number. Confirming a high value of a>0.9 would have
significant concequences for the theory of high-frequency quasiperiodic
oscillations (HF QPOs). Here we discuss its possible implications assuming
several commonly used orbital models of 3:2 HF QPOs. We show that the estimate
of a>0.9 is almost inconsistent with two hot-spot (relativistic precession and
tidal disruption) models and the warped disc resonance model. In contrast, we
demonstrate that the epicyclic resonance and discoseismic models assuming the
c- and g- modes are favoured. We extend our discussion to another two
microquasars that display the 3:2 HF QPOs. The frequencies of these QPOs scale
roughly inversely to the microquasar masses, and the differences in the
individual spins, such as a=0.9 compared to a=0.7, represent a generic problem
for most of the discussed geodesic 3:2 QPO models. To explain the observations
of all the three microquasars by one unique mechanism, the models would have to
accommodate very large non-geodesic corrections.Comment: 7 pages, 3 figures, 2 tables; v2: corrections in the introduction,
language corrections (Astronomy & Astrophysics proof-readed version
Black hole spin inferred from 3:2 epicyclic resonance model of high-frequency quasi-periodic oscillations
Estimations of black hole spin in the three Galactic microquasars GRS
1915+105, GRO J1655-40, and XTE J1550-564 have been carried out based on
spectral and timing X-ray measurements and various theoretical concepts. Among
others, a non-linear resonance between axisymmetric epicyclic oscillation modes
of an accretion disc around a Kerr black hole has been considered as a model
for the observed high-frequency quasi-periodic oscillations (HF QPOs).
Estimates of spin predicted by this model have been derived based on the
geodesic approximation of the accreted fluid motion. Here we assume accretion
flow described by the model of a pressure-supported torus and carry out related
corrections to the mass-spin estimates. We find that for dimensionless black
hole spin a<0.9, the resonant eigenfrequencies are very close to those
calculated for the geodesic motion. Their values slightly grow with increasing
torus thickness. These findings agree well with results of a previous study
carried out in the pseudo-Newtonian approximation. The situation becomes
different for a>0.9, in which case the resonant eigenfrequencies rapidly
decrease as the torus thickness increases. We conclude that the assumed
non-geodesic effects shift the lower limit of the spin, implied for the three
microquasars by the epicyclic model and independently measured masses, from
a~0.7 to a~0.6. Their consideration furthermore confirms compatibility of the
model with the rapid spin of GRS 1915+105 and provides highly testable
predictions of the QPO frequencies. Individual sources with a moderate spin
(a<0.9) should exhibit a smaller spread of the measured 3:2 QPO frequencies
than sources with a near-extreme spin (a~1). This should be further examined
using the large amount of high-resolution data expected to become available
with the next generation of X-ray instruments, such as the proposed Large
Observatory for X-ray Timing (LOFT).Comment: 6 pages, 4 figures, accepted by Astronomy & Astrophysic
Black hole tidal charge constrained by strong gravitational lensing
Spherically symmetric brane black holes have tidal charge, which modifies
both weak and strong lensing characteristics. Even if lensing measurements are
in agreement with a Schwarzschild lens, the margin of error of the detecting
instrument allows for a certain tidal charge. In this paper we derive the
respective constraint on the tidal charge of the supermassive black hole (SMBH)
in the center of our galaxy, from the radius of the first relativistic Einstein
ring, emerging in strong lensing. We find that even if general relativistic
predictions are confirmed by high precision strong lensing measurements, SMBHs
could have a much larger tidal charge, than the Sun or neutron stars