Publication date: Jun 2015
Abstract:
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
≡ cJ/GM2 ≲ 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).
Authors:
Šrámková, E.; Török, G.; Kotrlová, A.; Bakala, P.; Abramowicz, M. A.; Stuchlík, Z.; Goluchová, K.; Kluźniak, W.;