7,060 research outputs found
Quasi-Periodic Oscillations and energy spectra from the two brightest Ultra-Luminous X-ray sources in M82
Ultra-Luminous X-ray sources are thought to be accreting black holes that
might host Intermediate Mass Black Holes (IMBH), proposed to exist by
theoretical studies, even though a firm detection (as a class) is still
missing. The brightest ULX in M82 (M82 X-1) is probably one of the best
candidates to host an IMBH. In this work we analyzed the data of the recent
release of observations obtained from M82 X-1 taken by XMM-Newton. We performed
a study of the timing and spectral properties of the source. We report on the
detection of (46+-2) mHz Quasi-Periodic Oscillations (QPOs) in the power
density spectra of two observations. A comparison of the frequency of these
high-frequency QPOs with previous detections supports the 1:2:3 frequency
distribution as suggested in other studies. We discuss the implications if the
(46+-2) mHz QPO detected in M82 X-1 is the fundamental harmonic, in analogy
with the High-Frequency QPOs observed in black hole binaries. For one of the
observations we have detected for the first time a QPO at 8 mHz (albeit at a
low significance), that coincides with a hardening of the spectrum. We suggest
that the QPO is a milli-hertz QPO originating from the close-by transient ULX
M82 X-2, with analogies to the Low-Frequency QPOs observed in black hole
binaries.Comment: 9 pages (with 4 figures and 4 tables). Accepted for publication in
MNRAS (26/09/13
Ultraluminous X-ray sources with flat-topped noise and QPO
We analyzed the X-ray power density spectra of five ultraluminous X-ray
sources (ULXs) NGC5408 X-1, NGC6946 X-1, M82 X-1, NGC1313 X-1 and IC342 X-1
that are the only ULXs which display both flat-topped noise (FTN) and
quasi-periodic oscillations (QPO). We studied the QPO frequencies, fractional
root-mean-square (rms) variability, X-ray luminosity and spectral hardness. We
found that the level of FTN is anti-correlated with the QPO frequency. As the
frequency of the QPO and brightness of the sources increase, their fractional
variability decreases. We propose a simple interpretation using the
spherizarion radius, viscosity time and -parameter as basic properties
of these systems. The main physical driver of the observed variability is the
mass accretion rate which varies >3 between different observations of the same
source. As the accretion rate decreases the spherization radius reduces and the
FTN plus the QPO move toward higher frequencies resulting in a decrease of the
fractional rms variability. We also propose that in all ULXs when the accretion
rate is low enough (but still super-Eddington) the QPO and FTN disappear.
Assuming that the maximum X-ray luminosity depends only on the black hole (BH)
mass and not on the accretion rate (not considering the effects of either the
inclination of the super-Eddington disc nor geometrical beaming of radiation)
we estimate that all the ULXs have about similar BH masses, with the exception
of M82 X-1, which might be 10 times more massive.Comment: 15 pages, 7 figures, accepted for publication in MNRA
The INTEGRAL-OMC Scientific Archive
The Optical Monitoring Camera (OMC) on-board the INTEGRAL satellite has, as
one of its scientific goals, the observation of a large number of variable
sources previously selected. After almost 6 years of operations, OMC has
monitored more than 100 000 sources of scientific interest. In this
contribution we present the OMC Scientific Archive
(http://sdc.laeff.inta.es/omc/) which has been developed to provide the
astronomical community with a quick access to the light curves generated by
this instrument.We describe the main characteristics of this archive, as well
as important aspects for the users: object types, temporal sampling of light
curves and photometric accuracy.Comment: 4 pages, 5 figures. "Highlights of Spanish Astrophysics V"
Proceedings of the VIII Scientific Meeting of the Spanish Astronomical
Society (SEA) held in Santander, July 7-11, 200
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