92 research outputs found
The Periods Discovered by RXTE in Thermonuclear Flash Bursts
Oscillations in the X-ray flux of thermonuclear X-ray bursts have been
observed with RXTE from at least 6 low-mass binaries, at frequencies from 330
Hz to 589 Hz. There appear to be preferred relations between the frequencies
present during the bursts and those seen in the persistent flux. The amplitude
of the oscillations can exceed 50 % near burst onset. Except for a systematic
increase in oscillation frequency as the burst progresses, the frequency is
stable. Time resolved spectra track increases in the X-ray emitting area due to
propagation of the burning front over the neutron star surface, as well as
radiation driven expansion of the photosphere. The neutron star mass, radius,
and distance can be inferred when spectra are compared to theoretical
expectations.Comment: 6 pages, 5 figures, to be published in Proceedings of the Symposium
"The Active X-Ray Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy,
21-24 October, 1997, Nuclear Physics B Proceedings Supplements. Eds. L.
Scarsi, H. Bradt, P. Giommi, and F. Fior
High frequency oscillations during magnetar flares
The recent discovery of high frequency oscillations during giant flares from
the Soft Gamma Repeaters SGR 1806-20 and SGR 1900+14 may be the first direct
detection of vibrations in a neutron star crust. If this interpretation is
correct it offers a novel means of testing the neutron star equation of state,
crustal breaking strain, and magnetic field configuration. We review the
observational data on the magnetar oscillations, including new timing analysis
of the SGR 1806-20 giant flare using data from the Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) and the Rossi X-ray Timing Explorer (RXTE). We
discuss the implications for the study of neutron star structure and crust
thickness, and outline areas for future investigation.Comment: 5 pages, 1 figure, to appear in the proceedings of the conference
"Isolated Neutron Stars: from the Interior to the Surface" (April 2006,
London), eds. D. Page, R. Turolla, & S. Zane, Astrophysics & Space Science in
pres
A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105
(abridged) In this paper we extend the idea suggested previously by Petri
(2005a,b) that the high frequency quasi-periodic oscillations observed in
low-mass X-ray binaries may be explained as a resonant oscillation of the
accretion disk with a rotating asymmetric background (gravitational or
magnetic) field imposed by the compact object. Here, we apply this general idea
to black hole binaries. It is assumed that a test particle experiences a
similar parametric resonance mechanism such as the one described in paper I and
II but now the resonance is induced by the interaction between a spiral density
wave in the accretion disk, excited close to the innermost stable circular
orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry
to deduce the characteristic frequencies of this test particle. The response of
the test particle is maximal when the frequency ratio of the two strongest
resonances is equal to 3:2 as observed in black hole candidates. Finally,
applying our model to the microquasar GRS 1915+105, we reproduce the correct
value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz
features in the power spectrum time analysis is predicted. Moreover, based only
on the two HF-QPO frequencies, our model is able to constrain the mass and angular momentum of the accreting black hole.Comment: Accepted for publication in Astrophysics & Space Scienc
QPOs: Einstein's gravity non-linear resonances
There is strong evidence that the observed kHz Quasi Periodic Oscillations
(QPOs) in the X-ray flux of neutron star and black hole sources in LMXRBs are
linked to Einstein's General Relativity. Abramowicz&Klu\'zniak (2001) suggested
a non-linear resonance model to explain the QPOs origin: here we summarize
their idea and the development of a mathematical toy-model which begins to
throw light on the nature of Einstein's gravity non-linear oscillations.Comment: Proceeding of the Einstein's Legacy, Munich 200
Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
The X-ray spectral and timing properties of ultraluminous X-ray sources
(ULXs) have many similarities with the very high state of stellar-mass black
holes (power-law dominated, at accretion rates greater than the Eddington
rate). On the other hand, their cool disk components, large characteristic
inner-disk radii and low characteristic timescales have been interpreted as
evidence of black hole masses ~ 1000 Msun (intermediate-mass black holes). Here
we re-examine the physical interpretation of the cool disk model, in the
context of accretion states of stellar-mass black holes. In particular, XTE
J1550-564 can be considered the missing link between ULXs and stellar-mass
black holes, because it exhibits a high-accretion-rate, low-disk-temperature
state (ultraluminous branch). On the ultraluminous branch, the accretion rate
is positively correlated with the disk truncation radius and the bolometric
disk luminosity, while it is anti-correlated with the peak temperature and the
frequency of quasi-periodic-oscillations. Two prototypical ULXs (NGC1313 X-1
and X-2) also seem to move along that branch. We use a phenomenological model
to show how the different range of spectral and timing parameters found in the
two classes of accreting black holes depends on both their masses and accretion
rates. We suggest that ULXs are consistent with black hole masses ~ 50-100
Msun, moderately inefficiently accreting at ~20 times Eddington.Comment: 11 pages, accepted for publication in Astrophysics and Space Science.
Based on work presented at the Fifth Stromlo Symposium, Australian National
University, Dec 200
Plasma Magnetosphere Formation Around Oscillating Magnetized Neutron Stars
The notion of death line of rotating pulsars is applied to model of
oscillating neutron stars. It is shown that the magnetosphere of typical
non-rotating oscillating stars may not contain secondary plasma to support the
generation of radio emission in the region of open field lines of plasma
magnetosphere.Comment: Accepted for publication in Astrophysics & Space Scienc
Fast variability from black-hole binaries
Currently available information on fast variability of the X-ray emission
from accreting collapsed objects constitutes a complex phenomenology which is
difficult to interpret. We review the current observational standpoint for
black-hole binaries and survey models that have been proposed to interpret it.
Despite the complex structure of the accretion flow, key observational
diagnostics have been identified which can provide direct access to the
dynamics of matter motions in the close vicinity of black holes and thus to the
some of fundamental properties of curved spacetimes, where strong-field general
relativistic effects can be observed.Comment: 20 pages, 11 figures. Accepted for publication in Space Science
Reviews. Also to appear in hard cover in the Space Sciences Series of ISSI
"The Physics of Accretion onto Black Holes" (Springer Publisher
NICER observations of MAXI J1820+070 suggest a rapidly-brightening black hole X-ray binary in the hard state
NICER observed the new X-ray transient MAXI J1820+070 (ATel #11399, #11400, #11403, #11404, #11406, #11418, #11420, #11421) on multiple occasions from 2018 March 12 to 14. & nbsp;During this time the source brightened rapidly, from a total NICER mean count rate of 880 count/s on March 12 to 2800 count/s by March 14 17:00 & nbsp;UTC, corresponding to a change in 2-10 keV modelled flux (see below) from 1.9E-9 to 5E-9 erg cm-2 s-1. & nbsp; The broadband X-ray spectrum is absorbed by a low column density (fitting the model given below, we obtain 1.5E21 cm-2), in keeping with the low Galactic column in the direction of the source (ATel #11418; Dickey & Lockman, 1990, ARAA, 28, 215; Kalberla et al. 2005, A &A, 440, 775) and consists of a hard power-law component with weak reflection features (broad iron line and narrow 6.4 keV line core) and an additional soft X-ray component
Simultaneous NICER and NuSTAR observations of the Ultracompact X-ray Binary 4U 0614+091
We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary 4U 0614+091. This source shows quasiperiodic flux variations on the timescale of âŒdays. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios
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