Testing the rotating hot spot model using X-ray burst oscillations from 4U 1636-536. (arXiv:1303.0248v1 [astro-ph.HE])

Testing the rotating hot spot model using X-ray burst oscillations from 4U 1636-536. (arXiv:1303.0248v1 [astro-ph.HE]):
Precise and accurate measurements of neutron star masses and radii would
provide valuable information about the still uncertain properties of cold
matter at supranuclear densities. One promising approach to making such
measurements involves analysis of the X-ray flux oscillations often seen during
thermonuclear (type 1) X-ray bursts. These oscillations are almost certainly
produced by emission from hotter regions on the stellar surface modulated by
the rotation of the star. One consequence of the rotation is that the
oscillation should appear earlier at higher photon energies than at lower
energies. Ford (1999) found compelling evidence for such a hard lead in the
tail oscillations of one type 1 burst from Aql X-1. We have therefore analyzed
individually the oscillations observed in the tails of the four type 1 bursts
from 4U 1636-536 that, when averaged, provided the strongest evidence for a
soft lead in the analysis by Muno et al. (2003). We have also analyzed the
oscillation observed during the superburst from this star. We find that the
data from these five bursts, treated both individually and jointly, are fully
consistent with a rotating hot spot model. Unfortunately, the uncertainties in
these data are too large to provide interesting constraints on the mass and
radius of this star.

A soft X-ray reverberation lag in ESO 113-G010. (arXiv:1210.7874v1 [astro-ph.HE])

A soft X-ray reverberation lag in ESO 113-G010. (arXiv:1210.7874v1 [astro-ph.HE]):
Reverberation lags have recently been discovered in a handful of nearby,
variable AGN. Here, we analyze a ~100 ksec archival XMM-Newton observation of
the highly variable AGN, ESO 113-G010 in order to search for lags between hard
(1.5 - 4.5 keV) and soft (0.3 - 0.9 keV) energy bands. At the lowest
frequencies available in the lightcurve, we find hard lags where the power-law
dominated hard band lags the soft band where the reflection fraction is high.
However, at higher frequencies in the range 2E-4 - 3E-4 Hz we find a soft lag
of 325 +/- 89 seconds at greater than the 3.5-sigma level. The general
evolution from hard to soft lags as the frequency increases is similar to other
AGN where soft lags have been detected. We interpret this soft lag as due to
reverberation, with the reflection component responding to variability in the
power-law. For a black hole mass of 7E6 M_solar this corresponds to a
light-crossing time of ~9 GM/c^3, however, dilution effects mean that the
intrinsic lag is likely longer than this. Based on recent black hole
mass-scaling for lag properties, the lag amplitude and frequency are more
consistent with a black hole a few times more massive than the best estimates,
though flux-dependent effects could easily add scatter this large.

Low-frequency QPO from the 11 Hz accreting pulsar in Terzan 5: not frame dragging. (arXiv:1210.1494v1 [astro-ph.HE])

Low-frequency QPO from the 11 Hz accreting pulsar in Terzan 5: not frame dragging. (arXiv:1210.1494v1 [astro-ph.HE]):
We report on 6 RXTE observations taken during the 2010 outburst of the 11 Hz
accreting pulsar IGR J17480-2446 located in the globular cluster Terzan 5.
During these observations we find power spectra which resemble those seen in
Z-type high-luminosity neutron star low-mass X-ray binaries, with a
quasi-periodic oscillation (QPO) in the 35-50 Hz range simultaneous with a kHz
QPO and broad band noise. Using well known frequency-frequency correlations, we
identify the 35-50 Hz QPOs as the horizontal branch oscillations (HBO), which
were previously suggested to be due to Lense-Thirring precession. As IGR
J17480-2446 spins more than an order of magnitude more slowly than any of the
other neutron stars where these QPOs were found, this QPO can not be explained
by frame dragging. By extension, this casts doubt on the Lense-Thirring
precession model for other low-frequency QPOs in neutron-star and perhaps even
black-hole systems.

The effect of frame dragging on the iron K alpha line in X-ray binaries. (arXiv:1208.0728v1 [astro-ph.HE])

The effect of frame dragging on the iron K alpha line in X-ray binaries. (arXiv:1208.0728v1 [astro-ph.HE]):
The clear characteristic timescale picked out by the low frequency
quasi-periodic oscillations (QPOs) seen in many black hole and neutron star
binaries has the potential to provide a very powerful diagnostic of the inner
regions of the accretion flow. However, this potential cannot be realised
without a quantitative model for the QPO. We have recently shown that the same
truncated disc/hot inner flow geometry which is used to interpret the spectral
transitions can also directly produce the QPO from Lense-Thirring (vertical)
precession of the hot inner flow. This correctly predicts both the frequency
and spectrum of the QPO, and the tight correlation of these properties with the
total spectrum of the source via a changing truncation radius between the disc
and hot flow. This model predicts a unique iron line signature as a vertically
tilted flow illuminates different azimuths of the disc as it precesses. The
iron line arising from this rotating illumination is blue shifted when the flow
irradiates the approaching region of the spinning disc and red shifted when the
flow irradiates the receding region of the disc. This gives rise to a
characteristic rocking of the iron line on the QPO frequency which is a
necessary (and probably sufficient) test of a Lense-Thirring origin. This is
also an independent test of disc truncation models for the low/hard state, as
vertical precession cannot occur if there is a disc in the midplane.

We show that it may be possible to observe this effect using archival data
from the Rossi X-ray timing explorer (RXTE) or XMM Newton. However, a clean
test requires a combination of moderate resolution and good statistics, such as
would be available from a long XMM-Newton observation or with data from the
proposed ESA mission LOFT.

High-Frequency Quasi-Periodic Oscillations in black-hole binaries. (arXiv:1207.2311v1 [astro-ph.HE])

High-Frequency Quasi-Periodic Oscillations in black-hole binaries. (arXiv:1207.2311v1 [astro-ph.HE]):
We present the results of the analysis of a large database of X-ray
observations of 22 galactic black-hole transients with the Rossi X-Ray timing
explorer throughout its operative life for a total exposure time of ~12 Ms. We
excluded persistent systems and the peculiar source GRS 1915+105, as well as
the most recently discovered sources. The semi-automatic homogeneous analysis
was aimed at the detection of high-frequency (100-1000 Hz) quasi-periodic
oscillations (QPO), of which several cases were previously reported in the
literature. After taking into account the number of independent trials, we
obtained 11 detections from two sources only: XTE J1550-564 and GRO J1655-40.
For the former, the detected frequencies are clustered around 180 Hz and 280
Hz, as previously found. For the latter, the previously-reported dichotomy
300-450 Hz is found to be less sharp. We discuss our results in comparison with
kHz QPO in neutron-star X-ray binaries and the prospects for future timing
X-ray missions.