209 research outputs found
On the Optical -- X-ray correlation from outburst to quiescence in Low Mass X-ray Binaries: the representative cases of V404 Cyg and Cen X-4
Low mass X-ray binaries (LMXBs) show evidence of a global correlation of
debated origin between X-ray and optical luminosity. We study for the first
time this correlation in two transient LMXBs, the black hole V404 Cyg and the
neutron star Cen X-4, over 6 orders of magnitude in X-ray luminosity, from
outburst to quiescence. After subtracting the contribution from the companion
star, the Cen X-4 data can be described by a single power law correlation of
the form , consistent with disk reprocessing. We
find a similar correlation slope for V404 Cyg in quiescence (0.46) and a
steeper one (0.56) in the outburst hard state of 1989. However, V404 Cyg is
about times optically brighter, at a given keV X-ray
luminosity, compared to Cen X-4. This ratio is a factor of 10 smaller in
quiescence, where the normalization of the V404 Cyg correlation also changes.
We show that once the bolometric X-ray emission is considered and the known
main differences between V404 Cyg and Cen X-4 are taken into account (a larger
compact object mass, accretion disk size, and the presence of a strong jet
contribution in the hard state for the black hole system) the two systems lie
on the same correlation. In V404 Cyg, the jet dominates spectrally at
optical-infrared frequencies during the hard state, but makes a negligible
contribution in quiescence, which may account for the change in its correlation
slope and normalization. These results provide a benchmark to compare with data
from the 2015 outburst of V404 Cyg and, potentially, other transient LMXBs as
well.Comment: Accepted on ApJ, 12 pages, 4 figures, 4 table
Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR
Observational evidence has been accumulating that thermonuclear X-ray bursts
ignited on the surface of neutron stars influence the surrounding accretion
flow. Here, we exploit the excellent sensitivity of NuSTAR up to 79 keV to
analyze the impact of an X-ray burst on the accretion emission of the neutron
star LMXB 4U 1608-52. The ~200 s long X-ray burst occurred during a hard X-ray
spectral state, and had a peak intensity of ~30-50 per cent of the Eddington
limit with no signs of photospheric radius expansion. Spectral analysis
suggests that the accretion emission was enhanced up to a factor of ~5 during
the X-ray burst. We also applied a linear unsupervised decomposition method,
namely non-negative matrix factorization (NMF), to study this X-ray burst. We
find that the NMF performs well in characterizing the evolution of the burst
emission and is a promising technique to study changes in the underlying
accretion emission in more detail than is possible through conventional
spectral fitting. For the burst of 4U 1608-52, the NMF suggests a possible
softening of the accretion spectrum during the X-ray burst, which could
potentially be ascribed to cooling of a corona. Finally, we report a small (~3
per cent) but significant rise in the accretion emission ~0.5 h before the
X-ray burst, although it is unclear whether this was related to the X-ray burst
ignition.Comment: 10 pages, 10 figures, 1 table, to appear in MNRA
The origin of optical emission lines in the soft state of X-ray binary outbursts: the case of MAXI J1820+070
The optical emission line spectra of X-ray binaries (XRBs) are thought to be
produced in an irradiated atmosphere, possibly the base of a wind, located
above the outer accretion disc. However, the physical nature of - and physical
conditions in - the line-forming region remain poorly understood. Here, we test
the idea that the optical spectrum is formed in the transition region between
the cool, geometrically thin part of the disc near the mid-plane and a hot,
vertically extended atmosphere or outflow produced by X-ray irradiation. We
first present a VLT X-Shooter spectrum of XRB MAXI J1820+070 in the soft state
associated with its 2018 outburst, which displays a rich set of double-peaked
hydrogen and helium recombination lines. Aided by ancillary X-ray spectra and
reddening estimates, we then model this spectrum with the Monte Carlo radiative
transfer code Python, using a simple biconical disc wind model inspired by
radiation-hydrodynamic simulations of irradiation-driven outflows from XRB
discs. Such a model can qualitatively reproduce the observed features; nearly
all of the optical emission arising from the transonic 'transition region' near
the base of the wind. In this region, characteristic electron densities are on
the order of 10 cm, in line with the observed flat Balmer
decrement (H/H). We conclude that strong irradiation
can naturally give rise to both the optical line-forming layer in XRB discs and
an overlying outflow/atmosphere that produces X-ray absorption lines.Comment: 17 pages, 11 figures. This article has been accepted for publication
in MNRAS published by Oxford University Press on behalf of the Royal
Astronomical Societ
Events leading up to the June 2015 outburst of V404 Cyg
On 2015 June 15 the burst alert telescope (BAT) on board {\em Swift} detected
an X-ray outburst from the black hole transient V404 Cyg. We monitored V404 Cyg
for the last 10 years with the 2-m Faulkes Telescope North in three optical
bands (V, R, and i). We found that, one week prior to this outburst, the
optical flux was 0.1--0.3 mag brighter than the quiescent orbital modulation,
implying an optical precursor to the X-ray outburst. There is also a hint of a
gradual optical decay (years) followed by a rise lasting two months prior to
the outburst. We fortuitously obtained an optical spectrum of V404 Cyg 13 hours
before the BAT trigger. This too was brighter () than
quiescence, and showed spectral lines typical of an accretion disk, with
characteristic absorption features of the donor being much weaker. No He II
emission was detected, which would have been expected had the X-ray flux been
substantially brightening. This, combined with the presence of intense
H emission, about 7 times the quiescent level, suggests that the disk
entered the hot, outburst state before the X-ray outburst began. We propose
that the outburst is produced by a viscous-thermal instability triggered close
to the inner edge of a truncated disk. An X-ray delay of a week is consistent
with the time needed to refill the inner region and hence move the inner edge
of the disk inwards, allowing matter to reach the central BH, finally turning
on the X-ray emission.Comment: Accepted by ApJ Letter, 7 pages, 5 figure
Wind suppression by X-rays in Cygnus X-3
Context. The radiatively driven wind of the primary star in wind-fed X-ray binaries can be suppressed by the X-ray irradiation of the compact secondary star. This causes feedback between the wind and the X-ray luminosity of the compact star.Aims. We aim to estimate how the wind velocity on the face-on side of the donor star depends on the spectral state of the high-mass X-ray binary Cygnus X-3.Methods. We modeled the supersonic part of the wind by computing the line force (force multiplier) with the Castor, Abbott & Klein formalism and XSTAR physics and by solving the mass conservation and momentum balance equations. We computed the line force locally in the wind considering the radiation fields from both the donor and the compact star in each spectral state. We solved the wind equations at different orbital angles from the line joining the stars and took the effect of wind clumping into account. Wind-induced accretion luminosities were estimated using the Bondi-Hoyle-Lyttleton formalism and computed wind velocities at the compact star. We compared them to those obtained from observations.Results. We found that the ionization potentials of the ions contributing the most to the line force fall in the extreme-UV region (100-230 angstrom). If the flux in this region is high, the line force is weak, and consequently, the wind velocity is low. We found a correlation between the luminosities estimated from the observations for each spectral state of Cyg X-3 and the computed accretion luminosities assuming moderate wind clumping and a low mass of the compact star. For high wind clumping, this correlation disappears. We compared the XSTAR method used here with the comoving frame method and found that they agree reasonably well with each other.Conclusions. We show that soft X-rays in the extreme-UV region from the compact star penetrate the wind from the donor star and diminish the line force and consequently the wind velocity on the face-on side. This increases the computed accretion luminosities qualitatively in a similar manner as observed in the spectral evolution of Cyg X-3 for a moderate clumping volume filling factor and a compact star mass of a few (2-3) solar masses.Peer reviewe
Revealing the X-ray Variability of AGN with Principal Component Analysis
We analyse a sample of 26 active galactic nuclei with deep XMM-Newton
observations, using principal component analysis (PCA) to find model
independent spectra of the different variable components. In total, we identify
at least 12 qualitatively different patterns of spectral variability, involving
several different mechanisms, including five sources which show evidence of
variable relativistic reflection (MCG-6-30-15, NGC 4051, 1H 0707-495, NGC 3516
and Mrk 766) and three which show evidence of varying partial covering neutral
absorption (NGC 4395, NGC 1365, and NGC 4151). In over half of the sources
studied, the variability is dominated by changes in a power law continuum, both
in terms of changes in flux and power law index, which could be produced by
propagating fluctuations within the corona. Simulations are used to find unique
predictions for different physical models, and we then attempt to qualitatively
match the results from the simulations to the behaviour observed in the real
data. We are able to explain a large proportion of the variability in these
sources using simple models of spectral variability, but more complex models
may be needed for the remainder. We have begun the process of building up a
library of different principal components, so that spectral variability in AGN
can quickly be matched to physical processes. We show that PCA can be an
extremely powerful tool for distinguishing different patterns of variability in
AGN, and that it can be used effectively on the large amounts of high-quality
archival data available from the current generation of X-ray telescopes.Comment: 25 pages, 27 figures, accepted to MNRAS. Analysis code available on
request to lead author. Edit: Rogue table remove
The Reoccurrence of mHz QPOs in Cygnus X-3
We have re-analyzed archival RXTE data of the X-ray binary Cygnus X-3 with a
view to investigate the timing properties of the source. As compared to
previous studies, we use an extensive sample of observations that include all
the radio/X-ray spectral states that have been categorized in the source
recently. In this study we identify two additional instances of Quasi-Periodic
Oscillations that have centroid frequencies in the mHz regime. These events are
all associated to a certain extent with major radio flaring, that in turn is
associated with relativistic jet ejection events. We review briefly scenarios
whereby the Quasi-Periodic Oscillations may arise.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Orbital modulation of X-ray emission lines in Cygnus X-3
We address the problem where the X-ray emission lines are formed and
investigate orbital dynamics using Chandra HETG observations, photoionizing
calculations and numerical wind-particle simulations.The observed Si XIV (6.185
A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P
Cygni-type profiles consisting of an emission and a blue-shifted absorption
component. In the models, the emission originates in the photoionized wind of
the WR companion illuminated by a hybrid source: the X-ray radiation of the
compact star and the photospheric EUV-radiation from the WR star. The emission
component exhibits maximum blue-shift at phase 0.5 (when the compact star is in
front), while the velocity of the absorption component is constant (around -900
km/s). The simulated FeXXVI Ly alpha line (1.78 A) from the wind is weak
compared to the observed one. We suggest that it originates in the vicinity of
the compact star, with a maximum blue shift at phase 0.25 (compact star
approaching). By combining the mass function derived with that from the
infrared HeI absorption (arising from the WR companion), we constrain the
masses and inclination of the system. Both a neutron star at large inclination
(over 60 degrees) and a black hole at small inclination are possible solutions.Comment: 8 pages, accepted 19/04/2009 for publication in Astronomy and
Astrophysic
- …