66 research outputs found
The spectral-timing properties of upper and lower kHz QPOs
Soft lags from the emission of the lower kilohertz quasi-periodic
oscillations (kHz QPOs) of neutron star low mass X-ray binaries have been
reported from 4U1608-522 and 4U1636-536. Those lags hold prospects for
constraining the origin of the QPO emission. In this paper, we investigate the
spectral-timing properties of both the lower and upper kHz QPOs from the
neutron star binary 4U1728-34, using the entire Rossi X-ray Timing Explorer
archive on this source. We show that the lag-energy spectra of the two QPOs are
systematically different: while the lower kHz QPO shows soft lags, the upper
kHz QPO shows either a flat lag-energy spectrum or hard variations lagging
softer variations. This suggests two different QPO-generation mechanisms. We
also performed the first spectral deconvolution of the covariance spectra of
both kHz QPOs. The QPO spectra are consistent with Comptonized blackbody
emission, similar to the one found in the time-averaged spectrum, but with a
higher seed-photon temperature, suggesting that a more compact inner region of
the Comptonization layer (boundary/spreading layer, corona) is responsible for
the QPO emission. Considering our results together with other recent findings,
this leads us to the hypothesis that the lower kHz QPO signal is generated by
coherent oscillations of the compact boundary layer region itself. The upper
kHz QPO signal may then be linked to less-coherent accretion-rate variations
produced in the inner accretion disk, being detected when they reach the
boundary layer.Comment: 20 pages, 7 figures, accepted for publication in Ap
Timing the warm absorber in NGC 4051
We investigated, using spectral-timing analysis, the characterization of
highly ionized outflows in Seyfert galaxies, the so-called warm absorbers.
Here, we present our results on the extensive ~ 600 ks of XMM-Newton archival
observations of the bright and highly variable Seyfert 1 galaxy NGC 4051, whose
spectrum has revealed a complex multicomponent wind. Making use of both RGS and
EPIC-pn data, we performed a detailed analysis through a time-dependent
photoionization code in combination with spectral and Fourier spectral-timing
techniques. The source light curves and the warm absorber parameters obtained
from the data were used to simulate the response of the gas due to variations
in the ionizing flux of the central source. The resulting time variable spectra
were employed to predict the effects of the warm absorber on the time lags and
coherence of the energy dependent light curves. We have found that, in the
absence of any other lag mechanisms, a warm absorber with the characteristics
of the one observed in NGC 4051, is able to produce soft lags, up to 100 s, on
timescales of ~ hours. The time delay is associated with the response of the
gas to changes in the ionizing source, either by photoionization or radiative
recombination, which is dependent on its density. The range of radial distances
that, under our assumptions, yield longer time delays are comparable to the
existing estimates of the location of the warm absorber in NGC 4051. For this
reason, we suggest that it is likely that the observed X-ray time lags may
carry a signature of the warm absorber response time, to changes in the
ionizing continuum. These results highlight the importance of understanding the
contribution of the warm absorber to the AGN X-ray time lags, since it is also
vital information for interpreting the lags associated with propagation and
reverberation effects in the inner emitting regions.Comment: 13 pages, 12 figures, Accepted for publication by A&
Comparison of time/phase lags in the hard state and plateau state of GRS 1915+105
We investigate the complex behavior of energy- and frequency-dependent
time/phase lags in the plateau state and the radio-quiet hard state of GRS
1915+105. In our timing analysis, we find that when the source is faint in the
radio, QPOs are observed above 2 Hz and typically exhibit soft lags (soft
photons lag hard photons), whereas QPOs in the radio-bright plateau state are
found below 2.2 Hz and consistently show hard lags. The phase lag at the QPO
frequency is strongly anti-correlated with the QPO frequency, changing sign at
2.2 Hz. However, the phase lag at the frequency of the first harmonic is
positive and nearly independent of frequency at at ~0.172 rad, regardless of
the radio emission. The lag-energy dependence at the first harmonic is also
independent of radio flux. However, the lags at the QPO frequency are negative
at all energies during the radio-quiet state, but lags at the QPO frequency
during the plateau state are positive at all energies and show a
'reflection-type' evolution of the lag-energy spectra with respect to the
radio-quiet state. The lag-energy dependence is roughly logarithmic, but there
is some evidence for a break around 4-6 keV. Finally, the Fourier
frequency-dependent phase lag spectra are fairly flat during the plateau state,
but increase from negative to positive during the radio-quiet state. We discuss
the implications of our results in the light of some generic models.Comment: 9 pages, 7 figures, accepted for publication in Ap
A jet-dominated model for a broad-band spectral energy distribution of the nearby low-luminosity active galactic nucleus in M94
We have compiled a new multiwavelength spectral energy distribution (SED) for
the closest obscured low-ionization emission-line region active galactic
nucleus (AGN), NGC 4736, also known as M94. The SED comprises mainly
high-resolution (mostly sub-arcsecond, or, at the distance to M94, <23 pc from
the nucleus) observations from the literature, archival data, as well as
previously unpublished sub-millimetre data from the Plateau de Bure
Interferometer (PdBI) and the Combined Array for Research in Millimeter-wave
Astronomy, in conjunction with new electronic MultiElement Radio
Interferometric Network (e-MERLIN) L-band (1.5 GHz) observations. Thanks to the
e-MERLIN resolution and sensitivity, we resolve for the first time a double
structure composed of two radio sources separated by ~1 arcsec, previously
observed only at higher frequency. We explore this data set, which further
includes non-simultaneous data from the Very Large Array, the Gemini telescope,
the Hubble Space Telescope and the Chandra X-ray observatory, in terms of an
outflow-dominated model. We compare our results with previous trends found for
other AGN using the same model (NGC 4051, M81*, M87 and Sgr A*), as well as
hard- and quiescent-state X-ray binaries. We find that the nuclear broad-band
spectrum of M94 is consistent with a relativistic outflow of low inclination.
The findings in this work add to the growing body of evidence that the physics
of weakly accreting black holes scales with mass in a rather straightforward
fashion.Comment: 18 pages, 7 figure
A systematic study of the phase difference between QPO harmonics in black hole X-ray binaries
We perform a systematic study of the evolution of the waveform of black hole
X-ray binary low-frequency QPOs, by measuring the phase difference between
their fundamental and harmonic features. This phase difference has been studied
previously for small number of QPO frequencies in individual sources. Here, we
present a sample study spanning fourteen sources and a wide range of QPO
frequencies. With an automated pipeline, we systematically fit power spectra
and calculate phase differences from archival Rossi X-ray Timing Explorer
(RXTE) observations. We measure well-defined phase differences over a large
range of QPO frequencies for most sources, demonstrating that a QPO for a given
source and frequency has a persistent underlying waveform. This confirms the
validity of recently developed spectral-timing methods performing phase
resolved spectroscopy of the QPO. Furthermore, we evaluate the phase difference
as a function of QPO frequency. For Type-B QPOs, we find that the phase
difference stays constant with frequency for most sources. We propose a simple
jet precession model to explain these constant Type-B QPO phase differences.
The phase difference of the Type-C QPO is not constant but systematically
evolves with QPO frequency, with the resulting relation being similar for a
number of high inclination sources, but more variable for low-inclination
sources. We discuss how the evolving phase difference can naturally arise in
the framework of precession models for the Type-C QPO, by considering the
contributions of a direct and reflected component to the QPO waveform
Spectral and timing evolution of the black hole transient MAXI J1727-203 with NICER
MAXI J1727-203 is a new X-ray transient discovered on 5 June 2018. A hard-to-soft state transition at the beginning of the outburst led to the identification as a black hole candidate. MAXI J1727-203 was monitored with the Neutron Star Interior Composition Explorer (NICER) on an almost daily basis from the beginning of the outburst. We present a spectral and timing analysis of the full outburst of the source, which lasted approximately four months. A preliminary spectral analysis suggest that the accretion disk component can was detected throughout the entire outburst, with temperatures ranging from ~0.4 keV (in the soft state), down to ~0.2 keV near the end of the outburst when the source was in the hard state. The power spectrum in the hard state shows broadband noise up to 10 Hz, with no detection of any quasi-periodic oscillations. We argue that the system's characteristics are not consistent with those expected for a neutron star and that they are particularly reminiscent of the black hole X-ray binaries XTE J1118+480 and Cyg X-1
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