ABSTRACT
The X-ray light curves of accreting black holes and neutron stars in binary systems show various types of quasi-periodic oscillations (QPOs), the origin of which is still debated. The ...relativistic precession model identifies the QPO frequencies with fundamental time-scales from General Relativity, and has been proposed as a possible explanation of certain types of such oscillations. Under specific conditions (i.e. the detection of a particular QPOs triplet) such a model can be used to obtain self-consistent measurements of the mass and spin of the compact object. So far this has been possible only in the black hole binary GRO J1655-40. In the RXTE/PCA data from the 1999–2000 outburst of the black hole transient XTE J1859+226 we found a QPO triplet, and used the relativistic precession model to obtain high-precision measurements of the black hole mass and spin – M = (7.85 ± 0.46) M⊙, a* = 0.149 ± 0.005 – the former being consistent with the most recent dynamical mass determination from optical measurements. Similarly to what has been already observed in other black hole systems, the frequencies of the QPOs and broad-band noise components match the general relativistic frequencies of particle motion close to the compact object predicted by the model. Our findings confirm previous results and further support the validity of the relativistic precession model, which is the only electromagnetic-measurement-based method that so far has consistently yielded spins close to those from the gravitational waves produced by merging binary black holes.
Abstract
We studied five XMM–Newton observations of the neutron star binary 4U 1728–34 covering the hard, intermediate and soft spectral states. By jointly fitting the spectra with several reflection ...models, we obtained an inclination angle of 25º–53º and an iron abundance up to 10 times the solar. From the fits with reflection models, we found that the fluxes of the reflection and the Comptonized components vary inconsistently; since the latter is assumed to be the illuminating source, this result possibly indicates the contribution of the neutron star surface/boundary layer to the disc reflection. As the source evolved from the relatively soft to the intermediate state, the disc inner radius decreased, opposite to the prediction of the standard accretion disc model. We also explore the possible reasons why the supersolar iron abundance is required by the data and found that this high value is probably caused by the absence of the hard photons in the XMM–Newton data.
We present a systematic analysis of the complete set of observations of the black hole (BH) binary XTE J1550-564 obtained by the Rossi X-ray Timing Explorer. We study the fast time variability ...properties of the source and determine the spin of the BH through the relativistic precession model. Similarly to what is observed in the BH binary GRO J1655-40, the frequencies of the QPOs and broad-band noise components match the general relativistic frequencies of particle motion close to the compact object predicted by the relativistic precession model. The combination of two simultaneously observed quasi-periodic oscillation (QPO) frequencies together with the dynamical BH mass from optical/infrared observations yields a spin equal to a = 0.34 ± 0.01, consistent with previous determinations from X-ray spectroscopy. Based on the derived BH parameters, the low-frequency QPO emission radii vary from ∼30 gravitational radii (R
g) to the innermost stable orbit for this spin (∼5 R
g), where they sharply disappear as observed for the case of GRO J1655-40.
Context. XSS J1227.0-4859 is a peculiar, hard X-ray source recently positionally associated to the Fermi-LAT source 1FGL J1227.9-4852/2FGL J1227.7-4853. Multi-wavelength observations have added ...information on this source, indicating a low-luminosity low-mass X-ray binary (LMXB), but its nature is still unclear. Aims. To progress in our understanding, we present new X-ray data from a monitoring campaign performed in 2011 with the XMM-Newton, RXTE, and Swift satellites and combine them with new gamma-ray data from the Fermi and AGILE satellites. We complement the study with simultaneous near-UV photometry from XMM-Newton and with previous UV/optical and near-IR data. Methods. We analysed the temporal characteristics in the X-rays, near-UV, and gamma rays and studied the broad-band spectral energy distribution from radio to gamma rays. Results. The X-ray history of XSS J1227 over 7 yr shows a persistent and rather stable low-luminosity (6 × 1033 d1 kpc2 erg s-1) source, with flares and dips being peculiar and permanent characteristics. The associated Fermi-LAT source 2FGL J1227.7-4853 is also stable over an overlapping period of 4.7 yr. Searches for X-ray fast pulsations down to msec give upper limits to pulse fractional amplitudes of 15−25% that do not rule out a fast spinning pulsar. The combined UV/optical/near-IR spectrum reveals a hot component at ~13 kK and a cool one at ~4.6 kK. The latter would suggest a late-type K2−K5 companion star, a distance range of 1.4−3.6 kpc, and an orbital period of 7–9 h. A near-UV variability (≳6 h) also suggests a longer orbital period than previously estimated. Conclusions. The analysis shows that the X-ray and UV/optical/near-IR emissions are more compatible with an accretion-powered compact object than with a rotational powered pulsar. The X-ray to UV bolometric luminosity ratio could be consistent with a binary hosting a neutron star, but the uncertainties in the radio data may also allow an LMXB black hole with a compact jet. In this case, it would be the first associated with a high-energy gamma-ray source.
We present the discovery of fast infrared/X-ray correlated variability in the black hole transient GX 339−4. The source was observed with subsecond time resolution simultaneously with Very Large ...Telescope/Infrared Spectrometer And Array Camera and Rossi X-ray Timing Explorer/Proportional Counter Array in 2008 August, during its persistent low-flux highly variable hard state. The data show a strong correlated variability, with the infrared emission lagging the X-ray emission by 100ms. The short time delay and the nearly symmetric cross-correlation function, together with the measured brightness temperature of ∼2.5 × 106K, indicate that the bright and highly variable infrared emission most likely comes from a jet near the black hole. Under standard assumptions about jet physics, the measured time delay can provide us a lower limit of Γ > 2 for the Lorentz factor of the jet. This suggests that jets from stellar-mass black holes are at least mildly relativistic near their launching region. We discuss implications for future applications of this technique.
We analyzed all published frequencies, $\nu_1$ and $\nu_2$, of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in bright neutron star low-mass X-ray binaries. The two frequencies are well ...correlated but, contrary to recent suggestions, the frequency–frequency correlation is significantly different from a $\nu_2 = (3/2) \nu_1$ relation. To check whether, although not following the the 3/2 relation, the QPO frequencies cluster around a region where $\nu_2/\nu_1 \approx 3/2$, we re-analyzed the Sco X-1 data that were used to report that ratio and show that, because the distribution of ratios of linearly correlated measurements is intrinsically biased, although the significance of the clustering around $\nu_2/\nu_1 \approx 3/2$ previously reported in the case of Sco X-1 is formally correct, it does not provide any useful information about a possible underlying resonance mechanism in this source. Using the same data, we then show that the (unbiased) distribution of QPO frequencies is consistent with a uniform distribution at a $2.4\sigma$ level. To investigate this further, we analyzed a larger data set of Sco X-1 and four other sources, 4U 1608–52, 4U 1636–53, 4U 1728–34 and 4U 1820–30. We find that for all five sources the distribution of the kHz QPO frequencies is not uniform and has multiple peaks, which have no analogy in the distribution of points in the spectral color–color diagrams of these sources. Finally, we demonstrate that a simple random walk of the QPO frequencies can reproduce qualitatively the observed distributions in frequency and frequency ratio. This result weakens the support for resonance models of kHz QPOs in neutron stars.
Using archival Rossi X-ray Timing Explorer (RXTE) data, we studied the low-frequency quasi-periodic oscillations (LFQPOs) in the neutron star low-mass X-ray binary (LMXB) Cir X-1 and examined their ...contribution to frequency-frequency correlations for Z sources. We also studied the orbital phase effects on the LFQPO properties and found them to be phase independent. Comparing LFQPO frequencies in different classes of LMXBs, we found that systems that show both Z and atoll states form a common track with atoll/BH sources in the so-called WK correlation, while persistent Z systems are offset by a factor of about two. We found that neither source luminosity nor mass accretion rate is related to the shift of persistent Z systems. We discuss the possibility of a misidentification of fundamental frequency for horizontal branch oscillations from persistent Z systems and interpreted the oscillations in terms of models based on relativistic precession.
We present the spectral and timing analysis of X-ray observations performed on the Galactic black hole transient GRS 1716-249 during the 2016-2017 outburst. The source was almost continuously ...observed with the Neil Gehrels Swift Observatory from 2016 December until 2017 October. The X-ray hardness ratio and timing evolution indicate that the source approached the soft state three times during the outburst, even though it never reached the canonical soft spectral state. Thus, GRS 1716-249 increases the number of black hole transients showing outbursts with `failed' state transition. During the softening events, XRT and BAT broad-band spectral modelling, performed with thermal Comptonization plus a multicolour disc black-body, showed a photon index (Γ < 2) and an inner disc temperature (kTin = 0.2-0.5 keV) characteristic of the hard intermediate state. This is in agreement with the root mean square amplitude of the flux variability (rms > 10{{ per cent}}). We find that, coherently with a scenario in which the disc moves closer to the compact object, the accretion disc inner radius decreases with the increase of the inner disc temperature, until a certain point when the temperature starts to increase at constant radius. This, in addition with the spectral analysis results, suggests that either the accretion disc reached the innermost stable circular orbit during the hard intermediate state or the hot accretion flow might recondensate in an inner mini-disc. We report on the radio observations performed during the outburst finding that GRS 1716-249 is located on the radio-quiet `outlier' branch of the radio/X-ray luminosity plane.
ABSTRACT
The clumpy stellar wind from the companion star in high-mass X-ray binaries causes variable, partial absorption of the emission from the X-ray source. We studied XMM–Newton observations from ...a 7.22 d long monitoring campaign, in order to constrain the effects of the stellar wind on the short-time-scale X-ray spectral-timing properties of Cygnus X-1. We find these properties to change significantly in the presence of the wind. In particular, the longest sampled time-scales (corresponding to temporal frequencies of ν ∼ 0.1–1 Hz) reveal an enhancement of the fractional variability power, while on the shortest sampled time-scales (ν ∼ 1–10 Hz), the variability is suppressed. In addition, we observe a reduction (by up to a factor of ∼ 1.8) of the otherwise high coherence between soft and hard band light curves, as well as of the amplitude of the hard X-ray lags intrinsic to the X-ray continuum. The observed increase of low-frequency variability power can be explained in terms of variations of the wind column density as a consequence of motions of the intervening clumps. In this scenario (and assuming a terminal velocity of $v_{\infty }=2400\, {\rm km\ s^{-1}}$), we obtain an estimate of l ∼ 0.5–1.5 × 10−4R* for the average radial size of a clump. On the other hand, we suggest the behaviour at high frequencies to be due to scattering in an optically thicker medium, possibly formed by collision of the stellar wind with the edge of the disc.
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