Quasi-periodic Oscillations Amplitude Research Articles

The 2018 failed outburst of H 1743 – 322: Insight-HXMT, NuSTAR, and NICER views

ABSTRACT We studied the 2018 outburst of the black hole transient H 1743 − 322 with a series of Insight-HXMT, NICER, and NuSTAR observations, covering the 1–120 keV band. With our broad-band X-ray spectral modelling, we confirm that the source remained in the low/hard state throughout the month-long outburst, although it became marginally softer at peak flux. We detected Type-C quasi-periodic oscillations (QPOs) and followed the evolution of their properties. The QPO frequency increased from ∼0.1 to ∼0.4 Hz during the rising phase of the outburst and decreased again in the decline. Continuum X-ray flux, power-law photon index, QPO frequency, and QPO root-mean-square amplitude were positively correlated. The QPO amplitude was slightly higher in the soft X-ray band (typical values of 12–16 per cent, compared with 8–10 per cent in the hard band). Our spectral-timing results shed light on the initial rising phase in the low/hard state, which has rarely been monitored with such high cadence, time resolution, and broad-band coverage. Combining spectral and timing properties, we find that ‘failed’ (hard state only) and ‘successful’ outbursts follow the same initial evolutionary track, although the former class of outburst never reaches the threshold for a transition to softer (thermally dominated) accretion regimes.

Spectral Variability of a Soft-intermediate State QPO from MAXI J1820+070

Abstract Black Hole X-ray binaries are known to exhibit variability in their light curves on timescales as short as milliseconds and as long as months. The short-timescale variability can be in the form of quasi-periodic oscillations (QPOs), which may be produced by general relativistic effects. In this work, we looked at low-frequency QPOs from an exciting recent black hole transient, MAXI J1820+070, which first went into outburst in 2018. This source was observed in a multi-wavelength campaign that included the NICER mission, a soft X-ray telescope attached to the International Space Station. In our analysis, we applied X-ray timing and spectral-timing techniques to help place constraints on the QPO emission mechanism in the extreme environment surrounding the black hole. Although the QPO amplitude was too low to carry out phase-resolved spectroscopy, our work indicates that weak QPOs such as this one may be present in the light curves of many sources.

INSIGHT-HXMT Observations of the New Black Hole Candidate MAXI J1535−571: Timing Analysis

Abstract We present X-ray timing results of the new black hole candidate MAXI J1535−571 during its 2017 outburst from Hard X-ray Modulation Telescope (Insight-HXMT) observations taken from 2017 September 6 to 23. Following the definitions given by Belloni, we find that the source exhibits transitions from the low/hard state to the hard intermediate state, and eventually to the soft intermediate state. Quasi-periodic oscillations (QPOs) are found in the intermediate states, which suggest different types of QPOs. With the large effective area of Insight-HXMT at high energies, we are able to present the energy dependence of the QPO amplitude and centroid frequency up to 100 keV, which has rarely been explored by previous satellites. We also find that the phase lag at the type-C QPOs centroid frequency is negative (soft lag) and strongly correlated with the centroid frequency. Assuming a geometrical origin of type-C QPOs, the source is consistent with being a high-inclination system.

NICER Detects a Soft X-Ray Kilohertz Quasi-periodic Oscillation in 4U 0614+09

Abstract We report on the detection of a kilohertz quasi-periodic oscillation (QPO) with the Neutron Star Interior Composition Explorer (NICER). Analyzing approximately 165 ks of NICER exposure on the X-ray burster 4U 0614+09, we detect multiple instances of a single-peak upper kHz QPO, with centroid frequencies that range from 400 to 750 Hz. We resolve the kHz QPO as a function of energy, and measure, for the first time, the QPO amplitude below 2 keV. We find the fractional amplitude at 1 keV is on the order of 2% rms, and discuss the implications for the QPO emission process in the context of Comptonization models.

A VLT-ULTRACAM study of the fast optical quasi-periodic oscillations in the polar V834 Centauri

Quasi-periodic oscillations (QPOs) of a few seconds have been detected in some polars, the synchronised subclass of cataclysmic systems containing a strongly magnetised white dwarf which accretes matter from a red dwarf companion. The QPOs are thought to be related to instabilities of a shock formed in the accretion column, close to the white dwarf photosphere above the impact region. We present optical observations of the polar V834 Centauri performed with the fast ULTRACAM camera mounted on the ESO-VLT simultaneously in three filters (u′, He ii λ4686, r′) to study these oscillations and characterise their properties along the orbit when the column is seen at different viewing angles. Fast Fourier transforms and wavelet analysis have been performed and the mean frequency, rms amplitude, and coherence of the QPOs are derived; a detailed inspection of individual pulses has also been performed. The observations confirm the probable ubiquity of the QPOs for this source at all epochs when the source is in a high state, with observed mean amplitude of 2.1% (r′), 1.5% (He ii), and 0.6% (u′). The QPOs are present in the r′ filter at all phases of the orbital cycle, with a higher relative amplitude around the maximum of the light curve. They are also detected in the He ii and u′ filters but at a lower level. Trains of oscillations are clearly observed in the r′ light curve and can be mimicked by a superposition of damped sinusoids with various parameters. The QPO energy distribution is comparable to that of the cyclotron flux, consistent for the r′ and He ii filters but requiring a significant dilution in the u′ filter. New 1D hydrodynamical simulations of shock instabilities, adapted to the physical parameters of V834 Cen, can account for the optical QPO amplitude and X-ray upper limit assuming a cross section of the accretion column in the range ~(4 − 5) × 1014 cm2. However, the predicted frequency is larger than the observed one by an order of magnitude. This shortcoming indicates that the QPO generation is more complex than that produced in a homogeneous column and calls for a more realistic 3D treatment of the accretion flow in future modelling.

The type B QPO phenomena in the transient black hole candidate GX 339-4

We investigated the spectral and timing properties of the type B quasi-periodic oscillations (QPOs) showing up in the transient black hole binary GX 339-4 during its four outbursts observed by Rossi X-ray Timing Explorer (RXTE)/PCA and HEXTE in 2002, 2004, 2007 and 2010. We find that, the dependence on variability of the accretion flow turns out to be similar for the type B QPOs occurring in these four outbursts. We therefore take the results from the 2010 outburst for presentation. Our spectral results obtained from both the energy and time domains show that, the occurrence of the type B QPO is accompanied with sudden increase of hard component flux, relatively smaller inner disk radius, stable disk but variable corona. The latter may be understood in a scenario of variable input of seed photons for Comptonization in the corona. Further clues to probing the possible origination of the type B QPO come from our analysis of time lag and its energy dependence. The energy dependence of type B QPO amplitude suggests that the hard component dominate the variability and the time lag spectral analysis results suggests the type B QPO could be related to inverse Compton scattering. The time lag between hard and soft energy band is about 10 ms and found to depend on frequency ν in a form of ν−0.7 for type B QPOs in the rising phase. Finally we put these results in a context of a blob mechanism at work.

Spectral Properties of Low‐Frequency Quasi‐periodic Oscillations in GRS 1915+105

We report on the timing analysis of RXTE observations of the Galactic microquasar GRS 1915+105 performed in 2003. Out of a total of six ~20 ks observations, we focus here only on the three during which GRS 1915+105 is found in a steady C-state (referred to as class χ), resulting in a total of ~50 ks. During these observations, we detect low-frequency quasi-periodic oscillations (QPOs) with high (~14%) rms amplitude in the 2-40 keV energy range. Contrary to what is usually observed in GRS 1915+105, in most of our observations the QPO frequency presents no correlation with the RXTE PCA count rate, nor with the RXTE ASM count rate. We present, for the first time, high-resolution (22 spectral channels) 2-40 keV spectral fits of the energy dependence of the QPO amplitude (QPO spectra). The QPO spectra are well modeled with a cutoff power law except on one occasion in which a single power law gives a satisfactory fit (with no cutoff at least up to ~40 keV). The cutoff energy evolves significantly from one observation to another, from a value of ~21.8 to ~30 keV in the other observations in which it is detected. We discuss the possible origin of this behavior and suggest that the compact jet detected in the radio contributes to the hard X-ray (≥20 keV) mostly through synchrotron emission, whereas the X-rays emitted below 20 keV would originate through inverse Compton scattering. The dependence of the QPO amplitude on the energy can be understood if the modulation of the X-ray flux is contained in the Comptonized photons and not in the synchrotron ones.

Dependence of Kilohertz Quasi-periodic Oscillation Properties on the Normal-Branch Oscillation Phase in Scorpius X-1

We analysed RXTE data of Sco X-1 which show kHz quasi-periodic oscillations (QPOs) and the $\sim$6--8 Hz normal-branch oscillation (NBO) simultaneously. Using power spectra of 0.03--0.5 s data segments, we find that both the upper kHz QPO frequency $\nu_2$ and the ratio of lower to upper kHz QPO amplitude are anticorrelated to variations in the X-ray count-rate taking place on the NBO time scale. The frequency dependence is similar to (but probably weaker than) that found on longer time scales, but the power ratio dependence is opposite to it. A model where radiative stresses on the disk material, modulated at the NBO frequency, lead to changes in $\nu_2$ can explain the data; this implies some of the NBO flux changes originate from inside the inner disk radius. We discuss how these findings affect our understanding of kHz QPOs and of the low-frequency variability of low-mass X-ray binaries.

Correlations between Low‐Frequency Quasi‐periodic Oscillations and Spectral Parameters in XTE J1550−564 and GRO J1655−40

Utilizing observations obtained with the Rossi X-Ray Timing Explorer, we examine correlations between the properties of 0.08-22 Hz variable-frequency quasi-periodic oscillations (QPOs) and the X-ray spectral parameters for the black hole candidates XTE J1550-564 and GRO J1655-40. The spectra were fitted to a model including a multitemperature blackbody disk and a power-law component. We find that the QPO frequency and amplitude are well correlated with the spectral parameters, although the correlations found for XTE J1550-564 are generally opposite to those for GRO J1655-40. There is one exception: both sources exhibit a general increase in the QPO frequency as the disk flux increases (or as the rate of mass accretion through the disk increases). In addition, these QPOs are observed only when the power-law component contributes more than 20% of the 2-20 keV flux, which indicates that both the disk and the power-law components are linked to the QPO phenomenon.

Phase Lags of Quasi-periodic Oscillations in Microquasar GRS 1915+105

I report the discovery of hard X-ray phase lags associated with the famous 67 Hz quasi-periodic oscillation (QPO) in microquasar GRS 1915+105. The QPO is seen on multiple occasions. For this investigation, I have chosen one particular observation when the oscillation is the strongest. The measured hard lags show strong energy dependence. With respect to the 2-2.5 keV band, the phase lag increases from insignificant detection at 5.2-7.0 keV to as much as 2.3 radians (which corresponds a time lag of ~5.6 ms) above ~13 keV. In the same observation, one narrowly peaked, strong QPO at 67 mHz, along with its first three harmonics, one weak QPO at ~400 mHz, and one broad QPO at ~840 mHz are also detected. Similar cross spectral analyses have been performed on these QPOs (and the harmonics), in order to measure any phase lags associated with the features. Phase lags are detected in all, with similar energy dependence. For the 67 mHz QPO, the results are quite intriguing: only the first- and third-harmonic components show hard lags, while the fundamental and second-harmonic components actually display smaller soft lags. Coupled with the energy dependence of the QPO amplitude, the results seem to indicate a complicated change in the signal profile with photon energy. I will discuss the implication of the phase lags on possible origins of the QPOs.

X-Ray Nova XTE J1550−564: Discovery of a Quasi-periodic Oscillation near 185 H[CLC]z[/CLC

We have investigated the X-ray timing properties of XTE J1550-564 during 60 Rossi X-Ray Timing Explorer Proportional Counter Array observations made between 1998 September 18 and November 28. We detect quasi-periodic oscillations (QPOs) near 185 Hz during four time intervals. The QPO widths (FWHM) are ~50 Hz, and the rms amplitudes are ~1% of the mean flux at 2-30 keV. This is the third Galactic black hole candidate known to exhibit a transient X-ray timing signature above 50 Hz, following the 67 Hz QPO in GRS 1915+105 and the 300 Hz QPO in GRO J1655-40. However, unlike the previous cases, which appear to show stationary frequencies, the QPO frequency in XTE J1550-564 must vary by at least ~10% to be consistent with observations. The occurrences and properties of the QPO were insensitive to large changes in the X-ray intensity (1.5-6.8 crab). However, the QPO appearance was accompanied by changes in the energy spectrum, namely, an increase in the temperature and a decrease in the normalization of the thermal component. The QPO is also closely related to the hard X-ray power-law component of the energy spectrum since the fractional amplitude of the QPO increases with photon energy. The fast QPOs in accreting black hole binaries are thought to be effects of general relativity; however, the relevance of the specific physical models that have been proposed remains largely uncertain. Low-frequency QPOs in the range 3-13 Hz were often observed. Occasionally, at high luminosity, the rms QPO amplitude was ~15% of the flux, a level previously reached only by GRS 1915+105. These extraordinary oscillations have a coherence parameter (ν/Δν) in the range 4-12 and are tied to the power-law component in the energy spectrum.