Quasi-periodic Brightness Oscillations Research Articles

Dipper-like variability of the Gaia alerted young star V555 Ori

ABSTRACT V555 Ori is a T Tauri star, whose 1.5 mag brightening was published as a Gaia science alert in 2017. We carried out optical and near-infrared (NIR) photometric, and optical spectroscopic observations to understand the light variations. The light curves show that V555 Ori was faint before 2017, entered a high state for about a year, and returned to the faint state by mid-2018. In addition to the long-term flux evolution, quasi-periodic brightness oscillations were also evident, with a period of about 5 d. At optical wavelengths both the long-term and short-term variations exhibited colourless changes, while in the NIR they were consistent with changing extinction. We explain the brightness variations as the consequence of changing extinction. The object has a low accretion rate whose variation in itself would not be enough to reproduce the optical flux changes. This behaviour makes V555 Ori similar to the pre-main sequence star AA Tau, where the light changes are interpreted as periodic eclipses of the star by a rotating inner disc warp. The brightness maximum of V555 Ori was a moderately obscured (AV = 2.3 mag) state, while the extinction in the low state was AV = 6.4 mag. We found that while the Gaia alert hinted at an accretion burst, V555 Ori is a standard dipper, similar to the prototype AA Tau. However, unlike in AA Tau, the periodic behaviour was also detectable in the faint phase, implying that the inner disc warp remained stable in both the high and low states of the system.

On the Persistence of QPOs during the SGR 1806−20 Giant Flare

Abstract The discovery of quasi-periodic brightness oscillations (QPOs) in the X-ray emission accompanying the giant flares of the soft gamma-ray repeaters SGR 1806–20 and SGR 1900+14 has led to intense speculation about their nature and what they might reveal about the interiors of neutron stars. Here we take a fresh look at the giant flare data for SGR 1806–20, and in particular we analyze short segments of the post-peak emission using a Bayesian procedure, which has not previously been applied to these data. We find at best weak evidence that any QPO persists for more than ∼1 s; instead, almost all the data are consistent with a picture in which there are numerous independently excited modes that decay within a few tenths of a second. This has interesting implications for the rapidity of decay of the QPO modes, which could occur by the previously suggested mechanism of coupling to the MHD continuum. The strongest QPOs favor certain rotational phases, which might suggest special regions of the crust or of the magnetosphere. We also find several previously unreported QPOs in these data, which may help in tracking down their origin.

WAVELETS WITH RIDGES: A HIGH-RESOLUTION REPRESENTATION OF CATACLYSMIC VARIABLE TIME SERIES

Quasi-periodic oscillations and dwarf nova oscillations occur in dwarf novae and nova-like variables during outburst and occasionally during quiescence, and have analogues in high-mass X-ray binaries and black-hole candidates. The frequent low coherence of quasi-period oscillations and dwarf nova oscillations can make detection with standard time-series tools such as periodograms problematic. This paper develops tools to analyse quasi-periodic brightness oscillations. We review the use of time-frequency representations in the astronomical literature, and show that representations such as the Choi-Williams Distribution and Zhao-Atlas-Marks Representation, which are best suited to high signal-to-noise data, cannot be assumed a priori to be the best techniques for our data, which have a much higher noise level and lower coherence. This leads us to a detailed analysis of the time-frequency resolution and statistical properties of six time-frequency representations. We conclude that the wavelet scalogram, with the addition of wavelet ridges and maxima points, is the most effective time-frequency representation for analysing quasi-periodicities in low signal-to-noise data, as it has high time-frequency resolution, and is a minimum variance estimator. We use the wavelet ridges method to re-analyse archival data from VW Hyi, and find 62 new QPOs and 7 new long-period DNOs. Relative to previous analyses, our method substantially improves the detection rate for QPOs.

Bardeen‐Petterson Effect and Quasi‐periodic Oscillations in X‐Ray Binaries

The Bardeen-Petterson effect around a rapidly-rotating compact object causes a tilted accretion disk to warp into the equatorial plane of the rotating body. Viscous forces cause the accretion flow to divide into two distinct regions - an inner aligned accretion disk and an outer tilted accretion disk. The transition between these two occurs at a characteristic radius that depends on the mass and angular momentum of the central object and possibly on the accretion rate through the disk. We propose that accreting material passing through the transition region may generate quasi-periodic brightness oscillations (QPOs) such as have been observed in a number of X-ray binaries. We show that this effect may be present in the black-hole X-ray binary GRO J1655-40. We also argue that the QPO frequency range predicted by this model is consistent with observed QPO frequencies in both black-hole and neutron-star low-mass X-ray binaries.

Attenuation of Beaming Oscillations near Neutron Stars

Observations with RXTE have revealed kilohertz quasi-periodic brightness oscillations (QPOs) from nearly twenty different neutron-star low-mass X-ray binaries (LMXBs). These frequencies often appear as a pair of kilohertz QPOs in a given power density spectrum. In many models the higher-frequency of these QPOs is a beaming oscillation at the frequency of a nearly circular orbit at some radius near the neutron star. In such models it is expected that there will also be beaming oscillations at the stellar spin frequency and at overtones of the orbital frequency, but no strong QPOs have been detected at these frequencies. We therefore examine the processes that can attenuate beaming oscillations near neutron stars, and in doing so extend the work on this subject that was initiated by the discovery of lower-frequency QPOs from LMXBs. Among our main results are (1)in a spherical scattering cloud, all overtones of rotationally modulated beaming oscillations are attenuated strongly, not just the even harmonics, and (2)it is possible to have a relatively high-amplitude modulation near the star at, e.g., the stellar spin frequency, even if no peak at that frequency is detectable in a power density spectrum taken at infinity. We discuss the application of these results to modeling of kilohertz QPOs.

Implications of kHz quasi-periodic brightness oscillations in X-ray binaries for neutron star structure

Kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries (LMXBs) may represent the orbital frequencies of the innermost Keplerian orbits around accreting neutron stars. Attempts have recently been made to derive constraints on the mass and the equation of state of the neutron star, by identifying the highest observed QPO frequency with the Keplerian frequency at the marginally stable orbit given by the general theory of relativity. These estimates have either neglected the effect of neutron star rotation or used an approximate treatment of rotation in general relativity. We rederive these constraints using a fully general relativistic formalism including the effect of rapid rotation. We also present constraints corresponding to the case where the innermost stable orbit touches the stellar surface.

Sonic‐Point Model of Kilohertz Quasi‐periodic Brightness Oscillations in Low‐Mass X‐Ray Binaries

Quasi-periodic brightness oscillations (QPOs) with frequencies ranging from ~300 to ~1200 Hz have been discovered in the X-ray emission from 14 neutron stars in low-mass binary systems and from another neutron star in the direction of the Galactic center. These kilohertz QPOs are very strong, with rms relative amplitudes ranging up to ~15% of the total X-ray count rate, and are remarkably coherent, with frequency-to-FWHM ratios as large as ~200. Two simultaneous kilohertz QPOs differing in frequency by ~250-350 Hz have been detected in 12 of the 15 sources. Here we propose a model for these QPOs. In this model, the X-ray source is a neutron star with a surface magnetic field ~107-1010 G and a spin frequency of a few hundred hertz, accreting gas via a Keplerian disk. Some of the accreting gas is channeled by the stellar magnetic field but some remains in a Keplerian disk flow that penetrates to within a few kilometers of the stellar surface. The frequency of the higher frequency QPO in a kilohertz QPO pair is the Keplerian frequency at a radius near the sonic point at the inner edge of the Keplerian flow, whereas the frequency of the lower frequency QPO is the difference between the Keplerian frequency at a radius near the sonic point and the fundamental or first overtone of the stellar spin frequency. The difference between the frequencies of the pair of QPOs is therefore close to (but not necessarily equal to) the stellar spin frequency. The amplitudes of the QPOs at the sonic-point Keplerian frequency and at the beat frequency depend on the strength of the neutron star's magnetic field and the accretion rate, and hence one or both of these QPOs may sometimes be undetectable. Oscillations at the stellar spin frequency and its overtones are expected to be weak but may sometimes be detectable. This model is consistent with the magnetic field strengths, accretion rates, and scattering optical depths inferred from previous modeling of the X-ray spectra and rapid X-ray variability of the atoll and Z sources. It explains naturally the frequencies of the kilohertz QPOs and the similarity of these frequencies in sources with different accretion rates and magnetic fields. The model also explains the high coherence and large amplitudes of the kilohertz QPOs and the steep increase of QPO amplitude with photon energy. The increase in QPO frequency with inferred accretion rate seen in many sources is also understandable in this model. We show that if the frequency of the higher frequency QPO in a pair is an orbital frequency, as in the sonic-point model, the frequencies of these QPOs place interesting upper bounds on the masses and radii of the neutron stars in the kilohertz QPO sources and provide new constraints on the equation of state of matter at high densities. Further observations of these QPOs may provide compelling evidence for the existence of a marginally stable orbit, confirming a key prediction of general relativity in the strong-field regime.

Evidence for Antipodal Hot Spots During X-Ray Bursts from 4U 1636−536

The discovery of high-frequency brightness oscillations in thermonuclear X-ray bursts from several neutron star low-mass X-ray binaries has important implications for the beat frequency model of kilohertz quasi-periodic brightness oscillations, the propagation of nuclear burning, the structure of the subsurface magnetic fields in neutron stars, and the equation of state of high-density matter. These implications depend crucially on whether the observed frequency is the stellar spin frequency or its first overtone. Here we report an analysis of five bursts from 4U 1636-536 that exhibit strong oscillations at ~580 Hz. We show that combining the data from the first 0.75 s of each of the five bursts yields a signal at ~290 Hz that is significant at the 4×10−5 level when the number of trials is taken into account. This strongly indicates that ~290 Hz is the spin frequency of this neutron star and that ~580 Hz is its first overtone, in agreement with other arguments about this source but in contrast to suggestions in the literature that ~580 Hz is the true spin frequency. The method used here, which is similar to matched filtering, may be used for any source to search for weak oscillations that have frequencies related in a definite way to the frequency of a strong oscillation.

Effects of Radiation Forces on the Frequency of Gravitomagnetic Precession near Neutron Stars

Gravitomagnetic precession near neutron stars and black holes has received much recent attention, particularly as a possible explanation of 15--60 Hz quasi-periodic brightness oscillations (QPOs) from accreting neutron stars in low-mass X-ray binaries, and of somewhat higher-frequency QPOs from accreting stellar-mass black holes. Previous analyses of this phenomenon have either ignored radiation forces or assumed for simplicity that the radiation field is isotropic, and in particular that there is no variation of the radiation field with angular distance from the rotational equatorial plane of the compact object. However, in most realistic accretion geometries (e.g., those in which the accretion proceeds via a geometrically thin disk) the radiation field depends on latitude. Here we show that in this case radiation forces typically have an important, even dominant, effect on the precession frequency of test particles in orbits that are tilted with respect to the star's rotational equator. Indeed, we find that even for accretion luminosities only a few percent of the Eddington critical luminosity, the precession frequency near a neutron star can be changed by factors of up to $\sim 10$. Radiation forces must therefore be included in analyses of precession frequencies near compact objects, in such varied contexts as low-frequency QPOs, warp modes of disks, and trapped oscillation modes. We discuss specifically the impact of radiation forces on models of low-frequency QPOs involving gravitomagnetic precession, and show that such models are rendered much less plausible by the effects of radiation forces.