High Frequency Break Research Articles

Solar wind magnetic field background spectrum from fluid to kinetic scales

The solar wind is highly structured in fast and slow flows. These two dynamical regimes remarkably differ not only for the average values of magnetic field and plasma parameters but also for the type of fluctuations they transport. Fast wind is characterized by large amplitude, incompressible fluctuations, mainly Alfv\'{e}nic, slow wind is generally populated by smaller amplitude and less Alfv\'{e}nic fluctuations, mainly compressive. The typical corotating fast stream is characterized by a stream interface, a fast wind region and a slower rarefaction region formed by the trailing expansion edge of the stream. Moving {between these two regions}, from faster to slower wind, we observe the following behavior: a) the power level of magnetic fluctuations within the inertial range largely decreases, keeping the typical Kolmogorov scaling; b) at proton scales, for about one decade right beyond the high frequency break, the spectral index becomes flatter and flatter towards a value around -2.7; c) at higher frequencies, before the electron scales, the spectral index remains around -2.7 and, {based on suitable observations available for $4$ corotating streams}, the power level does not change, irrespective of the flow speed. All these spectral features, characteristic of high speed streams, suggest the existence of a sort of magnetic field background spectrum. This spectrum would be common to both faster and slower wind but, any time the observer would cross the inner part of a fluxtube channeling the faster wind into the interplanetary space, a turbulent and large amplitude Alfv\'{e}nic spectrum would be superposed to it.

An intermediate‐mass black hole candidate in M51?

AbstractWe present the current results of an investigation into M51 ULX‐7, using archival data from XMM‐Newton, Chandra and NuSTAR, and optical and radio data from HST and VLA. The source has a consistently hard power‐law X‐ray spectrum and high short‐term variability. This is unusual variability behaviour for a ULX, as we would expect highly variable ULXs to have soft energy spectra. The power spectrum features a break at ∼ 10–3 Hz, from low frequency spectral index α = 0.1 to high frequency spectral index α = 0.8, analogous to the low frequency break found in power spectra of black holes accreting in the low/hard state. We do not observe a corresponding high frequency break, however taking the white noise level as a frequency lower limit of the break, we can calculate a black hole mass upper limit of 9.12×104 M, assuming that the ULX is in the low/hard state. While there is no radio detection, we find a flux density upper limit of 87 µJy/beam. Using the X‐ray/radio fundamental plane, we calculate a black hole mass upper limit of 1.95×105 M®. Therefore, this ULX is consistent with being an IMBH accreting in the low/hard state. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

SPECTRAL ANALYSIS OF MAGNETIC FLUCTUATIONS AT PROTON SCALES FROM FAST TO SLOW SOLAR WIND

This Letter investigates the spectral characteristics of the interplanetary magnetic field fluctuations at proton scales during several time intervals chosen along the speed profile of a fast stream. The character of the fluctuations within the first frequency decade, beyond the high frequency break located between the fluid and the kinetic regime, strongly depends on the type of wind. While the fast wind shows a clear signature of both right handed and left handed polarized fluctuations, possibly associated with KAW and Ion-Cyclotron waves, respectively, the rarefaction region, where the wind speed and the Alfv\'{e}nicity of low frequency fluctuations decrease, shows a rapid disappearance of the ion-cyclotron signature followed by a more gradual disappearance of the KAWs. Moreover, also the power associated to perpendicular and parallel fluctuations experiences a rapid depletion, keeping, however, the power anisotropy in favour of the perpendicular spectrum.

Reversibility of time series: revealing the hidden messages in X-ray binaries and cataclysmic variables

We explore the non-linear, high-frequency, aperiodic variability properties in the three cataclysmic variables MV Lyr, KIC 8751494 and V1504 Cyg observed with Kepler, as well as the X-ray binary Cyg X-1 observed with RXTE. This is done through the use of a high-order Fourier statistic called the bispectrum and its related biphase and bicoherence, as well as the time-skewness statistic. We show how all objects display qualitatively similar biphase trends. In particular all biphase amplitudes are found to be smaller than $\pi/2$, suggesting that the flux distributions for all sources are positively skewed on all observed timescales, consistent with the log-normal distributions expected from the fluctuating accretion disk model. We also find that for all objects the biphases are positive at frequencies where the corresponding power spectral densities display their high frequency break. This suggests that the noise-like flaring observed is rising more slowly than it is falling, and thus not time-reversible. This observation is also consistent with the fluctuating accretion disk model. Furthermore, we observe the same qualitative biphase trends in all four objects, where the biphases display a distinct decrease at frequencies below the high-frequency break in their respective power spectral densities. This behaviour can also be observed in the time-skewness of all four objects. As far as we are aware, there is no immediate explanation for the observed biphase decreases. The biphase decreases may thus suggest that the fluctuating accretion disk model begins to break down at frequencies below the high frequency break.

A physical model for the continuum variability and quasi-periodic oscillation in accreting black holes

The power spectra of black hole binaries have been well studied for decades, giving a detailed phenomenological picture of the variability properties and their correlation with the energy spectrum (spectral state) of the source. Here we take the truncated disc/hot inner flow picture which can describe the spectral changes, and show that propagating mass accretion rate fluctuations in the hot flow can match the broad band power spectral properties seen in black hole binaries, i.e. give approximately band limited noise between a low and high frequency break. The low frequency break marks the viscous timescale at the outer edge of the hot inner flow, which is the inner edge of the truncated disc. The model also predicts the Lense-Thirring precession timescale of the hot flow, as this is set by the surface density of the flow which is self consistently calculated from the propagating fluctuations. We show that this naturally gives the observed relation between the low frequency break and QPO frequency as the outer radius of the flow moves inwards, and that this model predicts many of the observed QPO properties such as correlation of coherence with frequency, and of the recently discovered correlation of frequency with flux on short timescales. We fit this total model of the variability to a sequence of 5 observed power spectra from the bright black hole binary XTE J1550-564 as the source transitioned from a low/hard to very high state. This is the first time that a power spectrum from a black hole binary has been fit with a physical model for the variability. The data are well fit if the inner radius of the flow remains constant, while the outer radius sweeps inwards from ~60-12 Rg. This range of radii agrees with models of the energy spectral evolution, giving the first self consistent description of the evolution of both the spectrum and variability of BHB.

CAIXA: a catalogue of AGN in the XMM-Newton archive

Context. We report on the results of the first XMM-Newton systematic “excess variance” study of all the radio quiet, X-ray un-obscured AGN. The entire sample consist of 161 sources observed by XMM-Newton for more than 10 ks in pointed observations, which is the largest sample used so far to study AGN X-ray variability on time scales less than a day. Aims. Recently it has been suggested that the same engine might be at work in the core of every black hole (BH) accreting object. In this hypothesis, the same variability should be observed in all AGN, once rescaled by the MBH (MBH) and accretion rate (ṁ). Methods. We systematically compute the excess variance for all AGN, on different time-scales (10, 20, 40 and 80 ks) and in different energy bands (0.3–0.7, 0.7–2 and 2–10 keV). Results. We observe a highly significant and tight (∼0.7 dex) correlation between σrms and MBH. The subsample of reverberation mapped AGN shows an even smaller scatter (only a factor of 2–3) comparable to the one induced by the MBH uncertainties. This implies that X-ray variability can be used as an accurate tool to measure MBH and this method is more accurate than the ones based on single epoch optical spectra. This allows us to measure MBH for 65 AGN and estimate lower limits for the remaining 96 AGN. On the other hand, the σrms vs. accretion rate dependence is weaker than expected based on the PSD break frequency scaling. This strongly suggests that both the PSD high frequency break and the normalisation depend on accretion rate in such a way that they almost completely counterbalance each other (PSDamp ∝ ṁ−0.8). A highly significant correlation between σrms and 2–10 keV spectral index is observed. The highly significant correlations between σrms and both the LBol and the FWHMHβ are consistent with being just by-products of the σrms vs. MBH relation. The soft and medium σ 2 rms is very well correlated with the hard σ 2 rms, with no deviations from a linear one to one correlation. This suggests that the additional soft components (i.e. soft excess, warm absorber) add a minor contribution to the total variability. Once the variability is rescaled for MBH and ṁ, no significant difference between narrow-line and broad-line Seyfert 1 is observed. Conclusions. The results are in agreement with a picture where, to first approximation, all local AGN have the same variability properties once rescaled for MBH and ṁ.

Time Variabilities of Optically Thin Black Hole Accretion Disks Reproduced by 3D MHD Simulations

We analysed the results of long time-scale global three-dimensional resistive magnetohydrodynamic simulations of radiatively inefficient accretion flows onto a black hole. We found that the power spectral density (PSD) ofthe time variation of the mass accretion rate has a power law dependence f - a with index a ∼ 1.5 in the frequency range 1M - 1 1 0 Hz < f < 80M - 1 1 0 Hz, where M 1 0 = M/10M is the mass of the black hole. The PSD has a high frequency break at f ∼ 80M - 1 1 0 Hz, above which a ∼ 2.4, and another break in the lower frequency range at f ∼ 1M - 1 1 0 Hz. Numerical results indicate that the high frequency break is related to disk oscillation in the innermost region of the disk. The lower frequency break of the PSD is due to the quasi-periodic mass accretion from the outer region.

Detection of a high frequency break in the X-ray power spectrum of Ark 564

We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of ∼3.5 decades. We detect a high frequency break at ∼2 x 10 - 3 Hz. Thepower spectrum has an rms of ∼30% and a slope of ∼-1 and ∼-2 below and above the break frequency. When combined with the results from a long RXTE observation (Pounds et al. 2001), the observed power spectra of Ark 564 and Cyg X-1 (in the low/hard state) are almost identical, showing a similar shape and rms amplitude. However, the ratio of the high frequency breaks is very small (∼10 3 - 4 ), implying that these characteristic frequencies are not indicative of the black hole mass. This result supports the idea of a small black hole mass/high accretion rate in Ark 564.

X-ray variability of AGN and correlations with spectral properties

Rapid flux changes in the X-ray emission from active galactic nuclei are commonly observed. The power-density spectra show a pseudo power-law form with a turnover at low frequencies and a high frequency break, similar to galactic black-hole candidates. There have been a few claims for periodicities but these are not a well-established property of the class. The amplitude of variability depends on a number of source parameters, including luminosity and spectral properties. The variability amplitude is correlated with X-ray spectral index, and anti-correlated with the width of the permitted optical lines. In one particularly well-observed case, NGC 7469, we see a relationship between the X-ray and UV variability, which indicates that the dominant emission process for the X-rays is thermal Comptonization. Variations in the X-ray emission are related to changes in the UV seed photons, but it appears there must also be a mechanism - perhaps that which heats the Comptonizing corona - that induces rapid variability intrinsic to the X-rays.