High-frequency X-ray variability as a mass estimator of stellar and supermassive black holes

There is increasing evidence that supermassive black holes in active galactic nuclei (AGN) are scaled-up versions of Galactic black holes. We show that the amplitude of high-frequency X-ray variability in the hard spectral state is inversely proportional to the black hole mass over eight orders of magnitude. We have analysed all available hard-state data from RXTE of seven Galactic black holes. Their power density spectra change dramatically from observation to observation, except for the high-frequency (≳10 Hz) tail, which seems to have a universal shape, roughly represented by a power law of index -2. The amplitude of the tail, CM (extrapolated to 1 Hz), remains approximately constant for a given source, regardless of the luminosity, unlike the break or quasi-periodic oscillation frequencies, which are usually strongly correlated with luminosity. Comparison with a moderate-luminosity sample of AGN shows that the amplitude of the tail is a simple function of black hole mass, CM = C/M, where C ≈ 1.25 M⊙ Hz -1. This makes CM a robust estimator of the black hole mass which is easy to apply to low- to moderate-luminosity supermassive black holes. The high-frequency tail with its universal shape is an invariant feature of a black hole and, possibly, an imprint of the last stable orbit.</p

An absorption origin for the soft excess in Seyfert 1 active galactic nuclei

(abridged) The soft excess seen in the X-ray spectra of many high mass accretion rate AGN can be well modelled by reflection from a partially ionised accretion disc. However, the derived parameters are often extreme, both in terms of the underlying spacetime and the reflection geometry, and these models require that the disc is far from hydrostatic equilibrium. An alternative model uses similarly partially ionised, velocity smeared material but seen in absorption, though again the derived velocities are extreme, requiring magnetic driving (in the jet?) rather than a simple line driven disc wind. We find that while both models give comparably good fits to XMM--Newton data, we favour the absorption model as, unlike reflection, all the derived spectral indices are soft. This is as expected by analogy with the correspondingly high mass accretion rate stellar mass black holes. Furthermore, these X-ray spectra are consistent with a one--to--one mapping between AGN type and spectral state, with NLS1's having softer spectra corresponding to the very high state, while the broad line AGN have Gamma~2 as expected for the high/soft state. We also use the simultaneous OM data to derive the ratio of disc to total accretion power which is another tracer of spectral state in X-ray binaries. This does not always show that the disc in NLS1's contributes less than 80 per cent of the total power, as expected for a very high state. We suggest that this is an artifact of the standard disc models used to determine the disc luminosity in our fits. The disc seen in the very high state of black hole binaries is often observed to be distorted from the standard shape, and a similar effect in NLS1's could recover the correspondance between black hole binary spectral state and AGN type.Comment: 12 pages, 12 figures, submitted to MNRA

Universal spectral shape of AGN with high accretion rate

The spectra of radio quiet and NLS1 galaxies show suprising similarity in their shape. They seem to scale only with the accretion rate but not with central black hole mass. We consider two mechanisms modifying the disk spectrum. First, the outer parts of the disk are irradiated by the flux emerging from the inner parts. This is due to the scattering of the flux by the extended hot medium (warm absorber). Second process is connected with the development of the disk warm Comptonizing skin above the disk and/or coronae. Our scenario applies only to object with relatively high luminosity to the Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 4 pages, 5 figures, 1 table, Proc. of the meeting: "The Restless High-Energy Universe" (Amsterdam, The Netherlands

Compton scattering as the explanation of the peculiar X-ray properties of Cyg X-3

We consider implications of a possible presence of a Thomson-thick, low-temperature, plasma cloud surrounding the compact object in the binary system Cyg X-3. The presence of such a cloud was earlier inferred from the energy-independent orbital modulation of the X-ray flux and the lack of high frequencies in its power spectra. Here, we study the effect of Compton scattering by the cloud on the X-ray energy and power spectra, concentrating on the hard spectral state. The process reduces the energy of the high-energy break/cut-off in the energy spectra, which allows us to determine the Thomson optical depth. This, together with the observed cut-off in the power spectrum, determines the size of the plasma to be 2x10^9 cm. At this size, the cloud will be in thermal equilibrium in the photon field of the X-ray source, which yields the cloud temperature of 3 keV, which refines the determination of the Thomson optical depth to 7. At these parameters, thermal bremsstrahlung emission of the cloud becomes important as well. The physical origin of the cloud is likely to be collision of the very strong stellar wind of the companion Wolf-Rayet star with a small accretion disc formed by the wind accretion. Our model thus explains the peculiar X-ray energy and power spectra of Cyg X-3.Comment: MNRAS, the version as printed, the title and abstract change

What can we learn about quasars from alpha_OX measurements in galactic black hole binaries?

We draw a comparison between AGN and Galactic black hole binaries using a uniform description of spectral energy distribution of these two classes of accreting X-ray sources. We parametrize spectra of GBHs with an alpha_GBH parameter which we define as a slope of a nominal power law function between 3 and 20 keV. We show that this parameter can be treated as an equivalent of the X-ray loudness, alpha_OX, used to describe AGN spectra. We do not find linear correlation between the alpha_GBH and disc flux (similar to that between alpha_OX and optical/UV luminosity found in AGN). Instead, we show that alpha_GBH follows a well defined pattern during a GBH outburst. We find that alpha_GBH tend to cluster around 1, 1.5 and 2, which correspond to a hard, very high/intermediate and soft spectral state, respectively. We conclude that majority of the observed Type 1 radio quiet AGN are in a spectral state corresponding to a very high/intermediate state of GBHs. The same conclusion is valid for radio loud AGN. We also study variations of the spectral slopes (alpha_GBH and the X-ray photon index, Gamma) as a function of disc and Comptonization fluxes. We discuss these dependencies in the context of correlations of alpha_OX and Gamma with the optical/UV and X-ray 2 keV fluxes considered for AGN and quasars.Comment: 10 pages, 7 figures, accepted for publication in MNRA

Thermal proteome profiling of breast cancer cells reveals proteasomal activation by CDK4/6 inhibitor palbociclib

Palbociclib is a CDK4/6 inhibitor approved for metastatic estrogen receptor-positive breast cancer. In addition to G1 cell cycle arrest, palbociclib treatment results in cell senescence, a phenotype that is not readily explained by CDK4/6 inhibition. In order to identify a molecular mechanism responsible for palbociclib-induced senescence, we performed thermal proteome profiling of MCF7 breast cancer cells. In addition to affecting known CDK4/6 targets, palbociclib induces a thermal stabilization of the 20S proteasome, despite not directly binding to it. We further show that palbociclib treatment increases proteasome activity independently of the ubiquitin pathway. This leads to cellular senescence, which can be counteracted by proteasome inhibitors. Palbociclib-induced proteasome activation and senescence is mediated by reduced proteasomal association of ECM29. Loss of ECM29 activates the proteasome, blocks cell proliferation, and induces a senescence-like phenotype. Finally, we find that ECM29 mRNA levels are predictive of relapse-free survival in breast cancer patients treated with endocrine therapy. In conclusion, thermal proteome profiling identifies the proteasome and ECM29 protein as mediators of palbociclib activity in breast cancer cells

How many biological replicates are needed in an RNA-seq experiment and which differential expression tool should you use?

An RNA-seq experiment with 48 biological replicates in each of 2 conditions was performed to determine the number of biological replicates ($n_r$) required, and to identify the most effective statistical analysis tools for identifying differential gene expression (DGE). When $n_r=3$, seven of the nine tools evaluated give true positive rates (TPR) of only 20 to 40 percent. For high fold-change genes ($|log_{2}(FC)|\gt2$) the TPR is $\gt85$ percent. Two tools performed poorly; over- or under-predicting the number of differentially expressed genes. Increasing replication gives a large increase in TPR when considering all DE genes but only a small increase for high fold-change genes. Achieving a TPR $\gt85$% across all fold-changes requires $n_r\gt20$. For future RNA-seq experiments these results suggest $n_r\gt6$, rising to $n_r\gt12$ when identifying DGE irrespective of fold-change is important. For $6 \lt n_r \lt 12$, superior TPR makes edgeR the leading tool tested. For $n_r \ge12$, minimizing false positives is more important and DESeq outperforms the other tools.Comment: 21 Pages and 4 Figures in main text. 9 Figures in Supplement attached to PDF. Revision to correct a minor error in the abstrac

Statistical models for RNA-seq data derived from a two-condition 48-replicate experiment

High-throughput RNA sequencing (RNA-seq) is now the standard method to determine differential gene expression. Identifying differentially expressed genes crucially depends on estimates of read count variability. These estimates are typically based on statistical models such as the negative binomial distribution, which is employed by the tools edgeR, DESeq and cuffdiff. Until now, the validity of these models has usually been tested on either low-replicate RNA-seq data or simulations. A 48-replicate RNA-seq experiment in yeast was performed and data tested against theoretical models. The observed gene read counts were consistent with both log-normal and negative binomial distributions, while the mean-variance relation followed the line of constant dispersion parameter of ~0.01. The high-replicate data also allowed for strict quality control and screening of bad replicates, which can drastically affect the gene read-count distribution. RNA-seq data have been submitted to ENA archive with project ID PRJEB5348.Comment: 15 pages 6 figure

The MeV spectral tail in Cyg X-1 and optically-thin emission of jets

We study the average X-ray and soft gamma-ray spectrum of Cyg X-1 in the hard spectral state, using data from INTEGRAL. We compare these results with those from CGRO, and find a good agreement. Confirming previous studies, we find the presence of a high-energy MeV tail beyond a thermal-Comptonization spectrum; however, the tail is much softer and weaker than that recently published by Laurent et al. In spite of this difference, the observed high-energy tail could still be due to the synchrotron emission of the jet of Cyg X-1, as claimed by Laurent et al. To test this possibility, we study optically-thin synchrotron and self-Compton emission from partially self-absorbed jets. We develop formalisms for calculating both emission of the jet base (which we define here as the region where the jet starts its emission) and emission of the entire jet. We require the emission to match that observed at the turnover energy. The optically thin emission is dominated by that from the jet base, and it has to become self-absorbed within it at the turnover frequency. We find this implies the magnetic field strength at the jet base of B_0 prop. to z_0^4, where z_0 is the distance of the base from the black-hole centre. The value of B_0 is then constrained from below by the condition that the self-Compton emission is below an upper limit in the GeV range, and from above by the condition that the Poynting flux does not exceed the jet kinetic power. This yields B_0 of the order of ~10^4 G and the location of the jet base at ~10^3 gravitational radii. Using our formalism, we find the MeV tail can be due to jet synchrotron emission, but this requires the electron acceleration at a rather hard power-law index, p~1.3-1.6. For acceleration indices of p> 2, the amplitude of the synchrotron component is much below that of MeV tail, and its origin is likely to be due to hybrid Comptonization in the accretion flow.Comment: MNRAS, in press, 13 page

Evaluation of the diagnostic accuracy of prototype rapid tests for human African trypanosomiasis

Peer reviewedPublisher PD