arXiv subjects

Phil Uttley

Publications and source records attributed to Phil Uttley.

At least 55 records · Page 3Linked to original sources

arXiv2014

No Time for Dead Time: Timing analysis of bright black hole binaries with NuSTAR

Timing of high-count rate sources with the NuSTAR Small Explorer Mission requires specialized analysis techniques. NuSTAR was primarily designed for spectroscopic observations of sources with relatively low count-rates rather than for timing analysis of bright objects. The instrumental dead time per event is relatively long (~2.5 msec), and varies by a few percent event-to-event. The most obvious effect is a distortion of the white noise level in the power density spectrum (PDS) that cannot be modeled easily with the standard techniques due to the variable nature of the dead time. In this paper, we show that it is possible to exploit the presence of two completely independent focal planes and use the cross power density spectrum to obtain a good proxy of the white noise-subtracted PDS. Thereafter, one can use a Monte Carlo approach to estimate the remaining effects of dead time, namely a frequency-dependent modulation of the variance and a frequency-independent drop of the sensitivity to variability. In this way, most of the standard timing analysis can be performed, albeit with a sacrifice in signal to noise relative to what would be achieved using more standard techniques. We apply this technique to NuSTAR observations of the black hole binaries GX 339-4, Cyg X-1 and GRS 1915+105.

astro-ph.HE↗

arXiv2014

Flexible and Scalable Methods for Quantifying Stochastic Variability in the Era of Massive Time-Domain Astronomical Data Sets

We present the use of continuous-time autoregressive moving average (CARMA) models as a method for estimating the variability features of a light curve, and in particular its power spectral density (PSD). CARMA models fully account for irregular sampling and measurement errors, making them valuable for quantifying variability, forecasting and interpolating light curves, and for variability-based classification. We show that the PSD of a CARMA model can be expressed as a sum of Lorentzian functions, which makes them extremely flexible and able to model a broad range of PSDs. We present the likelihood function for light curves sampled from CARMA processes, placing them on a statistically rigorous foundation, and we present a Bayesian method to infer the probability distribution of the PSD given the measured lightcurve. Because calculation of the likelihood function scales linearly with the number of data points, CARMA modeling scales to current and future massive time-domain data sets. We conclude by applying our CARMA modeling approach to light curves for an X-ray binary, two AGN, a long-period variable star, and an RR-Lyrae star, in order to illustrate their use, applicability, and interpretation.

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arXiv2013

Comparison of time/phase lags in the hard state and plateau state of GRS 1915+105

We investigate the complex behavior of energy- and frequency-dependent time/phase lags in the plateau state and the radio-quiet hard state of GRS 1915+105. In our timing analysis, we find that when the source is faint in the radio, QPOs are observed above 2 Hz and typically exhibit soft lags (soft photons lag hard photons), whereas QPOs in the radio-bright plateau state are found below 2.2 Hz and consistently show hard lags. The phase lag at the QPO frequency is strongly anti-correlated with the QPO frequency, changing sign at 2.2 Hz. However, the phase lag at the frequency of the first harmonic is positive and nearly independent of frequency at at ~0.172 rad, regardless of the radio emission. The lag-energy dependence at the first harmonic is also independent of radio flux. However, the lags at the QPO frequency are negative at all energies during the radio-quiet state, but lags at the QPO frequency during the plateau state are positive at all energies and show a 'reflection-type' evolution of the lag-energy spectra with respect to the radio-quiet state. The lag-energy dependence is roughly logarithmic, but there is some evidence for a break around 4-6 keV. Finally, the Fourier frequency-dependent phase lag spectra are fairly flat during the plateau state, but increase from negative to positive during the radio-quiet state. We discuss the implications of our results in the light of some generic models.

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arXiv2013

New Methods for Timing Analysis of Transient Events, Applied to Fermi/GBM Magnetar Bursts

In order to discern the physical nature of many gamma-ray sources in the sky, we must look not only in spectral and spatial dimensions, but also understand their temporal variability. However, timing analysis of sources with a highly transient nature, such as magnetar bursts, is difficult: standard Fourier techniques developed for long-term variability generally observed, for example, from AGN often do not apply. Here, we present newly developed timing methods applicable to transient events of all kinds, and show their successful application to magnetar bursts observed with Fermi/GBM. Magnetars are a prime subject for timing studies, thanks to the detection of quasi-periodicities in magnetar Giant Flares and their potential to help shed light on the structure of neutron stars. Using state-of-the art statistical techniques, we search for quasi-periodicities (QPOs) in a sample of bursts from Soft Gamma Repeater SGR J0501+4516 observed with Fermi/GBM and provide upper limits for potential QPO detections. Additionally, for the first time, we characterise the broadband variability behaviour of magnetar bursts and highlight how this new information could provide us with another way to probe these mysterious objects.

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arXiv2013

Quasi-Periodic Oscillations and broadband variability in short magnetar bursts

The discovery of quasi-periodic oscillations (QPOs) in magnetar giant flares has opened up prospects for neutron star asteroseismology. However, with only three giant flares ever recorded, and only two with data of sufficient quality to search for QPOs, such analysis is seriously data limited. We set out a procedure for doing QPO searches in the far more numerous, short, less energetic magnetar bursts. The short, transient nature of these bursts requires the implementation of sophisticated statistical techniques to make reliable inferences. Using Bayesian statistics, we model the periodogram as a combination of red noise at low frequencies and white noise at high frequencies, which we show is a conservative approach to the problem. We use empirical models to make inferences about the potential signature of periodic and quasi-periodic oscillations at these frequencies. We compare our method with previously used techniques and find that although it is on the whole more conservative, it is also more reliable in ruling out false positives. We illustrate our Bayesian method by applying it to a sample of 27 bursts from the magnetar SGR J0501+4516 observed by the Fermi Gamma-ray Burst Monitor, and we find no evidence for the presence of QPOs in any of the bursts in the unbinned spectra, but do find a candidate detection in the binned spectra of one burst. However, whether this signal is due to a genuine quasi-periodic process, or can be attributed to unmodeled effects in the noise is at this point a matter of interpretation.

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arXiv2013

Swift XRT Timing Observations of the Black-Hole Binary SWIFTJ1753.5-0127: Disk-Diluted Fluctuations in the Outburst Peak

After a careful analysis of the instrumental effects on the Poisson noise to demonstrate the feasibility of detailed stochastic variability studies with the Swift X-Ray Telescope (XRT), we analyze the variability of the black hole X-ray binary SWIFT J1753.5-0127 in all XRT observations during 2005-2010. We present the evolution of the power spectral components along the outburst in two energy bands: soft (0.5-2 keV) and hard (2-10 keV), and in the hard band find results consistent with those from the Rossi X-ray Timing Explorer (RXTE). The advantage of the XRT is that we can also explore the soft band not covered by RXTE. The source has previously been suggested to host an accretion disk extending down to close to the black hole in the low hard state, and to show low frequency variability in the soft band intrinsic to this disk. Our results are consistent with this, with at low intensities stronger low-frequency variability in the soft than in the hard band. From our analysis we are able to present the first measurements of the soft band variability in the peak of the outburst and find it to be less variable than the hard band, especially at high frequencies, opposite to what is seen at low intensity. Both results can be explained within the framework of a simple two emission-region model where the hot flow is more variable in the peak of the outburst and the disk is more variable at low intensities.

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arXiv2012

Fundamental parameters of FR II radio galaxies and their impact on groups and clusters' environments

Radio galaxies are among the largest and most powerful single objects known and are found at variety of redshifts, hence they are believed to have had a significant impact on the evolving Universe. Their relativistic jets inject considerable amounts of energy into the environments in which the sources reside; thus the knowledge of the fundamental properties (such as kinetic luminosities, lifetimes and ambient gas densities) of these sources is crucial for understanding AGN feedback in galaxy clusters. In this work, we explore the intrinsic and extrinsic fundamental properties of Fanaroff-Riley II (FR II) objects through the construction of multidimensional Monte Carlo simulations which use complete, flux limited radio catalogues and semi-analytical models of FR IIs' time evolution to create artificial samples of radio galaxies. This method allows us to set better limits on the confidence intervals of the intrinsic and extrinsic fundamental parameters and to investigate the total energy produced and injected to the clusters' environments by populations of FR IIs at various cosmological epochs (0.0<z<2.0). We find the latter estimates to be strikingly robust despite the strong degeneracy between the fundamental parameters -- such a result points to a conclusive indicator of the scale of AGN feedback in clusters of galaxies.

astro-ph.CO↗

arXiv2012

Fundamental properties of Fanaroff-Riley II radio galaxies investigated via Monte Carlo simulations

[Abridged] Radio galaxies and quasars are among the largest and most powerful single objects known and are believed to have had a significant impact on the evolving Universe and its large scale structure. We explore the intrinsic and extrinsic properties of the population of FRII objects (kinetic luminosities, lifetimes, and the central densities of their environments). In particular, the radio and kinetic luminosity functions of FRIIs are investigated using the complete, flux limited radio catalogues of 3CRR and Best et al. We construct multidimensional Monte Carlo simulations using semi-analytical models of FRII radio source growth to create artificial samples of radio galaxies. Unlike previous studies, we compare radio luminosity functions found with both the observed and simulated data to explore the fundamental source parameters. We allow the source physical properties to co-evolve with redshift, and we find that all the investigated parameters most likely undergo cosmological evolution. Strikingly, we find that the break in the kinetic luminosity function must undergo redshift evolution of at least (1+z)^3. The fundamental parameters are strongly degenerate, and independent constraints are necessary to draw more precise conclusions. We use the estimated kinetic luminosity functions to set constraints on the duty cycles of these powerful radio sources. A comparison of the duty cycles of powerful FRIIs with those determined from radiative luminosities of AGN of comparable black hole mass suggests a transition in behaviour from high to low redshifts, corresponding to either a drop in the typical black hole mass of powerful FRIIs at low redshifts, or a transition to a kinetically-dominated, radiatively-inefficient FRII population.

astro-ph.CO↗

arXiv2012

Joint spectral-timing modelling of the hard lags in GX 339-4: constraints on reflection models

The X-ray variations of hard state black hole X-ray binaries above 2 keV show 'hard lags', in that the variations at harder energies follow variations at softer energies, with a time-lag τdepending on frequency νapproximately as τ\propto ν^{-0.7}. Several models have so far been proposed to explain this time delay, including fluctuations propagating through an accretion flow, spectral variations during coronal flares, Comptonisation in the extended hot corona or a jet, or time-delays due to large-scale reflection from the accretion disc. In principle these models can be used to predict the shape of the energy spectrum as well as the frequency-dependence of the time-lags, through the construction of energy-dependent response functions which map the emission as a function of time-delay in the system. Here we use this approach to test a simple reflection model for the frequency-dependent lags seen in the hard state of GX 339-4, by simultaneously fitting the model to the frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and 2009. Our model cannot simultaneously fit both the lag and spectral data, since the relatively large lags require an extremely flared disc which subtends a large solid angle to the continuum at large radii, in disagreement with the observed Fe Kαemission. Therefore, we consider it more likely that the lags > 2 keV are caused by propagation effects in the accretion flow, possibly related to the accretion disc fluctuations which have been observed previously.

astro-ph.HE↗

arXiv2011

CAIXA: a catalogue of AGN in the XMM-Newton archive III. Excess Variance Analysis

We report on the results of the first XMM systematic "excess variance" study of all the radio quiet, X-ray un-obscured AGN. The entire sample consist of 161 sources observed by XMM 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. We 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). We observe a highly significant and tight (~0.7 dex) correlation between excess variance and MBH. The subsample of reverberation mapped AGN shows an even smaller scatter (~0.45 dex) 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. The excess variance vs. accretion rate dependence is weaker than expected based on the PSD break frequency scaling, suggesting 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. A highly significant correlation between excess variance and 2-10 keV spectral index is observed. Both the variability vs. LBol and FWHM_Hbeta correlations are consistent with being just by-products of the correlation with MBH. The soft and medium variability is very well correlated with the hard variability, suggesting 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 mdot, no significant difference between narrow-line and broad-line Seyfert 1 is observed. 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 accretion rate.

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arXiv2011

Spectro-timing analysis of Cygnus X-1 during a fast state transition

We present the analysis of two long, quasi-uninterrupted RXTE observations of Cygnus X-1 that span several days within a 10 d interval. The spectral characteristics during this observation cover the region where previous observations have shown the source to be most dynamic. Despite that the source behavior on time scales of hours and days is remarkably similar to that on year time scales. This includes a variety of spectral/temporal correlations that previously had only been observed over Cyg X-1's long-term evolution. Furthermore, we observe a full transition from a hard to a soft spectral state that occurs within less than 2.5 hours - shorter than previously reported for any other similar Cyg X-1 transition. We describe the spectra with a phenomenological model dominated by a broken power law, and we fit the X-ray variability power spectra with a combination of a cutoff power law and Lorentzian components. The spectral and timing properties are correlated: the power spectrum Lorentzian components have an energy-dependent amplitude, and their peak frequencies increase with photon spectral index. Averaged over 3.2-10 Hz, the time lag between the variability in the 4.5-5.7 keV and 9.5-15 keV bands increases with decreasing hardness when the variability is dominated by the Lorentzian components during the hard state. The lag is small when there is a large power law noise contribution, shortly after the transition to the soft state. Interestingly, the soft state not only shows the shortest lags, but also the longest lags when the spectrum is at its softest and faintest. We discuss our results in terms of emission models for black hole binaries.

astro-ph.HE↗

arXiv2011

Searching for the trigger of the AGN QPO: 8 years of RE J1034+396

RE J1034+396 is one of the most extreme Narrow-line Seyfert 1s detected thus far, showing the only quasi-periodic oscillation (QPO) reliably detected in an Active Galactic Nucleus (AGN). Comparison with similar spectral and timing properties observed in the black hole X-ray binary (BHB) GRS 1915+105 suggests that RE J1034+396 is a super-Eddington accretor. A more complete understanding of the behaviour of RE J1034+396 can therefore lead to a unification of the accretion physics between such extreme AGN and super-Eddington BHBs. Here we report on our latest XMM-Newton observations of RE J1034+396, which no longer show the QPO, indicating that this source shows a non-stationary power-spectrum. We use spectral and temporal analysis across all five XMM-Newton observations of the source to probe the evolution of the object. The combination of the shape of the fractional variability with energy and the inferred velocity of absorbing material in the line-of-sight rules out an absorption-only method of creating the QPO. Instead the periodically changing absorption may be produced by the QPO causing a change in ionization state. We extend our analysis by including the covariance spectra which give much better signal to noise than an rms spectrum. These reveal a new aspect of the QPO, which is that there is also a small contribution from a soft component which is hotter than the soft excess seen in the mean spectrum. Folding the lightcurve on the QPO period shows that this component lags behind the hard X-rays. If this is due to re-processing then the lag corresponding to a light travel time across 30 R_g. Some of the remaining observations have similar energy spectra and covariance spectra, but none of them show a significant QPO, so we conclude that none of these features are the trigger for the appearance of the QPO in this object.

astro-ph.CO↗

arXiv2010

Radio and X-ray variability in the Seyfert galaxy NGC 4051

We present intensive quasi-simultaneous X-ray and radio monitoring of the narrow line Seyfert 1 galaxy NGC 4051, over a 16 month period in 2000-2001. Observations were made with the Rossi Timing X-ray Explorer (RXTE) and the Very Large Array (VLA) at 8.4 and 4.8 GHz. In the X-ray band NGC 4051 behaves much like a Galactic black hole binary (GBH) system in a `soft-state'. In such systems, there has so far been no firm evidence for an active, radio-emitting jet like those found in `hard state' GBHs. VLBI observations of NGC 4051 show three co-linear compact components. This structure resembles the core and outer hot spots seen in powerful, jet-dominated, extragalactic radio sources and suggests the existence of a weak jet. Radio monitoring of the core of NGC 4051 is complicated by the presence of surrounding extended emission and by the changing array configurations of the VLA. Only in the A configuration is the core reasonably resolved. We have carefully removed the contaminations of the core by extended emission in the various arrays. The resulting lightcurve shows no sign of large amplitude variability (i.e. factor 50 %) over the 16 month period. Within the most sensitive configuration (A array) we see marginal evidence for radio core variability of ~25% (~0.12 mJy at 8.4GHz) on a 2-week timescale, correlated with X-ray variations. Even if the radio variations in NGC 4051 are real, the percentage variability is much less than in the X-ray band. Within the B configuration observations, where sensitivity is reduced, there is no sign of correlated X-ray/radio variability. The lack of radio variability in NGC 4051, which we commonly see in `hard state' GBHs, may be explained by orientation effects. Another possibility is that the radio emission arises from the X-ray corona, although the linear structure of the compact radio components here is hard to explain.

astro-ph.HE↗

arXiv2010

On the use of structure functions to study blazar variability: caveats and problems

The extensive use of the structure function (SF) in the field of blazar variability suggests that characteristics time-scales are embedded in the light curves of these objects. We argue that for blazar variability studies, the SF results are sometimes erroneously interpreted leading to misconceptions about the actual source properties. Based on extensive simulations we caution that spurious breaks will appear in the SFs of almost all light curves, even though these light curves may contain no intrinsic characteristic time-scales. i.e. having a featureless underlying power-spectral-density (PSD). We show that the time-scales of the spurious SF-breaks depend mainly on the length of the artificial data set and also on the character of the variability i.e. the shape of the PSD. The SF is often invoked in the framework of shot-noise models to determine the temporal properties of individual shots. We caution that although the SF may be fitted to infer the shot parameters, the resultant shot-noise model is usually inconsistent with the observed PSD. As any model should fit the data in both the time and the frequency domain the shot-noise model, in these particular cases, can not be valid. Moreover, we show that the lack of statistical independence between adjacent SF points, in the standard SF formulation, means that it is not possible to perform robust statistical model fitting following the commonly used least-squares fitting methodology. The latter yields uncertainties in the fitting parameters (i.e. slopes, breaks) that are far too small with respect to their true statistical scatter. Finally, it is also commonly thought that SFs are immune to the sampling problems, such as data gaps, which affects the estimators of the PSDs. However we show that SFs are also troubled by gaps which can induce artefacts.

astro-ph.CO↗

arXiv2009

Constraints on the absorption-dominated model for the X-ray spectrum of MCG-6-30-15

Complexities in the X-ray spectrum of the nearby Seyfert 1.2 galaxy MCG-6-30-15 are commonly interpreted in terms of a broad iron line and the associated Compton reflection hump from the innermost relativistic regions of an accretion disk around a rapidly spinning black hole. However, an alternative model has recently been proposed in which these spectral features are caused entirely by complex (ionized and partial-covering) absorption. By considering the fluorescent emission that must accompany photoelectric absorption, we show that the absorption-dominated model over-predicts the 6.4keV iron line flux unless the marginally Compton-thick absorber responsible for the hard X-ray hump satisfies very restrictive geometric constraints. In the absence of a specific model that both obeys these geometrical constraints and is physically-plausible, the relativistic-reflection model is favoured.

astro-ph.HE↗

arXiv2007

A 100ks XMM-Newton view of the Seyfert 1.8 ESO113-G010. I. Discovery of large X-ray variability and study of the FeKalpha line complex

(Abridged) We present here a long (100ks) XMM-Newton follow-up of the Seyfert 1.8 galaxy ESO113-G010 performed in November 2005, in order to study over a longer time-scale its main X-ray properties. The source was found in a higher/softer time-averaged flux state, and timing analysis of this source reveals strong, rapid variability. The Power Spectral Density (PSD) analysis indicates (at 95% c.l.) a break at 3.7 x 10^-4 Hz. This cut-off frequency is comparable to those measured in some other rapidly-variable Seyferts, such as MCG-6-30-15 and NGC4051. From the mass-luminosity-time-scale, we infer that M_BH ranges from 4 x 10^6 - 10^7 M_odot and the source is accreting at or close to the Eddington rate (or even higher). The existing data cannot distinguish between spectral pivoting of the continuum and a two-component origin for the spectral softening, primarily because the data do not span a broad enough flux range. In the case of the two-component model, the fractional offsets measured in the flux-flux plots increase significantly toward higher energies (similar to what is observed in MCG-6-30-15) as expected if there exists a constant reflection component. Contrary to May 2001, no significant highly redshifted emission line is observed (which might be related to the source flux level), while two narrow emission lines at about 6.5keV and 7keV are observed. The S/N is not high enough to establish if the lines are variable or constant. As already suggested by the 2001 observation, no significant constant narrow 6.4keV FeK line (EW~32eV) is observed, hence excluding any dominant emission from distant cold matter such as a torus in this Seyfert type 1.8 galaxy.

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arXiv2007

Eight powers of ten: similarities in black hole accretion on all mass scales

In this paper we discuss the recent advances in the quantitative comparison of accretion, and the accretion:jet coupling, in accreting black holes in both X-ray binaries (where M ~ 10Msun) and Active Galactic Nuclei (10^5Msun < M < 10^9Msun). These similarities include the radiative efficiency and jet power as a function of accretion rate, which are themselves probably the origin of the `fundamental plane of black hole activity'. A second `fundamental plane' which connects mass, accretion rate and timing properties provides us with a further physical diagnostic. Patterns of radio loudness (i.e. jet production) as a function of luminosity and accretion state are shown to be similar for X-ray binaries and AGN. Finally we discuss how neutron stars are a useful control sample, and what the future may hold for this field.

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arXiv2007

Quasi-Periodic Variability in NGC 5408 X-1

We report the discovery with XMM-Newton of quasiperiodic variability in the 0.2 - 10 keV X-ray flux from the ultraluminous X-ray source NGC 5408 X-1. The average power spectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an average amplitude (rms) of 9%, and a coherence of Q~6. In a 33 ksec time interval when the 20 mHz QPO is strongest we also find evidence for a 2nd QPO peak at 15 mHz. This is the first indication for a close pair of QPOs in a ULX source. Interestingly, the frequency ratio of this QPO pair is inconsistent with 3:2 at the 3sigma level, but is consistent with a 4:3 ratio. A powerlaw noise component with slope near 1.5 is also present below 0.1 Hz with evidence for a break to a flatter slope at about 3 mHz. The source shows substantial broadband variability, with a total amplitude (rms) of about 30% in the 0.1 - 100 mHz frequency band, and there is strong energy dependence to the variability. We discuss the implications of these findings in the context of models for ULXs, and their implications for the object's mass.

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