W. N. Alston

False periodicities in quasar time-domain surveys

SV acknowledges support from STFC consolidated grant ST/K001000/1. WNA acknowledges support from the European Union Seventh Framework Programme (FP7/2013-2017) under grant agreement n.312789, StrongGravity. MJM acknowledges support from an STFC Ernest Rutherford fellowship. DH acknowledges support by the Moore–Sloan Data Science Environment at NYU. This research made use of NASA’s Astrophysics Data System. The CSS survey is funded by the National Aeronautics and Space Administration under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the US National Science Foundation under grants AST-0909182 and AST-1313422

A global look at X‐ray time lags in Seyfert galaxies

EK thanks Ari Laor for interesting discussions on this work and acknowledges support from the International Space Science Institute. This work is based on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. EK thanks the Gates Cambridge Scholarship and the Hubble Fellowship Program. Support for Program number HSTHF2-51360.001-A was provided by NASA through a Hubble Fellowship grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. EK, WNA, and ACF acknowledge support from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 312789, StrongGravity. CSR acknowledges support from NASA under grant NNX14AF86G. EMC gratefully acknowledges support from the National Science Foundation through CAREER award number 1351222.

The flux-dependent X-ray time lags in NGC 4051

We present an analysis of the X-ray time lags for the highly variable Seyfert 1 galaxy NGC 4051, based on a series of XMM-Newton observations taken in 2009. We investigate the Fourier frequency dependent time lags in the light curves between the 0.3-1.0 keV and 2.0-5.0 keV energy bands as a function of source flux, including simultaneous modelling of the resulting lag-frequency spectra. We find the shape of the lag-frequency spectra to vary significantly and systematically with source flux. We model the lag-frequency spectra using simple transfer functions, and find that two time lag components are required, one in each energy band. The simplest acceptable fits have only the relative contribution of the lagged component in the hard band varying with flux level, which can be associated with changes in the energy spectrum. We discuss the interpretation of these results in terms of the currently popular models for X-ray time lags.

The response of relativistic outflowing gas to the inner accretion disk of a black hole

The brightness of an active galactic nucleus is set by the gas falling onto it from the galaxy, and the gas infall rate is regulated by the brightness of the active galactic nucleus; this feedback loop is the process by which supermassive black holes in the centres of galaxies may moderate the growth of their hosts. Gas outflows (in the form of disk winds) release huge quantities of energy into the interstellar medium, potentially clearing the surrounding gas. The most extreme (in terms of speed and energy) of these—the ultrafast outflows—are the subset of X-ray-detected outflows with velocities higher than 10,000 kilometres per second, believed to originate in relativistic (that is, near the speed of light) disk winds a few hundred gravitational radii from the black hole. The absorption features produced by these outflows are variable, but no clear link has been found between the behaviour of the X-ray continuum and the velocity or optical depth of the outflows, owing to the long timescales of quasar variability. Here we report the observation of multiple absorption lines from an extreme ultrafast gas flow in the X-ray spectrum of the active galactic nucleus IRAS 13224−3809, at 0.236 ± 0.006 times the speed of light (71,000 kilometres per second), where the absorption is strongly anti-correlated with the emission of X-rays from the inner regions of the accretion disk. If the gas flow is identified as a genuine outflow then it is in the fastest five per cent of such winds, and its variability is hundreds of times faster than in other variable winds, allowing us to observe in hours what would take months in a quasar. We find X-ray spectral signatures of the wind simultaneously in both low- and high-energy detectors, suggesting a single ionized outflow, linking the low- and high-energy absorption lines. That this disk wind is responding to the emission from the inner accretion disk demonstrates a connection between accretion processes occurring on very different scales: the X-ray emission from within a few gravitational radii of the black hole ionizing the disk wind hundreds of gravitational radii further away as the X-ray flux rises.

NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk

We present simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR ) and Suzaku observations of the X-ray binary Cygnus X-1 in the hard state. This is the first time this state has been observed in Cyg X-1 with NuSTAR, which enables us to study the reflection and broadband spectra in unprecedented detail. We confirm that the iron line cannot be fit with a combination of narrow lines and absorption features, instead requiring a relativistically blurred profile in combination with a narrow line and absorption from the companion wind. We use the reflection models of Garcia et al. to simultaneously measure the black hole spin, disk inner radius, and coronal height in a self-consistent manner. Detailed fits to the iron line profile indicate a high level of relativistic blurring, indicative of reflection from the inner accretion disk. We find a high spin, a small inner disk radius, and a low source height and rule out truncation to greater than three gravitational radii at the 3σ confidence level. In addition, we find that the line profile has not changed greatly in the switch from soft to hard states, and that the differences are consistent with changes in the underlying reflection spectrum rather than the relativistic blurring. We find that the blurring parameters are consistent when fitting either just the iron line or the entire broadband spectrum, which is well modeled with a Comptonized continuum plus reflection model.

Revealing the X-ray variability of AGN with principal component analysis

We analyse a sample of 26 active galactic nuclei (AGN) with deep XMM–Newton observations, using principal component analysis (PCA) to find model-independent spectra of the different variable components. In total, we identify at least 12 qualitatively different patterns of spectral variability, involving several different mechanisms, including five sources which show evidence of variable relativistic reflection (MCG–6-30-15, NGC 4051, 1H 0707−495, NGC 3516 and Mrk 766) and three which show evidence of varying partial covering neutral absorption (NGC 4395, NGC 1365 and NGC 4151). In over half of the sources studied, the variability is dominated by changes in a power-law continuum, both in terms of changes in flux and power-law index, which could be produced by propagating fluctuations within the corona. Simulations are used to find unique predictions for different physical models, and we then attempt to qualitatively match the results from the simulations to the behaviour observed in the real data. We are able to explain a large proportion of the variability in these sources using simple models of spectral variability, but more complex models may be needed for the remainder. We have begun the process of building up a library of different principal components, so that spectral variability in AGN can quickly be matched to physical processes. We show that PCA can be an extremely powerful tool for distinguishing different patterns of variability in AGN, and that it can be used effectively on the large amounts of high-quality archival data available from the current generation of X-ray telescopes. We will make our PCA code available upon request to the lead author.

Discovery of an ∼2-h high-frequency X-ray QPO and iron Kα reverberation in the active galaxy MS 2254.9−3712

ABSTRACT We report the discovery of a ∼1.5×10 −4 Hz (∼2hr) X-ray quasi-periodic oscillation (QPO)in the active galaxy MS 2254.9–3712, using a ∼70ks XMM-Newtonobservation. The QPOis significantly detected ( ∼3.3σ) in the 1.2−5.0keV band only, connecting its origin withthe primary X-ray power-law continuum. We detect a highly coherent soft lag between the0.3−0.7keV and 1.2−5.0keV energy bands at the QPO frequency and at a frequency bandin a 3:2 ratio, strongly suggesting the presence of a QPO harmonic. An iron Kαreverbera-tion lag is found at the harmonic frequency, indicating the reflecting material subtends someangle to the primary continuum, which is modulated by the QPO mechanism. Frequencyresolved spectroscopy reveals the QPO and harmonic to have a hard energy dependence.These properties of the QPO variability, together with the current black hole mass estimate,M BH ∼4×10 6 M ⊙ , are consistent with the QPO originating from the same process as thehigh frequency QPO phenomenon observed in black hole X-ray binaries. Principle compo-nent analysis reveals the spectral variability in MS 2254.9–3712is similar to that of the activegalaxy RE J1034+396, a source which also displays an X-ray QPO. This suggests a distinctspectral variability pattern for accreting black holes when in a state where QPOs are present.Key words: galaxies: individual: MS 2254.9-3712 – galaxies: Seyfert – X-rays: galaxies

X-ray time delays in the narrow line Seyfert 1 galaxy PG 1244+026

We analyse the X-ray time-lags in the narrow line Seyfert 1 (NLS1) galaxy PG 1244+026 (MBH ∼ 107 M⊙, L/LEdd ∼ 1). The time delay between the soft (0.3–0.7 keV) and harder (1.2–4.0 keV) variations shows the well-established switch from hard lags at low frequencies to soft lags at high frequencies. The low-frequency hard lags are qualitatively consistent with the propagation of fluctuations model, with some long-time-scale response of the reflection component. The high-frequency soft lag appears to extend over a wide frequency band, that we divide this into two narrow frequency ranges, and examine the lag as a function of energy for each of these. At high frequencies the soft excess emission is delayed with respect to the harder energy bands, without any corresponding strong, hard X-ray reflection signature. At even higher frequencies a soft lag is seen at the softest energies, as well as tentative evidence for an iron Kα reverberation signal. These results point to the importance of reprocessing as well as reflection in determining the lags in NLS1s.

Ultraviolet and X-ray variability of NGC 4051 over 45 days with XMM-Newton and Swift

We analyse 15XMM–Newton observations of the Seyfert galaxy NGC 4051 obtained over 45 d to determine the ultraviolet (UV) light curve variability characteristics and search for correlated UV/X-ray emission. The UV light curve shows variability on all time-scales, however with lower fractional rms than the 0.2–10 keV X-rays. On days–weeks time-scales the fractional variability of the UV is Fvar ∼ 8 per cent, and on short (∼ hours) time-scales Fvar ∼ 2 per cent. The within-observation excess variance in 4 of the 15 UV observations was found to be much higher than the remaining 11. This was caused by large systematic uncertainties in the count rate masking the intrinsic source variance. For the four ‘good’ observations we fit an unbroken power-law model to the UV power spectra with slope −2.6 ± 0.5. We compute the UV/ X-ray cross-correlation function for the ‘good’ observations and find a correlation of ∼0.5 at a time lag of ∼3 ks, where the UV lags the X-rays. We also compute for the first time the UV/X-ray cross-spectrum in the range 0–28.5 ks and find a low coherence and an average time lag of ∼3 ks. Combining the 15 XMM–Newton and the Swift observations we compute the discrete correlation function over ±40 d but are unable to recover a significant correlation. The magnitude and direction of the lag estimate from the four ‘good’ observations indicate a scenario where ∼25 per cent of the UV variance is caused by thermal reprocessing of the incident X-ray emission.

Ultraviolet and X-ray variability of active galactic nuclei with Swift

ACF, AML and DJKB acknowledge support from the ERC Advanced Grant FEEDBACK 340442. WNA acknowledges support from the European Union Seventh Framework Programme (FP7/2013-2017) under grant agreement n.312789, StrongGravity. DB acknowledges an STFC studentship. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.