arXiv subjects

P. Uttley

Publications and source records attributed to P. Uttley.

At least 91 records · Page 5Linked to original sources

arXiv2012

Correlated X-ray/Ultraviolet/Optical variability in the very low mass AGN NGC 4395

We report the results of a one year Swift X-ray/UV/optical programme monitoring the dwarf Seyfert nucleus in NGC 4395 in 2008-2009. The UV/optical flux from the nucleus was found to vary dramatically over the monitoring period, with a similar pattern of variation in each of the observed UV/optical bands (spanning 1900 - 5500 Å). In particular, the luminosity of NGC 4395 in the 1900 Å band changed by more than a factor of eight over the monitoring period. The fractional variability was smaller in the UV/optical bands than that seen in the X-rays, with the X-ray/optical ratio increasing with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and each UV/optical band were well correlated, with cross correlation coefficients of >0.7, significant at 99.9 per cent confidence. Archival Swift observations from 2006 sample the intra-day X-ray/optical variability on NGC 4395. These archival data show a very strong correlation between the X-ray and b bands, with a cross-correlation coefficient of 0.84 (significant at >99 per cent confidence). The peak in the cross correlation function is marginally resolved and asymmetric, suggesting that X-rays lead the b band, but by 1 hour. In response to recent (August 2011) very high X-ray flux levels from NGC4395 we triggered Swift ToO observations, which sample the intra-hour X-ray/UV variability. These observations indicate, albeit with large uncertainties, a lag of the 1900 Å band behind the X-ray flux of ~400 s. The tight correlation between the X-ray and UV/optical lightcurves, together with the constraints we place on lag time-scale are consistent with the UV/optical variability of NGC 4395 being primarily due to reprocessing of X-ray photons by the accretion disc.

astro-ph.HE↗

arXiv2012

The Universal Nature of Accretion-induced Variability: The RMS-Flux Relation in an Accreting White Dwarf

We report the discovery of a linear relationship between the root-mean-square (rms) variability amplitude and the mean flux in the accreting white dwarf binary system MV Lyrae. Our lightcurve, obtained with the Kepler satellite, spans 633 days with quasi-continuous 58.8 second cadence resolution. We show, for the first time, how this cataclysmic variable displays linear rms-flux relations similar to those observed in many other black hole binaries, neutron star binaries and Active Galactic Nuclei. The phenomenological similarity between the rms-flux relation observed here and in other X-ray binaries suggests a common physical origin for the broad-band variability, independent of source type, mass or size of the compact accretor. Furthermore, we infer the viscosity parameter, alpha, and disk scale height, H/R, using two independent methods. In both cases, both values are found to be uncomfortably high to be accommodated by the disk instability model.

astro-ph.SR↗

arXiv2011

1H0707-495 in 2011: An X-ray source within a gravitational radius of the event horizon

The Narrow Line Seyfert 1 Galaxy 1H0707-495 went in to a low state from 2010 December to 2011 February, discovered by a monitoring campaign using the X-Ray Telescope on the Swift satellite. We triggered a 100 ks XMM-Newton observation of the source in 2011 January, revealing the source to have dropped by a factor of ten in the soft band, below 1 keV, and a factor of 2 at 5 keV, compared with a long observation in 2008. The sharp spectral drop in the source usually seen around 7 keV now extends to lower energies, below 6 keV in our frame. The 2011 spectrum is well fit by a relativistically-blurred reflection spectrum similar to that which fits the 2008 data, except that the emission is now concentrated solely to the central part of the accretion disc. The irradiating source must lie within 1 gravitational radius of the event horizon of the black hole, which spins rapidly. Alternative models are briefly considered but none has any simple physical interpretation.

astro-ph.HE↗

arXiv2011

PG 1211+143: probing high frequency lags in a high mass AGN

We present the timing analysis of the four archived XMM-Newton observations of PG 1211+143. The source is well-known for its spectral complexity, comprising a strong soft-excess and different absorption systems. Soft energy band (0.3-0.7 keV) lags are detected over all the four observations, in the frequency range $ν\lsim 6 \times 10^{-4}$ Hz, where hard lags, similar to those observed in black hole X-ray binaries, are usually detected in smaller mass AGN. The lag magnitude is energy-dependent, showing two distinct trends apparently connectable to the two flux levels at which the source is observed. The results are discussed in the context of disk- and/or corona-reprocessing scenarios, and of disk wind models. Similarities with the high-frequency negative lag of 1H 0707-495 are highlighted, and, if confirmed, they would support the hypothesis that the lag in PG 1211+143 represents the signature of the same underlying mechanism, whose temporal characteristics scale with the mass of the central object.

astro-ph.HE↗

arXiv2011

Optical and Near-IR long-term monitoring of NGC3783 and MR2251-178: evidence for variable near-IR emission from thin accretion discs

We present long term near-IR light curves for two nearby AGN: NGC3783 and MR2251-178. The near-IR data are complemented with optical photometry obtained over the same period of time. The light curves in all bands are highly variable and good correlations can be seen between optical and NIR variations. Cross-correlation analysis for NGC 3783 suggests that some disc near-IR emission is present in the J-band flux, while the H and K-bands are dominated by emission from a torus located at the dust sublimation radius. For MR2251-178 the cross-correlation analysis and the optical-near-IR flux-flux plots suggest that the near-IR flux is dominated by disc emission. We model the optical to near-IR Spectral Energy Distributions (SED) of both sources and find that disc flaring might be a necessary modification to the geometry of a thin disc in order to explain the observations. The SED of MR2251-178 gives some indications for the presence of NIR emission from a torus. Finally, we consider the implications of the standard alpha disc model to explain the different origin of the variable NIR emission in these AGN.

astro-ph.CO↗

arXiv2011

The connection between radio-loudness and central surface brightness profiles in optically-selected low-luminosity active galaxies

Recent results indicate a correlation between nuclear radio-loudness of active galaxies and their central stellar surface-brightness profiles, in that `core' galaxies (with inner logarithmic slope γ<0.3) are significantly more radio loud than `power-law' galaxies (γ>0.5). This connection, which indicates possible links between radio-loudness and galaxy formation history (e.g. through black hole spin) has so far only been confirmed for a radio-selected sample of galaxies. Furthermore, it has since been shown that the Nuker law, which was used to parameterise the brightness profiles in these studies, gives a poor description of the brightness profile. Here, we present an analysis of the central surface brightness profiles of the active galaxies of Hubble Type T<3, that were identified by the optically-selected Palomar spectroscopic survey of nearby galaxies. We fit the brightness profiles using Sersic, Core-Sersic and, where necessary, Double-Sersic models, which we fit to the semi-major axis brightness profiles extracted from high resolution images of the galaxies from the Hubble Space Telescope (HST). We use these fits to classify the galaxies as `Core', `Sersic' or `Double-Sersic' and compare this classification with the properties of the Active Galactic Nuclei (AGNs). We find that AGN hosted in Core galaxies are generally more radio-loud than those hosted in Sersic galaxies, although there is a large overlap between the two subsamples. The correlation between radio-loudness and brightness profile can partly be explained by a correlation between radio-loudness and black hole mass. Additionally, there is a significant (99 per cent confidence) partial correlation between radio-loudness and the Core/Sersic classification of the host galaxy, which lends support to the previous results based on the radio-selected sample.

astro-ph.GA↗

arXiv2011

The causal connection between disc and power-law variability in hard state black hole X-ray binaries

We use the XMM-Newton EPIC-pn instrument in timing mode to extend spectral time-lag studies of hard state black hole X-ray binaries into the soft X-ray band. We show that variations of the disc blackbody emission substantially lead variations in the power-law emission, by tenths of a second on variability time-scales of seconds or longer. The large lags cannot be explained by Compton scattering but are consistent with time-delays due to viscous propagation of mass accretion fluctuations in the disc. However, on time-scales less than a second the disc lags the power-law variations by a few ms, consistent with the disc variations being dominated by X-ray heating by the power-law, with the short lag corresponding to the light-travel time between the power-law emitting region and the disc. Our results indicate that instabilities in the accretion disc are responsible for continuum variability on time-scales of seconds or longer and probably also on shorter time-scales.

astro-ph.HE↗

arXiv2011

The rapid X-ray variability of NGC 4051

We present an analysis of the high frequency X-ray variability of NGC 4051 (M_BH ~ 1.7*10^6 M_sun) based on a series of XMM-Newton observations taken in 2009 with a total exposure of ~570 ks (EPIC pn). These data reveal the form of the power spectrum over frequencies from 10^-4 Hz, below the previously detected power spectral break, to > 10^-2 Hz, above the frequency of the innermost stable circular orbit (ISCO) around the black hole (nu_ISCO ~ 10^-3 - 10^-2 Hz, depending on the black hole spin parameter j). This is equivalent to probing frequencies >1 kHz in a stellar mass (M_BH ~ 10 Msun) black hole binary system. The power spectrum is a featureless power law over the region of the expected ISCO frequency, suggesting no strong enhancement or change in the variability at the fastest orbital period in the system. Despite the huge amplitude of the flux variations between the observations (peak-to-peak factor of >50) the power spectrum appears to be stationary in shape, and varies in amplitude at all observed frequencies following the previously established linear rms-flux relation. The rms-flux relation is offset in flux by a small and energy-dependent amount. The simplest interpretation of the offset is in terms of a very soft spectral component that is practically constant (compared to the primary source of variability). One possible origin for this emission is a circumnuclear shock energised by a radiatively driven outflow from the central regions, and emitting via inverse-Compton scattering of the central engine's optical-UV continuum (as inferred from a separate analysis of the energy spectrum). A comparison with the power spectrum of a long XMM-Newton observation taken in 2001 gives only weak evidence for non-stationarity in power spectral shape or amplitude. [Abridged]

astro-ph.HE↗

arXiv2010

Quasi-periodic Oscillations in XTE J1550-564: The rms-flux relation

We present an analysis of the short timescale variations in the properties of the strong (type "C") quasi-periodic oscillation observed in XTE J1550-564 during its 1998 outburst. In particular, the QPO shows a correlation between absolute rms amplitude and mean source flux over timescales shorter than ~ 3 ksec. A linear rms-flux relation has been observed to be a common property of broad-band noise but here we report the first detection of rms-flux dependence in a QPO. The gradient of the rms-flux relation is correlated with the QPO peak frequency: from a strong positive correlation when the QPO peak frequency is below ~ 4 Hz, through no correlation, to a strong negative correlation when the peak frequency is above 6 Hz. This is the first time a negative short term rms-flux relation has been observed in any component of the power spectrum. Previous work on both the broad-band noise and QPOs in a range of sources have suggested the presence of a filter reducing the amplitude of QPOs with increasing frequency. We attempt to remove the possible effects of this filter and find that the previously negative rms-flux relations above ~ 5 Hz become constant.

astro-ph.HE↗

arXiv2010

Understanding Reverberation Lags in 1H0707-495

The first reverberation lag from the vicinity of a supermassive black hole was recently detected in the NLS1 galaxy 1H0707-495. We interpreted the lag as being due to reflection from matter close to the black hole, within a few gravitational radii of the event horizon (an inner reflector). It has since been claimed by Miller et al that the lag can be produced by more distant matter, at hundreds of gravitational radii (an outer reflector). Here, we critically explore their interpretation of the lag. The detailed energy dependence of the time lags between soft and hard energy bands is well modelled by an inner reflector using our previously published spectral model. A contrary claim by Miller et al was obtained by neglecting the blackbody component in the soft band. Soft lags can be produced by a large-scale outer reflector if several, implausible, conditions are met. An additional transfer function is required in the soft band corresponding to a region that is physically close to the continuum source, or lies close to our line of sight and subtends a small solid angle at the source, challenging the production of the observed spectrum. We show that the original inner reflector interpretation of reverberation very close to the black hole provides a self-consistent and robust model which explains the energy spectrum and timing properties, including the time delays, power spectra and the shape of the coherence function. Several of these properties are opposite to the predictions from a simple large-scale outer reflection model.

astro-ph.HE↗

arXiv2010

X-ray and radio variability in the low luminosity Active Galactic Nucleus NGC 7213

We present the results of a ~ 3 year campaign to monitor the low luminosity active galactic nucleus (LLAGN) NGC 7213 in the radio (4.8 and 8.4 GHz) and X-ray bands (2-10 keV). With a reported X-ray Eddington ratio of 7 x 10^-4 L_Edd, NGC 7213 can be considered to be comparable to a hard state black hole X-ray binary. We show that a weak correlation exists between the X-ray and radio light curves. We use the cross-correlation function to calculate a global time lag between events in the X-ray and radio bands to be 24 +/- 12 days lag (8.4 GHz radio lagging X-ray), and 40 +/- 13 days lag (4.8 GHz radio lagging X-ray). The radio-radio light curves are extremely well correlated with a lag of 20.5 +/- 12.9 days (4.8 GHz lagging 8.4 GHz). We explore the previously established scaling relationship between core radio and X-ray luminosities and black hole mass L_{R} \propto M^{0.6-0.8} L_{X}^{0.6}, known as the `fundamental plane of black hole activity', and show that NGC 7213 lies very close to the best-fit `global' correlation for the plane as one of the most luminous LLAGN. With a large number of quasi-simultaneous radio and X-ray observations, we explore for the first time the variations of a single AGN with respect to the fundamental plane. Although the average radio and X-ray luminosities for NGC 7213 are in good agreement with the plane, we show that there is intrinsic scatter with respect to the plane for the individual data points.

astro-ph.GA↗

arXiv2010

Relativistic disc reflection in the extreme NLS1 IRAS13224-3809

We present a spectral variability study of the XMM-Newton and Suzaku observations of one of the most extreme Narrow Line Seyfert 1 galaxies, IRAS13224-3809. The X-ray spectrum is characterized by two main peculiar features, i) a strong soft excess with a steep rise below about 1.3 keV and ii) a deep drop in flux above 8.2 keV. We focus here on a reflection-based interpretation which interprets both features, as well as the large soft excess, in terms of partially ionized reflection off the inner accretion disc. We show that the two peculiar spectral features mentioned above can be reproduced by two relativistic emission lines due to Fe K and Fe L. The lines are produced in the inner accretion disc and independently yield consistent disc parameters. We argue that the high L/K intensity ratio is broadly consistent with expectations from an ionized accretion disc reflection, indicating that they belong to a single ionized reflection component. The spectral shape, X-ray flux, and variability properties are very similar in the XMM-Newton and Suzaku observations, performed about 5 years apart. The overall X-ray spectrum and variability can be described by a simple two-component model comprising a steep power law continuum plus its ionised reflection off the inner accretion disc. In this model, a rapidly rotating Kerr black hole and a steep emissivity profile are required to describe the data. The simultaneous detection of broad relativistic Fe L and K lines in IRAS 13224-3809 follows that in another extreme NLS1 galaxy, 1H0707-495. Although the data quality for IRAS13224-3809 does not allow us to rule out competing models as in 1H0707-495, we show here that our reflection-based interpretation describes in a self-consistent manner the available data and points towards IRAS13224-3809 being a very close relative of 1H0707-495 in terms of both spectral and variability properties.

astro-ph.CO↗

arXiv2009

Twelve years of X-ray and optical variability in the Seyfert galaxy NGC 4051

We discuss the origin of the optical variations in the Narrow line Seyfert 1 galaxy NGC 4051 and present the results of a cross-correlation study using X-ray and optical light curves spanning more than 12 years. The emission is highly variable in all wavebands, and the amplitude of the optical variations is found to be smaller than that of the X-rays, even after correcting for the contaminating host galaxy flux falling inside the photometric aperture. The optical power spectrum is best described by an unbroken power law model with slope $α=1.4^{+0.6}_{-0.2}$ and displays lower variability power than the 2-10 keV X-rays on all time-scales probed. We find the light curves to be significantly correlated at an optical delay of $1.2^{+1.0}_{-0.3}$ days behind the X-rays. This time-scale is consistent with the light travel time to the optical emitting region of the accretion disc, suggesting that the optical variations are driven by X-ray reprocessing. We show, however, that a model whereby the optical variations arise from reprocessing by a flat accretion disc cannot account for all the optical variability. There is also a second significant peak in the cross-correlation function, at an optical delay of $39^{+2.7}_{-8.4}$ days. The lag is consistent with the dust sublimation radius in this source, suggesting that there is a measurable amount of optical flux coming from the dust torus. We discuss the origin of the additional optical flux in terms of reprocessing of X-rays and reflection of optical light by the dust.

astro-ph.GA↗

arXiv2009

Broad iron L-line and X-ray reverberation in 1H0707-495

A detailed analysis of a long XMM-Newton observation of the Narrow Line Seyfert 1 galaxy 1H0707-495 is presented, including spectral fitting, spectral variability and timing studies. The two main features in the spectrum are the drop at ~ 7 keV and a complex excess below 1 keV. These are well described by two broad, K and L, iron lines. Alternative models based on absorption, although they may fit the high energy drop, cannot account for the 1 keV complexity and the spectrum as a whole. Spectral variability shows that the spectrum is composed of at least two components, which are interpreted as a power-law dominating between 1-4 keV, and a reflection component outside this range. The high count rate at the iron L energies has enabled us to measure a significant soft lag of ~ 30 s between 0.3-1 and 1-4 keV, meaning that the direct hard emission leads the reflected emissions. We interpret the lag as a reverberation signal originating within a few gravitational radii of the black hole.

astro-ph.HE↗

arXiv2009

The detection of Broad Iron K and L line emission in the Narrow-Line Seyfert 1 Galaxy 1H0707-495 using XMM-Newton

Since the discovery of the first broad iron-K line in 1995 from the Seyfert Galaxy MCG--6-30-15, broad iron-K lines have been found in several other Seyfert galaxies, from accreting stellar mass black holes and even from accreting neutron stars. The iron-K line is prominent in the reflection spectrum created by the hard X-ray continuum irradiating dense accreting matter. Relativistic distortion of the line makes it sensitive to the strong gravity and spin of the black hole. The accompanying iron-L line emission should be detectable when the iron abundance is high. Here we report the first discovery of both iron-K and L emission, using XMM-Newton observations of the Narrow-Line Seyfert 1 Galaxy 1H0707-495. The bright Fe-L emission has enabled us, for the first time, to detect a reverberation lag of 30 s between the direct X-ray continuum and its reflection from matter falling into the hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light-minute, from the event horizon of a rapidly-spinning, massive black hole.

astro-ph.HE↗

arXiv2009

Correlation and time delays of the X-ray and optical emission of the Seyfert Galaxy NGC3783

We present simultaneous X-ray and optical B and V band light curves of the Seyfert Galaxy NGC3783 spanning 2 years. The flux in all bands is highly variable and the fluctuations are significantly correlated. As shown before by Stirpe et al. the optical bands vary simultaneously, with a delay of less than 1.5 days but both B and V bands lag the X-ray fluctuations by 3-9 days. This delay points at optical variability produced by X-ray reprocessing and the value of the lag places the reprocessor close to the broad line region. A power spectrum analysis of the light curve, however, shows that the X-ray variability has a power law shape bending to a steeper slope at a time-scale ~2.9 days while the variability amplitude in the optical bands continues to grow towards the longest time-scale covered, ~ 300 days. We show that the power spectra together with the small value of the time delay is inconsistent with a picture where all the optical variability is produced by X-ray reprocessing, though the small amplitude, rapid optical fluctuations might be produced in this way. We detect larger variability amplitudes on long time-scales in the optical bands than in the X-rays. This behaviour adds to similar results recently obtained for at least three other AGN and indicates a separate source of long term optical variability, possibly accretion rate or thermal fluctuations in the optically emitting accretion disc.

astro-ph.CO↗

arXiv2009

The Behavior of Matter under Extreme Conditions

The cores of neutron stars harbor the highest matter densities known to occur in nature, up to several times the densities in atomic nuclei. Similarly, magnetic field strengths can exceed the strongest fields generated in terrestrial laboratories by ten orders of magnitude. Hyperon-dominated matter, deconfined quark matter, superfluidity, even superconductivity are predicted in neutron stars. Similarly, quantum electrodynamics predicts that in strong magnetic fields the vacuum becomes birefringent. The properties of matter under such conditions is governed by Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED), and the close study of the properties of neutron stars offers the unique opportunity to test and explore the richness of QCD and QED in a regime that is utterly beyond the reach of terrestrial experiments. Experimentally, this is almost virgin territory.

astro-ph.HE↗

arXiv2009

Stellar-Mass Black Holes and Their Progenitors

If a black hole has a low spin value, it must double its mass to reach a high spin parameter. Although this is easily accomplished through mergers or accretion in the case of supermassive black holes in galactic centers, it is impossible for stellar-mass black holes in X-ray binaries. Thus, the spin distribution of stellar-mass black holes is almost pristine, largely reflective of the angular momentum imparted at the time of their creation. This fact can help provide insights on two fundamental questions: What is the nature of the central engine in supernovae and gamma-ray bursts? and What was the spin distribution of the first black holes in the universe?

astro-ph.HE↗