arXiv subjects

P. Gandhi

Publications and source records attributed to P. Gandhi.

At least 181 records · Page 10Linked to original sources

arXiv2013

Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft State Observed with Suzaku

We present observations of a transient He-like Fe K alpha absorption line in Suzaku observations of the black hole binary Cygnus X-1 on 2011 October 5 near superior conjunction during the high/soft state, which enable us to map the full evolution from the start and the end of the episodic accretion phenomena or dips for the first time. We model the X-ray spectra during the event and trace their evolution. The absorption line is rather weak in the first half of the observation, but instantly deepens for ~10 ks, and weakens thereafter. The overall change in equivalent width is a factor of ~3, peaking at an orbital phase of ~0.08. This is evidence that the companion stellar wind feeding the black hole is clumpy. By analyzing the line with a Voigt profile, it is found to be consistent with a slightly redshifted Fe XXV transition, or possibly a mixture of several species less ionized than Fe XXV. The data may be explained by a clump located at a distance of ~10^(10-12) cm with a density of ~10^((-13)-(-11)) g cm^-3, which accretes onto and/or transits the line-of-sight to the black hole, causing an instant decrease in the observed degree of the ionization and/or an increase in density of the accreting matter. Continued monitoring for individual events with future X-ray calorimeter missions such as ASTRO-H and AXSIO will allow us to map out the accretion environment in detail and how it changes between the various accretion states.

astro-ph.HE↗

arXiv2013

SN 2009js at the crossroads between normal and subluminous Type IIP supernovae: optical and mid-infrared evolution

We present a study of SN 2009js in NGC 918. Multi-band Kanata optical photometry covering the first ~120 days show the source to be a Type IIP SN. Reddening is dominated by that due to our Galaxy. One-year-post-explosion photometry with the NTT, and a Subaru optical spectrum 16 days post-discovery, both imply a good match with the well-studied subluminous SN 2005cs. The plateau phase luminosity of SN 2009js and its plateau duration are more similar to the intermediate luminosity IIP SN 2008in. Thus, SN 2009js shares characteristics with both subluminous and intermediate luminosity SNe. Its radioactive tail luminosity lies between SN 2005cs and SN 2008in, whereas its quasi-bolometric luminosity decline from peak to plateau (quantified by a newly-defined parameter Delta[logL] measuring adiabatic cooling following shock breakout) is much smaller than both the others. We estimate the ejected mass of 56Ni to be low (~0.007 Msun). The SN explosion energy appears to have been small, similar to SN 2005cs. SN 2009js is the first subluminous SN IIP to be studied in the mid-infrared. It was serendipitously caught by Spitzer at very early times. In addition, it was detected by WISE 105 days later with a significant 4.6 micron flux excess above the photosphere. The infrared excess luminosity relative to the photosphere is clearly smaller than that of SN 2004dj extensively studied in the mid-infrared. The excess may be tentatively assigned to heated dust with mass ~3e-5 Msun, or to CO fundamental emission as a precursor to dust formation.

astro-ph.SR↗

arXiv2013

The Black Hole and Central Stellar Population of MCG--6-30-15

We present the first near-infrared integral field spectroscopy observations of the galaxy MCG--6-30-15. The H-band data studied in this paper cover the central 500 pc of the galaxy at the best resolution (0".1) so far. The spectra of the innermost regions are dominated by broad brackett series emission lines and non-stellar continuum, under which we are able to trace the distribution and kinematics of the stars and also the [Fe II] line emission. We find that there is a counter-rotating stellar core extending out to 125 pc, which appears to be associated with the [Fe II] emission. Based on the mass-to-light ratio, and the presence of this emission line, we estimate the age of the central stellar population to be of order of 65 Myr. We show that the gas needed to fuel the black hole is, at most, only 1 per cent of that needed to form these stars. We derive independent constraints on the black hole mass using the dynamical information and determine an upper limit for the black hole mass, Mbh < 6e7 Msun, that is consistent with other estimates.

astro-ph.CO↗

arXiv2013

Kepler observations of rapid optical variability in the BL Lac object W2R1926+42

We present the first Kepler monitoring of a strongly variable BL Lac, W2R1926+42. The light curve covers 181 days with ~0.2% errors, 30 minute sampling and >90% duty cycle, showing numerous delta I/I > 25% flares over timescales as short as a day. The flux distribution is highly skewed and non-Gaussian. The variability shows a strong rms-flux correlation with the clearest evidence to date for non-linearity in this relation. We introduce a method to measure periodograms from the discrete autocorrelation function, an approach that may be well-suited to a wide range of Kepler data. The periodogram is not consistent with a simple power-law, but shows a flattening at frequencies below 7x10-5 Hz. Simple models of the power spectrum, such as a broken power law, do not produce acceptable fits, indicating that the Kepler blazar light curve requires more sophisticated mathematical and physical descriptions than currently in use.

astro-ph.HE↗

arXiv2013

Giant Lobes of Centaurus A Radio Galaxy Observed with the Suzaku X-ray Satellite

[abridged] We report on Suzaku observations of selected regions within the Southern giant lobe of the radio galaxy Centaurus A. We focus on distinct X-ray features likely associated with fine radio structure of the lobe. We find that the spectral properties of the detected X-ray features are equally consistent with thermal emission from hot gas, or with a power-law radiation continuum. However, the plasma parameters implied by these different models favor a synchrotron origin for the analyzed X-ray spots, indicating that a very efficient acceleration of electrons is taking place within the giant structure of Centaurus A, albeit only in isolated and compact regions. We also present a detailed analysis of the diffuse X-ray emission, resulting in a tentative detection of a soft excess component best fitted by a thermal model with a temperature of 0.5 keV. The exact origin of the observed excess remains uncertain, although energetic considerations point to thermal gas filling the bulk of the volume of the lobe and mixed with the non-thermal plasma. The corresponding pressure of the thermal gas in such a case appears to be in almost exact equipartition with the pressure provided by the radio-emitting electrons and the magnetic field. Although tentative, our findings potentially imply that the structure of the extended lobes in active galaxies is likely to be highly inhomogeneous, with magnetic reconnection processes continuously converting magnetic energy to internal energy of the plasma particles, leading to spatial and temporal variations in the plasma parameters around the equilibrium condition.

astro-ph.CO↗

arXiv2013

The largest mid-infrared atlas of active galactic nuclei at sub-arcsecond spatial scales

We present the largest mid-infrared atlas of active galactic nuclei at sub-arcsecond spatial scales containing 249 objects. It comprises all ground-based HR MIR observations performed to date. This catalog includes a large number of new observations. The photometry in multiple filters allows for characterizing the properties of the dust emission for most objects. Because of its size and characteristics, this sample is very well-suited for AGN unification studies. In particular, we discuss the enlarged MIR--X-ray correlation which extends over six orders of magnitude in luminosity and potentially probes different physical mechanisms. Finally, tests for intrinsic differences between the AGN types are presented and we discuss dependencies of MIR--X-ray properties with respect to fundamental AGN parameters such as accretion rate and the column density and covering factor of obscuring material.

astro-ph.CO↗

arXiv2013

Confirmation of the nature of the absorber in IRAS 09104+4109

We present the first long \emph{Suzaku} observation of the hyperluminous infrared galaxy IRAS 09104+4109 which is dominated by a Type 2 AGN. The infrared to X-ray SED indicates that the source is an obscured quasar with a Compton-thin absorber. However, the 3$σ$ hard X-ray detection of the source with the \emph{BeppoSAX} PDS suggested a reflection-dominated, Compton-thick view. The high-energy detection was later found to be possibly contaminated by another Type 2 AGN, NGC 2785, which is only 17 arcmin away. Our new \emph{Suzaku} observation offers simultaneous soft and hard X-ray coverage and excludes contamination from NGC 2785. We find that the hard X-ray component is not detected by the \emph{Suzaku} HXD/PIN (effective energy band 14-45 keV). Both reflection and transmission models have been tested on the latest \emph{Suzaku} and \emph{Chandra} data. The 0.5-10 keV spectrum can be well modelled by the two scenarios. In addition, our analysis implied that the absorption column required in both models is $N_{\rm H} \sim 5 \times 10^{23}$ cm$^{-2}$. Unless IRAS 09104+4109 is a "changing-look" quasar, we confirm that it is a Compton-thin AGN. Although the lack of detection of X-ray emission above 10 keV seems to favour the transmission scenario, we found that the two models offer fairly similar flux predictions over the X-ray band below $\sim$ 40 keV. We also found that the strong iron line shown in the \emph{Suzaku} spectrum is in fact a blend of two emission lines, in which the 6.4 keV one is mostly contributed from the AGN and the 6.7 keV from the hot cluster gas. This implies that the neutral line is perhaps caused by disc reflection, and the reflection-dominated model is more likely the explanation. The transmission model should not be completely ruled out, but a deeper hard X-ray spectrum observation is needed to discriminate between the two scenarios.

astro-ph.HE↗

arXiv2012

Jet spectral breaks in black hole X-ray binaries

In X-ray binaries, compact jets are known to commonly radiate at radio to infrared frequencies, whereas at optical to gamma-ray energies, the contribution of the jet is debated. The total luminosity, and hence power of the jet is critically dependent on the position of the break in its spectrum, between optically thick (self-absorbed) and optically thin synchrotron emission. This break, or turnover, has been reported in just one black hole X-ray binary (BHXB) thus far, GX 339-4, and inferred via spectral fitting in two others, A0620-00 and Cyg X-1. Here, we collect a wealth of multiwavelength data from the outbursts of BHXBs during hard X-ray states, in order to search for jet breaks as yet unidentified in their spectral energy distributions. In particular, we report the direct detection of the jet break in the spectrum of V404 Cyg during its 1989 outburst, at nu_b = (1.8 +- 0.3) x 10^14 Hz (1.7 +- 0.2 microns). We increase the number of BHXBs with measured jet breaks from three to eight. Jet breaks are found at frequencies spanning more than two orders of magnitude, from nu_b = (4.5 +- 0.8) x 10^12 Hz for XTE J1118+480 during its 2005 outburst, to nu_b > 4.7 x 10^14 Hz for V4641 Sgr in outburst. A positive correlation between jet break frequency and luminosity is expected theoretically; nu_b \propto L_nu,jet^~0.5 if other parameters are constant. With constraints on the jet break in a total of 12 BHXBs including two quiescent systems, we find a large range of jet break frequencies at similar luminosities and no obvious global relation (but such a relation cannot be ruled out for individual sources). We speculate that different magnetic field strengths and/or different radii of the acceleration zone in the inner regions of the jet are likely to be responsible for the observed scatter between sources. (abridged)

astro-ph.HE↗

arXiv2012

Angular correlation functions of X-ray point-like sources in the full exposure XMM-LSS field

Our aim is to study the large-scale structure of different types of AGN using the medium-deep XMM-LSS survey. We measure the two-point angular correlation function of ~ 5700 and 2500 X-ray point-like sources over the ~ 11 sq. deg. XMM-LSS field in the soft (0.5-2 keV) and hard (2-10 keV) bands. For the conversion from the angular to the spatial correlation function we used the Limber integral equation and the luminosity-dependent density evolution model of the AGN X-ray luminosity function. We have found significant angular correlations with the power-law parameters gamma = 1.81 +/- 0.02, theta_0 = 1.3" +/- 0.2" for the soft, and gamma = 2.00 +/- 0.04, theta_0 = 7.3" +/- 1.0" for the hard bands. The amplitude of the correlation function w(theta) is higher in the hard than in the soft band for f_x < 10^-14 erg s^-1 cm^-2 and lower above this flux limit. We confirm that the clustering strength theta_0 grows with the flux limit of the sample, a trend which is also present in the amplitude of the spatial correlation function, but only for the soft band. In the hard band, it remains almost constant with r_0 = 10h^-1$ Mpc, irrespective of the flux limit. Our analysis of AGN subsamples with different hardness ratios shows that the sources with a hard-spectrum are more clustered than soft-spectrum ones. This result may be a hint that the two main types of AGN populate different environments. Finally, we find that our clustering results correspond to an X-ray selected AGN bias factor of ~ 2.5 for the soft-band sources (at a median z = 1.1) and ~ 3.3 for the hard-band sources (at a median z = 1), which translates into a host dark matter halo mass of ~ 10^13 h^-1 M_o and ~ 10^13.7 h^-1 M_o for the soft and hard bands, respectively.

astro-ph.CO↗

arXiv2012

The near-infrared counterpart of 4U 1636-53

The optical counterpart of the neutron star X-ray binary and well known X-ray burster, 4U 1636-53 (= 4U 1636-536 = V801 Ara) has been well studied for three decades. However to date, no infrared studies have been reported. Our aims are to identify and investigate the near-infrared (NIR) counterpart of 4U 1636-53. We present deep, K_s-band (2.2 micron) imaging of the region of 4U 1636-53 taken with the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope. Archival optical and UV data are used to infer the 0.2-2.2 micron spectral energy distribution (SED). One star is located at coordinates alpha =16:40:55.57, delta =-53:45:05.2 (J2000; 1 sigma positional uncertainty of ~ 0.3 arcsec) which is consistent with the known optical position of 4U 1636-53; its magnitude is K_s = 16.14 +- 0.12. This star is also detected in the 2MASS survey in J-band and has a magnitude of J = 16.65 +- 0.22. Under the assumption that the persistent emission is largely unvarying, the 0.4-2.2 micron de-reddened SED can be described by a power law; F_nu propto nu^(1.5 +- 0.3), with some possible curvature (F_nu propto nu^(<= 1.5)) at 0.2-0.4 microns. The SED can be approximated by a blackbody of temperature ~ 27 000 K. This is typical for an active low-mass X-ray binary, and the emission can be explained by the outer regions of a (likely irradiated) accretion disc. We therefore interpret this K_s-band star as the NIR counterpart.

astro-ph.HE↗

arXiv2011

Nuclear mid-infrared properties of nearby low-luminosity AGN

We present ground-based high-spatial resolution mid-infrared (MIR) observations of 20 nearby low-luminosity AGN (LLAGN) with VLT/VISIR and the preliminary analysis of a new sample of 10 low-luminosity Seyferts observed with Gemini/Michelle. LLAGN are of great interest because these objects are the most common among active galaxies, especially in the nearby universe. Studying them in great detail makes it possible to investigate the AGN evolution over cosmic timescale. Indeed, many LLAGN likely represent the final stage of an AGN's lifetime. We show that even at low luminosities and accretion rates nuclear unresolved MIR emission is present in most objects. Compared to lower spatial resolution Spitzer/IRS spectra, the high-resolution MIR photometry exhibits significantly lower fluxes and different PAH emission feature properties in many cases. By using scaled Spitzer/IRS spectra of typical starburst galaxies, we show that the star formation contribution to the 12 micron emission is minor in the central parsecs of most LLAGN. Therefore, the observed MIR emission in the VISIR and Michelle data is most likely emitted by the AGN itself, which, for higher luminosity AGN, is interpreted as thermal emission from a dusty torus. Furthermore, the 12 micron emission of the LLAGN is strongly correlated with the absorption corrected 2-10 keV luminosity and the MIR--X-ray correlation found previously for AGN is extended to a range from 10^40 to 10^45 erg/s. This correlation is independent of the object type, and in particular the low-luminosity Seyferts observed with Michelle fall exactly on the power-law fit valid for brighter AGN. In addition, no dependency of the MIR--X-ray ratio on the accretion rate is found. These results are consistent with the unification model being applicable even in the probed low-luminosity regime.

astro-ph.CO↗

arXiv2011

The Suzaku View of the Disk-Jet Connection in the Low Excitation Radio Galaxy NGC 6251

We present results from an 87-ks Suzaku observation of the canonical low-excitation radio galaxy (LERG) NGC 6251. We have previously suggested that LERGs violate conventional AGN unification schemes: they may lack an obscuring torus and are likely to accrete in a radiatively inefficient manner, with almost all of the energy released by the accretion process being channeled into powerful jets. We model the 0.5-20 keV Suzaku spectrum with a single power law of photon index $Γ=1.82^{+0.04}_{-0.05}$, together with two collisionally ionized plasma models whose parameters are consistent with the known galaxy- and group-scale thermal emission. Our observations confirm that there are no signatures of obscured, accretion-related X-ray emission in NGC 6251, and we show that the luminosity of any such component must be substantially sub-Eddington in nature.

astro-ph.HE↗

arXiv2011

Can we measure the accretion efficiency of Active Galactic Nuclei?

The accretion efficiency for individual black holes is very difficult to determine accurately. There are many factors that can influence each step of the calculation, such as the dust and host galaxy contribution to the observed luminosity, the black hole mass and more importantly, the uncertainties on the bolometric luminosity measurement. Ideally, we would measure the AGN emission at every wavelength, remove the host galaxy and dust, reconstruct the AGN spectral energy distribution and integrate to determine the intrinsic emission and the accretion rate. In reality, this is not possible due to observational limitations and our own galaxy line of sight obscuration. We have then to infer the bolometric luminosity from spectral measurements made in discontinuous wavebands and at different epochs. In this paper we tackle this issue by exploring different methods to determine the bolometric luminosity. We first explore the trend of accretion efficiency with black hole mass (efficiency proportional to M^{\sim 0.5}) found in recent work by Davis & Laor and discuss why this is most likely an artefact of the parameter space covered by their PG quasar sample. We then target small samples of AGN at different redshifts, luminosities and black hole masses to investigate the possible methods to calculate the accretion efficiency. For these sources we are able to determine the mass accretion rate and, with some assumptions, the accretion efficiency distributions. Even though we select the sources for which we are able to determine the parameters more accurately, there are still factors affecting the measurements that are hard to constrain. We suggest methods to overcome these problems based on contemporaneous multi-wavelength data measurements and specifically targeted observations for AGN in different black hole mass ranges.

astro-ph.CO↗

arXiv2011

Mid-infrared properties of nearby low-luminosity AGN at high angular resolution

High spatial resolution mid-infrared (MIR) 12 \mum continuum imaging of low-luminosity active galactic nuclei (LLAGN) obtained by VLT/VISIR is presented. The goal of this investigation is to determine if the nuclear MIR emission of LLAGN is consistent with the existence of a dusty obscuring torus. A sample of 17 nearby LLAGN was selected and combined with archival VISIR data of 9 additional LLAGN with available X-ray measurements. Of the 17 observed LLAGN, 7 are detected, while upper limits are derived for the 10 non-detections. All detections except NGC 3125 appear point-like on a spatial scale of \sim 0.35". The detections do not significantly deviate from the known MIR-X-ray correlation but extend it by a factor of \sim 10 down to luminosities < 10^41 erg/s with a narrow scatter. The latter is dominated by the uncertainties in the X-ray luminosity. Interestingly, a similar correlation with comparable slope but with a normalization differing by \sim 2.6 orders of magnitude has been found for local starburst galaxies. In addition, the VISIR data are compared with lower spatial resolution data from Spitzer/IRS and IRAS. By using a scaled starburst template SED and the PAH 11.3 \mum emission line the maximum nuclear star formation contamination to the VISIR photometry is restricted to < 30% for 75% of the LLAGN. Exceptions are NGC 1097 and NGC 1566, which may possess unresolved strong PAH emission. Furthermore, within the uncertainties the MIR-X-ray luminosity ratio is unchanged over more than 4 orders of magnitude in accretion rate. These results are consistent with the existence of the dusty torus in all observed LLAGN, although the jet or accretion disk as origin of the MIR emission cannot be excluded. Finally, the fact that the MIR-X-ray correlation holds for all LLAGN and Seyferts makes it a very useful empirical tool for converting between the MIR and X-ray powers of these nuclei.

astro-ph.CO↗

arXiv2011

A variable mid-infrared synchrotron break associated with the compact jet in GX 339-4

Many X-ray binaries remain undetected in the mid-infrared, a regime where emission from their compact jets is likely to dominate. Here, we report the detection of the black hole binary GX 339-4 with the Wide-field Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength dataset, clear spectral curvature is found in the infrared, associated with the peak flux density expected from the compact jet. An optically-thin slope of ~-0.7 and a jet radiative power of >6x10^{35} erg/s (d/8 kpc)^2 are measured. A ~24 h WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period ~95 mins. There is also significant change during one pair of observations spaced by only 11 s. These variations imply that the spectral break associated with the transition from self-absorbed to optically-thin jet synchrotron radiation must be varying across the full wavelength range of ~3-22 microns that WISE is sensitive to, and more. Based on four-band simultaneous mid-infrared detections, the break lies at ~5x10^{13} Hz in at least two epochs of observation, consistent with a magnetic field B~1.5x10^4 G assuming a single-zone synchrotron emission region. The observed variability implies that either B, or the size of the acceleration zone above the jet base, are being modulated by factors of ~10 on relatively-short timescales.

astro-ph.HE↗

arXiv2011

A late jet rebrightening revealed from multi-wavelength monitoring of the black hole candidate XTE J1752-223

We present optical monitoring of the black hole candidate XTE J1752-223 during its 2009 - 2010 outburst and decay to quiescence. The optical light curve can be described by an exponential decay followed by a plateau, then a more rapid fade towards quiescence. The plateau appears to be due to an extra component of optical emission that brightens and then fades over ~ 40 days. We show evidence for the origin of this optical 'flare' to be the synchrotron jet during the decaying hard state, and we identify and isolate both disc and jet components in the spectral energy distributions. The optical flare has the same morphology and amplitude as a contemporaneous X-ray rebrightening. This suggests a common origin, but no firm conclusions can be made favouring or disfavouring the jet producing the X-ray flare. The quiescent optical magnitudes are B >= 20.6, V >= 21.1, R >= 19.5, i' >= 19.2. From the optical outburst amplitude we estimate a likely orbital period of < 22 h. We also present near-infrared (NIR) photometry and polarimetry and rare mid-infrared imaging (8 - 12 microns) when the source is nearing quiescence. The fading jet component, and possibly the companion star may contribute to the NIR flux. We derive deep mid-IR flux upper limits and NIR linear polarization upper limits. With the inclusion of radio data, we measure an almost flat jet spectral index between radio and optical; F_nu ~ nu^(~ +0.05). The data favour the jet break to optically thin emission to reside in the infrared, but may shift to frequencies as high as the optical or UV during the peak of the flare.

astro-ph.HE↗

arXiv2011

Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core

We present new imaging at 12.81 and 11.7 microns of the central ~40"x30" (~0.7x0.5 kpc) of the starburst galaxy M82. The observations were carried out with the COMICS mid-infrared (mid-IR) imager on the 8.2m Subaru telescope, and are diffraction-limited at an angular resolution of <0".4. The images show extensive diffuse structures, including a 7"-long linear chimney-like feature and another resembling the edges of a ruptured bubble. This is the clearest view to date of the base of the kpc-scale dusty wind known in this galaxy. These structures do not extrapolate to a single central point, implying multiple ejection sites for the dust. In general, the distribution of dust probed in the mid-IR anticorrelates with the locations of massive star clusters that appear in the near-infrared. The 10-21 micron mid-IR emission, spatially-integrated over the field of view, may be represented by hot dust with temperature of ~160 K. Most discrete sources are found to have extended morphologies. Several radio HII regions are identified for the first time in the mid-IR. The only potential radio supernova remnant to have a mid-IR counterpart is a source which has previously also been suggested to be a weak active galactic nucleus. This source has an X-ray counterpart in Chandra data which appears prominently above 3 keV and is best described as a hot (~2.6 keV) absorbed thermal plasma with a 6.7 keV Fe K emission line, in addition to a weaker and cooler thermal component. The mid-IR detection is consistent with the presence of strong [NeII]12.81um line emission. The broad-band source properties are complex, but the X-ray spectra do not support the active galactic nucleus hypothesis. We discuss possible interpretations regarding the nature of this source.

astro-ph.GA↗

arXiv2010

Rapid optical and X-ray timing observations of GX 339-4: multi-component optical variability in the low/hard state

A rapid timing analysis of VLT/ULTRACAM and RXTE observations of the black hole binary GX 339-4 in its 2007 low/hard state is presented. The optical light curves in the r', g' and u' filters show slow (~20 s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (~50 ms) and with maximum strengths of more than twice the local mean. Power spectral analysis over ~0.004-10 Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical vs. X-ray flux ratio, higher than that seen when the source was fully jet-dominated. Such a large ratio cannot be easily explained with a disc alone. The optical:X-ray cross-spectrum shows a markedly different behaviour above and below ~0.2 Hz. The peak of the coherence function above this threshold is associated with a short optical time lag, also seen as the dominant feature in the time-domain cross-correlation at ~150 ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal disc fraction of 20% at ~5000 A. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency QPO is seen in the optical but not in X-rays. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ~10 s and strong coherence in the blue u' filter. Simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is unclear. But canonical disc reprocessing cannot dominate the optical power easily, nor explain the fast variability. (abridged)

astro-ph.HE↗