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Previous observations with the Rossi X-ray Timing Explorer (RXTE) have suggested that the power spectral density (PSD) of NGC 3783 flattens to a slope near zero at low frequencies, in a similar manner to that of Galactic black hole X-ray binary systems (GBHs) in the `hard' state. The low radio flux emitted by this object, however, is inconsistent with a hard state interpretation. The accretion rate of NGC 3783 (~7% of the Eddington rate) is similar to that of other AGN with `soft' state PSDs and higher than that at which the GBH Cyg X-1, with which AGN are often compared, changes between `hard' and `soft' states (~2% of the Eddington rate). If NGC 3783 really does have a `hard' state PSD, it would be quite unusual and would indicate that AGN and GBHs are not quite as similar as we currently believe. Here we present an improved X-ray PSD of NGC 3783, spanning from ~10^{-8} to ~10^{-3} Hz, based on considerably extended (5.5 years) RXTE observations combined with two orbits of continuous observation by XMM-Newton. We show that this PSD is, in fact, well fitted by a `soft' state model which has only one break, at high frequencies. Although a `hard' state model can also fit the data, the improvement in fit by adding a second break at low frequency is not significant. Thus NGC 3783 is not unusual. These results leave Arakelian 564 as the only AGN which shows a second break at low frequencies, although in that case the very high accretion rate implies a `very high', rather than `hard' state PSD. The break frequency found in NGC 3783 is consistent with the expectation based on comparisons with other AGN and GBHs, given its black hole mass and accretion rate., Comment: MNRAS accepted

The aim of this paper is to study the evolution of the broadband spectrum of one of the brightest and nearest quasars 3C 273. We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM monitoring of the blazar 3C 273 in 2003--2005 in the UV, X-ray and soft gamma-ray bands and study the results in the context of the long-term evolution of the source. The 0.2-100 keV spectrum of the source is well fitted by a combination of a soft cut-off power law and a hard power law. No improvement of the fit is achieved if one replaces the soft cut-off power law by either a blackbody, or a disk reflection model. During the observation period the source has reached the historically softest state in the hard X-ray domain with a photon index $\Gamma=1.82\pm 0.01$. Comparing our data with available archived X-ray data from previous years, we find a secular evolution of the source toward softer X-ray emission (the photon index has increased by $\Delta\Gamma\simeq 0.3-0.4$ over the last thirty years). We argue that existing theoretical models have to be significantly modified to account for the observed spectral evolution of the source., Comment: 11 pages, accepted to A&A

A long-standing question is whether active galactic nuclei (AGN) vary like Galactic black hole systems when appropriately scaled up by mass (refs 1-3). If so, we can then determine how AGN should behave on cosmological timescales by studying the brighter and much faster varying Galactic systems. As X-ray emission is produced very close to the black holes, it provides one of the best diagnostics of their behaviour. A characteristic timescale, which potentially could tell us about the mass of the black hole, is found in the X-ray variations from both AGN and Galactic black holes (refs 1-6), but whether it is physically meaningful to compare the two has been questioned (ref 7). Here we report that, after correcting for variations in the accretion rate, the timescales can be physically linked, revealing that the accretion process is exactly the same for small and large black holes. Strong support for this linkage comes, perhaps surprisingly, from the permitted optical emission lines in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1 galaxies) correlate strongly with the characteristic X-ray timescale, exactly as expected from the AGN black hole masses and accretion rates. So AGN really are just scaled-up Galactic black holes., Comment: 10 pages of main paper, 3 figures and 16 pages of Supplementary Information, all in one PDF file

Discerning the exact nature of the faint (sub-mJy) radio population has been historically difficult due to the low luminosity of these sources at most wavelengths. Using deep observations from Chandra/XMM-Newton/Spitzer and ground based follow up we are able to disentangle the AGN and star-forming populations for the first time in a deep multi-frequency GMRT/VLA/MERLIN Survey. The many diagnostics include radio luminosity, morphology, radio to mid-IR flux density ratios, radio to optical flux density ratios and radio spectral indices. Further diagnostics, e.g. optical spectra X-ray spectra/hardness ratios, IR colours indicate the presence of the AGN {\em independent} of whether the radio emission is powered by AGN or star-formation. We are able to examine the star-formation history of the universe up to z=2.5 in a unique way based on an unbiased star-formation rate indicator, radio luminosity. This work provides an alternative perspective on the distribution of star-formation by mass, ``downsizing'' and allows us to examine the prevalence of AGN in star-bursts., Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of the Universe' (9-13 October 2006, Sintra, Portugal)

We use the data from a recent, 100 ksec XMM-Newton observation of the Narrow Line Seyfert 1 galaxy Ark 564 to obtain its time average, X-ray spectrum. The 3-11 keV spectrum is well fitted by a power-law of slope 2.43. We detect a weak (equivalent width ~80 eV) emission line at ~6.7 keV, which implies emission from ionized iron. There is no compelling evidence for significant broadening of the line.We also detect a possible Doppler shifted absorption line at 8.1 keV. At energies lower than 2 keV, the spectrum is dominated by a smooth soft excess component which can be well fitted either by a two black body components (kT~0.15 and 0.07 keV) or by a black body plus a relativistically blurred photoionized disc reflection model. We detect a broad, shallow flux deficit in the 0.65-0.85 keV band, reminiscent of the iron unresolved transition array (UTA) features. We do not detect neither a strong absorption edge around 0.7 keV nor an emission line around 1 keV. The soft excess emission is consistent with being reflected emission from a geometrically flat disc, with solar abundances, illuminated by an isotropic source. The weakness of the iron line emission can be explained by relativistic blurring. The UTA feature implies the presence of warm material with a column density of 2-5*10^{20} cm^{-2}. If the absorption line at 8.1 keV corresponds to FeXXVI Kalpha, it suggests the presence of a highly ionized absorbing material with N_H > 10^{23} cm^{-2}, outflowing at a high velocity of ~0.17c., Comment: 15 pages, 12 figures, accepted for publication in A&A

We use a 100 ks long XMM-Newton observation of the Narrow-Line Seyfert 1 galaxy Ark 564 and combine it with the month-long monitoring of the same source produced by ASCA, to calculate the phase lags and coherence between different energy bands, over frequencies ~ 10^{-6} to 10^{-3} Hz. This is the widest frequency range for which these spectral-timing properties have been calculated accurately for any AGN. The 0.7-2 and 2--0 keV ASCA light curves, and the XMM-Newton light curves in corresponding energy bands, are highly coherent (~ 0.9) over most of the frequency range studied. We observe time lags between the energy bands, increasing both with time-scale and with energy separation of the bands. The time lag spectrum shows a broad peak in the 10^{-5} - 5x10^{-4} Hz frequency range, where the time lags follow a power law slope ~ -0.7. Above ~ 5 x 10^{-4} Hz the lags drop below this relation significantly. This change in slope resembles the shape of the lag spectra of black hole X-ray binaries (BHXRB) in the very high or intermediate state. The lags increase linearly with the logarithm of the separation of the energy bands, which poses one more similarity between this AGN and BHXRBs., Comment: accepted for publication in MNRAS

We present the X-ray spectra of 86 optically-identified sources in the 13H XMM-Newton/Chandra deep field which have >70 X-ray counts. The sample consists of 50 broad line AGN, 25 narrow emission line galaxies, 6 absorption line galaxies, and 5 Galactic stars. The majority (42/50) of the broad line AGN have X-ray spectra which are consistent with a power law shape. They have a mean photon index of Gamma = 2.0 +- 0.1 and an intrinsic dispersion sigma = 0.4 +- 0.1. Five BLAGN show a deficit of soft X-rays, indicating absorption. Significant absorption is more common in the narrow emission line galaxies (13/25) and absorption line galaxies (2/6) than in the broad line AGN (5/50), but is not universal in any of these classes of object. The majority of the 20 absorbed sources have X-ray spectra consistent with a simple cold photoelectric absorber, but 6/20 require more complex models with either an additional component of soft X-ray emitting plasma, or an ionised absorber. Of the 16 galaxies which do not show evidence for X-ray absorption, only 2 objects are likely to be powered by star formation, and both have 2-10 keV X-ray luminosities of <= 10^40 cgs. The X-ray emission in the other 14 unabsorbed NELGs and galaxies is most likely powered by AGN, which are not detected in the optical because they are outshone by their luminous host galaxies. The Galactic stars show multi-temperature thermal spectra which peak between 0.5 and 1 keV. Star/AGN discrimination is possible for 4 of the 5 stars solely from their X-ray spectra., Comment: Accepted for publication in MNRAS

Aims. The aim of this work is to identify and study spectral features in the quasar 3C 273 usually blended by its strong jet emission. Method. A historic minimum in the sub-millimetre emission of 3C 273 triggered coordinated multi-wavelength observations in June 2004. X-ray observations from the INTEGRAL, XMM-Newton and RXTE satellites are complemented by ground-based optical, infrared, millimetre and radio observations. The overall spectrum is used to model the infrared and X-ray spectral components. Results. Three thermal dust emission components are identified in the infrared. The dust emission on scales from 1 pc to several kpc is comparable to that of other quasars, as expected by AGN unification schemes. The observed weakness of the X-ray emission supports the hypothesis of a synchrotron self-Compton origin for the jet component. There is a clear soft-excess and we find evidence for a very broad iron line which could be emitted in a disk around a Kerr black hole. Other signatures of a Seyfert-like X-ray component are not detected., Comment: 4 pages. Accepted for publication in A&A Letters

We use X-ray monitoring data obtained over a broad range of time-scales to measure the broadband power spectral density functions (PSDs) of two Seyfert galaxies, the broad line Seyfert 1 NGC 3227 and the Seyfert 2 NGC 5506, which has recently been identified as an obscured Narrow Line Seyfert 1 (NLS 1). Using a Monte-Carlo fitting technique we demonstrate that both PSDs are reminiscent of the PSD of black hole X-ray binaries (BHXRBs) in the high/soft state, and specifically rule out a low/hard state PSD shape in NGC 3227. This result demonstrates that, at least where variability is concerned, broad line Seyferts with hard X-ray spectra (photon index~1.6) are not simply the analogues of the low/hard state in BHXRBs, and the dichotomy of NLS 1 and broad line Seyferts cannot be simply interpreted in terms of the two states. We show that the PSD normalisation in NGC 3227 is strongly energy dependent, with larger variability amplitudes at lower energies, unlike NGC 5506 which shows little energy-dependence of variability. We demonstrate that this difference is caused by spectral pivoting of the continuum in NGC 3227 at high energies, which is probably also related to the large amplitude of variability seen in the 2-10 keV band in this AGN. Using the new PSD data and new results in the literature, we replot the PSD break time-scale versus mass plot for all AGN with PSD breaks measured so far, and demonstrate that higher accretion-rate AGN appear to have relatively shorter break time-scales for their black hole mass than lower-accretion rate AGN., Comment: 13 pages, 13 figures, accepted for publication in MNRAS

We present the results of a deep X-ray survey conducted with XMM-Newton, centred on the UK ROSAT 13H deep field area. This region covers 0.18 deg^2 and is the first of two areas covered with XMM-Newton as part of an extensive multi-wavelength survey designed to study the nature and evolution of the faint X-ray source population. We have produced detailed Monte-Carlo simulations to obtain a quantitative characterisation of the source detection procedure and to assess the reliability of the resultant sourcelist. We use the simulations to establish a likelihood threshold above which we expect less than 7 (3%) of our sources to be spurious. We present the final catalogue of 225 sources. Within the central 9 arcmin, 68 per cent of source positions are accurate to 2 arcsec, making optical follow-up relatively straightforward. We construct the N(>S) relation in four energy bands: 0.2-0.5 keV, 0.5-2 keV, 2-5 keV and 5-10 keV. In all but our highest energy band we find that the source counts can be represented by a double powerlaw with a bright end slope consistent with the Euclidean case and a break around 10^-14 cgs. Below this flux the counts exhibit a flattening. Our source counts reach densities of 700, 1300, 900 and 300 deg^-2 at fluxes of 4.1x10^-16, 4.5x10^-16, 1.1x10^-15 and 5.3x10^-15 cgs in the 0.2-0.5, 0.5-2, 2-5 and 5-10 keV energy bands respectively. We have compared our source counts with those in the two Chandra deep fields and Lockman hole and find our source counts to be amongst the highest of these fields in all energy bands. We resolve >51% (>50%) of the X-ray background emission in the 1-2 keV (2-5 keV) energy bands., Comment: 27 pages, 18 figures, MNRAS accepted

We present an analysis of the X-ray properties of sources detected in the 13H XMM-Newton deep (200ks) field. In order to constrain the absorbed AGN population, we use extensive Monte Carlo simulations to directly compare the X-ray colours of observed sources with those predicted by several model distributions. We have tested the simplest form of the AGN unified scheme, whereby the intrinsic XLF of absorbed AGN is set to be the same as that of their unabsorbed brethren, coupled with various model distributions of absorption. The best fitting of these models sets the fraction of AGN with absorbing column NH, proportional to (logNH)^8. We have also tested two extensions to the unified scheme: an evolving absorption scenario, and a luminosity dependent model distribution. Both of these provide poorer matches to the observed X-ray colour distributions than the best fitting simple unified model. We find that a luminosity dependent density evolution XLF reproduces poorly the 0.5-2 keV source counts seen in the 13H field. Field to field variations could be the cause of this disparity. Computing the simulated X-ray colours with a simple absorbed power-law + reflection spectral model is found to over-predict, by a factor of two, the fraction of hard sources that are completely absorbed below 0.5 keV, implying that an additional source of soft-band flux must be present for a number of the absorbed sources. Finally, we show that around 40% of the 13H sample are expected to be AGN with NH>10^22 cm^-2., Comment: 13 pages, 9 figures, Accepted for publication in MNRAS

We present an analysis of the submillimetre/X-ray properties of 19 X-ray absorbed, Compton-thin quasi-stellar objects (QSOs) selected to have luminosities and redshifts which represent the peak of cosmic QSO activity. i.e. ~L* objects at 11.5, the high star formation rates are consistent with a scenario in which the QSOs evolve to become local luminous elliptical galaxies. Combining these results with previously published data for X-ray unabsorbed QSOs and submillimetre-selected galaxies we propose the following evolutionary sequence: the forming galaxy is initially far-infrared luminous but X-ray weak similar to the sources discovered by the Submillimetre Common-User Bolometer Array; as the black hole and spheroid grow with time a point is reached when the central QSO becomes powerful enough to terminate the star formation and eject the bulk of the fuel supply (the Compton-thin absorbed QSO phase); this transition is followed by a period of unobscured QSO activity which subsequently declines to leave a quiescent spheroidal galaxy., Comment: MNRAS in press, 9 pages, 4 figs

We present a detailed study of the long-timescale X-ray variability of the Narrow Line Seyfert 1 (NLS1) Galaxy MCG-6-30-15, based on 8 years of frequent observations with the Rossi X-ray Timing Explorer. Combined with short timescale XMM-Newton observations, we derive the powerspectral density (PSD) covering 6 decades from ~10^{-8} to ~10^{-2} Hz. As with NGC4051, another NLS1, the PSD of MCG-6-30-15 is a close analogue of the PSD of a Galactic Black Hole X-ray binary system (GBH) in a `high' rather than a `low' state. Like high state GBHs, its PSD is better fitted by a smoothly bending rather than sharply breaking powerlaw model and the break frequency is 7.6^{+10}_{-3} x 10^{-5} Hz. Assuming linear scaling of break frequency with mass, we estimate the black hole (BH) mass in MCG-6-30-15 to be ~2.9^{+1.8}_{-1.6} X 10^{6} solar masses. We also derive the BH mass using a variety of new optical observations and find a value between 3.6 and 6 x 10^{6} solar masses, consistent with the mass derived from the PSD. Combining these data with revised reverberation masses for other AGN we update the BH mass/break timescale diagram. The observations are consistent with NLS1s having shorter break timescales, for a given mass, than broad line AGN, probably reflecting a higher accretion rate. However the data are also consistent with all of the X-ray bright AGN being high state objects. This result may simply be a selection effect, based on their selection from X-ray all sky catalogues, and their consequent typically high X-ray/radio ratios, which indicate high state systems., Comment: 13 pages, 14 figures. Accepted for publication in Mon. Not. R. astron. Soc

We show that the rms-flux relation recently discovered in the X-ray light curves of Active Galactic Nuclei (AGN) and X-ray binaries (XRBs) implies that the light curves have a formally non-linear, exponential form, provided the rms-flux relation applies to variations on all time-scales (as it appears to). This phenomenological model implies that stationary data will have a lognormal flux distribution. We confirm this result using an observation of Cyg X-1, and further demonstrate that our model predicts the existence of the powerful millisecond flares observed in Cyg X-1 in the low/hard state, and explains the general shape and amplitude of the bicoherence spectrum in that source. Our model predicts that the most variable light curves will show the most extreme non-linearity. This result can naturally explain the apparent non-linear variability observed in some highly variable Narrow Line Seyfert 1 (NLS1) galaxies, as well as the low states observed on long time-scales in the NLS1 NGC 4051, as being nothing more than extreme manifestations of the same variability process that is observed in XRBs and less variable AGN. That variability process must be multiplicative (with variations coupled together on all time-scales) and cannot be additive (such as shot-noise), or related to self-organised criticality, or result from completely independent variations in many separate emitting regions. Successful models for variability must reproduce the observed rms-flux relation and non-linear behaviour, which are more fundamental characteristics of the variability process than the power spectrum or spectral-timing properties. Models where X-ray variability is driven by accretion rate variations produced at different radii remain the most promising., Comment: 20 pages, 9 figures, accepted for publication in MNRAS

In order to determine the relationship between the faint X-ray and faint radio source populations, and hence to help understand the X-ray and radio emission mechanisms in those faint source populations, we have made a deep 1.4GHz Very Large Array radio survey of the 13h +38deg XMM/ROSAT X-ray Survey Area. From a combined data set of 10hours B configuration data and 14hours A configuration data, maps with 3.35'' resolution and a noise limit of 7.5uJy were constructed. A complete sample of 449 sources was detected within a 30' diameter region above a 4sigma detection limit of 30uJy, at the map centre, making this one of the deepest radio surveys at this frequency. The differential source count shows a significant upturn at sub-milliJansky flux densities, similar to that seen in other deep surveys at 1.4GHz (eg the Phoenix survey), but larger than that seen in the HDF which may have been selected to be under-dense. This upturn is well modelled by the emergence of a population of medium redshift starforming galaxies which dominate at faint flux densities. The brighter source counts are well modelled by AGNs., Comment: 13 pages, 5 figures (one only available from http://www2.iap.fr/users/seymour/ due to size), 2 tables, accepted for publication in MNRAS

We show from the recent extrapolation of the radio-FIR correlation to the MIR that the 20 cm and 15 um differential source counts are likely to come from the same parent population., Comment: 2 pages, 2 figures, to appear in proceedings of 'Multi-wavelength AGN surveys', Cozumel, 2003

We present XMM-Newton RGS observations of the soft X-ray spectrum of NGC 4051, and explore their implications for the inner accretion disk and ionized outflow in the active galactic nucleus. We fit the soft X-ray excess with a relativistically broadened O VIII recombination spectrum, including the entire line series and recombination continuum. This plus an underlying power law continuum provides a much better fit to the soft X-ray spectrum than a single temperature or disk blackbody plus power law. The emission line profiles, computed for a Kerr metric around a maximally rotating black hole, reveal a sharply peaked disk emissivity law and inner radius smaller than 1.7 R_G. The spectrum also includes narrow absorption and emission lines from C, N, O, Ne, and Fe in an ionized outflow. Outflow column densities are relatively low and do not create significant edges in the spectrum. The small amount of absorption bolsters confidence in the detection of relativistic emission line features. The narrow-line emitter has a large (76%) global covering fraction, leading to strong forbidden lines and filling in of the resonance absorption lines. We also find broad C VI Ly-alpha and very broad O VII emission from the broad-line region. The narrow and broad-line regions span large ranges in ionization parameter, and may arise in a disk outflow. The ionized absorber has a large ionization range which is inconsistent with pressure equilibrium in a multiphase medium. The mass outflow rate exceeds the accretion rate by a factor of one thousand., Comment: 43 pages, 12 figures, Accepted for publication in the Astrophysical Journal

We present over 6.5 years of frequent X-ray observations of the NLS1 galaxy NGC4051 by RXTE together with a >100ks observation by XMM-Newton and so derive a 2-10 keV PSD covering an unprecedent frequency range of over 6.5 decades from <10^-8 to >10^-2 Hz. This PSD is a very good match to the PSD of the galactic black hole binary system (GBH) Cyg X-1 when in a `high', rather than `low', state providing the first definite confirmation of an AGN in a `high' state. A bending, rather than sharply broken, powerlaw is a better description of the PSDs of Cyg X-1 in the high state and of NGC4051. At low frequencies the PSD of NGC4051 has a slope of -1.1 bending, at nu_B = 8 x 10^-4 Hz, to a slope of alpha_H ~ -2 (steeper at lower energies). If nu_B scales with mass, we imply a black hole mass of 3 x 10^5 Msolar in NGC4051, consistent with the 5 x 10^5 Msolar reverberation value. Hence NGC4051 is emitting at ~30% L_Edd. NGC4051 follows the same rms-flux relationship as GBHs. Phase lags and coherence between bands are also the same as in GBHs. The lag of soft by hard photons increases, and coherence decreases, as band separation increases. The lag, and coherence, are greater for variations of longer Fourier period. This behaviour suggests that higher photon energies and shorter variability timescales are associated with smaller radii. Using also data from the literature, we cannot fit all AGN to the same linear scaling of break timescale with black hole mass. However broad line AGN are consistent with a linear scaling from Cyg X-1 in its low state and NLS1 galaxies scale better with Cyg X-1 in its high state. We suggest that the relationship between black hole mass and break timescale is a function of at least one other underlying parameter, which may be accretion rate or black hole spin., Comment: Accepted for publication in MNRAS; 22 pages, 19 figures

The Narrow Line Seyfert 1 galaxy NGC 4051 was observed in one of its prolonged low-lux states by XMM-Newton in November 2002. Here we present the results of an analysis of EPIC-pn data obtained during the observation. Within the low state, the source shows complex spectral variability which cannot easily be explained by any simple model. However, by making a `flux-flux' plot which combines the low state data with data obtained during a normal flux state, we demonstrate that the extremely hard spectrum observed above 2 keV results from a continuation of the spectral variability seen in the normal state, which is caused by spectral pivoting of the power-law continuum. The pivoting power-law appears to be attached to a Comptonised thermal component of variable flux (blackbody temperature kT~0.1 keV, consistent with the small black hole mass in NGC 4051) which dominates the soft X-ray band in the low state, and is probably the source of seed photons for Comptonisation. Additional constant thermal and reflection components, together with absorption by ionised gas, seem to be required to complete the picture and explain the complex X-ray spectral variability seen in the low state of NGC 4051., Comment: Accepted for publication in MNRAS. 13 pages, 10 figs. A higher resolution eps version of Fig. 8 is included in the source files

The Narrow Line Seyfert 1 galaxy NGC4051 shows unusual low flux states, lasting several months, when the 2-10 keV X-ray spectrum becomes unusually hard (photon index<1) while the spectrum at lower X-ray energies is dominated by a large soft excess. A Chandra TOO of the low state has shown that the soft excess and hard components are variable and well-correlated. The variability of the hard component rules out an origin in a distant reflector. Here we present results from a recent XMM-Newton TOO of NGC4051 in the low state, which allows a much more detailed examination of the nature of the hard and soft spectral components in the low state. We demonstrate that the spectral shape in the low state is consistent with the extrapolation of the spectral pivoting observed at higher fluxes. The XMM-Newton data also reveals the warm absorbing gas in emission, as the drop in the primary continuum flux unmasks prominent emission lines from a range of ion species., Comment: 4 pages, 4 figures. Proc. of the meeting: "The Restless High-Energy Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Eds

We report the results from a Chandra TOO observation of the low-luminosity Narrow Line Seyfert 1 galaxy NGC 4051, obtained during a 6-week duration low-flux state in 2001 February. During the Chandra observation, the 2-10 keV source flux was 7e-12 erg/cm^2/s, corresponding to a 2-10 keV luminosity of 8e40 erg/s. We confirm the absence of strong extended soft X-ray emission in NGC 4051, and that the low state spectrum is dominated by the central point source. The X-ray spectrum has an unusual, hybrid shape, very soft below ~3 keV (and which we model with a black body of temperature 0.14 keV) and very hard at higher energies (power-law slope of Gamma~1). The lightcurves in both soft and hard bands are significantly variable and correlated, implying a connection between hard and soft components and proving that the hard component is dominated by primary continuum emission and is not due to pure reflection from distant cold material. However, a comparison with RXTE data obtained during the same 2001 low state suggests the presence of unusually prominent disk reflection features, which may help to explain the apparent upwards curvature and extreme hardness of the Chandra spectrum above a few keV. The shape of the 2001 low state spectrum is consistent with that observed in a brief (<3 d) low flux excursion during the source's normal state in April 2000, suggesting that emission processes during the low state are not significantly different to those in the normal state. The unusual spectral shape observed in the low state may be a continuation to low fluxes of the normal, flux-dependent spectral variability of the source., Comment: 27 pages (preprint format), 9 figures. Accepted for publication in ApJ

This paper presents the results of a dense and intensive X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051 carried out in 2000. Results of the optical analysis are consistent with previous measurements. The amplitude of optical emission line variability is a factor of two larger than that of the underlying optical continuum, but part or all of the difference can be due to host-galaxy starlight contamination or due to the lines being driven by the unseen UV continuum, which is more variable than the optical continuum. We measured the lag between optical lines and continuum and found a lower, more accurate broad line region size of 3.0+-1.5 light days in this object. The implied black hole mass is M_BH=5(+6,-3)x10^5 M_sun; this is the lowest mass found, so far, for an active nucleus. We find significant evidence for an X-ray-optical (XO) correlation with a peak lag of about <1 day, although the centroid of the asymmetric correlation function reveals that part of the optical flux varies in advance of the X-ray flux by 2.4+-1.0 days. This complex XO correlation is explained as a possible combination of X-ray reprocessing and perturbations propagating from the outer (optically emitting) parts of the accretion disc into its inner (X-ray emitting) region., Comment: 8 pages, 4 figures included, LaTeX mn.sty, accepted for publication in MNRAS

We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed an unusual hardening of the 2-10 keV X-ray spectrum which lasted several months. The spectral hardening was not accompanied by any correlated variation in flux above 8 keV. We therefore interpret the spectral change as transient absorption by a gas cloud of column density 2.6 10^23 cm^-2 crossing the line of sight to the X-ray source. A spectrum obtained by XMM-Newton during an early phase of the hard-spectrum event confirms the obscuration model and shows that the absorbing cloud is only weakly ionised. The XMM-Newton spectrum also shows that ~10% of the X-ray flux is not obscured, but this unabsorbed component is not significantly variable and may be scattered radiation from a large-scale scattering medium. Applying the spectral constraints on cloud ionisation parameter and assuming that the cloud follows a Keplerian orbit, we constrain the location of the cloud to be R~10-100 light-days from the central X-ray source, and its density to be n_H~10^8cm^-3, implying that we have witnessed the eclipse of the X-ray source by a broad line region cloud., Comment: 5 pages, 6 figures, accepted for publication in MNRAS letters

By combining complementary monitoring observations spanning long, medium and short time scales, we have constructed power spectral densities (PSDs) of six Seyfert~1 galaxies. These PSDs span $\gtrsim$4 orders of magnitude in temporal frequency, sampling variations on time scales ranging from tens of minutes to over a year. In at least four cases, the PSD shows a "break," a significant departure from a power law, typically on time scales of order a few days. This is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass compact systems with breaks on time scales of seconds. NGC 3783 shows tentative evidence for a doubly-broken power law, a feature that until now has only been seen in the (much better-defined) PSDs of low-state XRBs. It is also interesting that (when one previously-observed object is added to make a small sample of seven), an apparently significant correlation is seen between the break time scale $T$ and the putative black hole mass $M_{\rm BH}$, while none is seen between break time scale and luminosity. The data are consistent with the linear relation $ T = M_{\rm BH}/10^{6.5} \Msun$; extrapolation over 6--7 orders of magnitude is in reasonable agreement with XRBs. All of this strengthens the case for a physical similarity between Seyfert~1s and XRBs., Comment: 27 pages, 13 figures. Accepted for publication in ApJ. Typo corrected

We present the results of a Chandra ACIS-I survey of a high latitude region at 13h +38 which was earlier observed with ROSAT (McHardy et al. 1998) and which has recently been observed by XMM-Newton for 200ksec. XMM-Newton will provide good quality X-ray spectra for over 200 sources with fluxes around the knee of the logN/logS, which are responsible for the bulk of the X-ray background (XRB). The main aim of the Chandra observations is to provide arcsecond, or better, positions, and hence reliable identifications, for the XMM-Newton sources. The ACIS-I observations were arranged in a mosaic of four 30ksec pointings, covering almost all of the 15' radius XMM-Newton/ROSAT field. We detect 214 Chandra sources above 5 sigma significance, to a limiting flux of \~1.3e-15 erg/cm2/s (0.5-7 keV). Optical counterparts are derived from a Subaru SuprimeCam image reaching to R~27. The very large majority of the Chandra sources have an optical counterpart, with the distribution peaking at 23

The 13hr XMM-Newton/Chandra deep survey is the first of two extremely deep XMM-Newton fields observed by the XMM-OM consortium. A 120 ks Chandra mosaic, covering 0.2 square degrees, provides sensitive, confusion-free point source detection with sub-arcsecond positions, while the 200 ks XMM-Newton observation provides high quality X-ray spectroscopy over the same sky area. We have optical spectroscopic identifications for 70 X-ray sources. Of these, 42 are broad emission-line AGN with a wide range of redshifts. The optical counterparts of a further 23 sources are narrow emission line galaxies and absorption line galaxies. These 23 sources all lie at z < 1 and typically have lower X-ray luminosities than the broad-line AGN. About half of them show significant X-ray absorption and are almost certainly intrinsically absorbed AGN. However some of them have unabsorbed, AGN-like, power-law components in their X-ray spectra, but do not show broad emission lines in their optical spectra. These sources may be weak, unobscured AGN in bright galaxies and their existence at low redshifts could be a consequence of the strong cosmological evolution of AGN characteristic luminosities., Comment: Proceedings of the "X-ray surveys, in the light of the new observatories" workshop, Astronomische Nachrichten, in press (4 pages, 4 figures)

Our VLA observations of the XMM-Newton/Chandra 13hr deep survey field (see Page et al., this proceedings) result in one of the two deepest 1.4GHz radio maps ever made. Within the 15' radius field covered by the deep X-ray data (0.19 sq deg), a total of 556 radio sources are detected, down to a 4 sigma flux limit of 28uJy. Of the 214 Chandra sources, 55 have radio counterparts. The sub-arcsecond accuracy of the VLA and Chandra positions enable us to determine with high confidence the sources common to both surveys. Here we present the relationship between the X-ray and radio source populations at the faintest radio flux limits yet probed by such a study. We discuss how the X-ray/radio relationship differs as a function of optical morphology, ie between unresolved `stellar' objects and well resolved galaxies. We then discuss the origin of the X-ray and radio emission, ie AGN, starburst or a mixture of both, in these two classes of object., Comment: Astronomische Nachrichten, in press (Feb 2003). Proceedings of "X-ray Surveys, in the Light of New Observatories", 4-6 September, Santander, Spain

We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051 observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period between May 1996 and March 1999. The spectra were obtained as part of monitoring observations and from two long observations using the RXTE Proportional Counter Array (PCA). During the monitoring period the 2-10 keV flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE PCA observations from a distinct low state in May 1998 using the latest background and detector response models. The RXTE and BeppoSAX observations of NGC 4051 during the low state show a very hard spectrum with a strong unresolved fluorescence line. This emission, probably due to reflection from a molecular torus, is likely to be constant over long time-scales and is therefore assumed as an underlying component at all flux states. By subtracting the torus component we are able to determine the spectral variability of the primary continuum. In the variable component we observe a strong anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We show that the changes in hardness are caused by slope variability of the primary power law spectrum rather than by changing reflection or variable photoelectric absorption. The primary spectral index varies between Gamma=1.6 for the faintest states and Gamma=2.3 during the brightest states, at which level the spectral index approaches an asympotic value. We find that the response of the flux of the 6.4 keV iron fluorescence line to changes in the continuum flux depends on the timescale of the observation. The profile of the line is very broad and indicates an origin in the innermost regions of the accretion disk., Comment: accepted for publication in MNRAS

Competing models for broad spectral features in the soft X-ray spectrum of the Seyfert I galaxy Mrk766 are tested against data from a 130 ks XMM-Newton observation. A model including relativistically broadened Ly-alpha emission lines of OVIII, NVII and CVI is a better fit to 0.3-2 keV XMM RGS data than a dusty warm absorber. Moreover, the measured depth of neutral iron absorption lines in the spectrum is inconsistent with the magnitude of the iron edge required to produce the continuum break at 17-18Angstroms in the dusty warm absorber model. The relativistic emission line model can reproduce the broad-band (0.1-12 keV) XMM-EPIC data with the addition of a fourth line to represent emission from ionized iron at 6.7 keV and an excess due to reflection at energies above the iron line. The profile of the 6.7 keV iron line is consistent with that measured for the low energy lines. There is evidence in the RGS data at the 3sigma level for spectral features that vary with source flux. The covering fraction of warm absorber gas is estimated to be ~12%. Iron in the warm absorber is found to be overabundant with respect to CNO compared to solar values., Comment: Accepted for publication in ApJ Letters

In order to study systematically the soft X-ray emission of Active Galactic Nuclei (AGNs) at medium to high redshifts, we have analyzed ROSAT PSPC and HRI data of QSOs at 0.26 3.0), one of which is a narrow-line QSO - a high luminosity version of narrow-line Seyfert 1 galaxies. The average photon index () is 2.40+/-0.09 (with a dispersion of 0.57) in the 0.1-2 keV band. The average QSO spectra in four redshift bins flatten from an average photon index of ~ 2.53 at 0.25 < z < 1 to ~ 2 at 2 < z < 3.4. The flattening of the average photon index can be understood in terms of the redshift effect of the mean intrinsic QSO spectrum consisting of two components - a soft X-ray excess and a power-law component. We have also studied optical spectra of 12 of the 22 QSOs., Comment: 22 pages, uses mn2e.cls, To appear in MNRAS

We describe some of the first X-ray detections of groups of galaxies at high redshifts (z~0.4), based on the UK deep X-ray survey of McHardy et al (1998). Combined with other deep ROSAT X-ray surveys with nearly complete optical identifications, we investigate the X-ray evolution of these systems. We find no evidence for evolution of the X-ray luminosity function up to z=0.5 at the low luminosities of groups of galaxies and poor clusters (Lx>3e42 erg/s), although the small sample size precludes very accurate measurements. This result confirms and extends to lower luminosities current results based on surveys at brighter X-ray fluxes. The evolution of the X-ray luminosity function of these low luminosity systems is more sensitive to the thermal history of the intra-group medium (IGM) than to cosmological parameters. Energy injection into the IGM (from for example supernovae or AGN winds) is required to explain the X-ray properties of nearby groups. The observed lack of evolution suggests that the energy injection occured at redshifts z>0.5., Comment: 13 pages, MNRAS accepted

We present results from RXTE data obtained during a systematic monitoring program of four Seyfert galaxies (NGC5548, NGC5506, MCG-6-30-15 and NGC4051). We studied the variability of three hardness ratios derived from the light curves in four energy bands. All the objects show similar spectral variations in all ratios. In order to interpret the results we computed the hardness ratios corresponding to a simple spectral model of a power law plus iron line plus reflection component, considering two possibilities: a)variations with constant line equivalent width and reflection parameter R and b) variations with constant line and reflection flux. The overall, mean observed trends can be explained by spectral slope variations and a constant flux Fe line and reflection component, although the existence of a line component which is variable on short time scales cannot be excluded. Finally, we find that the data are not consistent with an increase of R with flux for individual sources, indicating that, as a single source varies, softer spectra do not correspond to larger R values., Comment: 24 pages, 8 figures, accepted for publication in ApJ

We have developed a Monte Carlo technique to test models for the true power spectra of intermittently sampled lightcurves against the noisy, observed power spectra, and produce a reliable estimate of the goodness of fit of the given model. We apply this technique to constrain the broadband power spectra of a sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer (RXTE) over three years. We show that the power spectra of three of the AGN in our sample (MCG-6-30-15, NGC5506 and NGC3516) flatten significantly towards low frequencies, while the power spectrum of NGC5548 shows no evidence of flattening. We fit two models for the flattening, a `knee' model, analogous to the low-frequency break seen in the power spectra of BHXRBs in the low state (where the power-spectral slope flattens to \alpha=0) and a `high-frequency break' model (where the power-spectral slope flattens to \alpha=1), analogous to the high-frequency break seen in the high and low-state power spectra of the classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of all four AGN. For both models, the characteristic frequency for flattening is significantly higher in MCG-6-30-15 than in NGC 3516 (by factor ~10) although both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming linear scaling of characteristic frequencies with black hole mass, the high accretion rate implied for MCG-6-30-15 favours the high-frequency break model for this source and further suggests that MCG-6-30-15 and possibly NGC 5506, may be analogues of Cyg X-1 in the high state [ABRIDGED]., Comment: 23 pages, accepted for publication in MNRAS

Light curves of 3C 279 are presented in optical (R-band), X-rays (RXTE/PCA), and gamma rays (CGRO/EGRET) for 1999 Jan-Feb and 2000 Jan-Mar. During both of those epochs the gamma-ray levels were high, and all three observed bands demonstrated substantial variation, on time scales as short as one day. Correlation analyses provided no consistent pattern, although a rather significant optical/gamma-ray correlation was seen in 1999, with a gamma-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the gamma-ray and X-ray light curves during the large gamma ray flare in 1996 Feb and the two gamma-bright weeks leading up to it; the correlation at that time was strong, with a gamma-ray/X-ray offset of no more than 1 day., Comment: 20 pages, including 7 figures; accepted by The Astrophysical Journal

We report the discovery of a narrow-line QSO (NLQSO) RX J1334.2+3759 with a steep soft X-ray spectrum. Soft X-ray emission from the NLQSO is highly variable. Changes in the intensity by a factor of about 2 have been detected in the ROSAT PSPC observations of RX J1334.2+3759 on time scales of 20000-40000s. Rapid variability events have also been observed from RX J1334.2+3759. The most extreme variable event has Delta L/Delta t = (1.95+/-1.02)E42 erg/s/s corresponding to a change in intensity by a factor of about 4 within 400s. The PSPC spectra of the NLQSO can be well represented by a power-law of photon index 3.8, modified by an absorbing column local to the source over and above that due to our own Galaxy. The intrinsic soft X-ray luminosity of RX J1334.2+3759 is estimated to be 2.2E44 erg/s in the energy band of 0.1-2.0 keV. The optical spectrum of RX J1334.2+3759 is typical of the NLS1 galaxies showing strong Balmer Hbeta, Halpha, and forbidden line [O III]5007. Fe II multiplets, usually present in the optical spectra of NLS1 galaxies, are also detected from RX J1334.2+3759. Decomposition of the Hbeta and Halpha line profiles shows the presence of narrow (FWHM = 880 km/s) and broad (FWHM = 2850 km/s) components in the spectrum of RX J1334.2+3759. The narrow-line region of RX J1334.2+3759 appears to be significantly different from those of normal Seyfert galaxies. A possible explanation for the observed properties of the narrow line region and the broad line region is suggested in terms of density enhancements., Comment: 13 pages with 9 figures, LaTex. To appear in MNRAS

We report on some results from a three-year program of RXTE observations of 4 Seyfert galaxies: NGC 4051, NGC 5548, MCG-6-30-15 and NGC 5506. We focus here on color-color and color-flux diagrams obtained using the count-rates of the PCA instrument in different energy ranges: 3--5, 5--7 and 7--10 keV. The data of the different sources show interesting trends in the different color-color and color-flux planes. These trends are quite similar from one source to the other, even in the case of the narrow line Seyfert 1 NGC 4051 which exhibits strong flux and spectral variability. We discuss these results in term of a simple spectral model: a power law with high energy cut-off + reflection + Iron line, with the scope of understanding the relations between the observed variations of different components and the physical causes of the variability., Comment: 6 pages, 3 figures (uses newpasp.sty included). To appear in Proc. of "X-ray astronomy 2000",(Palermo Sep. 2000), Eds. R. Giacconi, L. Stella, S. Serio, ASP Conf. Series, in press

We present multiwaveband photometric and optical spectropolarimetric observations of the R=15.9 narrow emission line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infra-red and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray errorbox and so conclude that R117_A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117_A., Comment: 8 pages, 9 figures, MNRAS accepted

We report on observations of the narrow line Seyfert galaxy NGC 5506 with the Rossi X-ray TIming Explorer. The observations cover a time interval of ~1000 days during which the source showed strong flux and spectral variability. The spectrum clearly shows iron fluorescence emission at 6.4 keV and significant reflection features. Both the equivalent width of the iron line and the relative strength of the reflected continuum are higher during low flux states. The variability of the reflection features can be explained by the presence of two reflected components: one that responds rapidly to flux changes of the primary continuum and a second, slowly variable, component originating from a distant reflector, e.g. a molecular torus., Comment: 7 pages, 7 figures. Accepted for publication in MNRAS

We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the time-averaged X-ray and optical continuum fluxes are well-correlated. Variations in the flux of the broad Hbeta line are found to lag behind the optical continuum variations by 6 days (with an uncertainty of 2-3 days), and combining this with the line width yields a virial mass estimate of about 1.1 million solar masses, at the very low end of the distribution of AGN masses measured by line reverberation. Strong variability of He II 4686 is also detected, and the response time measured is similar to that of Hbeta, but with a much larger uncertainty. The He II 4686 line is almost five times broader than Hbeta, and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archival spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low to moderate inclination disk-like configuration, and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. Previous observations of the narrow-line region morphology of this source suggest that the system is inclined by about 50 degrees, and if this is applicable to the broad Hbeta-emitting region, a central mass of about 1.4 million solar masses can be inferred. During the third year of monitoring, both the X-ray continuum and the He II 4686 line went into extremely low states, although the optical continuum and the Hbeta broad line were both still present and variable. We suggest that the inner part of the accretion disk may have gone into an advection-dominated state, yielding little radiation from the hotter inner disk., Comment: 29 pages, 10 figures. accepted for publication in The Astrophysical Journal

We report on a three-year program of coordinated X-ray and optical monitoring of the Narrow-Line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the X-ray and optical continuum fluxes are correlated on time scales of many weeks and longer. Variations in the flux of the broad H-beta line are found to lag behind the optical continuum variations by approximately 6 days (with an uncertainty of 2-3 days), and combining this with the line width yields a virial mass estimate of about 1.1e6 solar masses, at the very low end of the distribution of AGN masses measured by line reverberation. Strong variability of He II lambda 4686 is also detected, and the response time measured is similar to that of H-beta, but with a much larger uncertainty. The He II lambda 4686 line is almost five times broader than H-beta, and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archive spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low-inclination (nearly face-on) disk-like configuration, and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. During the third year of monitoring, both the X-ray continuum and the He II lambda 4686 line went into extremely low states, although the optical continuum and the H-beta broad line were both still present and variable. We suggest that the inner part of the accretion disk may have gone into an advection-dominated state, yielding little radiation from the hotter inner disk., Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-workshop

The VLBI observations at 22 GHz and 43 GHz of the quasar 3C273 obtained during a multi-frequency campaign in late 1992 in the radio, millimetre and X-ray bands allow us to derive the components' angular sizes, their peak fluxes and turnover frequencies. Lower limits to the Doppler factors have been derived by comparing the observed X-ray fluxes with those predicted by the Synchrotron Self-Compton model. Independent estimates of the Doppler factors were obtained through the assumption of the energy equipartition between the particles and the magnetic field. Of the five components used to model the first two milli-arcseconds of the jet, apart from the core, two components are in equipartition and the remaining two, at larger distances from the core, have large Doppler factors and are mainly responsible for the X-ray emission due to the Synchrotron Self-Compton process., Comment: 12 pages, plain tex, plus 2 .ps figures, accepted by Astronomy and Astrophysics, Main Journal, also available at http://www.bo.astro.it/bap/BAPhome/bap99-32-text.ps

We present a flux variability study of simultaneous RXTE and EUVE observations of the highly variable Seyfert galaxy NGC4051. We find a strong correlation between variability in the EUV and medium energy X-ray bands,indicating that both are sampling the same power-law continuum. The lag between the two bands is less than 20 ks and, depending on model assumptions, may be <1 ks. We examine the consequences of such a small lag in the context of simple Comptonisation models for the production of the power-law continuum. A lag of <1 ks implies that the size of the Comptonising region is less than 20 Schwarzschild radii for a black hole of mass >1E6 solar masses., Comment: 8 pages, accepted for publication in MNRAS

The BL Lacertae object PKS 1413+135 is associated with a disk dominated galaxy which heavily absorbs the BL Lac nucleus at optical and X-ray wavelengths. It has been argued whether this galaxy is actually the host galaxy of PKS 1413+135 or whether the BL Lac is a background QSO, gravitationally lensed by the apparent host galaxy. We have obtained deep high resolution H-band images of this unusual BL Lac object using the UKIRT IRCAM3. Our observations show that the BL Lac nucleus is centered within < 0.05 arcsec of the galaxy. Based on this result we assess the probability for the lensing scenario and come to the conclusion that the disk galaxy is indeed the host of PKS 1413+135. The galaxy shows peanut-shaped isophotes, suggesting the presence of a central bar which is a common feature of AGN, Comment: accepted for publication in MNRAS, 5 pages with 2 Postscript figures

The gamma-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT and EGRET for three weeks between December 1992 and January 1993. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during the above multiwavelength campaign. A maximum UV variation of ~50% is detected, while during the same period the X-ray flux varied by no more than 13%. At the lowest UV flux level the average spectrum in the 1230-2700 A interval is unusually flat for this object (~1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or could be due to the presence of a thermal component at ~20000 K possibly associated with an accretion disk. The presence of an accretion disk in this blazar object, likely observable only in very low states and otherwise hidden by the beamed, variable synchrotron component, would be consistent with the scenario in which the seed photons for the inverse Compton mechanism producing the gamma-rays are external to the relativistic jet. We further discuss the long term correlation of the UV flux with the X-ray and gamma-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X- and gamma-ray fluxes are generally well correlated with the UV flux, approximately with square root and quadratic dependences, respectively., Comment: 22 pages, Latex, 7 PostScript figures, to appear in The Astrophysical Journal

At the faint end of the deepest X-ray surveys, a population of X-ray luminous galaxies is seen. In this paper, we present the results of a cross-correlation between the residual, unresolved X-ray photons in a very deep X-ray survey and the positions of faint galaxies, in order to examine the importance of these objects at even fainter flux levels. We measure a significant correlation on all angular scales up to ~1 arcmin. This signal could account for a significant fraction of the unresolved X-ray background, approximately 35 per cent if the clustering is similar to optically selected galaxies. However, the angular form of the correlation is seen to be qualitatively similar to that expected for clusters of galaxies and the X-ray emission could be associated with hot gas in clusters or with QSOs within galaxy clusters rather than emission from individual faint galaxies. The relative contribution from each of these possibilities cannot be determined with the current data., Comment: 7 pages LaTeX including 9 EPS figures. Uses mn.sty. Accepted for publication by MNRAS

On 9-11 May 1998, the highly-variable, low luminosity Seyfert 1 galaxy NGC4051 was observed in an unusual low flux state by BeppoSAX (Guainazzi et al. 1998) RXTE and EUVE. We present fits of the 4-15 keV RXTE spectrum and BeppoSAX MECS spectrum obtained during this observation, which are consistent with the interpretation that the source had switched off, leaving only the spectrum of pure reflection from distant cold matter. We place this result in context by showing the X-ray lightcurve of NGC4051 obtained by our RXTE monitoring campaign over the past two and a half years, which shows that the low state lasted for ~150 days before the May observations (implying that the reflecting material is > 10^17 cm from the continuum source) and forms part of a lightcurve showing distinct variations in long-term average flux over timescales > months. We show that the long-timescale component to X-ray variability is intrinsic to the primary continuum and is probably distinct from the variability at shorter timescales, possibly associated with variations in the accretion flow of matter onto the central black hole. As the source approaches the low state, the variability process becomes non-linear. NGC4051 may represent a microcosm of all X-ray variability in radio quiet active galactic nuclei (AGNs), displaying in a few years a variety of flux states and variability properties which more luminous AGNs may pass through on timescales of decades to thousands of years., Comment: 5 pages, accepted for publication in MNRAS

The results of VLBI observations of the quasar 3C273, obtained during a multi-frequency campaign in late 1992 in the radio, millimeter and X-ray bands are presented and discussed. The VLBI observations were made at 22 GHz with a Global Array and at 43 GHz with the Very Long Baseline Array. Hybrid maps and modelfits were made in order to look for any short time scale structural variations of the inner part of the radio jet. In 42 days 3C273 was observed 5 times at roughly 10 day intervals. The jet structure did not show dramatic changes in that period. However, we were able to follow the changes in the positions of the components along the first 2 mas of the jet with respect to the core. The resolution achieved by these observations allows the decomposition of the apparent velocity vector of each component into two orthogonal directions. We find evidence of reverse motion for a component along the jet. The jet starts wiggling well inside the first parsec from the core. The spatial period of the quasi-sinusoidal oscillating jet path about the jet major axis was derived for distances between 2 and 8 mas from the core., Comment: 17 pages, A&A LaTeX v3, plus 15 .ps figures. Accepted for publications by A&A Main Journal

We present the results of a three-week daily monitoring campaign on the quasar 3C 279 by the X-ray satellites RXTE and ROSAT. A cross correlation provides no evidence for any time lag between the very similar soft and hard X-ray light curves, and the source shows no significant spectral variability over the observing period. There is no evidence to support the presence of a soft excess, with a 99 per cent upper limit on any such component of 25 per cent of the total observed luminosity in the 0.1-2 keV band (< 3 x 10^38 W). This fraction (but not the luminosity) is significantly less than that of the soft excess observed in 3C 273., Comment: 4 pages, 3 figures, accepted for publication in MNRAS. Style files needed: epsf, mn.sty and new_cite.sty (this last is included in submission)

We present timing and spectral analysis of the data collected by the Extreme Ultraviolet Explorer Satellite (EUVE) for the Seyfert 1 galaxy NGC 4051 during 1996. NGC 4051 was observed twice in May 1996 and again in December 1996 for a total of more than 200 ks. The observations were always simultaneous with hard X-ray observations conducted with the Rossi X-Ray Timing Explorer (RXTE). The EUVE light curves are extremely variable during each observation, with the maximum variability of more than a factor of 15 from peak to minimum. We detected signal in the EUVE spectrograph in the 75-100 A range which is well fitted by absorbed power law models. We illustrate the results of our spectral and detailed power spectrum analysis performed on EUVE data and the comparison with RXTE lightcurves and discuss the constraint we can place on the mass of the central object and on the size of the emitting region., Comment: 4 pages, 2 figures, latex-macros memsait.sty, epsf.sty, to be published in the proceedings of the conference "Dal nano al Tera eV: tutti i colori degli AGN", third italian conference on AGNs, Roma, May 18-21 1998, Memorie S.A.It