Search

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 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

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

We present a wide multi-wavelength database of most observations of the quasar 3C 273 obtained during the last 30 years. This database is the most complete set of observations available for an active galactic nucleus (AGN). It contains nearly 20'000 observations grouped together into 70 light curves covering 16 orders of magnitude in frequency from the radio to the gamma-ray domain. The database is constituted of many previously unpublished observations and of most publicly available data gathered in the literature and on the World Wide Web (WWW). It is complete to the best of our knowledge, except in the optical (UBV) domain where we chose not to add all observations from the literature. In addition to the photometric data, we present the spectra of 3C 273 obtained by the International Ultraviolet Explorer (IUE) satellite. In the X-ray domain, we used the spectral fit parameters from the literature to construct the light curves. Apart from describing the data, we show the most representative light curves and the average spectrum of 3C 273. The database is available on the WWW in a homogeneous and clear form and we wish to update it regularly by adding new observations., Comment: 12 pages, 6 figures, to be published in A&AS, data available at: http://obswww.unige.ch/3c273/

Archival IUE SWP and HST FOS spectra show the presence of a relatively strong, broad Lyman alpha emission line superposed onto the UV spectral continuum of the blazar 3C 279. As opposed to a factor ~50 variation of the continuum flux during eight years, the emission line did not exhibit significant intensity changes. Simultaneous IUE SWP and LWP spectra of 3C 279 in low emission state are fitted by power-laws of index alpha_nu ~1 (f_nu ~ nu^{-alpha_nu}), significantly flatter than measured during higher states. Our observations suggest that the Lyman alpha line is not powered by the beamed, anisotropic synchrotron radiation which produces the observed continuum in 3C 279, but rather by an unbeamed component characterized by slower and lower amplitude variability. The latter may account for the UV continuum observed during the very low state of January 1993., Comment: 4 pages, LaTex, esapub format, 4 postscript figures, to appear in the Proceedings of the Conference 'Ultraviolet Astrophysics Beyond the IUE Final Archive', held in Sevilla, Spain (1997 November 11-14)

We present deep K-band imaging at the positions of four very faint X-ray sources found in the UK ROSAT Deep Survey to have no optical counterpart brighter than R~23. Likely identifications are found within the ROSAT error circle in all four fields with R-K colours of between 3.2 +/- 0.4 and 6.4 +/- 0.6. From a consideration of the R-K colours and X-ray to optical luminosity ratios of the candidate identifications, we tentatively classify two of the X-ray sources as very distant (z ~ 1) clusters of galaxies, one as a narrow emission line galaxy and one as an obscured QSO., Comment: 7 pages, LaTeX, 7 figures (1 postscript file each). Uses mn.sty and epsf.sty. Accepted by MNRAS. For more information see http://www.astro.soton.ac.uk/~amn/UKdeep

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of 1.6x10^-15 erg cm^-2 s^-1 (0.5--2.0 keV). Above a flux limit of 2x10^-15 erg cm^-2 s^-1 we define a complete sample of 70 sources of which 59 are identified. Some (5) other sources have tentative identifications and in a further 4 the X-ray error-boxes are blank to R=23 mag. At the brighter flux levels (>= 10^-14 erg cm^-2 s^-1) we confirm the results of previous less deep X-ray surveys with 84% of the sources begin QSOs. At fainter fluxes, however, the survey is dominated by a population of galaxies with narrow optical emission lines (NELGs). In addition, a number of groups and clusters of galaxies are found at intermediate fluxes. Most of these are poor systems of low X-ray luminosity and are generally found at redshifts of > 0.3. Their numbers are consistent with a zero evolutionary scenario, in contrast to the situation for high luminosity clusters at the same redshift. We discuss the significance of these results to the determination of the cosmic soft X-ray background (XRB) and show that at 2x10^-15 erg cm^-2 s^-1, we have resolved more than 50% of the background. We also briefly consider the probable importance of NELG objects to the residual background and look at some of the properties of these unusual objects., Comment: 28 pages, LaTeX, 17 figures from 32 postscript files. Uses mn.sty and epsf.sty. Accepted by MNRAS. Revised version now contains data table. For more information see http://www.astro.soton.ac.uk/~amn/UKdeep

In the deepest optically identified X-ray survey yet performed, we have identified 32 X-ray selected QSOs to a flux limit of 2x10^{-15} erg cm^{-2} s^{-1} (0.5-2 keV). The survey, performed with the ROSAT PSPC, has 89% spectroscopic completeness. The QSO log(N)-log(S) relation is found to have a break to a flat slope at faint fluxes. The surface density of QSOs at the survey limit is 230+/-40 per square degree, the largest so far of any QSO survey. We have used this survey to measure the QSO X-ray luminosity function at low luminosities (Lx<10^{44.5} erg s^{-1}) and high redshifts (1

We analyse the ROSAT PSPC spectrum of 19 X-ray selected Narrow Emission Line Galaxies (NELGs) discovered during the optical identification of sources in the ROSAT UK Deep Survey. Their properties are compared to those of broad line Active Galactic Nuclei (AGN) in the same sample. Counts in three spectral bands have been extracted for all the sources, and have been fitted with a power-law model assuming the Galactic value for NH. The average slope of NELGs is \alpha=0.45+-0.09, whilst for the AGN it is \alpha=0.96+-0.03. The power-law model is a good fit for ~90% of NELGs and ~75% of AGN. Recent work shows that the fractional surface density of NELGs increases with respect to AGN at faint fluxes. Thus they are expected to be an important component of the residual soft (<2 keV) X--ray background. The slope of the X--ray background (\alpha~0.4, 1-10 keV) is harder than that of AGN (\alpha~1) but our results show that it is consistent with the summed spectrum of the NELGs in the deep survey (\alpha~0.4). This may finally reconcile the spectrum of the background with the properties of the sources that constitute it., Comment: Uuencoded compressed LaTex file and three PS figures plus mn.sty. Accepted in MNRAS

We conduct a multi-wavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 Angstrom to 9157 Angstrom) combine simultaneous Hubble Space Telescope, Swift, and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination i = 36deg +/- 10deg, temperature T(sub 1) = (44+/-6) times 10 (exp 3)K at 1 light day from the black hole, and a temperature radius slope (T proportional to r (exp -alpha)) of alpha = 0.99 +/- 0.03. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/L(sub Edd) = 0.1.

The Large Observatory For x-ray Timing (LOFT) is a mission concept which was proposed to ESA as M3 and M4 candidate in the framework of the Cosmic Vision 2015-2025 program. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument and the uniquely large field of view of its wide field monitor, LOFT will be able to study the behaviour of matter in extreme conditions such as the strong gravitational field in the innermost regions close to black holes and neutron stars and the supra-nuclear densities in the interiors of neutron stars. The science payload is based on a Large Area Detector (LAD, is greater than 8m2 effective area, 2-30 keV, 240 eV spectral resolution, 1 degree collimated field of view) and a Wide Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g., GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the current technical and programmatic status of the mission.