On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure

We use the method of Press, Rybicki & Hewitt (1992) to search for time lags and time leads between different energy bands of the RXTE data for MCG-6-30-15. We tailor our search in order to probe any reverberation signatures of the fluorescent iron Kalpha line that is thought to arise from the inner regions of the black hole accretion disk. In essence, an optimal reconstruction algorithm is applied to the continuum band (2-4keV) light curve which smoothes out noise and interpolates across the data gaps. The reconstructed continuum band light curve can then be folded through trial transfer functions in an attempt to find lags or leads between the continuum band and the iron line band (5-7keV). We find reduced fractional variability in the line band. The spectral analysis of Lee et al. (1999) reveals this to be due to a combination of an apparently constant iron line flux (at least on timescales of few x 10^4s), and flux correlated changes in the photon index. We also find no evidence for iron line reverberation and exclude reverberation delays in the range 0.5-50ksec. This extends the conclusions of Lee et al. and suggests that the iron line flux remains constant on timescales as short as 0.5ksec. The large black hole mass (>10^8Msun) naively suggested by the constancy of the iron line flux is rejected on other grounds. We suggest that the black hole in MCG-6-30-15 has a mass of M_BH~10^6-10^7Msun and that changes in the ionization state of the disk may produce the puzzling spectral variability. Finally, it is found that the 8-15keV band lags the 2-4keV band by 50-100s. This result is used to place constraints on the size and geometry of the Comptonizing medium responsible for the hard X-ray power-law in this AGN.Comment: 11 pages, 13 postscript figures. Accepted for publication in Ap

The implementation and use of Ada on distributed systems with reliability requirements

The issues involved in the use of the programming language Ada on distributed systems are discussed. The effects of Ada programs on hardware failures such as loss of a processor are emphasized. It is shown that many Ada language elements are not well suited to this environment. Processor failure can easily lead to difficulties on those processors which remain. As an example, the calling task in a rendezvous may be suspended forever if the processor executing the serving task fails. A mechanism for detecting failure is proposed and changes to the Ada run time support system are suggested which avoid most of the difficulties. Ada program structures are defined which allow programs to reconfigure and continue to provide service following processor failure

Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN

Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we study correlations between X-ray spectral features. The sample consists of 350 time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to the central black hole that is reprocessed and absorbed by material around the black hole. To test the robustness of our results, we performed Monte Carlo simulations of the spectral sample. We find a complex relationship between the iron line equivalent width (EW) and the underlying power law index (Gamma). The data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2, but finds a weak anti-correlation for steeper photon indices. We propose that this relationship is driven by dilution of a disk spectrum (which includes the narrow iron line) by a beamed jet component and, hence, could be used as a diagnostic of jet-dominance. In addition, our sample shows a strong correlation between the reflection fraction (R) and Gamma, but we find that it is likely the result of modeling degeneracies. We also see the X-ray Baldwin effect (an anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample as a whole, but not for the individual galaxies and galaxy types.Comment: Accepted for publication in ApJ, 14 page

Occultation Mapping of the Central Engine in the Active Galaxy MCG -6-30-15

The colossal power output of active galactic nuclei (AGN) is believed to be fueled by the accretion of matter onto a supermassive black hole. This central accreting region of AGN has hitherto been spatially unresolved and its structure therefore unknown. Here we propose that a previously reported `deep minimum' in the X-ray intensity of the AGN MCG-6-30-15, was due to a unique X-ray occultation event and that it probes structure of the central engine on scales < 1e14 cm, or 1.4e-7 arcseconds. The data are consistent with a bright central source surrounded by a less intense ring, which we identify with the inner edge of an accretion disk. These may be the first direct measurements of the spatial structure and geometry of the accreting black-hole system in an active galaxy.If the ring of X-ray emission is identified with the inner edge of an accretion disk, upper limits on the BH mass can be derived. Our occultation interpretation is controversial in the sense that X-ray variability in AGNs is normally attributed to intrinsic physical changes in the X-ray emission region, such as disk or coronal instabilities.Comment: 15 pages, 2 Figures. Latex with separate postscript figure files. Accepted for publication in ApJ Letter

Atomic Deuterium Adsorbed on the Surface of Liquid Helium

We investigate deuterium atoms adsorbed on the surface of liquid helium in equilibrium with a vapor of atoms of the same species. These atoms are studied by a sensitive optical method based on spectroscopy at a wavelength of 122 nm, exciting the 1S-2P transition. We present a direct measurement of the adsorption energy of deuterium atoms on helium and show evidence for the existence of resonantly enhanced recombination of atoms residing on the surface to molecules.Comment: 6 pages 4 figure

Weak Reprocessed Features in the Broad Line Radio Galaxy 3C382

We present a detailed X-ray study of the Broad Line Radio Galaxy 3C382, observed with the BeppoSAX satellite in a very bright state. The continuum emission is well modeled with a power law that steepens at high energies, with an e-folding energy of about 120 keV. At soft energies a clear excess of emission is detected, which can not be explained solely by the extended thermal halo seen in a ROSAT HRI image. A second, more intense soft X-ray component, possibly related to an accretion disk, is required by the data. Both a reflection component (R=0.3) and an iron line (EW \sim 50) are detected, at levels much weaker than in Seyfert galaxies, suggesting a common origin. Combining our measurements with results from the literature we find that the iron line has remained approximately constant over 9 years while the continuum varied by a factor of 5. Thus the fluorescent gas does not respond promptly to the variations of the X-ray primary source, suggesting that the reprocessing site is located away, likely at parsec distances. While the continuum shape indicates that X-rays derive from a thermal Comptonization process, the weakness of other spectral features implies that either the upper layers of the optically thick accretion disk are completely ionized or the corona above the disk is outflowing with mildly relativistic velocity.Comment: 11 pages, 4 figures. Accepted for publication in the Astrophysical Journa

On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs

We investigate properties of iron fluorescent line arising as a result of illumination of a black hole accretion disc by an X-ray source located above the disc surface. We study in details the light-bending model of variability of the line, extending previous work on the subject. We indicate bending of photon trajectories to the equatorial plane, which is a distinct property of the Kerr metric, as the most feasible effect underlying reduced variability of the line observed in several objects. A model involving an X-ray source with a varying radial distance, located within a few central gravitational radii around a rapidly rotating black hole, close to the disc surface, may explain both the elongated red wing of the line profile and the complex variability pattern observed in MCG--6-30-15 by XMM-Newton. We point out also that illumination by radiation which returns to the disc (following the previous reflection) contributes significantly to formation of the line profile in some cases. As a result of this effect, the line profile always has a pronounced blue peak (which is not observed in the deep minimum state in MCG--6-30-15), unless the reflecting material is absent within the innermost 2--3 gravitational radii.Comment: 24 pages, 22 figures. Accepted for publication in MNRA

Jet power extracted from ADAF and the applications to X-ray binaries and radio galaxy FR dichotomy

We calculate the jet power of the classical Blandford-Znajek(BZ) model and hybrid model developed by Meier based on the global solutions of advection dominated accretion flows (ADAFs) surrounding Kerr black holes. We find that the jet power of the hybrid model is larger than that of the pure BZ model. The jet power will dominate over the accretion power, and the objects will enter into "jet-power-dominated advective systems", when the accretion rate is less than a critical value mdot_c=Mdot_c/Mdot_Edd, where 3*10^-4 < mdot_c < 5*10^-3 is a function of black hole spin parameter. The accretion power will be dominant when mdot<mdot_c and the objects will enter into "accretion-power-dominated advective systems." This is roughly consistent with that constrained from the low/hard-state black hole X-ray binaries (e.g., Fender et al.). We calculate the maximal jet power as a function of black hole mass with the hybrid jet formation model, and find it can roughly reproduce the dividing line of the Ledlow-Owen relation for FR I/FR II dichotomy in the jet power-black hole(BH) mass plane (Q_jet-M_BH) if the dimensionless accretion rate mdot~0.01 and BH spin parameter j~0.9-0.99 are adopted. This accretion rate mdot~0.01 is consistent with that of the critical accretion rate for the accretion mode transition of a standard disk to an ADAF constrained from the state transition of X-ray binaries. Our results imply that most FR I galaxies may be in the ADAF accretion mode similar to the low/hard-state XRBs.Comment: 6 pages, 3 color figures, ApJ in press, edited with ApJ style and English is improved as suggested by Edito

On The Reddening in X-ray Absorbed Seyfert 1 Galaxies

There are several Seyfert galaxies for which there is a discrepancy between the small column of neutral hydrogen deduced from X-ray observations and the much greater column derived from the reddening of the optical/UV emission lines and continuum. The standard paradigm has the dust within the highly ionized gas which produces O~VII and O~VIII absorption edges (i.e., a ``dusty warm absorber''). We present an alternative model in which the dust exists in a component of gas in which hydrogen has been stripped, but which is at too low an ionization state to possess significant columns of O~VII and O~VIII (i.e, a ``lukewarm absorber''). The lukewarm absorber is at sufficient radial distance to encompass much of the narrow emission-line region, and thus accounts for the narrow-line reddening, unlike the dusty warm absorber. We test the model by using a combination of photoionization models and absorption edge fits to analyze the combined ROSAT/ASCA dataset for the Seyfert 1.5 galaxy, NGC 3227. We show that the data are well fit by a combination of the lukewarm absorber and a more highly ionized component similar to that suggested in earlier studies. We predict that the lukewarm absorber will produce strong UV absorption lines of N V, C IV, Si IV and Mg II. Finally, these results illustrate that singly ionized helium is an important, and often overlooked, source of opacity in the soft X-ray band (100 - 500 eV).Comment: 17 pages, Latex, includes 1 figure (encapsulated postscript), one additional table in Latex (landscape format), to appear in the Astrophysical Journa

The ASCA X-Ray Spectrum Of The Broad-Line Radio Galaxy Pictor A: A Simple Power Law With No Fe K-alpha Line

We present the X-ray spectrum of the broad-line radio galaxy Pictor A as observed by ASCA in 1996. The main objective of the observation was to detect and study the profiles of the Fe~K$\alpha$ lines. The motivation was the fact that the Balmer lines of this object show well-separated displaced peaks, suggesting an origin in an accretion disk. The 0.5-10 keV X-ray spectrum is described very well by a model consisting of a power law of photon index 1.77 modified by interstellar photoelectric absorption. We find evidence for neither a soft nor a hard (Compton reflection) excess. More importantly, we do not detect an Fe K-alpha line, in marked contrast with the spectra of typical Seyfert galaxies and other broad-line radio galaxies observed by ASCA. The 99%-confidence upper limit on the equivalent width of an unresolved line at a rest energy of 6.4 keV is 100 eV, while for a broad line (FWHM of approximately 60,000 km/s) the corresponding upper limit is 135 eV. We discuss several possible explanations for the weakness of the Fe K-alpha line in Pictor~A paying attention to the currently available data on the properties of Fe K-alpha lines in other broad-line radio galaxies observed by ASCA. We speculate that the absence of a hard excess (Compton reflection) or an Fe K-alpha line is an indication of an accretion disk structure that is different from that of typical Seyfert galaxies, e.g., the inner disk may be an ion torus.Comment: To appear in the Astrophysical Journal (18 pages, including 8 postscript figures; uses psfig.tex