Seyfert Galaxies and BeppoSAX

The contributions that BeppoSAX is expected to give and, after one and a half year of operation, has already given to our knowledge of both type 1 and type 2 Seyfert galaxies are outlined and reviewed.Comment: Invited review, to appear in "The Active X-ray sky: Results from BepppoSAX and Rossi-XTE", conference held in Rome, Italy, October 1997. 10 pages LaTeX, using espcrc2 and epsfig. 5 postscript figures inserte

Effects of Kerr Spacetime on Spectral Features from X-Ray Illuminated Accretion Discs

We performed detailed calculations of the relativistic effects acting on both the reflection continuum and the iron line from accretion discs around rotating black holes. Fully relativistic transfer of both illuminating and reprocessed photons has been considered in Kerr spacetime. We calculated overall spectra, line profiles and integral quantities, and present their dependences on the black hole angular momentum. We show that the observed EW of the lines is substantially enlarged when the black hole rotates rapidly and/or the source of illumination is near above the hole. Therefore, such calculations provide a way to distinguish among different models of the central source.Comment: 9 pages, latex, 13 figures, 3 Tables; accepted for publication in MNRA

Iron fluorescence from within the innermost stable orbit of black hole accretion disks

The fluorescent iron Ka line is a powerful observational probe of the inner regions of black holes accretion disks. Previous studies have assumed that only material outside the radius of marginal stability can contribute to the observed line emission. Here, we show that fluorescence by material inside the radius of marginal stability, which is in the process of spiralling towards the event horizon, can have a observable influence on the iron line profile and equivalent width. For concreteness, we consider the case of a geometrically thin accretion disk, around a Schwarzschild black hole, in which fluorescence is excited by an X-ray source placed at some height above the disk and on the axis of the disk. Fully relativistic line profiles are presented for various source heights and efficiencies. It is found that the extra line flux generally emerges in the extreme red wing of the iron line, due to the large gravitational redshift experienced by photons from the region within the radius of marginal stability. We apply our models to the variable iron line seen in the ASCA spectrum of the Seyfert nucleus MCG-6-30-15. It is found that the change in the line profile, equivalent width, and continuum normalization, can be well explained as being due to a change in the height of the source above the disk. We discuss the implications of these results for distinguishing rapidly-rotating black holes from slowly rotating holes using iron line diagnostics.Comment: 20 pages, LaTeX. Accepted for publication in Astrophysical Journal. Figures 3 to 7 replaced with corrected versions (previous figures affected by calculational error). Some changes in the best fitting parameter

Iron Line Spectroscopy of NGC4593 with XMM-Newton: Where is the Black Hole Accretion Disk?

We present an analysis of the 2-10keV XMM-Newton/EPIC-pn spectrum of the Seyfert-1 galaxy NGC4593. Apart from the presence of two narrow emission lines corresponding to the Kalpha lines of cold and hydrogen-like iron, this spectrum possesses a power-law form to within 3-5%. There is a marked lack of spectral features from the relativistic regions of the black hole accretion disk. We show that the data are, however, consistent with the presence of a radiatively-efficient accretion disk extending right down to the radius of marginal stability if it possesses low iron abundance, an appropriately ionized surface, a very high inclination, or a very centrally concentrated emission pattern (as has been observed during the Deep Minimum State of the Seyfert galaxy MCG-6-30-15). Deeper observations of this source are required in order to validate or reject these models.Comment: 6 pages, 3 postscript figures. Accepted for publication in the Monthly Notices of the Royal Astronomical Societ

The XMM-Newton view of the relativistic spectral features in AXJ0447-0627

The XMM-Newton observation of the optically Type 1 AGN AXJ0447-0627 (z=0.214) unambiguously reveals a complex, bright and prominent set of lines in the 4-8 keV rest frame energy range. Although, from a phenomenological point of view, the observed properties can be described by a simple power law model plus 5 narrow Gaussian lines (at rest frame energies of nearly 4.49, 5.55, 6.39, 7.02 and 7.85 keV), we find that a model comprising a power law (Gamma of the order of 2.2), a reflected relativistic continuum, a narrow Fe I Kalpha line from neutral material as well as a broad Fe Kalpha relativistic line from a ionized accretion disk represents a good physical description of the data. The ''double horned'' profile of the relativistic line implies an inclination of the accretion disk of the order of 45 degree, and an origin in a narrow region of the disk, from R_in of the order of 19 GM/c^2 to R_out of the order of 30 GM/c^2. The narrow Fe I Kalpha line from neutral material is probably produced far from the central black hole, most likely in the putative molecular torus. Although some of these properties have been already found in other Type 1 AGN and discussed in the literature, at odd with the objects reported so far we measure high equivalent widths (EWs) of the observed lines: nearly 1.4 keV for the ``double horned'' relativistic line and nearly 0.4 keV for the narrow line.Comment: 16 pages, 3 figures, Latex manuscript; accepted for publication in Ap

Extremal energy shifts of radiation from a ring near a rotating black hole

Radiation from a narrow circular ring shows a characteristic double-horn profile dominated by photons having energy around the maximum or minimum of the allowed range, i.e. near the extremal values of the energy shift. The energy span of a spectral line is a function of the ring radius, black hole spin, and observer's view angle. We describe a useful approach to calculate the extremal energy shifts in the regime of strong gravity. Then we consider an accretion disk consisting of a number of separate nested annuli in the equatorial plane of Kerr black hole, above the innermost stable circular orbit (ISCO). We suggest that the radial structure of the disk emission could be reconstructed using the extremal energy shifts of the individual rings deduced from the broad wings of a relativistic spectral line.Comment: 9 pages, 5 figures, ApJ accepte

Are the Nuclei of Seyfert 2 Galaxies Viewed Face-On?

We show from modeling the Fe Kalpha line in the ASCA spectra of four X-ray bright narrow emission line galaxies (Seyfert types 1.9 and 2) that two equally viable physical models can describe the observed line profile. The first is discussed by Turner et al. (1998) and consists of emission from a nearly pole-on accretion disk. The second, which is statistically preferred, is a superposition of emission from an accretion disk viewed at an intermediate inclination of about 48 degrees and a distinct, unresolved feature that presumably originates some distance from the galaxy nucleus. The intermediate inclination is entirely consistent with unified schemes and our findings challenge recent assertions that Seyfert 2 galaxies are preferentially viewed with their inner regions face-on. We derive mean equivalent widths for the narrow and disk lines of =60 eV and = 213 eV, respectively. The X-ray data are well described by a geometry in which our view of the active nucleus intersects and is blocked by the outer edges of the obscuring torus, and therefore do not require severe misalignments between the accretion disk and the torus.Comment: 19 pages, 3 postscript figures. Accepted for publication in ApJ

Fe K\alpha emission from photoionized slabs: the impact of the iron abundance

Iron K\alpha emission from photoionized and optically thick material is observed in a variety of astrophysical environments including X-ray binaries, active galactic nuclei, and possibly gamma-ray bursts. This paper presents calculations showing how the equivalent width (EW) of the Fe K line depends on the iron abundance of the illuminated gas and its ionization state -- two variables subject to significant cosmic scatter. Reflection spectra from a constant density slab which is illuminated with a power-law spectrum with photon-index \Gamma are computed using the code of Ross & Fabian. When the Fe K EW is measured from the reflection spectra alone, we find that it can reach values greater than 6 keV if the Fe abundance is about 10 times solar and the illuminated gas is neutral. EWs of about 1 keV are obtained when the gas is ionized. In contrast, when the EW is measured from the incident+reflected spectrum, the largest EWs are ~800 keV and are found when the gas is ionized. When \Gamma is increased, the Fe K line generally weakens, but significant emission can persist to larger ionization parameters. The iron abundance has its greatest impact on the EW when it is less than 5 times solar. When the abundance is further increased, the line strengthens only marginally. Therefore, we conclude that Fe K lines with EWs much greater than 800 eV are unlikely to be produced by gas with a supersolar Fe abundance. These results should be useful in interpreting Fe K emission whenever it arises from optically thick fluorescence.Comment: 5 pages, 5 figures, accepted by MNRAS Letter

Does science need computer science?

IBM Hursley Talks Series 3An afternoon of talks, to be held on Wednesday March 10 from 2:30pm in Bldg 35 Lecture Room A, arranged by the School of Chemistry in conjunction with IBM Hursley and the Combechem e-Science Project.The talks are aimed at science students (undergraduate and post-graduate) from across the faculty. This is the third series of talks we have organized, but the first time we have put them together in an afternoon. The talks are general in nature and knowledge of computer science is certainly not necessary. After the talks there will be an opportunity for a discussion with the lecturers from IBM.Does Science Need Computer Science?Chair and Moderator - Jeremy Frey, School of Chemistry.- 14:00 "Computer games for fun and profit" (*) - Andrew Reynolds - 14:45 "Anyone for tennis? The science behind WIBMledon" (*) - Matt Roberts - 15:30 Tea (Chemistry Foyer, Bldg 29 opposite bldg 35) - 15:45 "Disk Drive physics from grandmothers to gigabytes" (*) - Steve Legg - 16:35 "What could happen to your data?" (*) - Nick Jones - 17:20 Panel Session, comprising the four IBM speakers and May Glover-Gunn (IBM) - 18:00 Receptio

The BeppoSAX X-ray view of reflection-dominated Seyfert Galaxies

We present new results from BeppoSAX observations of reflection-dominated Seyfert galaxies, and namely: 1) the Compton-thick Seyfert 2s NGC1068 and Circinus Galaxy; 2) the Seyfert 1 NGC4051, whose nucleus was observed on May 1998 to have switched off, leaving only a residual reflection component as an echo of its past activity. Our main focus in this paper is on the soft X-ray continuum properties and on the X-ray line spectroscopy.Comment: 6 Latex pages, 5 figures, Accepted for publication in Advances in Space Research, Proceedings of 32nd Sci. Ass. of COSPA