The Distribution and Cosmic Density of Relativistic Iron Lines in Active Galactic Nuclei

X-ray observations of several active galactic nuclei show prominent iron K-shell fluorescence lines that are sculpted due to special and general relativistic effects. These observations are important because they probe the space-time geometry close to distant black holes. However, the intrinsic distribution of Fe line strengths in the cosmos has never been determined. This uncertainty has contributed to the controversy surrounding the relativistic interpretation of the emission feature. Now, by making use of the latest multi-wavelength data, we show theoretical predictions of the cosmic density of relativistic Fe lines as a function of their equivalent width and line flux. We are able to show unequivocally that the most common relativistic iron lines in the universe will be produced by neutral iron fluorescence in Seyfert galaxies and have equivalent widths < 100 eV. Thus, the handful of very intense lines that have been discovered are just the bright end of a distribution of line strengths. In addition to validating the current observations, the predicted distributions can be used for planning future surveys of relativistic Fe lines. Finally, the predicted sky density of equivalent widths indicate that the X-ray source in AGNs can not, on average, lie on the axis of the black hole.Comment: 12 pages, 3 figures, accepted by ApJ Letter

High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres

We show that Compton scattering of soft photons with energies near 100 eV in thermally stratified black-hole accretion plasmas with temperatures in the range 100 keV - 1 MeV can give rise to an X-ray spectral hardening near 10 keV. This could produce the hardening observed in the X-ray spectra of black holes, which is generally attributed to reflection or partial covering of the incident continuum source by cold optically thick matter. In addition, we show that the presence of very hot (kT=1 MeV) cores in plasmas leads to spectra exibiting high energy tails similar to those observed from Galactic black-hole candidates.Comment: 11 pages, uuencoded gziped postscript, ApJ Letters in pres

X-ray Line Emitting Objects in XMM-Newton Observations: the Tip of the Iceberg

We present preliminary results from a novel search for X-ray Line Emitting Objects (XLEOs) in XMM-Newton images. Three sources have been detected in a test-run analysis of 13 XMM-Newton observations. The three objects found are most likely extremely absorbed AGN characterized by a column density NH~10^24cm^-2. Their redshift has been directly determined from the X-ray data, by interpreting the detected emission line as the 6.4 keV Fe line. The measured equivalent width of the X-ray line is, in all three cases, several keV. This pilot study demonstrates the success of our search method and implies that a large sample of XLEOs can be obtained from the public XMM-Newton data archive.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter

ASCA observations of type-2 Seyfert galaxies: II. The Importance of X-ray Scattering and Reflection

We discuss the importance of X-ray scattering and Compton reflection in type-2 Seyfert galaxies, based upon the analysis of ASCA observations of 25 such sources. Consideration of the iron Kalpha, [O III] line and X-ray variability suggest that NGC 1068, NGC 4945, NGC 2992, Mrk 3, Mrk 463E and Mrk 273 are dominated by reprocessed X-rays. We examine the properties of these sources in more detail. We find that the iron Kalpha complex contains significant contributions from neutral and high-ionization species of iron. Compton reflection, hot gas and starburst emission all appear to make significant contributions to the observed X-ray spectra. Mrk 3 is the only source in this subsample which does not have a significant starburst contamination. The ASCA spectrum below 3 keV is dominated by hot scattering gas with U_X ~ 5, N_H ~ 4 x 10^23 cm^-2. This material is more highly ionized than the zone of material comprising the warm absorber seen in Seyfert~1 galaxies, but may contain a contribution from shock-heated gas associated with the jet. Estimates of the X-ray scattering fraction cover 0.25 - 5%. The spectrum above 3 keV appears to be dominated by a Compton reflection component although there is evidence that the primary continuum component becomes visible close to 10 keV.Comment: 27 pages, 6 figures. LaTeX with encapsulated postscript. To appear in the Astrophysical Journal. Also available via http://lheawww.gsfc.nasa.gov/~george/papers/gnt_s2p2/abstract.htm

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

Chandra observations of NGC4698: a Seyfert-2 with no absorption

We present Chandra ACIS-S observations of the enigmatic Seyfert-2 galaxy NGC4698. This object together with several other bona-fide Seyfert-2 galaxies show no absorption in the low spatial resolution ASCA data, in contrast to the standard unification models. Our Chandra observations of NGC4698 probe directly the nucleus allowing us to check whether nearby sources contaminate the ASCA spectrum. Indeed, the Chandra observations show that the ASCA spectrum is dominated by two nearby AGN. The X-ray flux of NGC4698 is dominated by a nuclear source with luminosity L(0.3-8 keV) ~ 10^39, erg s-1 coincident with the radio nucleus. Its spectrum is well represented by a power-law, ~ 2.2, obscured by a small column density of 5x10^20 cm-2 suggesting that NGC4698 is an atypical Seyfert galaxy. On the basis of its low luminosity we then interpret NGC4698 as a Seyfert galaxy which lacks a broad-line region.Comment: 12 pages, to appear in Ap

Extranuclear X-ray Emission in the Edge-on Seyfert Galaxy NGC 2992

We found several extranuclear (r >~ 3") X-ray nebulae within 40" (6.3 kpc at 32.5 Mpc) of the nucleus of the Seyfert galaxy NGC 2992. The net X-ray luminosity from the extranuclear sources is ~2-3 E39 erg/s (0.3-8.0 keV). The X-ray core itself (r <~ 1") is positioned at 9:45:41.95 -14:19:34.8 (J2000) and has a remarkably simple power-law spectrum with photon index Gamma=1.86 and Nh=7E21 /cm2. The near-nuclear (3" <~ r <~ 18") Chandra spectrum is best modelled by three components: (1) a direct AGN component with Gamma fixed at 1.86, (2) cold Compton reflection of the AGN component, and (3) a 0.5 keV low-abundance (Z < 0.03 Zsolar) "thermal plasma," with ~10% of the flux of either of the first two components. The X-ray luminosity of the 3rd component (the "soft excess") is ~1.4E40 erg/s, or ~5X that of all of the detected extranuclear X-ray sources. We suggest that most (~75-80%) of the soft excess emission originates from 1" < r < 3", which is not imaged in our observation due to severe CCD pile-up. We also require the cold reflector to be positioned at least 1" (158 pc) from the nucleus, since there is no reflection component in the X-ray core spectrum. Much of the extranuclear X-ray emission is coincident with radio structures (nuclear radio bubbles and large-scale radio features), and its soft X-ray luminosity is generally consistent with luminosities expected from a starburst-driven wind (with the starburst scaled from L_FIR). However, the AGN in NGC 2992 seems equally likely to power the galactic wind in that object. Furthermore, AGN photoionization and photoexcitation processes could dominate the soft excess, especially the \~75-80% which is not imaged by our observations.Comment: 34 pages AASTEX, 9 (low-res) PS figures, ApJ, in press. For full-resolution postscript file, visit http://www.pha.jhu.edu/~colbert/n2992_chandra.ps.g

The XMM-Newton view of the central degrees of the Milky Way

The deepest XMM-Newton mosaic map of the central 1.5 deg of the Galaxy is presented, including a total of about 1.5 Ms of EPIC-pn cleaned exposures in the central 15" and about 200 ks outside. This compendium presents broad-band X-ray continuum maps, soft X-ray intensity maps, a decomposition into spectral components and a comparison of the X-ray maps with emission at other wavelengths. Newly-discovered extended features, such as supernova remnants (SNRs), superbubbles and X-ray filaments are reported. We provide an atlas of extended features within +-1 degree of Sgr A*. We discover the presence of a coherent X-ray emitting region peaking around G0.1-0.1 and surrounded by the ring of cold, mid-IR-emitting material known from previous work as the "Radio Arc Bubble" and with the addition of the X-ray data now appears to be a candidate superbubble. Sgr A's bipolar lobes show sharp edges, suggesting that they could be the remnant, collimated by the circumnuclear disc, of a SN explosion that created the recently discovered magnetar, SGR J1745-2900. Soft X-ray features, most probably from SNRs, are observed to fill holes in the dust distribution, and to indicate a direct interaction between SN explosions and Galactic center (GC) molecular clouds. We also discover warm plasma at high Galactic latitude, showing a sharp edge to its distribution that correlates with the location of known radio/mid-IR features such as the "GC Lobe". These features might be associated with an inhomogeneous hot "atmosphere" over the GC, perhaps fed by continuous or episodic outflows of mass and energy from the GC region.Comment: MNRAS published online. See www.mpe.mpg.de/heg/gc/ for a higher resolution version of the figure

A river model of space

Within the theory of general relativity gravitational phenomena are usually attributed to the curvature of four-dimensional spacetime. In this context we are often confronted with the question of how the concept of ordinary physical three-dimensional space fits into this picture. In this work we present a simple and intuitive model of space for both the Schwarzschild spacetime and the de Sitter spacetime in which physical space is defined as a specified set of freely moving reference particles. Using a combination of orthonormal basis fields and the usual formalism in a coordinate basis we calculate the physical velocity field of these reference particles. Thus we obtain a vivid description of space in which space behaves like a river flowing radially toward the singularity in the Schwarzschild spacetime and radially toward infinity in the de Sitter spacetime. We also consider the effect of the river of space upon light rays and material particles and show that the river model of space provides an intuitive explanation for the behavior of light and particles at and beyond the event horizons associated with these spacetimes.Comment: 22 pages, 5 figure