K. Nandra

ASCA observations of Seyfert 1 galaxies. 2. Relativistic iron K-alpha emission

We present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by ASCA, 14 show an iron K? line that is resolved, with mean width ?K? = 0.43 ? 0.12 keV for a Gaussian profile (full width at half-maximum, FWHM ~ 50,000 km s-1). However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole, and accretion disk models provide an excellent description of the data. The peak energy of the line is 6.4 keV, which indicates that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30?, consistent with orientation-dependent unification schemes. Differences in the line profiles from source to source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Although a small contribution to the emission from a region other than the disk is not ruled out, it is not generally required and has little effect on our conclusions regarding the disk line. Our data are fit equally well with rotating (Kerr) and nonrotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of ?3-10 = 1.91 ? 0.07 after accounting for the effects of reflection. Such observations probe the innermost regions of active galactic nuclei and arguably provide the best evidence yet obtained for the existence of supermassive black holes in the centers of active galaxies.

An XMM–Newton survey of broad iron lines in Seyfert galaxies

We present an analysis of the X-ray spectra of a sample of 37 observations of 26 Seyfert galaxies observed by XMM–Newton in order to characterize their iron Kα emission. All objects show evidence for iron line emission in the 6–7 keV band. A narrow ‘core’ at 6.4 keV is seen almost universally in the spectra, and we model this using a neutral Compton reflection component, assumed to be associated with distant, optically thick material such as the molecular torus. Once this, and absorption by a zone of ionized gas in the line of sight is accounted for, less than half of the sample observations show an acceptable fit. Approximately two-thirds of the sample shows evidence for further, broadened emission in the iron K band. When modelled with a Gaussian, the inferred energy is close to that expected for neutral iron, with a slight redshift, and an average velocity width of ∼0.1c. The mean parameters are consistent with previous ASCA results and support the idea that the broad components can be associated with the accretion disc. Before proceeding to that conclusion, we test an alternative model comprising a blend of three to four narrow, unshifted emission lines (including the 6.4-keV core), together with one to two zones of highly ionized gas in the line of sight. Around one-third of the objects are not adequately fitted by this model, and in general better fits are obtained with a relativistic disc line model, which has fewer free parameters. None the less we find that absorption by ionized gas affects the spectrum above 2.5 keV in approximately half the sample. There is evidence for multiple ionized zones in at least three objects, but in all those cases a blurred reflector is required in addition to the complex absorption. We also identify a number of narrow emission and absorption features around the Fe complex, and the significance and interpretation of these lines is discussed. After accounting for these additional complexities, we determine the typical parameters for the broad reflection. The emission is found to come, on average, from a characteristic radius ∼15 rg and the average disc inclination is ∼ 40°. The broad reflection is on average significantly weaker, by a factor of ∼2, than that expected from a flat disc illuminated by a point source. Notwithstanding these average properties, the objects exhibit a significant and wide range of reflection parameters. We find that 30 per cent of the sample observations can be explained solely with narrow-line components, with no evidence for broadened emission at all. A further 25 per cent show evidence for significant broad emission, but at a characteristic radius relatively far from the black hole. The remaining ∼45 per cent are best fitted with a relativistically blurred reflection model. In 12/37 observations the characteristic emission radius is constrained to be <50 rg, where the gravitational redshift is measurable. For at least this subsample, our observations verify the potential for X-ray spectroscopy to diagnose the strong-gravity regime of supermassive black holes.

ASCA Observations of Seyfert 1 Galaxies. I. Data Analysis, Imaging, and Timing

We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by ASCA. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the active galactic nuclei. All but one of the Seyferts show evidence for variability on timescales of minutes to hours, with the amplitude anticorrelated with the source luminosity, confirming previous results. In at least eight sources there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power law in the soft band. Ultrarapid variability, implying significant power at frequencies greater than 10-3 Hz is detected in at least five sources but is difficult to detect in most cases, because of the sampling and signal-to-noise ratio. In Mrk 766 and MCG -6-30-15 there is also an indication that the high-frequency power spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151 but on longer timescales.

The Herschel ATLAS

The Herschel ATLAS is the largest open-time key project that will be carried out on the Herschel Space Observatory. It will survey 570 deg2 of the extragalactic sky, 4 times larger than all the other Herschel extragalactic surveys combined, in five far-infrared and submillimeter bands. We describe the survey, the complementary multiwavelength data sets that will be combined with the Herschel data, and the six major science programs we are undertaking. Using new models based on a previous submillimeter survey of galaxies, we present predictions of the properties of the ATLAS sources in other wave bands.

The All-wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a panchromatic data set in the Extended Groth Strip region of the sky. Our survey, the All-Wavelength Extended Groth Strip International Survey (AEGIS), aims to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray, GALEX ultraviolet, CFHT/MegaCam Legacy Survey optical, CFHT/CFH12K optical, Hubble Space Telescope/ACS optical and NICMOS near-infrared, Palomar/WIRC near-infrared, Spitzer/IRAC mid-infrared, Spitzer/MIPS far-infrared, and VLA radio continuum. In addition, this region of the sky has been targeted for extensive spectroscopy using the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

A long hard look at MCG–6-30-15 with XMM-Newton

We present the first results from a 325-ks observation of the Seyfert 1 galaxy MCG–6-30-15 with XMM-Newton and BeppoSAX. The strong, broad, skewed iron line is clearly detected and is well characterized by a steep emissivity profile within 6rg (i.e. 6GM/c2) and a flatter profile beyond. The inner radius of the emission appears to lie at about 2rg, consistent with results reported from both an earlier XMM-Newton observation of MCG–6-30-15 by Wilms et al. and part of an ASCA observation by Iwasawa et al. when the source was in a lower flux state. The radius and steep emissivity profile do depend however on an assumed incident power-law continuum and a lack of complex absorption above 2.5 keV. The blue wing of the line profile is indented, either by absorption at about 6.7 keV or by a hydrogenic iron emission line. The broad iron line flux does not follow the continuum variations in a simple manner.

ASCA Observations of Type 2 Seyfert Galaxies. I. Data Analysis Results

We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by ASCA. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the AGN. All but one of the Seyferts show evidence for variability on timescales of minutes-hours, with the amplitude anti-correlated with the source luminosity, confirming previous results. In at least 8 sources, there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power-law in the soft band. Ultra-rapid variability, implying significant power at frequencies > 10 −3 Hz is detected in at least 5 sources, but is difficult to detect in most cases, due to the sampling and signal-to-noise ratio. In Mrk 766 and MCG-6-30-15 there is also an indication that the high–frequency power–spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151, but on longer time scales.We present ASCA spatial, temporal, and spectral data for a sample of 26 observations of 25 type 2 active galactic nuclei (AGNs), composed of 17 Seyfert 2 galaxies and eight narrow emission line galaxies (NELGs). Twenty-four of the 25 sources were detected. The ASCA images are generally consistent with emission from point sources at energies above ~3 keV. We use archival ROSAT data to examine each field at high spatial resolution and to check for the presence of sources that would contaminate the ASCA data. Of the five sources bright enough for 128 s temporal analysis, three are variable at >99% confidence, with characteristics consistent with those observed in Seyfert 1 galaxies. Analysis on a timescale of 5760 s reveals six sources variable at >99% confidence, and comparison with previous X-ray results shows most of the sample to be variable in hard X-ray flux on timescales of years. Simple continuum models are fitted to the sample spectra to characterize the variety of spectral forms and hence to determine the fundamental nature of the X-ray spectrum of each source. No single spectral model provides an adequate fit to all the sample sources. Thirty-six percent of the sample cannot be adequately fitted by any of our test models (all rejected at >95% confidence). Approximately half of the sample have an iron Kα line with an equivalent width consistent with an origin in the line-of-sight absorber; the remaining lines must be produced in material out of the line of sight. Absorbing columns up to 1024 atoms cm-2 are detected, and even larger columns are inferred for some sources. The mean underlying hard X-ray power-law index is Γ ~ 2. Many X-ray emission lines were detected at high levels of confidence. The iron K-shell regime is dominated by emission from neutral material. Many data sets also show evidence for complexity in the iron Kα profile, which may be interpreted as evidence for broad-line profiles, including flux both redward and blueward of the line peak, and/or for the existence of hydrogen-like and helium-like iron K lines. Hydrogen-like and helium-like lines are detected from Fe, Ne, Si, S, and Ar in addition to Mg lines. While almost half of the sample have an estimated starburst contribution of >30% in the 0.5-4.5 keV bandpass, the soft X-ray emission lines are not solely associated with a strong starburst component.

The evolution of the hard X-ray luminosity function of AGN

We present new observational determinations of the evolution of the 2–10 keV X-ray luminosity function (XLF) of active galactic nuclei (AGN). We utilize data from a number of surveys including both the 2 Ms Chandra Deep Fields and the AEGIS-X 200 ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z 50 per cent of black hole growth takes place at z > 1 , with around half in L_X < 10^(44) erg s^(−1) AGN.

The Variable iron k emission line in MCG-6-30-15

We report on the variability of the iron K emission line in the Seyfert 1 galaxy MCG--6-30-15 during a four-day ASCA observation. The line consists of a narrow core at an energy of about 6.4 keV, and a broad red wing extending to below 5 keV, which are interpreted as line emission arising from the inner parts of an accretion disk. The narrow core correlates well with the continuum flux whereas the broad wing weakly anti-correlates. When the source is brightest, the line is dominated by the narrow core, whilst during a deep minimum, the narrow core is very weak and a huge red tail appears. However, at other times when the continuum shows rather rapid changes, the broad wing is more variable than the narrow core, and shows evidence for correlated changes contrary to its long time scale behaviour. The peculiar line profile during the deep minimum spectrum suggests that the line emitting region is very close to a central spinning (Kerr) black hole where enormous gravitational effects operate.

X-Ray Observations of Optically Selected, Radio-quiet Quasars. I. The ASCA Results

We present the result of 27 ASCA observations of 26 radio-quiet quasars (RQQs) from the Palomar-Green (PG) survey. The sample is not statistically complete, but it is reasonably representative of RQQs in the PG survey. For many of the sources, the ASCA data are presented here for the first time. All the RQQs were detected except for two objects, both of which contain broad absorption lines in the optical band. We find the variability characteristics of the sources to be consistent with Seyfert 1 galaxies. A power law offers an acceptable description of the time-averaged spectra in the 2-10 keV (quasar frame) band for all but one data set. The best-fitting values of the photon index vary from object to object over the range 1.5 ?2-10 3, with a mean ?2-10 2 and dispersion ?(?2-10) 0.25. The distribution of ?2-10 is therefore similar to that observed in other RQ active galactic nuclei (AGNs) and seems to be unrelated to X-ray luminosity. No single model adequately describes the full 0.6-10 keV (observed frame) continuum of all the RQQs. Approximately 50% of the sources can be adequately described by a single power law or by a power law with only very subtle deviations. All but one of the remaining data sets were found to have convex spectra (flattening as one moves to higher energies). The exception is PG 1411+442, in which a substantial column density (NH,z ~ 2 ? 1023 cm-2) obscures ~98% of the continuum. We find only five (maybe six) of 14 objects with z 0.25 to have soft excesses at energies 1 keV, but we find no universal shape for these spectral components. The spectrum of PG 1244+026 contains a rather narrow emission feature centered at an energy ~1 keV (quasar frame). The detection rate of absorption due to ionized material in these RQQs is lower than that seen in Seyfert 1 galaxies. In part, this may be due to selection effects. However, when detected, the absorbers in the RQQs exhibit a similar range of column density and ionization parameter as Seyfert 1 galaxies. We find evidence of Fe K-shell emission in at least eight RQQs. These are all low-luminosity objects, and the line parameters are consistent with those of other low-luminosity RQ AGNs. However the construction of the mean data/model ratios for various luminosity ranges reveals a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.