BBXRT observations of supernova remnants

The Broad Band X-Ray Telescope (BBXRT) was designed to perform sensitive, moderate resolution spectroscopy of cosmic X-ray sources in the 0.3–10 keV band from the Space Shuttle. During its nine-day flight in December, 1990, the BBXRT observed a variety of supernova remnants and related objects. We present results from some of these observations, emphasizing the ability of the BBXRT to perform spatially-resolved spectroscopy. The improved spectral resolution and efficiency over previous instruments makes possible measurements of previously undetectable lines, and the broad bandpass allows simultaneous measurements of lines from oxygen through iron.     

Ginga observations of Seyfert galaxies

We observed twenty-eight Seyfert 2 galaxies with the Japanese X-ray satellite, Ginga, and found Seyfert 2 galaxies, in general, have the X-ray spectral characteristics of obscured Seyfert 1 nuclei. This result agrees with the predictions from Unified Seyfert model proposed by Antonucci and Miller /1/. However, among the observed Seyfert 2 galaxies, there are a few galaxies with no evidence of an obscuration, contrary to the general predictions of the unified model. We note that type 2 AGN will contribute to the Cosmic Diffuse X-ray Background, if the unified Seyfert model can be extended to the far distant AGN such as quasars.     

HEAO-1 and BBXRT non-source X-ray observations

A more appropriate title for this talk would have been “Measurements of Large Scale Structure from X-ray Background Fluctuations”. While it has long been recognized that the X-ray Background (XRB) is primarily of a cosmological origin (with z < a few), it has recently become apparent that surface brightness fluctuations in the surveys of the XRB can be used to trace the distribution of matter in much the same way as complete catalogs of individual objects. The distance which is probed is related to the angular resolution of the detector; for the HEAO-1 A2 experiment, which provides the best all-sky data base for the XRB in the 2–20 keV band, the effective depth is a few 100 Mpc.     

The structure of the IC443 supernova remnant complex derived from ROSAT and Ginga observations

Radio observations have shown that the emission from IC443 can be attributed to three, partially incomplete shells. The two smaller diameter shells are bright in X-rays, whereas the X-ray extent of the largest one has been discovered only recently in the ROSAT All-Sky-Survey. This new X-ray shell — G189.6+3.3 — is well explained by a separate old ( 10⁵yr) supernova remnant (SNR), in contrast to the IC443 proper SNR, the age of which has been estimated to 10³yr. Spatial structure of the X-ray absorption observed with ROSAT suggests that G189.6+3.3 is located in front of a molecular cloud while the two smaller subshells previously known as IC443 are behind it. This geometry is confirmed by the observation of an additional absorption ridge across IC443, which is produced by a shell of cool and dense matter associated with G189.6+3.3.     

Polarization VLBI observations of AGN jets now and into the future

The history of Very Long Baseline Interferometry (VLBI) observations has been characterized predominantly by an ongoing quest for increasingly high resolution and sensitivity. VLBI monitoring of relatively large samples of Active Galactic Nuclei (AGNs) with uniform quality and linear polarization sensitivity are now available at the moderately high frequencies of 15 and 43 GHz. This has enabled considerable advances in our understanding of the relativistic jets of AGNs, but gaps in the available observational material remain, which must be addressed in future VLBI polarization observations. Linear polarization observations at frequencies above 43 GHz remain non-routine, and the availability of multi-frequency and circular polarization measurements is still limited. It is of interest both to push inward toward the jet base and to study details of the jets themselves on parsec scales, such as magnetic field structures along and across the jets, which are intrinsically related to their formation, launching and propagation. Requirements for future VLBI polarization observations are considered, highlighting the key role that can be played by space VLBI observations. Unique opportunities are offered by relatively low-frequency space VLBI observations that are sensitive to effects such as Faraday rotation, opacity, and low-frequency absorption.     

Star formation and AGN fuelling

We present new observational data that tackle the issues of the star formation in Seyfert galaxies, the obscuration and fuelling mechanisms of active galactic nuclei, and the connection between these phenomena. New ISOCAM mid-IR images of nearby Seyfert 2 galaxies confirm that these systems are characterized by enhanced star forming activity. In barred systems the star forming activity occurs preferentially along the bar, indicating that these bars have formed recently and are still in the process of transporting gas towards the center. New X-ray data indicate that the gaseous column density absorbing Sy2 nuclei is a function of the bar strength, therefore indicating that stellar bars play an important role in obscuring AGNs. We speculate that non-axisymmetric disturbances (interaction/bars) both enhance the star forming activity in host galaxy and drive gas into the nuclear region to obscure the AGN, thus making the observed starburst-Sy2 connection. On smaller scales (10–100 pc), we report the discovery of a nuclear gaseous bar in the nearby Sy2 Circinus galaxy. The molecular gas kinematics indicates that this bar causes the gas to flow into the nuclear 10 pc. In the nuclear 10 pc we detect a young nuclear stellar cluster. We show that the post-main-sequence mass loss of this young nuclear stellar population could account for the fuelling of the active nucleus.     

Obscured AGN and the X-ray background

There is growing evidence that the hard X-ray background (XRB) can be explained by a large population of obscured AGN. I review some of the results of recent deep X-ray surveys, and in particular I discuss the nature of the X-ray luminous emission-line galaxies which have emerged at the faintest X-ray fluxes. If obscured AGN do explain the XRB, a direct implication is that the majority of the energy produced by accretion in the universe is absorbed and not emitted directly. Deep submillimetre surveys with SCUBA have recently attracted a lot of attention, with the potential to allow us an unobscured view dust-enshrouded starformation at high redshift. It has generally been assumed that these sources are purely high redshift starforming galaxies, but if models for the XRB are correct then a significant fraction (∼20%) could contain a luminous AGN.     

Simultaneous COMPTON GRO and optical monitoring of AGN

During two 14-day periods in January 1992 and March 1992, several AGN which have been in the wide field of view of COMPTEL and EGRET at that time were monitored optically for variations in intensity and polarization. We report on first results of this correlated optical and CGRO observations.     

High energy emission from AGN

The recent detection of radio loud quasars with a large high energy γ-ray luminosity is discussed, in the framework of non-thermal synchrotron self Compton models. These observations are strong evidence of relativistic beaming of the radiation, due to the absence of strong photon-photon opacity. If γ-rays are self Compton emission, these sources can be the first observed example of a Compton catastrophe, resulting from the dominance of the radiation over the magnetic energy density. Inhomogeneous and relativistic jet models can in this case explain the observed spectrum from the far infrared to the γ-rays. If, on the other hand, the magnetic energy density is dominant, then the γ-rays must be due to direct synchrotron emission, implying very large particle energies. Pure synchrotron models which take into account electron-positron pair production are then discussed. Multifrequency simultaneous observations are a powerful test of the models.     

AGN activity triggered by circumnuclear starbursts

We present a radiative/hydrodynamical mechanism for triggering AGN activity; the intensive radiation from a circumnuclear starburst drives the nuclear fueling due to the Poynting-Robertson (radiation drag) effects. When the starburst is in an early and therefore super-Eddington phase, the radiative flux force is likely to obstruct severely the mass accretion onto the nucleus (radiative blizzard phase). But, in a later sub-Eddington phase, the radiation flux force builds up a wall of dusty gas. The wall absorbs the radiation from the starburst regions and re-emits infrared radiation, which causes the disk accretion due to the Poynting-Robertson effect, consequently leading to nuclear fueling (radiative avalanche phase). The radiative avalanche could link to an advection-dominated accretion flow (ADAF) onto a putative supermassive black hole. The radiatively triggered nuclear activity diminishes as the starburst dims. In this context, the AGN type could be discriminated not only by the viewing angles but also by the evolution of a circumnuclear starbursts. Based on such a picture, we reconsider the AGN activity in luminous IRAS galaxies.     

Interstellar scintillation,ISS, and intrinsic variability of radio AGN

We investigate the relationship between the 5 GHz interstellar scintillation (ISS) and the 15 GHz intrinsic variability of the compact, radio-selected active galactic nuclei (AGN) common to the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory blazar flux density monitoring program. As part of this investigation, we also re-examine the reported intrinsic nature of the February 1990 VLA observations of the blazar S5 0716+714. We are also examining the presence of IDV/ISS in the Owens Valley 15 GHz flux density monitoring data. We find a significant relationship between the Owens Valley 15 GHz modulation index and the MASIV modulation index. We also discuss the implications of these findings for RadioAstron.     

Ultraluminous infrared galaxies: Starburst vs. AGN

We have obtained ISOPHOT-S low resolution mid-infrared spectra of a sample of 60 Ultraluminous Infrared Galaxies (ULIRGs). We use the strength of the ‘PAH’ mid-infrared features as a discriminator between starburst and AGN activity, and to probe for evolutionary effects. Observed ratios of PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. We suggest that such PAH ratio changes relate to the conditions in the interstellar medium in these galaxies, and in particular to extinction.     

Catalogue of ionized emission line spectra in obscured AGN

High-energy, high-resolution in the spatial and energy domains is crucial to derive the geometrical distribution and the physical properties of gas and dust surrounding Active Galactic Nuclei (AGN). These are, in turn, basic ingredients of any serious attempt to build an AGN structure model. We present in this paper CIELO-AGN, the first catalogue of ionized emission lines detected in the soft X-ray spectra of a sample of 69 nearby obscured AGN. This catalogue has been implemented according to an IVOA data model, and can be accessed through VO-compatible applications.     

The AGN/normal galaxy connection: Summary

The connection between normal and active galaxies is reviewed, by summarizing our progress on answering nine key questions. (1) Do all galaxies contain massive dark objects (MDOs)? (2) Are these MDOs actually supermassive black holes? (3) Why are the dark objects so dark? (4) Do all galaxies contain an Active Galactic Nucleus (AGN)? (5) Are the “dwarf AGN” really AGN? (6) Does AGN activity correlate with host galaxy properties? (7) How are AGN fuelled? (8) Is AGN activity related to starburst activity? (9) How do quasars relate to galaxy formation?     

Hydrodynamical evolution of AGN driven by radiation from circumnuclear starbursts

We explore the hydrodynamical evolution of dusty gas around active galactic nuclei (AGN) driven by the radiation from circumnuclear starbursts. For this purpose, we calculate the temporal equilibrium states between the radiation force by starburst regions and the gravity in galactic nuclei. As a result, we find that the equilibrium patterns between the radiation force and the gravity are roughly characterized by three types. The first is the situation where the starburst luminosity is larger than the Eddington luminosity. In this case, the dusty gas is blown out like a wind. We may detect intense infrared (IR) radiation from the starburst regions screened by blown-out dusty gas. The second is the situation when the radiation force is comparable to the gravity. In this case, the equilibrium surface surrounds the nuclear regions as well as starburst regions. Since the dusty gas absorbs UV or soft X-rays from the center and re-emits IR radiation, we may recognize it as a Seyfert 2 galaxy. The last is the situation where the starburst luminosity is small. In this case, the dusty wall of equilibrium would be built up only in the vicinity of starburst regions. The radiation from central regions is rarely obscured, because the dusty regions have only small angular extension. So, it would look like a Seyfert 1 galaxy which is characterized by intense soft X-rays. When we consider the stellar evolution in starburst regions, the starburst luminosity decreases with time. Therefore, we can recognize the above three types as time evolution; a starburst galaxy (first stage), a Seyfert 2 galaxy (second stage), and a Seyfert 1 galaxy (third stage). Note that we present here an alternative scenario for explaining the relation between Sy 1's and Sy 2's to the standard “Unified Scheme”.     

High-density and high-temperature molecular gas around the AGN in M51

We investigated the physical properties of molecular gas in the nuclear region of M51 (Seyfert 2). We obtained an aperture synthesis ¹³CO(J = 1 − 0) image using the Nobeyama Millimeter Array (NMA), and compared it with NMA ¹²CO(J = 1 − 0) and HCN(J = 1 − 0) maps at similar spatial resolutions. Within a radius of 180 pc from the center, the ¹³CO(1 − 0) integrated intensity was found to be 3 times weaker than that of HCN(1 − 0). Large-Velocity-Gradient (LVG) calculations suggest that the observed high HCN(1 − 0)/¹³CO(1 − 0) intensity ratio would arise from dense (n_H2 ∼ 10⁵ cm⁻³) and hot (T_kin ≳ 300 K) molecular clouds in the nuclear molecular disk. We also observed in the ¹²CO(1 − 0), (3 − 2), ¹³CO(1 − 0), and (3 − 2) lines using the Nobeyama 45m and JCMT 15m telescopes. We detected weak ¹³CO lines as well as strong ¹²CO lines. The LVG calculations assuming a two-component model suggest that there is a large amount of low-density (n_H2 ∼ 3 − 6 × 10² cm⁻³), low-temperature (T_kin ∼ 20 – 50 K) gas, and a small amount of high-density (n_H2 ≳ 10⁴ cm⁻³), high-temperature (T_kin ≳ 500 K) gas. The existence of the high-density and high-temperature component, although having a quite small beam filling factor, supports the aperture synthesis observation results mentioned above. Since this dense, hot gas is located in the nuclear molecular disk around the Active Galactic Nucleus (AGN), it may be heated by the strong X-ray radiation and/or by the shock induced by the radio jet.     

A study of a complete soft X-ray selected sample of AGN

We present preliminary results from analyses of hard X-ray and optical observations of a soft X-ray selected sample. We created a small but complete sample with 20 of the softest and brightest objects with low Galactic absorption from the ROSAT bright soft X-ray selected radio-quiet AGN sample. This sample consists of 10 narrow-line Seyfert 1 galaxies and 10 broad-line Seyfert galaxies. We analyze ASCA data in the 0.6–10 keV band and optical spectra from ground-based telescopes. We investigate the photon indices in the hard X-ray band, soft excesses in the ASCA band, and optical emission line properties. The photon indices in the 2–10 keV band are nominal for both narrow-line Seyfert 1 galaxies and broad-line Seyfert 1 galaxies in each class compared with other heterogeneous samples. All of the narrow-line Seyfert 1 galaxies show soft excesses, but this component seems to be less significant for broad-line Seyfert 1 galaxies. There seems to be a trend of steeper X-ray spectra to be accompanied by narrower Hβ for narrow-line Seyfert 1 galaxies, but this is not extended to the larger velocity width regime of broad-line Seyfert 1 galaxies, and no clear trend is seen among them.     

X-ray spectroscopy and variability of AGN detected in the 2 Ms Chandra Deep Field-North Survey

We investigate the nature of the faint X-ray source population through X-ray spectroscopy and variability analyses of 136 active galactic nuclei (AGN) detected in the 2 Ms Chandra Deep Field-North Survey with >200 background-subtracted 0.5–8.0 keV counts [F_{0.5–8.0 keV} = (1.4−200) × 10⁻¹⁵ erg cm⁻² s⁻¹]. Our preliminary spectral analyses yield median spectral parameters of Γ = 1.61 and intrinsic N_H = 6.2 × 10²¹ cm⁻² (z = 1 assumed when no redshift available) when the AGN spectra are fitted with a simple absorbed power-law model. However, considerable spectral complexity is apparent (e.g., reflection, partial covering) and must be taken into account to model the data accurately. Moreover, the choice of spectral model (i.e., free vs. fixed photon index) has a pronounced effect on the derived JVH distribution and, to a lesser extent, the X-ray luminosity distribution. We also find that among the 136 AGN, 10 (≈7%) show significant Fe K emission-line features with equivalent widths in the range 0.1–1.3 keV. Two of these emission-line AGN could potentially be Compton thick (i.e., Γ < 1.0 and large Fe K equivalent width). Finally, we find that 81 (≈60%) of the 136 AGN show signs of variability, and that this fraction increases significantly (≈80–90%) when better photon statistics are available.     

On ultra-high energy cosmic rays: Origin in AGN jets and transport in expanding universe

The cosmic ray source spectrum produced by AGN (active galactic nucleus) jets is calculated. A distinctive feature of these calculations is the account for the jet distribution on kinetic energy. The expected cosmic ray spectrum at the Earth is determined with the use of a simple numerical code which takes into account interactions of ultra-high energy protons and nuclei with the background radiation in an expanding universe.