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NASA NTRS1992

Nonthermal electron-positron pairs and cold matter in the central engines of active galactic nuclei

The nonthermal e(+/-) pair model of the central engine of active galactic nuclei (AGNs) is discussed. The model assumes that nonthermal e(+/-) pairs are accelerated to highly relativistic energies in a compact region close to the central black hole and in the vicinity of some cold matter. The model has a small number of free parameters and explains a large body of AGN observations from EUV to soft gamma-rays. In particular, the model explains the existence of the UV bump, the soft X-rays excess, the canonical hard X-ray power law, the spectral hardening above about 10 keV, and some of the variability patterns in the soft and hard X-rays. In addition, the model explains the spectral steepening above about 50 keV seen in NGC 4151.

Zdziarski, Andrzej A.↗
NASA NTRS1994

Multifrequency emission from hot ion disks

The discovery of a large number of gamma-emitting active galactic nuclei (AGNs) by the EGRET instrument on the Compton Gamma Ray Observatory (CGRO) has spawned a lot of theoretical interest in the high-energy and multifrequency emission from these objects. Since most of them show evidence for relativistic outflow, jet models have received most of the attention so far. However, the presence of soft photons at the center of the active nucleus and the resulting Compton drag make it difficult to produce the observed amount of MeV/GeV emission. We explore hot, two-temperature accretion disks around Kerr black holes as an alternative to relativistic beam models for the production of the high-enerty emission. The decay of neutral pions created in the hot region produces photons with energies up to several hundred MeV. Relativistic pairs created as a result of charged pion decays produce additional inverse-Compton radiation in the range of approx. 1 keV-4 MeV if the pairs are exposed to UV radiation, or in the range of approx. 40 keV-150 MeV if the pairs are exposed to soft X-rays. This suggests that high-energy flares in AGNs may be triggered by changes in the disk structure (such as phase transitions or the development of electron scattering coronae) that temporarily shield the hot inner region from UV photons emitted at larger radii, thereby reducing the optical depth for MeV/GeV gamma-rays. Stochastic processes may also play a role in accelerating the utrarelativistic electrons responsible for producing the highest energy (GeV) emission.

Maisack, Michael↗
NASA NTRS1994

Gamma-ray bright quasars and BL Lacertae objects: EGRET data

The Compton Gamma-Ray Observatory (CGRO) Instruments, in particular, EGRET, have recently discovered new, gamma-ray bright active galactic nuclei (AGNs)(fluxes approximately 0.5-3 times as bright as 3C 273). Yet current sample sizes are modest (for example, EGRET as of 1993 January has detected 17 quasars and five BL Lac objects), and the sources probe only a small range in flux. Only by using a large, unbiased sample of AGNs can we answer key questions about source evolution, contributions to the gamma-ray background, and the different gamma-ray properties of quasars and BL Lac objects. Therefore, we need gamma-ray data on a much larger number of AGNs. We are currently pursuing two approaches toward remedying this situation: first, new (phase 2) OSSE, COMPTEL, and EGRET observations of X-ray selected AGNs predicted to be gamma-ray bright on simple assumptions; even an upper limit can set constraints on continuum models (e.g., pair production, synchrotron self-Compton); second, searches for radio-selected AGNs in wide-field EGRET data obtained during the pointings. For these AGN, we expect a large number of faint sources (i.e., mainly low signal-to-noise ratio detections and upper limits), a continuous progression down to the limiting EGRET sensitivity. Although special techniques must be employed to deal carefully with low-significance sources, the end product will be a much larger sample than currently available on which to base deductions about the gamma-ray properties of AGNs. Here we report the first results of this program. We find an upper limit for 3C 382 and demonstrate, using the 3C 382 field, how serendipitous blazars can systematically searched for in existing EGRET public (Phase 1) data. Preliminary results indicate that synchrotron self-Compton processes dominate.

Schachter, Jonathan↗
NASA NTRS1995

Low-state disks and low-beta disks

Stellar black hole candidates (BHCs) exhibit bimodal spectral states. We calculate nonthermal disk spectra, demonstrating that a large photon index (alpha (sub x) approximately 2-3) observed in the soft (high) state is due to a copious soft photon supply, whereas soft photon starvation leads to a smaller index (alpha (sub x) approximately 1.5-2) in the hard (low) state. Thus, the absence of the soft component flux in the low state cannot be due to obscuration. A possible disk configuration during the low state is discussed. We proposed that a low-state disk may be a low-beta disk in which magnetic pressure may exceed gas pressure becuase of the suppression of field escape by a strong shear. As a result, disk material will take the form of blobs constricted by mainly toroidal magnetic fields. Fields are dissipated mainly by occasional reconnection events with a huge energy release. This will account for large-amplitude, aperiodic X-ray variations (flickering) and high-energy radiation with small alpha(sub x) from hard state BHCs and possibly from active galactic nuclei. Further, we propose a hysteretic relation between the mass-flow rate and plasma-beta, a ratio of gas pressure to magnetic pressure, for the spectral evolution of transient BHCs. The disk is in the low-beta state in quiescence and early rise. The low-beta disk is optically thin and affected by advection. A hard-to-soft transition occurs before the peak luminosity, since there is no advection-dominated branch at higher luminosities. An optically thick, high-beta disk appears at small radii. In the decay phase of the light curve, the standard-type disk becomes effectively optically thin, when a soft-hard transition is triggered. High-beta plasmas in the main body shrink to form minute blobs, and low-beta coronal plasma fills interblob space.

Mineshige, Shin↗
NASA NTRS1996

Current Perspectives in High Energy Astrophysics

High energy astrophysics is a space-age discipline that has taken a quantum leap forward in the 1990s. The observables are photons and particles that are unable to penetrate the atmosphere and can only be observed from space or very high altitude balloons. The lectures presented as chapters of this book are based on the results from the Compton Gamma-Ray Observatory (CGRO) and Advanced Satellite for Cosmology and Astrophysics (ASCA) missions to which the Laboratory for High Energy Astrophysics at NASA's Goddard Space Flight Center made significant hardware contributions. These missions study emissions from very hot plasmas, nuclear processes, and high energy particle interactions in space. Results to be discussed include gamma-ray beaming from active galactic nuclei (AGN), gamma-ray emission from pulsars, radioactive elements in the interstellar medium, X-ray emission from clusters of galaxies, and the progress being made to unravel the gamma-ray burst mystery. The recently launched X-ray Timing Explorer (XTE) and prospects for upcoming Astro-E and Advanced X-ray Astronomy Satellite (AXAF) missions are also discussed.

Ormes, Jonathan F.↗
NASA NTRS1997

Multiwavelength Blazar Studies

The present report concerns the following projects 'High energy emission from hot accretion disks from active galactic nuclei'; 'OSSE spectral timing and monitoring observations of Cygnus X- 1'; 'OSSE observations of blazars'; and 'Search for correlated time variations of bright EGRET blazars'. The most favored model for the central engine inAGNs is the supermassive black hole hypothesis (Rees 1978). The observed luminosities range from approx. 10 logical and 44-45 erg /s for objects such as Mk 421 and Seyferts to as much as approx. 10logical and 47 for powerful QSOs such as 3C 273 and 3C 279, a large fraction of the observed bolometric luminosity being, in all likelihood, beamed. As such, these objects are strong X-ray emitters (cf. Makino et al. 1987) and often undergo gamma-ray flaring detected at the EGRET range (Hartman et al. 1992) and in the case of one of two known nearby BL Lacs, Mk 421, at TeV energies as well (Punch et al. 1992, Macomb et al. 1995). Previous campaigns emphasizing radio through X-ray and even gamma-ray observations have generally found that the multiwavelength spectrum is adequately fit by a standard synchrotron self-Compton (SSC-cf. Jones et al. 1974) model of a relativistic jet (e.g. Makino et al. 1987 and Macomb et al. 1995 for the BL Lac object Mk 421) or inhomogeneous relativistic jet (Mufson et al 1990). It also was examined the gamma-gamma transperency constraints inblazars.

Kafatos, Menas↗
NASA NTRS1997

The High Throughput X-ray Spectroscopy (HTXS) Mission

The HTXS mission concept combines large effective area (approximately 15,000 sq cm at 1 keV), high spectral resolution (E/Delta(E) approximately 300-3000), and broad energy bandpass (0.25-40 keV and possibly up to 100 keV) by using replicated optics together with a complement of spectroscopic instrumentation including reflection gratings readout by charge-coupled device detectors (CCDs), quantum micro-calorimeters, and cadmium zinc telluride (CZT) or comparable high energy detectors. An essential feature of this concept involves minimization of cost (approximately $350M for development and approximately $500-600M including launches) and risk by building six identical modest satellites to achieve the large area. Current mission and technology studies are targeted towards a new start in the 2002 timeframe, with first launch around 2005-2006. The HTXS mission represents a major advance, providing as much as a factor of 100 increase in sensitivity over currently planned high resolution X ray spectroscopy missions. HTXS will mark the start of a new era when high quality X ray spectra will be obtained for all classes of X ray sources, over a wide range of luminosity and distance. With its increased capabilities, HTXS will address many fundamental astrophysics questions such as the origin and distribution of the elements from carbon to zinc, the formation and evolution of clusters of galaxies, the validity of general relativity in the strong gravity limit, the evolution of supermassive black holes in active galactic nuclei, the details of supernova explosions and their aftermath, and the mechanisms involved in the heating of stellar coronae and driving of stellar winds.

White, N. E.↗
NASA NTRS1995

Granularity of the Diffuse Background Observed

First results are reported from a program for measuring the field-to-field fluctuation level of the cosmic diffuse background by using differences between the two background positions of each deep exposure with the High Energy X-ray Timing Experiment (HEXTE) instrument on the Remote X Ray Timing Explorer (RXTE). With 8 million live seconds accumulated to date a fluctuation level on the 15-25 keV band is observed which is consistent with extrapolations from the High Energy Astrophysical Observatory-1 (HEAO-1) measurements. Positive results are expected eventually at higher energies. Models of (active galactic nuclei) AGN origin will eventually be constrained by this program.

Gruber, D. E.↗
NASA NTRS2001

BATSE Observations of the Piccinotti Sample of AGN II: Variability and Spectral Analysis

The Piccinotti sample is the hard x-ray-selected sample of active galactic nuclei (AGNs) best studied at energies below approximately 10 keV. It represents the only unbiased and complete high-energy survey of the sky down to a limiting flux of 3.1 x 10(exp -11) per square erg per second. As such, it has been used to study AGN properties such as X-ray spectral characteristics, log N-log S relation, and luminosity function. The BATSE data provide, for the first time, a systematic coverage of the whole sample at high energies. BATSE data from nearly four years of observations (1993 November - 1997 September) were analyzed using standard BATSE occultation analysis software to extract a signal from sources in the Piccinotti sample. Although the BATSE sensitivity for individual source measurements is relatively poor on short time scales, the near all-sky coverage and long CGRO lifetime allows us to improve our sensitivity considerably by sumrr~ng data over many years. This significantly reduces our statistical errors at the expense of loss of temporal information. However, the systematic errors associated with the summation of data over such a long period are not negligible, and the study of systematic effects was a major part of our effort. We evaluated two types of systematic error: those affecting the overall normalization (which are important for comparison with other instruments) and those affecting the size of the fluctuations (which are relevant for estimating the confidence level of a detection). The former was studied by comparison of BATSE data with other instruments, primarily CGRO/OSSE. The results indicate that our absolute flux values maybe overestimated by as much as 35% for some sources. However, since this does not affect our estimation of the detection confidence, we made no corrections to our flux estimates.

Paciesas, William S.↗
NASA NTRS2002

Chandra Observations of the Evening Core of the Starburst Galaxy NGC 253

Chandra observations of the core of the nearby starburst galaxy NGC 253 reveal a heavily absorbed source of hard X-rays embedded within the nuclear starburst region. The source has an unabsorbed, 2 to 10 keV luminosity of greater than or equal to 10(exp 39) erg per s and photoionizes the surrounding gas. We observe this source through a dusty torus with a neutral absorbing column density of N(sub eta) approximately 2 x 10(exp 23)cm (exp -2). The torus is hundreds of pc across and collimates the starburst-driven nuclear outflow. We suggest that the ionizing source is an intermediate-mass black hole or a weakly accreting supermassive black hole, which may signal the beginnings or endings of AGN (active galactic nuclei) activity.

Weaver, K. A.↗
NASA NTRS2003

Science with the Space Infrared Telescope Facility

The Space Infrared Telescope Facility (SIRTF), the fourth and final member of NASA's series of Great Observatories, is scheduled to launch on April 15,2003. Together with the Hubbie Space Telescope, the Compton Gamma ray Telescope, and the Chandra X-Ray Telescope this series of observatories offers observational capabilities across the electromagnetic spectrum from the infrared to high-energy gamma rays. SIRTF is based on three focal plane instruments - an infrared spectrograph and two infrared imagers - coupled to a superfluid-helium cooled telescope to achieve unprecedented sensitivity from 3 to 180 microns. Although SIRTF is a powerful general-purpose infrared observatory, its design was based on the capability to address four broad science themes: (1) understanding the structure and composition of the early universe, (2) understanding the nature of brown dwarfs and super-planets, (3) probing protostellar, protoplanetary, and planetary debris disk systems, and (4) understanding the origin and structure of ultraluminous infrared galaxies and active galactic nuclei. This talk will address the design and capabilities of the SIRTF observatory, provide an overview of some of the initial science investigations planned by the SIRTF Guaranteed Time Observers, and give a brief overview of the General Observer proposal process.

Roellig, Thomas L.↗
NASA NTRS2007

Interferometry from Space

Space-based interferometric observatories will be challenging projects, equal at least to that of building the Great Observatories (the Hubble Space Telescope (HST), Spitzer Space Telescope (SST), Chandra X-ray Observatory, and the Gamma Ray Observatory), if not the Pyramids of Eygpt - but they represent the next logical step in examining our Universe at substantially higher angular resolution. Increasing our resolving power by factors of 100 or more (as is needed to make meaningful improvements in this observational arena) over existing facilities such as HST and SST requires mirror diameters (100's to 1000's of meters) much larger than can be supported by single or segmented mirrors - and thus the design and construction of sparse aperture, inteferometric arrays such as those described herein will be required. But just imagine the rewards of being able to see, for the first time, the surfaces of other stars, the location and type of extrasolar planets and even pictures of those same planets, the inner workings of Active Galactic Nuclei, the close-in details of supernovae explosions, black hole event horizons, and the infrared universe at the same resolution of the UV-optical Hubble Deep Fields. As a slight variation on the "Star Trek: Enterprise" theme song might say, it'll be a "long road, getting from here to there", but it will one well-worth taking.

Carpenter, Kenneth↗
NASA NTRS2008

An Axisymmetric, Hydrodynamical Model for the Torus Wind in Active Galactic Nuclei

We report on time-dependent axisymmetric simulations of an X-ray-excited flow from a parsec-scale, rotating, cold torus around an active galactic nucleus. Our simulations account for radiative heating and cooling and radiation pressure force. The simulations follow the development of a broad biconical outflow induced mainly by X-ray heating. We compute synthetic spectra predicted by our simulations. The wind characteristics and the spectra support the hypothesis that a rotationally supported torus can serve as the source of a wind which is responsible for the warm absorber gas observed in the X-ray spectra of many Seyfert galaxies.

Dorodnitsyn, A.↗
NASA NTRS2011

Investigating the Nuclear Activity of Barred Spiral Galaxies: The Case of NGC 1672

We have performed an X-ray study of the nearby barred spiral galaxy NGC 1672, primarily to ascertain the effect of the bar on its nuclear activity. We use both Chandra and XMM-Newton observations to investigate its X-ray properties, together with supporting high-resolution optical imaging data from the Hubble Space Telescope (HST) infrared imaging from the Spitzer Space Telescope, and Australia Telescope Compact Array ground-based radio data. We detect 28 X-ray sources within the D25 area of the galaxy; many are spatially correlated with star formation in the bar and spiral arms, and two are identified as background galaxies in the HST images. Nine of the X-ray sources are ultraluminous X-ray sources, with the three brightest (LX 5 * 10(exp 39) erg s(exp -1)) located at the ends of the bar. With the spatial resolution of Chandra, we are able to show for the first time that NGC 1672 possesses a hard (1.5) nuclear X-ray source with a 2-10 keV luminosity of 4 * 10(exp 38) erg s(exp -1). This is surrounded by an X-ray-bright circumnuclear star-forming ring, comprised of point sources and hot gas, which dominates the 2-10 keV emission in the central region of the galaxy. The spatially resolved multiwavelength photometry indicates that the nuclear source is a low-luminosity active galactic nucleus (LLAGN), but with star formation activity close to the central black hole. A high-resolution multiwavelength survey is required to fully assess the impact of both large-scale bars and smaller-scale phenomena such as nuclear bars, rings, and nuclear spirals on the fueling of LLAGN.

COLOR FIGURES↗
NASA NTRS2012

PKS 2123-463: A Confirmed Gamma-ray Blazar at High Redshift

The flat spectrum radio quasar (FSRQ) PKS 2123−463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1−4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123−463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123−463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123−463. A new photometric redshift has been estimated as z = 1.46 +/- 0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emis- sion detected by Swift/UVOT. This disk has a luminosity of 1.8x1046 erg s−1, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of 2 x 109 M(solar mass). This is the first black hole mass estimate for this source.

Gamma-ray blazar↗
NASA NTRS2014

Unusual Flaring Activity in the Blazar PKS 1424-418 During 2008-2011

Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission. Aims. We report peculiar behavior in the bright gamma-ray blazar PKS 1424-418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet. Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous -ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar. Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424-418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low -ray activity. Although the optical/gamma-ray behavior of PKS 1424-418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters. Conclusions. Detailed studies of individual blazars like PKS 1424-418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN.

flaring↗
NASA NTRS2015

Determining the Covering Factor of Compton-Thick Active Galactic Nuclei with NuSTAR

The covering factor of Compton-thick (CT) obscuring material associated with the torus in active galactic nuclei (AGNs) is at present best understood through the fraction of sources exhibiting CT absorption along the line of sight (N(sub H) greater than 1.5 x 10(exp 24) cm(exp −2)) in the X-ray band, which reveals the average covering factor. Determining this CT fraction is difficult, however, due to the extreme obscuration. With its spectral coverage at hard X-rays (greater than 10 keV), Nuclear Spectroscopic Telescope Array (NuSTAR) is sensitive to the AGNs covering factor since Compton scattering of X-rays off optically thick material dominates at these energies. We present a spectral analysis of 10 AGNs observed with NuSTAR where the obscuring medium is optically thick to Compton scattering, so-called CT AGNs. We use the torus models of Brightman and Nandra that predict the X-ray spectrum from reprocessing in a torus and include the torus opening angle as a free parameter and aim to determine the covering factor of the CT gas in these sources individually. Across the sample we find mild to heavy CT columns, with N(sub H) measured from 10(exp 24) to 10(exp 26) cm(exp −2), and a wide range of covering factors, where individual measurements range from 0.2 to 0.9. We find that the covering factor, f(sub c), is a strongly decreasing function of the intrinsic 2-10 keV luminosity, L(sub X), where f(sub c) = (−0.41 +/- 0.13)log(sub 10)(L(sub X)/erg s(exp −1))+18.31 +/- 5.33, across more than two orders of magnitude in L(sub X) (10(exp 41.5) - 10(exp 44) erg s(exp −1)). The covering factors measured here agree well with the obscured fraction as a function of LX as determined by studies of local AGNs with L(sub X) greater than 10(exp 42.5) erg s(exp −1).

NGC 4945↗
NASA NTRS2016

IC 3639 - A New Bona Fide Compton-Thick AGN Unveiled By NuSTAR

We analyze high-quality NuSTAR observations of the local (z = 0.011) Seyfert 2 active galactic nucleus (AGN) IC 3639, in conjunction with archival Suzaku and Chandra data. This provides the first broadband X-ray spectral analysis of the source, spanning nearly two decades in energy (0.5-30 keV). Previous X-ray observations of the source below 10 keV indicated strong reflection/obscuration on the basis of a pronounced iron fluorescence line at 6.4 keV. The hard X-ray energy coverage of NuSTAR, together with self-consistent toroidal reprocessing models, enables direct broadband constraints on the obscuring column density of the source. We find the source to be heavily Compton-thick (CTK) with an obscuring column in excess of 3.6 x 10(exp 24) cm(exp −2), unconstrained at the upper end. We further find an intrinsic 2-10 keV luminosity of log(sub 10) (L(sub 2-10 keV) [erg s(exp -1)])= 43.4(+0.6/-1.1) to 90% confidence, almost 400 times the observed flux, and consistent with various multiwavelength diagnostics. Such a high ratio of intrinsic to observed flux, in addition to an Fe-K(alpha) fluorescence line equivalent width exceeding 2 keV, is extreme among known bona fide CTK AGNs, which we suggest are both due to the high level of obscuration present around IC 3639. Our study demonstrates that broadband spectroscopic modeling with NuSTAR enables large corrections for obscuration to be carried out robustly and emphasizes the need for improved modeling of AGN tori showing intense iron fluorescence.

galaxies: active – galaxies: nuclei – galaxies↗