Nuclear Flux Research Articles

Response of the warm absorber cloud to a variable nuclear flux in active galactic nuclei

Recent modeling of the warm absorber in active galactic nuclei has proved the usefulness of constant total (gas plus radiation) pressure models, which are highly stratified in temperature and density. We explore the consistency of those models when the typical variation of the flux from the central source is taken into account. We have performed a variability study of the warm absorber response, based on timescales and our photoionization code titan. We show that the ionization and recombination timescales are much shorter than the dynamical timescale. Clouds very close to the central black hole will maintain their equilibrium since the characteristic variability timescales of the nuclear source are longer than cloud timescales. For more distant clouds, the density structure has no time to vary in response to the variations in the temperature or ionization structure, and such clouds will show the departure from the constant pressure equilibrium. We explore the impact of this departure on the observed properties of the transmitted spectrum and soft X-ray variability: (i) non uniform velocities, on the order of sound speed, appear due to pressure gradients, up to typical values of 100 km s-1. These velocities lead to the broadening of lines. This broadening is usually observed and very difficult to explain otherwise. (ii) Energy-dependent fractional variability amplitude in soft X-ray range has a broader hump around ~1–2 keV, and (iv) the plot of the equivalent hydrogen column density vs. ionization parameter is steeper than for equilibrium clouds. The results have the character of a preliminary study and should be supplemented in the future with full time-dependent radiation transfer and dynamical computations.

Comparison of neutron activation analysis methods

This work shows that nuclear data and neutron flux parameters can be applied to calculate directly the elemental concentrations. Techniques for extracting neutron flux parameters pertaining to the irradiation positions and nuclear data pertaining to the isotopes concerned from measured reaction rates have been previously developed. This method is compared to the comparator and relative methods of activation analysis. The principles, advantages and disadvantages of each method and sources of errors are discussed. It also briefly discusses other factors such as accuracy and precision, sensitivity, detection limits and limit of quantitative determination. The three methods are applied to the analysis of five environmental reference materials. The concentrations of more than 20 elements are determined. The results show a good agreement with the certified and/or literature values.

Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1

We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29–2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of ( 1.4 ± 0.2 ) × 10 32 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190 ± 20 km 2 .

Iron Line and Diffuse Hard X‐Ray Emission from the Starburst Galaxy M82

We examine the properties of the diffuse hard X-ray emission in the classic starburst galaxy M82. We use new Chandra ACIS-S observations in combination with reprocessed archival Chandra ACIS-I and XMM-Newton observations. We find E~6.7 keV Fe He-alpha emission is present in the central |r| < 200 pc, |z| < 100 pc of M82 in all datasets at high statistical significance, in addition to a possibly non-thermal X-ray continuum and marginally significant E=6.4 keV Fe K-alpha line emission. No statistically significant Fe emission is found in the summed X-ray spectra of the point-like X-ray sources or the ULX in the two epochs of Chandra observation. The total nuclear region iron line fluxes in the 2004 April 21 XMM-Newton observation are consistent with those of the Chandra-derived diffuse component, but in the 2001 May 6 XMM-Newton observation they are significantly higher and also both E=6.4 and E=6.9 keV iron lines are detected. We attribute the excess iron line emission to the Ultra-Luminous X-ray source in its high state. In general the iron K-shell luminosity of M82 is dominated by the diffuse component. The total X-ray luminosity of the diffuse hard X-ray emission (corrected for emission by unresolved low luminosity compact objects) is L_X ~ 4.4 x 10^39 erg/s in the E=2-8 keV energy band, and the 6.7 keV iron line luminosity is L_X ~ (1.1 -- 1.7) x 10^38 erg/s. The 6.7 keV iron line luminosity is consistent with that expected from the previously unobserved metal-enriched merged supernova ejecta that is thought to drive the larger-scale galactic superwind. The iron line luminosity implies a thermal pressure within the starburst region of P/k ~ 2 x 10^7 K/cm^3, which is consistent with independent observational estimates of the starburst region pressure [Abstract abridged].

Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1

We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29–2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of ( 1.4 ± 0.2 ) × 10 32 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190 ± 20 km 2 .

A study of radiation resistance of silica optical fibers under conditions of reactor irradiation

A measuring system has been developed to investigate the characteristics of radiation-induced optical absorption and radioluminescence in the spectral range of 400–1000 nm for a set of optical fibers being irradiated by fast neutrons and γ rays. This system has been used on the IR-8 research nuclear reactor to perform comparative tests of eight types of silica optical fibers from Heraeus (Germany), Fujikura (Japan), Mitsubishi (Japan), and the Fiber Optics Research Center of the Russian Academy of Sciences’ Prokhorov General Physics Institute. The measured dependences of the induced absorption and radioluminescence intensity versus the nuclear radiation wavelength, flux, and fluence, as well as the temperature, are analyzed. The microscopic mechanisms of induced absorption and radioluminescence are discussed. The values of induced optical absorption and radioluminescence in glass fibers containing H2 gas and having a hermetic coating (the technology for producing such fibers has been developed by the Fiber Optics Research Center) is found to be many times smaller than in other fibers.

Activity-dependent and -independent nuclear fluxes of HDAC4 mediated by different kinases in adult skeletal muscle

Class II histone deacetylases (HDACs) may decrease slow muscle fiber gene expression by repressing myogenic transcription factor myocyte enhancer factor 2 (MEF2). Here, we show that repetitive slow fiber type electrical stimulation, but not fast fiber type stimulation, caused HDAC4-GFP, but not HDAC5-GFP, to translocate from the nucleus to the cytoplasm in cultured adult skeletal muscle fibers. HDAC4-GFP translocation was blocked by calmodulin-dependent protein kinase (CaMK) inhibitor KN-62. Slow fiber type stimulation increased MEF2 transcriptional activity, nuclear Ca2+ concentration, and nuclear levels of activated CaMKII, but not total nuclear CaMKII or CaM-YFP. Thus, calcium transients for slow, but not fast, fiber stimulation patterns appear to provide sufficient Ca2+-dependent activation of nuclear CaMKII to result in net nuclear efflux of HDAC4. Nucleocytoplasmic shuttling of HDAC4-GFP in unstimulated resting fibers was not altered by KN-62, but was blocked by staurosporine, indicating that different kinases underlie nuclear efflux of HDAC4 in resting and stimulated muscle fibers.

Ultraviolet Light from Young Stars in GEMS Quasar Host Galaxies at 1.8 &lt; z &lt; 2.75

We have performed HST imaging of a sample of 23 high-redshift (1.8<z<2.75) AGN, drawn from the COMBO-17 survey. The sample contains moderately luminous quasars (M_B ~ -23). The data are part of the GEMS imaging survey that provides high resolution optical images obtained with ACS in two bands (F606W and F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN images into nuclear and resolved (host galaxy) components we use a PSF subtraction technique that is strictly conservative with respect to the flux of the host galaxy. We resolve the host galaxies in both filter bands in 9 of the 23 AGN, whereas the remaining 14 objects are considered non-detections, with upper limits of less than 5% of the nuclear flux. However, when we coadd the unresolved AGN images into a single high signal-to-noise composite image we find again an unambiguously resolved host galaxy. The recovered host galaxies have apparent magnitudes of 23.0<F606W<26.0 and 22.5<F850LP<24.5 with rest-frame UV colours in the range -0.2<(F606W-F850LP)_obs<2.3. The rest-frame absolute magnitudes at 200 nm are -20.0<M_200nm<-22.2. The photometric properties of the composite host are consistent with the individual resolved host galaxies. We find that the UV colors of all host galaxies are substantially bluer than expected from an old population of stars with formation redshift z=5, independent of the assumed metallicities. These UV colours and luminosities range up to the values found for Lyman-break galaxies (LBGs) at z=3. Our results agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts. [abridged]

Testing Radiatively Inefficient Accretion Flow Theory: AnXMM‐NewtonObservation of NGC 3998

We present the results of a 10 ks XMM-Newton observation of NGC 3998, a ``type-I'' LINER galaxy. Our goal is to test the extent to which radiatively-inefficient accretion flow (RIAF) models and/or scaled-down AGN models are consistent with the observed properties of NGC 3998. The upper-limit for narrow Fe-K emission derived from a combined fit of the XMM-Newton and BeppoSAX spectra is 25 eV, which is one of the strictest limits to date for any AGN. This significantly rules out Fe-K emission as is expected to be observed in typical Seyfert 1 galaxies. The lack of any reflection features suggests that any optically-thick, geometrically-thin accretion disk must be truncated, probably at a radius of order 100-300 R_s. RIAF models fit the UV to X-ray spectral energy distribution of NGC 3998 reasonably well. In these models the mid-IR flux also constrains the emission from any outer thin disk component that might be present. The UV to X-ray SED is also consistent with a Comptonized thin disk with a very low accretion rate, in which case the lack of Fe-K emission may be due to an ionized accretion disk. Accretion models in general do not account for the observed radio flux of NGC 3998, and the radio flux may be due to a jet. Recent jet models may also be consistent with the nuclear fluxes of NGC 3998 in general, including the X-ray, optical/UV and mid-IR bands. We also derive nuclear fluxes using archival HST WFPC2 data to constrain the SED of NGC 3998. We discuss a possible OM U band and USNO-B detection of the NGC 3998 ULX.

Follow‐UpChandraObservations of Three Candidate Tidal Disruption Events

Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT All-Sky Survey in several galaxies with no evidence of Seyfert activity in their ground-based optical spectra. These flares had the properties predicted for a tidal disruption of a star by a central supermassive black hole. We report Chandra observations of three of these galaxies taken a decade after their flares that reveal weak nuclear X-ray sources that are from 240 to 6000 times fainter than their luminosities at peak, supporting the theory that these were special events and not ongoing active galactic nucleus (AGN) variability. The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the (t-t_D)^(-5/3) decay predicted for the fall-back phase of a tidal disruption event, but only if ROSAT was lucky enough to catch the event exactly at its peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also consistent with (t-t_D)^(-5/3). In the H II galaxy NGC 5905 we find only resolved, soft X-ray emission that is undoubtedly associated with starburst activity. When accounting for the starburst component, the ROSAT observations of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are consistent with a (t-t_D)^(-5/3) decay by at least a factor of 1000. Although we found weak Seyfert~2 emission lines in Hubble Space Telescope spectra of NGC 5905, indicating that a low-luminosity AGN was present prior to the X-ray flare, we favor a tidal disruption explanation for the flare itself.

The Radiation-Measuring Complex of the СПРУТ-VI System

The radiation-measuring complex of the СПРУТ-VI system is designed to determine the absorbed dose and values of heavy-nuclei fluxes under a program for investigating isolated failures in microcircuits. The complex is composed of active and passive elements. It is used to measure the total absorbed dose and the radiation doses received by exposed microelectronic elements behind shields of 1 g/cm2 (across the range of 10–4–5 Gy), establish a link between isolated failures and the actual radiation environment, detect nuclear fluxes with a charge of Z > 2 and energies of 30–200 MeV/nucleon, and determine the fluence of heavy charged particles across the range of 1 to 105 cm–2.

Star Formation History and Extinction in the Central Kiloparsec of M82‐like Starbursts

We report on the star formation histories and extinction in the central kiloparsec region of a sample of starburst galaxies that have similar far-infrared (FIR), 10 μm, and K-band luminosities as those of the archetype starburst M82. Our study is based on new optical spectra and previously published K-band photometric data, both sampling the same area around the nucleus. Model starburst spectra were synthesized as a combination of stellar populations of distinct ages formed over the Hubble time and were fitted to the observed optical spectra and K-band flux. The model is able to reproduce simultaneously the equivalent widths of emission and absorption lines, the continuum fluxes between 3500 and 7000 A, and the K-band and FIR flux. A good fit requires a minimum of three populations: (1) a young population of age ≤8 Myr, with its corresponding nebular emission, (2) an intermediate-age population (age <500 Myr), and (3) an old population that forms part of the underlying disk or/and bulge population. The birthrate parameter, which is defined as the ratio of the current star formation rate to the average past rate, is found to be in the range 1-12. The contribution of the old population to the K-band luminosity depends on the birthrate parameter and remains above 60% in the majority of the sample galaxies. Even in the blue band, the intermediate-age and old populations contribute more than 40% of the total flux in all the cases. A relatively high contribution from the old stars to the K-band nuclear flux is also apparent from the strength of the 4000 A break and the Ca II K line. The extinction of the old population is found to be around half that of the young population. The contribution to the continuum from the relatively old stars has the effect of diluting the emission equivalent widths below the values expected for young bursts. The mean dilution factors are found to be 5 and 3 for the Hα and Hβ lines, respectively.

The XMM-Newton view of NGC 6251

We report on the nuclear X-ray properties of the radio galaxy NGC 6251 observed with XMM-Newton . NGC 6251 is a well-known radio galaxy with intermediate FRI/II radio properties. It is optically classified as a Seyfert 2 and hosts a supermassive black hole with mass ~$ 6\times10^8~ M_{\odot}$. The 0.4–10 keV EPIC pn continuum is best fitted by two thermal components ($kT \sim 0.5$ and 1.4 keV, respectively), plus a power law with photon index $\Gamma \sim 1.9$ absorbed by a column density $N_{\rm H} \sim 5 \times 10^{20}$ cm -2 . We confirm the previous ASCA detection of a strong iron line. The line, resolved in the EPIC pn spectrum, is adequately fitted with a broad ($\sigma\sim 0.6$ keV) Gaussian at rest-frame energy 6.4 keV with EW ~ 220 eV. We also detect, for the first time, short-term, low-amplitude variability of the nuclear flux on a timescale of a few ks. The spectral properties argue in favor of the presence of a standard accretion disk, ruling out the base of the jet as the sole origin of the X-rays. The moderate X-ray luminosity and lack of strong intrinsic absorption suggest that NGC 6251 is a “pure” type 2 AGN which lacks a broad-line region.

Physical Implications of RHESSI Neutron-Capture Line Measurements

We report high spectral resolution measurements of the 2.223 MeV neutron-capture line obtained with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) from the 2002 July 23 solar flare. The time history of this line is affected by both the photospheric 3He abundance and the angular distribution of the interacting flare-accelerated particles producing the neutrons. The measured time history is compared with predicted time histories calculated using a magnetic loop model with a magnetic field perpendicular to the solar surface at the footpoints. The model includes energy losses due to Coulomb collisions, removal by nuclear reactions, magnetic mirroring in the convergent flux tube, and MHD pitch-angle scattering in the corona. This is the first analysis of the neutron-capture line using such a physically based model. The derived constraints on the interacting angular distribution require that the accelerated particles had to suffer pitch-angle scattering during their transport through the coronal portion of the loop. The derived photospheric 3He/H ratio was not well constrained, primarily because of uncertainties associated with the measured nuclear de-excitation line flux used to represent the neutron-production time profile.

Compact Nuclear Starbursts in Seyfert 2 Galaxies from the CfA and 12 Micron Samples

We present infrared 2.8-4.1 μm slit spectra of 32 Seyfert 2 galaxies in the CfA and 12 μm samples. The 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature was used to estimate the absolute magnitude of a compact nuclear starburst (less than a few hundred parsecs in size) that is presumed to have occurred in the outer region of an obscuring dusty molecular torus around a central supermassive black hole. We detected 3.3 μm PAH emission in 11 of the 32 Seyfert 2 nuclei in our sample, providing evidence for the presence of compact nuclear starbursts in a significant fraction of Seyfert 2 nuclei. However, the rest-frame equivalent widths of the 3.3 μm PAH emission and the 3.3 μm PAH-to-infrared luminosity ratios measured in this study suggest that compact nuclear starbursts generally do not contribute significantly to the observed 3-4 μm nuclear fluxes or to the infrared luminosities of Seyfert 2 galaxies. Absorption features at 3.4 μm from bare dust were clearly detected in only two of the nuclei, and features at 3.1 μm from ice-covered dust were detected in only one nucleus. If the dust properties in the direction of these Seyfert 2 nuclei do not differ significantly from the Galactic interstellar medium, then these small absorption optical depths suggest that dust extinction toward the 3-4 μm continuum emitting region in the innermost part of the obscuring dusty torus is modest: AV < 50-60 mag. Finally, the 3.3 μm PAH emission luminosities measured in this study were found to be significantly correlated with IRAS 12 and 25 μm and nuclear N-band (10.6 μm) luminosities. If these three luminosities trace the power of the active galactic nucleus (AGN), then the luminosities of compact nuclear starbursts and AGNs are correlated. This correlation is in agreement with theories predicting that the presence of a compact nuclear starburst in the torus leads to an enhancement of the mass accretion rate onto the central supermassive black hole.

Unveiling the powerful quasar hidden in the nucleus of the FR II radio galaxy 3C 265

The radio source 3C 265, located at z=0.811, is one of the most luminous radio galaxies in the 3CR catalogue. Indirect evidence for the presence of a powerful quasar in the nucleus of 3C 265 was provided by optical, infrared, and radio studies. We present preliminary results from new Chandra X-ray observations of the radio galaxy 3C 265, allowing us to test the presence of such a powerful quasar, and at the same time to study the effect of the intense nuclear flux on the relativistic electron population in the radio lobes of 3C 265.

Simultaneous Optical and Near‐Infrared Spectropolarimetry of Type 2 Seyfert Galaxies

We present optical and near-infrared spectropolarimetry of the nuclei of four type 2 Seyfert galaxies, Mrk 463E, Mrk 1210, NGC 1068, and NGC 4388. The data were obtained simultaneously, covering the wavelength range of 0.46-2.5 μm. We model the polarizations from two dust-scattering components: (1) scattering in dusty regions in ionization cones and (2) scattering in a torus surrounding a type 1 nucleus. The polarizations from electron scattering in the cones and dichroic absorption by aligned dust grains in the torus are also compared with the observations. We confirmed that a combination of electron and dust scattering in the ionization cones is the preferred mechanism for the optical continuum polarization. For the near-infrared, dichroic absorption by aligned grains can explain the continuum polarization of Mrk 463E and Mrk 1210 as well as NGC 1068. Visual optical depths of the order of 10-20 are estimated for dichroic absorption in these nuclei. Dust scattering in the torus, whose grain size distribution is assumed to be the same as in the Galactic diffuse interstellar medium, cannot reproduce the observed spectral slope of the near-infrared polarization and total nuclear flux simultaneously. However, this might only indicate that the grain size distribution in the torus of active galactic nuclei (AGNs) is different, and dust scattering with moderate optical depth and dominated by large grains might provide a reasonable explanation for the near-infrared radiation from AGNs.

Unifying B2 radio galaxies with BL Lacertae objects

In an earlier paper we presented nuclear X-ray flux densities , measured with ROSAT, for the B2 bright sample of nearby low-luminosity radio galaxies. In this paper we construct a nuclear X-ray luminosity function for the B2 radio galaxies, and discuss the consequences of our results for models in which such radio galaxies are the parent population of BL Lac objects. Based on our observations of the B2 sample, we use Monte Carlo techniques to simulate samples of beamed radio galaxies, and use the selection criteri a of existing samples of BL Lac objects to compare our simulated results to what is observed. We find that previous analytical results are not applicable since the BL Lac samples are selected on beamed flux density. A simple model in which BL Lacs are the moderately beamed (γ � 3) counterparts of radio galaxies, with some random dispersion (� 0.4 decades) in the intrinsic radio-X-ray relationship, can reproduce many of the features of the radio-selected and X-ray-selected BL Lac samples, including their radio and X-ray luminosity functions and the distributions of their radio-to-X-ray spectral indices. In contrast, models in wh ich the X-ray and radio emission have systematically different beaming parameters cannot reproduce important features of the radio-galaxy and BL Lac populations, and recently proposed models in which the radio-toX-ray spectral index is a function of source luminosity cannot in themselves account for the differences in the slopes of the radio and X-ray-selected BL Lac luminosity functions. The redshift distribution and number counts of the X-ray-selected EMSS sample are well reproduced by our best models, supporting a picture in which these objects are beamed FRI radio galaxies with intrinsic luminosities similar to those of th e B2 sample. However, we cannot match the redshift distribution of the radio-selected 1-Jy sample, and it is likely that a population of FRII radio galaxies is responsible for the high-r edshift objects in this sample, in agreement with previously reported results on the sample’s radio and optical-emission-line properties.

Balmer and H[CLC]e[/CLC] [CSC]i[/CSC] Absorption in the Nuclear Spectrum of NGC 4151

Spectra taken with the Space Telescope Imaging Spectrograph allow accurate location and extraction of the nuclear spectrum of NGC 4151, with minimal contamination by extended line emission and circumnuclear starlight. Spectra since 1997 show that the P Cygni Balmer and He I absorption seen previously in low nuclear states is present in higher states, with outflow velocity that changes with the nuclear flux. The phenomenon is discussed in terms of some of the absorbers seen in the UV resonance lines, and in terms of outflows from the central source and surrounding torus.

An X-ray view of the active nucleus in NGC 4258

XMM-Newton observed the Seyfert 1.9 galaxy NGC 4258 in December 2000. At energies above 2 keV a hard nuclear point source is resolved that can be fitted by a highly absorbed power-law spectrum (= () cm-2, photon index ) with an unabsorbed luminosity of 7.5 in the (2–10) keV band. No narrow iron Kα emission line is detected (90% upper limit of equivalent width eV). The nuclear emission flux was observed to remain constant over the observation. A short archival Chandra observation taken in March 2000 further constrains the hard emission to a point source coincident with the radio nucleus. A point source ~3´´ southwest of the nucleus does not contribute significantly. Spectral results of the Chandra nuclear source are comparable (within the limited statistics) to the XMM-Newton parameters. The comparison of our iron line upper limit with reported detections indicates variability of the line EW. These results can be explained by the relatively low nuclear absorption of NGC 4258 (which is in the range expected for its intermediate Seyfert type) and some variability of the absorbing material. Reflection components as proposed to explain the large iron line EW of highly absorbed Seyfert 2 galaxies and/or variations in the accretion disk are however imposed by the time variability of the iron line flux.