arXiv subjects

T. J. Turner

Publications and source records attributed to T. J. Turner.

At least 127 records · Page 7Linked to original sources

arXiv2001

The Detection of Circumnuclear X-ray Emission from the Seyfert Galaxy NGC 3516

We present the first high-resolution, X-ray image of the circumnuclear regions of the Seyfert 1 galaxy NGC 3516, using the Chandra X-ray Observatory (CXO). All three of the CXO observations reported were performed with one of the two grating assemblies in place, and here we restrict our analysis to undispersed photons (i.e. those detected in the zeroth-order). A previously-unknown X-ray source is detected approx 6 arcsec (1.1 h_{75}^{-1} kpc) NNE of the nucleus (position angle approx 29 degrees) which we designate CXOU 110648.1+723412. Its spectrum can be characterized as a power law with a photon index Gamma ~ 1.8--2.6, or as thermal emission with a temperature kT~0.7--3 keV. Assuming a location within NGC 3516, isotropic emission implies a luminosity L ~ 2--8e39 h_{75}^{-2} erg s^{-1} in the 0.4--2 keV band. If due to a single point source, the object is super-Eddington for a 1.4 M_sun neutron star. However, multiple sources or a small, extended source cannot be excluded using the current data. Large-scale extended X-ray emission is also detected out to ~10 arcsec (~1.7 h_{75}^{-1} kpc) from the nucleus to the NE and SW, and is approximately aligned with the morphologies of the radio emission and extended narrow emission line region (ENLR). The mean luminosity of this emission is 1--5e37 h_{75}^{-2} erg s^{-1} arcsec^{-2}, in the 0.4--2 keV band. Unfortunately the current data cannot usefully constrain its spectrum. These results are consistent with earlier suggestions of circumnuclear X-ray emission in NGC 3516. If the extended emission is due to scattering of the nuclear X-ray continuum, then the pressure in the X-ray emitting gas is at least two orders of magnitude too small to provide the confining medium for the ENLR clouds.

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arXiv2001

The Spectral Energy Distribution of the Seyfert galaxy Ton S180

We present spectral results from a multi-satellite, broad-band campaign on the Narrow-line Seyfert 1 galaxy Ton S180 performed at the end of 1999. We discuss the spectral-energy distribution of the source, combining simultaneous Chandra, ASCA and EUVE data with contemporaneous FUSE, HST, and ground-based optical and infra-red data. The resulting SED shows that most of the energy is emitted in the 10 -- 100 eV regime, which must be dominated by the primary energy source. No spectral turnover is evident in the UV regime. This, the strong soft X-ray emission, and the overall shape of the SED indicate that emission from the accretion disk peaks between 15 and 100 eV. High resolution FUSE spectra showing UV absorption due to O VI and the lack of detectable X-ray absorption in the Chandra spectrum demonstrate the presence of a low column density of highly ionized gas along our line-of-sight. The highly-ionized state of the circumnuclear gas is most likely linked to the high luminosity and steep spectrum of the active nucleus. Given the strong ionizing flux in Ton S180, it is possible that the clouds within a few tens of light days of the central source are too highly ionized to produce much line emission. Thus the narrow width of the emission lines in Ton S180 is due to the emission arising from large radii.

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arXiv2001

Subtleties in Measuring Iron K Lines in AGN

Chandra and XMM observations are showing that the Fe-K emission lines in type 1 AGN are composite, in general consisting of a narrow and a broad component. We review the latest Chandra HETG results and compare the line profiles with those measured by ASCA. The narrow Fe-K line components necessitate re-modeling of the line profiles measured previously and revision of the parameters of the relativistic broad components, as well re-interpretation of variability studies. We also explicitly demonstrate the effect of changes in the ASCA calibration on the measurements of the Fe-K lines in AGN. We find that the differences in measured parameters are insignificant. Both the intrinsic width and EW change by less than 8%. Fe-K line studies which claim much larger differences do not adequately model the line emission. We point out that it is incorrect to compare different calibrations, and indeed make astrophysical inferences, using models which do not describe the data. We then present some new results for the highest S/N broad Fe-K line profile in the entire ASCA archive, from NGC 4151.

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arXiv2001

Chandra Grating Observations of Seyfert 1 Galaxies

We present new results from a Chandra HETG observation of NGC 5548 and give a comparison of absorption and emission features found in other Seyfert 1 galaxies using Chandra grating observations. Deep soft X-ray edges are seen in Mkn 509 and NGC 3783, consistent with ASCA data. In NGC 5548 and NGC 4501 the edges are weak but consistent with the low column densities. We show that the detection of a narrow, probably non-disk, component of the iron line is very common. However, just as importantly, it is not detected in some cases. We show the effect of removing this narrow component from the ASCA Fe-K line profile in NGC 4151, revealing the underlying true shape of the relativistic Fe-K line component.

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arXiv2001

Reddening, Emission-Line, and Intrinsic Absorption Properties in the Narrow-Line Seyfert 1 Galaxy Akn 564

We use Hubble Space Telescope UV and optical spectra of the narrow-line Seyfert 1 (NLS1) galaxy Akn 564 to investigate its internal reddening and properties of its emission-line and intrinsic UV absorption gas. We find that the extinction curve of Akn 564, derived from a comparison of its UV/optical continuum to that of an unreddened NLS1, lacks a 2200 A bump and turns up towards the UV at a longer wavelength (4000 A) than the standard Galactic, LMC, and SMC curves. However, it does not show the extremely steep rise to 1200 A that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The emission-lines and continuum experience the same amount of reddening, indicating the presence of a dust screen that is external to the narrow-line region (NLR). Echelle spectra from the Space Telescope Imaging Spectrograph show intrinsic UV absorption lines due to Ly-alpha, N V, C IV, Si IV, and Si III, centered at a radial velocity of -190 km/s (relative to the host galaxy). Photoionization models of the UV absorber indicate that it has a sufficient columnand is at a sufficient distance from the nucleus (D > 95 pc) to be the source of the dust screen. Thus, Akn 564 contains a dusty ``lukewarm absorber'' similar to that seen in NGC 3227.

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arXiv2001

X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180

The bright, soft X-ray spectrum Seyfert 1 galaxies Ark 564 and Ton S180 were monitored for 35 days and 12 days with ASCA and RXTE (and EUVE for Ton S180). The short time scale (hours-days) variability patterns were very similar across energy bands, with no evidence of lags between any of the energy bands studied. The fractional variability amplitude was almost independent of energy band. It is difficult to simultaneously explain soft Seyferts stronger variability, softer spectra, and weaker energy-dependence of the variability relative to hard Seyferts. The soft and hard band light curves diverged on the longest time scales probed, consistent with the fluctuation power density spectra that showed relatively greater power on long time scales in the softest bands. The simplest explanation is that a relatively hard, rapidly-variable component dominates the total X-ray spectrum and a slowly-variable soft excess is present in the lowest energy channels of ASCA. Although it would be natural to identify the latter with an accretion disk and the former with a corona surrounding it, a standard thin disk could not get hot enough to radiate significantly in the ASCA band, and the observed variability time scales are much too short. The hard component may have a more complex shape than a pure power-law. The most rapid factor of 2 flares and dips occurred within ~1000 sec in Ark 564 and a bit more slowly in Ton S180. The speed of the luminosity changes rules out viscous or thermal processes and limits the size of the individual emission regions to <~15 Schwarzschild radii (and probably much less), that is, to either the inner disk or small regions in a corona.

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arXiv2001

A 12-day ASCA Observation of the Narrow-Line Seyfert 1 Galaxy Ton S180: Time-Selected Spectroscopy

We present an analysis of the X-ray variability properties of the NLS1 galaxy Ton S180, based upon a 12-day continuous ASCA observation. We observe flux variations of a factor of 3.5 in the 0.7-1.3 keV band and 3.9 in the 2-10 keV band.Time-resolved spectroscopy (~1d sampling) reveals that the broad `soft hump' component at energies <2 keV shows flux variations on timescales down to 1 day that are well correlated with the photon index and the 2-10 keV band flux. A broad Fe K$α$ emission is detected. There is a evidence for a narrow Fe K$α$ line at ~6.8 keV, indicating an origin in ionized material. We do not detect significant variations of the line flux or EW on ~1 day-1 week timescales. The softness ratio reveals spectral variability on timescales down to ~1000 s, indicating that the power-law continuum and soft hump fluxes are not well correlated on this timescale. It also shows a slow decline across the observation, due to a combination of the different time-variability of the power-law continuum and soft hump flux on timescales of ~1 week. The X-ray emission originates within 12 Schwarzschild radii, and the amplitudes and timescales of the rapid variations we observed are consistent with those expected within disk-corona models. The soft hump variability timescale rules out an origin in large scale components (circumnuclear starburst). The $Γ$-soft hump correlation is consistent with the soft hump being produced by up-scattering of the accretion disk radiation within a flaring disk corona.

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arXiv2001

Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. III. Optical Observations and the Optical--UV--X-ray Connection

We present the results of a two-year long optical monitoring program of the narrow-line Seyfert 1 galaxy Akn 564. The majority of this monitoring project was also covered by X-ray observations (RXTE) and for a period of ~50 days, we observed the galaxy in UV (HST) and X-rays (RXTE & ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly time-scale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (one day) or long (several months) time-scales. The only significant optical variations can be described as two 2--4 day events with ~10% flux variations. We detect no significant optical line variations and thus cannot infer a reverberation size for the broad-line region. Similarly, the large X-ray variations seem to vanish when the light curve is smoothed over a period of 30 days. The UV continuum follows the X-rays with a lag of ~0.4 days, and the optical band lags the UV band by ~2 days. No significant correlation was found between the entire X-ray dataset and the optical band. Focusing on a 20-day interval around the strongest optical event we detect a significant X-ray--optical correlation with similar events seen in the UV and X-rays. Our data are consistent with reprocessing models on the grounds of the energy emitted in this single event. However, several large X-ray flares produced no corresponding optical emission.

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arXiv2001

Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. II. Ultraviolet Continuum and Emission-line Variability

We present results of an intensive two-month campaign of approximately daily spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with HST. The fractional variability amplitude of the continuum variations between 1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1 galaxies over a similar period of time. However, large amplitude, short time-scale flaring behavior is evident, with trough-to-peak flux changes of about 18% in approximately 3 days. We present evidence for wavelength-dependent continuum time delays, with the variations at 3000 A lagging behind those at 1365 A by about 1 day. These delays may be interpreted as evidence for a stratified continuum reprocessing region, possibly an accretion-disk structure. The Lyman-alpha 1216 emission-line exhibits flux variations of about 1% amplitude.

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arXiv2001

Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. I. ASCA Observations and the Variability of the X-ray Spectral Components

We present a 35 day ASCA observation of the NLS1 Akn 564, which was part of a multiwavelength AGN Watch monitoring campaign. Akn 564 shows a photon index varying across the range 2.45--2.72. The presence of the soft hump component below 1 keV, previously detected in ASCA data, is confirmed. Time-resolved spectroscopy with ~daily sampling reveals a distinction in the variability of the soft hump and power-law components over a timescale of weeks, with the hump varying by a factor of 6 across the 35-day observation compared to a factor 4 in the power-law. Flux variations in the power-law component are measured down to a timescale of ~1000s and accompanying spectral variability suggests the soft hump is not well-correlated with the power-law on such short timescales. We detect Fe Ka and a blend of Fe Kb plus Ni Ka, indicating an origin in highly ionized gas. Variability measurements constrain the bulk of the Fe Ka to originate within a light week of the nucleus. The large EW of the emission lines may be due to high metallicity in NLS1s, supporting some evolutionary models for AGN.

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arXiv2001

The X-ray Emission from the Nucleus of the Dwarf Elliptical Galaxy NGC 3226

We present the first high resolution X-ray image of the dwarf elliptical galaxy NGC 3226. The data were obtained during an observation of the nearby Seyfert Galaxy NGC 3227 using the Chandra X-ray Observatory. We detect a point X-ray source spatially consistent with the optical nucleus of NGC 3226 and a recently-detected, compact, flat-spectrum, radio source. The X-ray spectrum can be measured up to ~10 keV and is consistent with a power law with a photon index 1.7 <~ Gamma <~ 2.2, or thermal bremmstrahlung emission with 4 <~ kT <~ 10 keV. In both cases the luminosity in the 2--10 keV band ~10^{40} h_{75}^{-1} erg/s. We find marginal evidence that the nucleus varies within the observation. These characteristics support evidence from other wavebands that NGC 3226 harbors a low-luminosity, active nucleus. We also comment on two previously-unknown, fainter X-ray sources <~ 15 arcsec from the nucleus of NGC 3226. Their proximity to the nucleus (with projected distances <~ 1.3/h_{75} kpc) suggests both are within NGC 3226, and thus have luminosities (~few x 10^{38} -- few x 10^{39} erg/s) consistent with black-hole binary systems.

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arXiv2000

Chandra Grating Spectroscopy of the Seyfert Galaxy Ton S180

This paper presents results from spectral observations of Ton S180 using Chandra and ASCA. The data confirm the presence of the soft excess but the Chandra LETG spectrum reveals it to be broad and smooth, rather than resolved into individual emission lines. This excess may represent either a primary or reprocessed continuum component or a blend of broad lines from an ionized accretion disk. The occurrence of a similar feature in five other NLSy1s leads us to conclude that this soft X-ray component may be a characteristic of sources accreting at a very high rate. The X-ray spectrum shows no evidence for absorption lines, indicating that if gas exists in the line-of-sight then it is in a very high ionization-state or has an extremely broad velocity distribution. The new ASCA data confirm that the narrow component of the Fe Kalpha line peaks close to a rest-energy of 7 keV, indicating the presence of a significant amount of highly-ionized material in the nuclear environs.

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arXiv2000

Physical Diagnostics from a Narrow Fe-Kalpha Emission Line Detected by Chandra in the Seyfert 1 Galaxy NGC 5548

We report the detection of a narrow Fe-K emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Gratings. In the galaxy frame we measure a center energy of 6.40 \pm 0.03 keV, a FWHM of 4515 (+3525,-2645) km/s, and an equivalent width of 133 (+62,-54) eV. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ~Fe XVIII are not ruled out. Assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light-month from the central X-ray source. We cannot rule out some contribution to the Fe-K line from a putative, parsec-scale obscuring torus. The continuum intensity during the Chandra observation was a factor ~2 less than typical historical levels. If the X-ray continuum was > than a factor 2 higher in the recent past before the Chandra observation and the narrow Fe-K line intensity had not yet responded, then the predicted line equivalent width and intensity for an origin in the BLR is within the 90% measurement errors. Retroactive spectral fitting of archival ASCA data shows evidence that the narrow Fe-K has a variable intensity. It will be important to measure the non-disk component of the Fe-K line in other AGN to obtain a more complete understanding of the total Fe-K line emission. Measurement of the narrow-line parameters provides powerful diagnostics of conditions beyond the accretion disk and is necessary to properly deconvolve the broad, relativistic Fe-K line.

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arXiv2000

The Origin of the X-ray and Ultraviolet Emission in NGC 7469

We present a spectral analysis of a 30d, near-continuous observation of the Seyfert 1 galaxy NGC 7469 with RXTE. Daily integrations show strong spectral changes during the observation. Ourmain result is that we find the X-ray spectral index to be correlated with the UV flux. Furthermore, the broad-band X-ray photon flux is also correlated with the UV continuum. These correlations point towards a model in which the X-rays originate via thermal Comptonization of UV seed photons. Furthermore, the UV is also correlated with the extrapolation of the X-ray power law into the soft X-ray/EUV region. Our data analysis therefore re-opens the possibility the the UV photons and their variability arise from reprocessing, as long as the primary source of heating is photo-electric absorption in the reprocessor, rather than Compton downscattering, A coherent picture of the X-ray/UV variability can therefore be constructed whereby absorption and reprocessing of EUV/soft X-rays in a standard accretion disk produce a variable seed photon distribution which are in turn up-scattered into the X-ray band. We also find a significant correlation between the 2-10 keV flux and the 6.4 keV iron K-line suggesting that at least some portion of the line originates within ~1 lt day of the X-ray continuum source. Neither the power law photon index nor the Compton reflection component are correlated with the 2-10 keV flux. The latter is not correlated with the iron K line flux either. We do find an apparent correlation between the X-ray spectral index and the strength of the Compton reflection component. In an Appendix we show, however, that this can be produced by a combination of statistical and systematic errors. We conclude the apparent variations in the Compton reflection component may be an artifact of these effects.

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arXiv2000

On The Reddening in X-ray Absorbed Seyfert 1 Galaxies

There are several Seyfert galaxies for which there is a discrepancy between the small column of neutral hydrogen deduced from X-ray observations and the much greater column derived from the reddening of the optical/UV emission lines and continuum. The standard paradigm has the dust within the highly ionized gas which produces O~VII and O~VIII absorption edges (i.e., a ``dusty warm absorber''). We present an alternative model in which the dust exists in a component of gas in which hydrogen has been stripped, but which is at too low an ionization state to possess significant columns of O~VII and O~VIII (i.e, a ``lukewarm absorber''). The lukewarm absorber is at sufficient radial distance to encompass much of the narrow emission-line region, and thus accounts for the narrow-line reddening, unlike the dusty warm absorber. We test the model by using a combination of photoionization models and absorption edge fits to analyze the combined ROSAT/ASCA dataset for the Seyfert 1.5 galaxy, NGC 3227. We show that the data are well fit by a combination of the lukewarm absorber and a more highly ionized component similar to that suggested in earlier studies. We predict that the lukewarm absorber will produce strong UV absorption lines of N V, C IV, Si IV and Mg II. Finally, these results illustrate that singly ionized helium is an important, and often overlooked, source of opacity in the soft X-ray band (100 - 500 eV).

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arXiv1999

BeppoSAX Observation of NGC 7582: Constraints on the X-ray absorber

This paper presents a BeppoSAX observation of NGC 7582 made during 1998 November and an optical spectrum taken in 1998 October. The new X-ray data reveal a previously unknown hard X-ray component in NGC 7582, peaking close to 20 keV. Rapid variability is observed with correlated changes in the 5-10 and 13-60 keV bands indicating that a single continuum component, produced by the active nucleus, provides the dominant flux across both bands. Changes in the nuclear X-ray flux appear unrelated to the gradual decline in optical flux noted since the high-state in 1998 July. The X-ray continuum is attenuated by a thick absorber of N_H ~ 1.6 x 10E24 cm^-2 covering ~60% of the nucleus, plus a screen with N_H ~ 1.4 x 10E23 cm^-2 covering the entire nucleus. Comparison of the BeppoSAX and ASCA spectra shows an increase in the full screen by N_H ~7 x 10E22 cm^-2 since 1994, confirming the absorption variability found by Xue et al. The increase in soft X-ray flux between 1994 and 1998 is consistent with the appearance of holes in the full screen producing some clear lines-of-sight to the broad-line-region.

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arXiv1999

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 PG survey. All the RQQs were detected except 2 BALQSOs. We find the variability characteristics of the sources to be consistent with Seyf-1s. A powerlaw offers an acceptable description of the time-averaged spectra in the 2-10 keV (RQQ-frame) band for all but 1 dataset. The photon indices vary from object to object over the range 1.5~<Gamma(2-10 keV)~<3 with a mean Gamma(2-10keV)~2 and dispersion ~=0.25. The distribution of Gamma(2-10 keV is therefore similar to that observed in other RQ AGN, and seems unrelated to X-ray luminosity. No single model adequately describes the full 0.6-10 keV (obs-frame) continuum of all the RQQs. ~50% of the sources can be described by a single powerlaw (or with only very subtle deviations). All but one of the remaining datasets have convex spectra (flattening to higher energies). The exception is PG 1411+442, in which a substantial column density obscures ~98% of the continuum. We find only 5 (or 6) of 14 objects with z<~0.25 to have a 'soft excess' at energies <~1 keV, but find no universal shape for these spectral components. The spectrum of PG 1244+026 contains an emission feature at ~1 keV. The detection rate of absorption due to ionized material in these RQQs is lower than in Seyf-1s. 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 Seyf-1s. We find evidence of Fe K-shell emission in at least 8 RQQs. These are all low-luminosity objects, and the line parameters are consistent with other low-luminosity RQ AGN. However we find a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.

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arXiv1999

A Highly Doppler Blueshifted Fe-K Emission Line in the High-Redshift QSO PKS 2149-306

We report the results from an \asca observation of the QSO PKS 2149-306 (z=2.345). We detect an emission line centered at $\sim 17$ keV in the quasar frame. Line emission at this energy has not been observed in any other active galaxy or quasar to date. We present evidence rejecting the possibility that this line is the result of instrumental artifacts, or a serendipitous source. The most likely explanation is blueshifted Fe-K emission (the EW is 300+/-200 eV, QSO frame). Bulk velocities of the order of 0.75c are implied by the data. We show that Fe-K line photons originating in an accretion disk and Compton-scattering off a leptonic can account for the emission line. Curiously, if the emission-line feature recently discovered in another quasar PKS 0637$-$752, $z=0.654$, is blueshifted Ovii, the Doppler factor is the same (~2.7) for both.

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