arXiv subjects

G. A. Kriss

Publications and source records attributed to G. A. Kriss.

At least 37 records · Page 2Linked to original sources

arXiv2019

Anomalous behaviour of the UV-optical continuum bands in NGC 5548

During the 2014 HST/Swift and ground-based multi-wavelength monitoring campaign of NGC 5548 (AGN STORM), the UV-optical broad emission lines exhibited anomalous, decorrelated behaviour relative to the far-UV continuum flux variability. Here, we use key diagnostic emission lines (Ly-alpha and He II) for this campaign to infer a proxy for the all important, variable driving EUV continuum incident upon BLR clouds. The inferred driving continuum provides a crucial step towards the recovery of the broad emission line response functions in this AGN. In particular, the ionising continuum seen by the BLR was weaker and softer during the anomalous period than during the first third of the campaign, and apparently less variable than exhibited by the far-UV continuum. We also report the first evidence for anomalous behaviour in the longer wavelength (relative to 1157A) continuum bands. This is corroborative evidence that a significant contribution to the variable UV-optical continuum emission arises from a diffuse continuum emanating from the same gas that emits the broad emission lines.

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arXiv2019

Space Telescope and Optical Reverberation Mapping Project. X. Understanding the Absorption-Line Holiday in NGC 5548

The flux variations in the emission lines in active galactic nuclei (AGNs) are driven by variations in the ionizing continuum flux --which are usually reflected in the observable UV-optical continuum. The "Reverberation mapping" technique measures the delay between line and continuum variations to determine the size of the line emitting region, this is the basis for measurements of the central black hole mass in AGNs. The Space Telescope and Optical Reverberation Mapping Project (AGN STORM) on NGC 5548 in 2014 is the most intensive multi-wavelength AGN monitoring campaign ever. For most of the campaign, the emission-line variations followed changes in the continuum with a time lag, as expected. However, the lines varied independently of the observed UV-optical continuum during a 60 -- 70 day "holiday." To understand this remarkable phenomenon, we study the intrinsic absorption lines present in NGC 5548. We identify a novel cycle that reproduces the absorption line variability and thus identify the physics that allows the holiday to occur. In our model, variations in this obscurer's line-of-sight covering factor modify the soft X-ray continuum. This leads to changes in the ionization of helium gas in the broad-line region. Ionizing radiation produced by recombining helium then affects the ionization of other species as observed during the AGN STORM holiday. It is likely that any other model which selectively changes the soft X-ray part of the continuum during the holiday can also explain the anomalous emission line behavior observed.

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arXiv2019

Evidence for an emerging disc wind and collimated outflow during an X-ray flare in the narrow-line Seyfert 1 galaxy Mrk 335

A triggered 140 ks XMM-Newton observation of the narrow-line Seyfert 1 (NLS1) Mrk 335 in December 2015 caught the active galaxy at its lowest X-ray flux since 2007. The NLS1 is relatively quiescent for the first ~120 ks of the observation before it flares in brightness by a factor of about five in the last 20 ks. Although only part of the flare is captured before the observation is terminated, the data reveal significant differences between the flare and quiescent phases. During the low-flux state, Mrk 335 demonstrates a reflection-dominated spectrum that results from a compact corona around a Kerr black hole. In addition to the rapid brightening, the flare is further described by spectral softening and a falling reflection fraction that are consistent with previous observations advocating at least part of the corona in Mrk 335 could be the base of an aborted jet. The spectrum during the flaring interval reveals several residuals between the 2-3 sigma level that could be attributed to absorption lines from a highly ionised plasma that is moving outward at v~0.12c. It could be that the increased luminosity during the flare enhances the radiation pressure sufficiently to launch a possible wind. If the wind is indeed responding to the change in corona luminosity then it must be located within ~80 Rg. The escape velocity at this distance is comparable to the estimated wind velocity. If confirmed, this is the first example of a radio-quiet AGN exhibiting behaviour consistent with both diffuse and collimated outflow.

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arXiv2019

Multiwavelength campaign on Mrk 509. XVI. Continued HST/COS monitoring of the far-ultraviolet spectrum

To elucidate the location, physical conditions, mass outflow rate, and kinetic luminosity of the outflow from the active nucleus of the Seyfert 1 galaxy Mrk 509 we used coordinated ultraviolet and X-ray spectral observations in 2012 to follow up our lengthier campaign conducted in 2009. We observed Mrk 509 with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) on 2012-09-03 and 2012-10-11 coordinated with X-ray observations using the High Energy Transmission Grating on the Chandra X-ray Observatory. Our far-ultraviolet spectra used grating G140L on COS to cover wavelengths from 920--2000 Å at a resolving power of $\sim2000$, and gratings G130M and G160M to cover 1160--1750 Å at a resolving power of $\sim15,000$. We detect variability in the blue-shifted UV absorption lines on timescales spanning 3--12 years. The inferred densities in the absorbing gas are greater than log $n \rm~cm^{-3} \sim 3$. For ionization parameters ranging over log $U = -1.5 \rm~to~-0.2$, we constrain the distances of the absorbers to be closer than 220 pc to the active nucleus. The impact on the host galaxy appears to be confined to the nuclear region.

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arXiv2018

Photoionized emission and absorption features in the high-resolution X-ray spectra of NGC 3783

Our Swift monitoring program triggered two joint XMM-Newton, NuSTAR and HST observations on 11 and 21 December 2016 targeting NGC 3783, as its soft X-ray continuum was heavily obscured. Consequently, emission features, including the O VII radiative recombination continuum, stand out above the diminished continuum. We focus on the photoionized emission features in the December 2016 RGS spectra and compare them to the time-averaged RGS spectrum obtained in 2000--2001 when the continuum was unobscured. A two-phase photoionized plasma is required to account for the narrow emission features. These narrow emission features are weakly varying between 2000--2001 and December 2016. We also find a statistically significant broad emission component in the time-averaged RGS spectrum in 2000--2001. This broad emission component is significantly weaker in December 2016, suggesting that the obscurer is farther away than the X-ray broad-line region. In addition, by analyzing the archival high-resolution X-ray spectra, we find that nine photoionized absorption components with different ionization parameters and kinematics are required for the warm absorber in X-rays.

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arXiv2018

HST/COS observations of the newly discovered obscuring outflow in NGC 3783

To understand the nature of transient obscuring outflows in active galactic nuclei, we observed the Seyfert 1 galaxy NGC 3783 on two occasions in December 2016 triggered by Swift monitoring indicating strong soft X-ray absorption in November. We obtained ultraviolet spectra using COS on HST and optical spectra using FEROS on the MPG/ESO 2.2-m telescope that were simultaneous with X-ray spectra from XMM-Newton and NuSTAR. We find new components of broad, blue-shifted absorption associated with Ly$α$, \ion{N}{v}, \ion{Si}{iv}, and \ion{C}{iv} in our COS spectra. The absorption extends from velocities near zero in the rest-frame of the host galaxy to $-6200$ $\rm km~s^{-1}$. These features appear for the first time in NGC 3783 at the same time as the heavy soft X-ray absorption seen in the XMM-Newton X-ray spectra. The X-ray absorption has a column density of $\sim 10^{23}~\rm cm^{-2}$, and it partially covers the X-ray continuum source. The X-ray absorption becomes more transparent in the second observation, as does the UV absorption. Combining the X-ray column densities with the UV spectral observations yields an ionization parameter for the obscuring gas of log $ξ=1.84^{+0.4}_{-0.2}$ $\rm erg~cm~s^{-1}$. Despite the high intensity of the UV continuum in NGC 3783, F(1470 Å)=$8 \times 10^{-14}~\rm erg~cm^{-2}~s^{-1}~Å^{-1}$, the well known narrow UV absorption lines are deeper than in earlier observations in unobscured states, and low ionization species such as \ion{C}{iii} appear, indicating that the narrow-line gas is more distant from the nucleus and is being shadowed by the gas producing the obscuration. Despite the high continuum flux levels in our observations of NGC 3783, moderate velocities in the UV broad line profiles have substantially diminished. We suggest that a collapse of the broad line region has led to the outburst and triggered the obscuring event.

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arXiv2018

The variability of the warm absorber in I Zwicky 1 as seen by XMM-Newton

We present new XMM-Newton observations of the intriguing warm absorber in I Zwicky 1. This luminous and nearby narrow-line Seyfert 1 galaxy shows ionized absorption by two components of outflowing gas; a low and a high-ionization phase with log $ξ$~0 and log $ξ$~2 respectively. Detailed modelling of these data reveal a complex and variable multi-phase warm absorber. However, we find the changes in the ionization state of the gas not to be straightforwardly correlated with the variability of the intrinsic continuum source, in apparent contrast with photoionization equilibrium. The observed variability hints instead at a close connection between the two gas components, possibly both directly connected to the accretion disc activity. We thus suggest a phenomenological model capable of explaining these observations, consisting of a clumpy outflow where the high and the low-ionization components are closely linked. Changes in ionization over the years are mainly driven by the different densities of the clumps crossing the observer's line-of-sight, in which the `skin' layer facing the source accounts for the more ionized component.

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arXiv2018

Recurring obscuration in NGC 3783

Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (> 1000 km/s) and column density (>3E25 per m2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we have witnessed such an event in NGC 3783. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008-2017. In August 2016, NGC 3783 also showed evidence for obscuration. While the column density of the obscuring material is ten times lower than in December 2016, the opacity is still sufficient to block a significant fraction of the ionising X-ray and EUV photons. From the Swift hardness ratio behaviour we find several other epochs with obscuration. Obscuration with columns >1E26 per m2 may take place in about half of the time. Also in archival X-ray data taken by ASCA in 1993 and 1996 we find evidence for obscuration. Obscuration of the ionising photons in NGC 3783 occurs more frequently than previously thought. This may not always have been recognised due to low spectral resolution observations, too limited spectral bandwidth or confusion with underlying continuum variations.

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arXiv2018

A Search for H I Lyman $α$ Counterparts to Ultra-Fast X-ray Outflows

Prompted by the H I Ly$α$ absorption associated with the X-ray ultra-fast outflow at -17,300 $\rm km~s^{-1}$ in the quasar PG~1211+143, we have searched archival UV spectra at the expected locations of H I Ly$α$ absorption for a large sample of ultra-fast outflows identified in XMM-Newton and Suzaku observations. Sixteen of the X-ray outflows have predicted H I Ly$α$ wavelengths falling within the bandpass of spectra from either the Far Ultraviolet Spectroscopic Explorer or the Hubble Space Telescope, although none of the archival observations were simultaneous with the X-ray observations in which UFOs were detected. In our spectra broad features with full-width at half-maximum of 1000 $\rm km~s^{-1}$ have 2-$σ$ upper limits on the H I column density of generally <$2\times10^{13}~\rm cm^{-2}$. Using grids of photoionization models covering a broad range of spectral energy distributions, we find that producing Fe XXVI Ly$α$ X-ray absorption with equivalent widths $>30$ eV and associated H I Ly$α$ absorption with $\rm N_{HI}<2\times10^{13}~cm^{-2}$ requires total absorbing column densities $\rm N_{H}>5\times10^{22}~cm^{-2}$ and ionization parameters log $ξ$ > 3.7. Nevertheless, a wide range of SEDs would predict observable H I Ly$α$ absorption if ionization parameters are only slightly below peak ionization fractions for Fe XXV and Fe XXVI. The lack of Ly$α$ features in the archival UV spectra indicates that either the UFOs have very high ionization parameters, very hard UV-ionizing spectra, or that they were not present at the time of the UV spectral observations due to variability.

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arXiv2018

Multi-wavelength campaign on NGC 7469: III. Spectral energy distribution and the AGN wind photoionisation modelling, plus detection of diffuse X-rays from the starburst with Chandra HETGS

We investigate the physical structure of the AGN wind in the Seyfert-1 galaxy NGC 7469 through high-resolution X-ray spectroscopy with Chandra HETGS and photoionisation modelling. Contemporaneous data from Chandra, HST, and Swift are used to model the optical-UV-X-ray continuum and determine the spectral energy distribution (SED) at two epochs, 13 years apart. For our investigation we use new observations taken in December 2015-January 2016, and historical ones taken in December 2002. We study the impact of a change in the SED shape, seen between the two epochs, on the photoionisation of the wind. The HETGS spectroscopy shows that the AGN wind in NGC 7469 consists of four ionisation components, with their outflow velocities ranging from -400 to -1800 km/s. From our modelling we find that the change in the ionising continuum shape between the two epochs results in some variation in the ionisation state of the wind components. However, for the main ions detected in X-rays, the sum of their column densities over all four components, remains in practice unchanged. For two of the four components, which are found to be thermally unstable in both epochs, we obtain 2 < r < 31 pc and 12 < r < 29 pc using the cooling and recombination timescales. For the other two thermally stable components, we obtain r < 31 pc and r < 80 pc from the recombination timescale. The results of our photoionisation modelling and thermal stability analysis suggest that the absorber components in NGC 7469 are consistent with being a thermally-driven wind from the AGN torus. Finally, from analysis of the zeroth-order ACIS/HETG data, we discover that the X-ray emission between 0.2-1 keV is spatially extended over 1.5-12". This diffuse soft X-ray emission is explained by coronal emission from the nuclear starburst ring in NGC 7469.

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arXiv2018

Multi-wavelength campaign on NGC 7469 IV. The broad-band X-ray spectrum

We conducted a multi-wavelength six-month campaign to observe the Seyfert galaxy NGC~7469, using the space-based observatories \textit{HST}, \textit{Swift}, \textit{XMM-Newton} and \textit{NuSTAR}. Here we report the results of the spectral analysis of the 7 simultaneous \textit{XMM-Newton} and \textit{NuSTAR} observations. The sources shows significant flux variability within each observation, but the average flux is less variable among the different pointings of our campaign. Our spectral analysis reveals a prominent narrow neutral \ion{Fe} K$α$ emission line in all the spectra, with weaker contributions from Fe K$β$, neutral Ni K$α$ and ionised iron. We find no evidence for variability or relativistic effects acting on the emission lines, which indicates that they originate from distant material. Analysing jointly \textit{XMM-Newton} and \textit{NuSTAR} data a constant photon index is found ($Γ$=$1.78\pm0.02$), together with a high energy cut-off $E_{\rm{cut}}=170^{+60}_{-40}$ keV. Adopting a self-consistent Comptonization model, these values correspond to an average coronal electron temperature of kT=$45^{+15}_{-12}$ keV and, assuming a spherical geometry, an optical depth $τ=2.6\pm0.9$. The reflection component is consistent with being constant, with a reflection fraction in the range $R=0.3-0.6$. A prominent soft excess dominates the spectra below 4 keV. This is best fit with a second Comptonization component, arising from a \virg{warm corona} with an average $kT=0.67\pm0.03$ keV and a corresponding optical depth $τ=9.2\pm0.2$.

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arXiv2017

Anatomy of the AGN in NGC 5548 IX. Photoionized emission features in the soft X-ray spectra

The X-ray narrow emission line region (NELR) of the archetypal Seyfert 1 galaxy NGC\,5548 has been interpreted as a single-phase photoionized plasma that is absorbed by some of the warm absorber components. This scenario requires those overlaying warm absorber components to have larger distance (to the central engine) than the X-ray NELR, which is not fully consistent with the distance estimates found in the literature. Therefore, we reanalyze the high-resolution spectra obtained in 2013--2014 with the Reflection Grating Spectrometer (RGS) aboard \textit{XMM}-Newton to provide an alternative interpretation of the X-ray narrow emission features. We find that the X-ray narrow emission features in NGC\,5548 can be described by a two-phase photoionized plasma with different ionization parameters ($\log ξ=1.3$ and $0.1$) and kinematics ($v_{\rm out}=-50$ and $-400~{\rm km~s^{-1}}$), and no further absorption by the warm absorber components. The X-ray and optical NELR might be the same multi-phase photoionized plasma. Both X-ray and optical NELR have comparable distances, asymmetric line profiles, and the underlying photoionized plasma is turbulent and compact in size. The X-ray NELR is not the counterpart of the UV/X-ray absorber outside the line of sight because their distances and kinematics are not consistent. In addition, X-ray broad emission features that we find in the spectrum can be accounted for by a third photoionized emission component. The RGS spectrum obtained in 2016 is analyzed as well, where the luminosity of most prominent emission lines (the \ion{O}{vii} forbidden line and \ion{O}{viii} Ly$α$ line) are the same (at a 1 $σ$ confidence level) as in 2013--2014.

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arXiv2017

Multi-wavelength campaign on NGC7469 II. Column densities and variability in the X-ray spectrum

We investigate the ionic column density variability of the ionized outflows associated with NGC$\sim$7469, to estimate their location and power. This could allow a better understanding of galactic feedback of AGNs to their host galaxies. Analysis of seven XMM-Newton grating observations from 2015 is reported. We use an individual-ion spectral fitting approach, and compare different epochs to accurately determine variability on time-scales of years, months, and days. We find no significant column density variability in a 10 year period implying that the outflow is far from the ionizing source. The implied lower bound on the ionization equilibrium time, 10 years, constrains the lower limit on the distance to be at least 12 pc, and up to 31 pc, much less but consistent with the 1 kpc wide starburst ring. The ionization distribution of column density is reconstructed from measured column densities, nicely matching results of two 2004 observations, with one large high ionization parameter ($ξ$) component at $2<\log ξ<3.5$, and one at $0.5<\log ξ<1$ in cgs units. The strong dependence of the expression for kinetic power, $\propto1/ξ$, hampers tight constraints on the feedback mechanism of outflows with a large range in ionization parameter, which is often observed and indicates a non-conical outflow. The kinetic power of the outflow is estimated here to be within 0.4 and 60 \% of the Eddington luminosity, depending on the ion used to estimate $ξ$.

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arXiv2017

Revealing structure and evolution within the corona of the Seyfert galaxy I Zw 1

X-ray spectral timing analysis is presented of XMM-Newton observations of the narrow line Seyfert 1 galaxy I Zwicky 1 (I Zw 1) taken in 2015 January. After exploring the effect of background flaring on timing analyses, X-ray time lags between the reflection-dominated 0.3-1.0keV energy and continuum-dominated 1.0-4.0keV band are measured, indicative of reverberation off the inner accretion disc. The reverberation lag time is seen to vary as a step function in frequency; across lower frequency components of the variability, 3e-4 to 1.2e-3Hz a lag of 160s is measured, but the lag shortens to (59 +/- 4)s above 1.2e-3Hz. The lag-energy spectrum reveals differing profiles between these ranges with a change in the dip showing the earliest arriving photons. The low frequency signal indicates reverberation of X-rays emitted from a corona extended at low height over the disc while at high frequencies, variability is generated in a collimated core of the corona through which luminosity fluctuations propagate upwards. Principal component analysis of the variability supports this interpretation, showing uncorrelated variation in the spectral slope of two power law continuum components. The distinct evolution of the two components of the corona is seen as a flare passes inwards from the extended to the collimated portion. An increase in variability in the extended corona was found preceding the initial increase in X-ray flux. Variability from the extended corona was seen to die away as the flare passed into the collimated core leading to a second sharper increase in the X-ray count rate.

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arXiv2017

Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the UV anomaly in NGC 5548 with X-Ray Spectroscopy

During the Space Telescope and Optical Reverberation Mapping Project (STORM) observations of NGC 5548, the continuum and emission-line variability became de-correlated during the second half of the 6-month long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as a part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than being due to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all the three observations: the UV emission line flux decrease, the soft-excess increase, and the emission line anomaly.

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arXiv2017

Chasing obscuration in type-I AGN: discovery of an eclipsing clumpy wind at the outer broad-line region of NGC 3783

In 2016 we carried out a Swift monitoring program to track the X-ray hardness variability of eight type-I AGN over a year. The purpose of this monitoring was to find intense obscuration events in AGN, and thereby study them by triggering joint XMM-Newton, NuSTAR, and HST observations. We successfully accomplished this for NGC 3783 in December 2016. We found heavy X-ray absorption produced by an obscuring outflow in this AGN. As a result of this obscuration, interesting absorption features appear in the UV and X-ray spectra, which are not present in the previous epochs. Namely, the obscuration produces broad and blue-shifted UV absorption lines of Ly$α$, C IV, and N V, together with a new high-ionisation component producing Fe XXV and Fe XXVI absorption lines. In soft X-rays, only narrow emission lines stand out above the diminished continuum as they are not absorbed by the obscurer. Our analysis shows that the obscurer partially covers the central source with a column density of few $10^{23}$ cm$^{-2}$, outflowing with a velocity of few thousand km s$^{-1}$. The obscuration in NGC 3783 is variable and lasts for about a month. Unlike the commonly-seen warm-absorber winds at pc-scale distances from the black hole, the eclipsing wind in NGC 3783 is located at about 10 light days. Our results suggest the obscuration is produced by an inhomogeneous and clumpy medium, consistent with clouds in the base of a radiatively-driven disk wind at the outer broad-line region of the AGN.

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arXiv2017

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-Line Analysis for NGC 5548

We present the results of an optical spectroscopic monitoring program targeting NGC 5548 as part of a larger multi-wavelength reverberation mapping campaign. The campaign spanned six months and achieved an almost daily cadence with observations from five ground-based telescopes. The H$β$ and He II $λ$4686 broad emission-line light curves lag that of the 5100 $Å$ optical continuum by $4.17^{+0.36}_{-0.36}$ days and $0.79^{+0.35}_{-0.34}$ days, respectively. The H$β$ lag relative to the 1158 $Å$ ultraviolet continuum light curve measured by the Hubble Space Telescope is roughly $\sim$50% longer than that measured against the optical continuum, and the lag difference is consistent with the observed lag between the optical and ultraviolet continua. This suggests that the characteristic radius of the broad-line region is $\sim$50% larger than the value inferred from optical data alone. We also measured velocity-resolved emission-line lags for H$β$ and found a complex velocity-lag structure with shorter lags in the line wings, indicative of a broad-line region dominated by Keplerian motion. The responses of both the H$β$ and He II $λ$4686 emission lines to the driving continuum changed significantly halfway through the campaign, a phenomenon also observed for C IV, Ly $α$, He II(+O III]), and Si IV(+O IV]) during the same monitoring period. Finally, given the optical luminosity of NGC 5548 during our campaign, the measured H$β$ lag is a factor of five shorter than the expected value implied by the $R_\mathrm{BLR} - L_\mathrm{AGN}$ relation based on the past behavior of NGC 5548.

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arXiv2016

Space Telescope and Optical Reverberation Mapping Project VI: reverberating Disk Models for NGC 5548

We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 to 9157 angstroms) combine simultaneous HST , Swift , and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination, i, temperature T1 at 1 light day from the black hole, and a temperature-radius slope, alpha. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/LEdd = 0.1

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