arXiv subjects

Andrew Lobban

Publications and source records attributed to Andrew Lobban.

At least 19 recordsLinked to original sources

arXiv2024

Revealing the burning and soft heart of the bright bare AGN ESO 141-G55: X-ray broadband and SED analysis

[Abridged] ESO 141-G55 is a nearby X-ray bright BLS1, which has been classified as a bare AGN due to the lack of warm absorption along its line-of-sight, providing an unhampered view into its disc-corona system. We aim to probe its disc-corona system thanks to the first simultaneous XMM-Newton and NuSTAR observation obtained on October 1-2, 2022. We carry out the X-ray broadband spectral analysis to determine the dominant process(es) at work, as well as the SED analysis to determine the disc-corona properties. The simultaneous broadband X-ray spectrum of ESO 141-G55 is characterised by the presence of a prominent smooth soft X-ray excess, a broad Fe K emission line and a significant Compton hump. The RGS spectra confirmed the lack of intrinsic warm-absorbing gas along our line of sight in the AGN rest frame, confirming that it is still in a bare state. However, soft X-ray emission lines are observed indicating substantial warm gas out of our line of sight. The intermediate inclination of the disc-corona system, ~43{\deg}, may offer us a favourable configuration to observe UFOs from the disc, but none is found in this 2022 observation, contrary to a previous 2007 XMM-Newton one. Relativistic reflection alone on a standard disc is ruled out from the X-ray broadband analysis, while a combination of soft and hard Comptonisation by a warm and hot corona (relagn), plus relativistic reflection (reflkerrd) reproduces its SED quite well. The hot corona temperature is very hot, ~140 keV, much higher than about 80% of the AGNs, whereas the warm corona temperature, ~0.3 keV, is similar to the values found in other sub-Eddington AGNs. ESO 141-G55 is accreting at a moderate Eddington accretion rate (~10--20%). Our analysis points to a significant contribution of an optically-thick warm corona to both the soft X-ray and UV emission in ESO 141-G55.

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arXiv2023

Probing the face-on disc-corona system of the bare AGN Mrk 110 from UV to hard X-rays: a moderate changing-state AGN?

[Abridged] The X-ray broadband spectra of the bare AGN Mrk 110, obtained by simultaneous XMM-Newton and NuSTAR observations (Nov 2019 and April 2020), are characterised by the presence of a prominent and absorption-free smooth soft X-ray excess, moderately broad OVII and Fe Kalpha emission lines, and a lack of a strong Compton hump. While relativistic reflection as the sole emission is ruled out, a simplified combination of soft and hard Comptonisation from a warm and a hot coronae, plus mild relativistic disc reflection reproduces the data very well. We aim to confirm the physical origin of the soft X-ray excess of Mrk 110 and to determine its disc-corona system properties from its energetics using two new sophisticated models: reXcor and relagn, respectively. At both epochs, the inferred high-values of the warm-corona heating from the X-ray broadband spectral analysis using reXcor confirm that the soft X-ray excess originates mainly from a warm corona rather than relativistic reflection. The intrinsic best-fit SED determined at both epochs using relagn show a high X-ray contribution relative to the UV and are very well reproduced by a warm and hot coronae plus mild relativistic reflection. The outer radii of the hot and warm coronae are located at a few 10s and ~100 Rg, respectively. Moreover, combining the inferred low Eddington ratio (~ a few %) from this work, and previous multi-wavelength spectral and timing studies suggests that Mrk 110 could be classified as a moderate changing-state AGN. Our analysis confirms the existence of a warm corona as a significant contribution to the soft X-ray excess and UV emission in Mrk 110, adding to growing evidence that AGN accretion deviates from standard disc theory. This strengthens the importance of long-term multi-wavelength monitoring on both single targets and large AGN surveys to reveal the real nature of disc-corona system in AGN.

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arXiv2023

Rapid Variability of the accretion disk wind in the narrow line Seyfert 1, PG 1448+273

PG 1448+273 is a luminous, nearby ($z=0.0645$), narrow line Seyfert 1 galaxy, which likely accretes close to the Eddington limit. XMM-Newton observations of PG 1448+273 in 2017 revealed the presence of an ultra fast outflow, as seen through its blueshifted iron K absorption profile, with an outflow velocity of about $0.1c$. Here, the first NuSTAR observation of PG 1448+273, performed in 2022 and coordinated with XMM-Newton is presented, which shows remarkable variability of its ultra fast outflow. The average count rate is a factor of 2 lower during the last 60 ks of the NuSTAR observation, where a much faster component of the ultra fast outflow was detected with a terminal velocity of $0.26\pm0.04c$. This is significantly faster than the outflow component which was initially detected in 2017, when overall PG 1448+273 was observed at a lower X-ray flux and which implies an order of magnitude increase in the wind kinetic power between the 2017 and 2022 epochs. Furthermore, the rapid variability of the ultra fast outflow in 2022, on timescales down to 10 ks, suggests we are viewing through a highly inhomogeneous disk wind in PG 1448+273, where the passage of a denser wind clump could account for the increase in obscuration in the last 60 ks of the NuSTAR observation.

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arXiv2023

Origins of the UV/X-ray Relation in Arakelian 120

We explore the accretion geometry in Arakelian 120 using intensive UV and X-ray monitoring from \textit{Swift}. The hard X-rays ($1-10$ keV) show large amplitude, fast (few-day) variability, so we expect reverberation from the disc to produce UV variability from the varying hard X-ray illumination. We model the spectral energy distribution including an outer standard disc (optical), an intermediate warm Comptonisation region (UV and soft X-ray) and a hot corona (hard X-rays). Unlike the lower Eddington fraction AGN (NGC 4151 and NGC 5548 at $L/L_{Edd}\sim 0.02$ and $0.03$ respectively), the SED of Akn 120 ($L\sim 0.05L_{Edd}$) is dominated by the UV, restricting the impact of reverberating hard X-rays by energetics alone. Illumination from a hard X-ray corona with height $\sim10 R_g$ produces minimal UV variability. Increasing the coronal scale height to $\sim 100 R_g$ improves the match to the observed amplitude of UV variability as the disc subtends a larger solid angle, but results in too much fast variability to match the UV data. The soft X-rays (connected to the UV in the warm Comptonisation model) are more variable than the hard, but again contain too much fast variability to match the observed smoother variability seen in the UV. Results on lower Eddington fraction AGN have emphasised the contribution from reverberation from larger scales (the broad line region), but reverberation induces lags on similar timescales to the smoothing, producing a larger delay than is compatible with the data. We conclude that the majority of the UV variability is therefore intrinsic, connected to mass accretion rate fluctuations in the warm Comptonisation region.

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arXiv2022

AGN Light Echoes and the Accretion Disc Self-Gravity Limit

Accretion disc theory predicts that an AGN disc becomes self-gravitating and breaks up into stars at an outer radius $R_{\rm sg}$ ~ 12 light-days, with effectively no free parameter. We present evidence that the longer observed AGN light echoes are all close to 12d in the AGN rest frames. These observations give a stringent test of AGN disc theory. Further monitoring should offer insight into the formation angular momentum of the gas forming the disc. For distant AGN, observed lags significantly longer than 12d give lower limits on their redshifts.

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arXiv2021

Possible evidence for shock-cooling in the accretion flow of the luminous Seyfert galaxy PG1211+143

Short-term variability and multiple velocity components in the powerful highly ionized wind of the archetypal UFO PG1211+143 are indicative of inner disc instabilities or short-lived accretion events. The recent detection of a high velocity inflow offered the first direct observational support for the latter scenario, where matter approaching at a high inclination to the black hole spin plane may result in warping and tearing of the inner accretion disc, with subsequent inter-ring collisions producing shocks, loss of rotational support and rapid mass infall. Here we identify a variable continuum component in the same data set, well-modelled by a hot thermal Comptonised spectrum that could represent cooling radiation from the shocked gas.

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arXiv2020

The flaring X-ray corona in the quasar PDS 456

New Swift monitoring observations of the variable, radio-quiet quasar, PDS 456, are presented. A bright X-ray flare was captured in September 2018, the flux increasing by a factor of 4 and with a doubling time-scale of 2 days. From the light crossing argument, the coronal size is inferred to be about 30 gravitational radii for a black hole mass of $10^{9} {\rm M}_{\odot}$ and the total flare energy exceeds $10^{51}$ erg. A hardening of the X-ray emission accompanied the flare, with the photon index decreasing from $Γ=2.2$ to $Γ=1.7$ and back again. The flare is produced in the X-ray corona, the lack of any optical or UV variability being consistent with a constant accretion rate. Simultaneous XMM-Newton and NuSTAR observations were performed, $1-3$ days after the flare peak and during the decline phase. These caught PDS 456 in a bright, bare state, where no disc wind absorption features are apparent. The hard X-ray spectrum shows a high energy roll-over, with an e-folding energy of $E_{\rm fold}=51^{+11}_{-8}$ keV. The deduced coronal temperature, of $kT=13$ keV, is one of the coolest measured in any AGN and PDS 456 lies well below the predicted pair annihilation line in X-ray corona. The spectral variability, becoming softer when fainter following the flare, is consistent with models of cooling X-ray coronae. Alternatively, an increase in a non-thermal component could contribute towards the hard X-ray flare spectrum.

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arXiv2018

Resolving the X-ray obscuration in a low flux observation of the quasar PDS 456

Simultaneous XMM-Newton, NuSTAR and HST observations, performed in March 2017, of the nearby ($z=0.184$) luminous quasar PDS 456 are presented. PDS 456 had a low X-ray flux compared to past observations, where the first of the two new XMM-Newton observations occurred during a pronounced dip in the X-ray lightcurve. The broad-band X-ray spectrum is highly absorbed, attenuated by a soft X-ray absorber of column density $N_{\rm H}=6\times10^{22}$ cm$^{-2}$. An increase in obscuration occurs during the dip, which may be due to an X-ray eclipse. In addition, the persistent, fast Fe K outflow is present, with velocity components of $-0.25c$ and $-0.4c$. The soft absorber is less ionized ($\logξ=3$) compared to the iron K outflow ($\logξ=5$) and is outflowing with a velocity of approximately $-0.2c$. A soft X-ray excess is present below 1 keV against the highly absorbed continuum and can be attributed to the re-emission from a wide angle wind. The complex X-ray absorption present in PDS 456 suggests that the wind is inhomogeneous, whereby the soft X-ray absorber originates from denser clumps or filaments which may form further out along the outflow. In contrast to the X-ray observations, the simultaneous UV spectrum of PDS 456 is largely unabsorbed, where only a very weak broad absorption trough is present bluewards of Ly$α$, compared to a past observation in 2000 when the trough was significantly stronger. The relative weakness of the UV absorption may be due to the soft X-ray absorber being too highly ionized and almost transparent in the UV band.

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arXiv2018

An ultra-fast inflow in the luminous Seyfert PG1211+143

Blueshifted absorption lines in the X-ray spectra of AGN show that ultra-fast outflows with typical velocities $v \sim 0.1c$ are a common feature of these luminous objects. Such powerful AGN winds offer an explanation of the observed M-$σ$ relation linking the mass of the supermassive black hole and the velocity dispersion in the galaxy's stellar bulge. An extended XMM-Newton study of the luminous Seyfert galaxy PG1211+143 recently revealed a variable multi-velocity wind. Here we report the detection of a short-lived, ultra-fast inflow during the same observation. Previous reports of inflows used single absorption lines with uncertain identifications, but this new result identifies an array of resonance absorption lines of highly ionised Fe, Ca, Ar, S and Si, sharing a common redshift when compared with a grid of realistic photoionization spectra. The redshifted absorption arises in a column of highly ionized matter close to the black hole, with a line-of-sight velocity, $v \sim 0.3c$, inconsistent with the standard picture of a plane circular accretion disc. This may represent the first direct evidence for chaotic accretion in AGN, where accretion discs are generally misaligned to the black hole spin. For sufficient inclinations, the Lense-Thirring effect can break the discs into discrete rings, which then precess, collide and shock, causing near free-fall of gas towards the black hole. The observed accretion rate for the reported infall is comparable to the hard X-ray luminosity in PG1211+143, suggesting that direct infall may be a significant contributor to inner disc accretion.

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arXiv2018

A new relativistic component of the accretion disk wind in PDS 456

Past X-ray observations of the nearby luminous quasar PDS 456 (at $z=0.184$) have revealed a wide angle accretion disk wind (Nardini et al. 2015), with an outflow velocity of $\sim-0.25c$. Here we unveil a new, relativistic component of the wind through hard X-ray observations with NuSTAR and XMM-Newton, obtained in March 2017 when the quasar was in a low flux state. This very fast wind component, with an outflow velocity of $-0.46\pm0.02c$, is detected in the iron K band, in addition to the $-0.25c$ wind zone. The relativistic component may arise from the innermost disk wind, launched from close to the black hole at radius of $\sim10$ gravitational radii. The opacity of the fast wind also increases during a possible obscuration event lasting for 50 ks. We suggest that the very fast wind may only be apparent during the lowest X-ray flux states of PDS 456, becoming overly ionized as the luminosity increases. Overall, the total wind power may even approach the Eddington value.

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arXiv2018

X-ray time lags in PG 1211+143

We investigate the X-ray time lags of a recent ~630ks XMM-Newton observation of PG 1211+143. We find well-correlated variations across the XMM-Newton EPIC bandpass, with the first detection of a hard lag in this source with a mean time delay of up to ~3ks at the lowest frequencies. We find that the energy-dependence of the low-frequency hard lag scales approximately linearly with log(E) when averaged over all orbits, consistent with the propagating fluctuations model. However, we find that the low-frequency lag behaviour becomes more complex on timescales longer than a single orbit, suggestive of additional modes of variability. We also detect a high-frequency soft lag at ~10^{-4}Hz with the magnitude of the delay peaking at <0.8ks, consistent with previous observations, which we discuss in terms of small-scale reverberation.

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arXiv2018

The variable, fast soft X-ray wind in PG 1211+143

The analysis of a series of seven observations of the nearby ($z=0.0809$) QSO, PG 1211+143, taken with the Reflection Grating Spectrometer (RGS) on-board XMM-Newton in 2014, are presented. The high resolution soft X-ray spectrum, with a total exposure exceeding 600 ks, shows a series of blue-shifted absorption lines, from the He and H-like transitions of N, O and Ne, as well as from L-shell Fe. The strongest absorption lines are all systematically blue-shifted by $-0.06c$, originating in two absorption zones, from low and high ionization gas. Both zones are variable on timescales of days, with the variations in absorber opacity effectively explained by either column density changes or by the absorber ionization responding directly to the continuum flux. We find that the soft X-ray absorbers probably exist in a two-phase wind, at a radial distance of $10^{17}-10^{18}$ cm from the black hole, with the lower ionization gas as denser clumps embedded within a higher ionization outflow. The overall mass outflow rate of the soft X-ray wind may be as high as $2{\rm M}_{\odot}$ yr$^{-1}$, close to the Eddington rate for PG 1211+143 and similar to that previously deduced from the Fe K absorption.

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arXiv2017

A deep X-ray view of the bare AGN Ark 120. III. X-ray timing analysis and multiwavelength variability

We present the spectral/timing properties of the bare Seyfert galaxy Ark 120 through a deep ~420ks XMM-Newton campaign plus recent NuSTAR observations and a ~6-month Swift monitoring campaign. We investigate the spectral decomposition through fractional rms, covariance and difference spectra, finding the mid- to long-timescale (~day-year) variability to be dominated by a relatively smooth, steep component, peaking in the soft X-ray band. Additionally, we find evidence for variable FeK emission red-ward of the FeK-alpha core on long timescales, consistent with previous findings. We detect a clearly-defined power spectrum which we model with a power law with a slope of alpha ~ 1.9. By extending the power spectrum to lower frequencies through the inclusion of Swift and RXTE data, we find tentative evidence of a high-frequency break, consistent with existing scaling relations. We also explore frequency-dependent Fourier time lags, detecting a negative ('soft') lag for the first time in this source with the 0.3-1 keV band lagging behind the 1-4 keV band with a time delay of ~900s. Finally, we analyze the variability in the optical and UV bands using the Optical/UV Monitor on-board XMM-Newton and the UVOT on-board Swift and search for time-dependent correlations between the optical/UV/X-ray bands. We find tentative evidence for the U-band emission lagging behind the X-rays with a time delay of 2.4 +/- 1.8 days, which we discuss in the context of disc reprocessing.

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arXiv2017

High Resolution X-ray Spectroscopy of the Seyfert 1, Mrk 1040. Revealing the Failed Nuclear Wind with Chandra

High resolution X-ray spectroscopy of the warm absorber in the nearby X-ray bright Seyfert 1 galaxy, Mrk 1040 is presented. The observations were carried out in the 2013-2014 timeframe using the Chandra High Energy Transmission Grating with a total exposure of 200 ks. A multitude of absorption lines from Ne, Mg and Si are detected from a wide variety of ionization states. In particular, the detection of inner K-shell absorption lines from Ne, Mg and Si, from charge states ranging from F-like to Li-like ions, suggests the presence of a substantial amount of low ionization absorbing gas, illuminated by a steep soft X-ray continuum. The observations reveal at least 3 warm absorbing components ranging in ionization parameter from $\logξ= 0-2$ and with column densities of $N_{\rm H} =1.5-4.0 \times 10^{21}$cm$^{-2}$. The velocity profiles imply that the outflow velocities of the absorbing gas are low and within $\pm100$ km s$^{-1}$ of the systemic velocity of Mrk 1040, which suggests any outflowing gas may have stalled in this AGN on large enough scales. The warm absorber is likely located far from the black hole, within 300 pc of the nucleus and is spatially coincident with emission from an extended Narrow Line Region as seen in the HST images. The iron K band spectrum reveals only narrow emission lines, with Fe K$α$ at 6.4 keV consistent with originating from reflection off Compton thick pc-scale reprocessing gas.

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arXiv2017

Exploring accretion disc physics and black hole growth with regular monitoring of ultrafast AGN winds

15 years of XMM-Newton observations have established that ultra-fast, highly ionized winds are common in radio-quiet AGN. A simple theory of Eddington-limited accretion correctly predicts the typical velocity (~0.1c) and high ionization of such winds, with observed flow energy capable of ejecting star-forming gas. With a recent extended XMM-Newton observation of the archetypal UFO, PG1211+143, revealing a more complex flow pattern, we suggest that targetted observations over the next decade offer unique potential for probing the inner accretion disc structure and SMBH growth.

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arXiv2016

Testing relativistic reflection and resolving outflows in PG 1211+143 with XMM-Newton and NuSTAR

We analyze the broad-band X-ray spectrum (0.3-50 keV) of the luminous Seyfert 1 / quasar PG 1211+143 - the archetypal source for high-velocity X-ray outflows - using near-simultaneous XMM-Newton and NuSTAR observations. We compare pure relativistic reflection models with a model including the strong imprint of photoionized emission and absorption from a high-velocity wind (Pounds16a,16b), finding a spectral fit that extrapolates well over the higher photon energies covered by NuSTAR. Inclusion of the high S/N XMM-Newton spectrum provides much tighter constraints on the model parameters, with a much harder photon index / lower reflection fraction compared to that from the NuSTAR data alone. We show that pure relativistic reflection models are not able to account for the spectral complexity of PG 1211+143 and that wind absorption models are strongly required to match the data in both the soft X-ray and Fe K spectral regions. In confirming the significance of previously reported ionized absorption features, the new analysis provides a further demonstration of the power of combining the high throughput and resolution of long-look XMM-Newton observations with the unprecedented spectral coverage of NuSTAR.

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arXiv2016

A deep X-ray view of the bare AGN Ark 120. I. Revealing the Soft X-ray Line Emission

The Seyfert 1 galaxy, Ark 120, is a prototype example of the so-called class of bare nucleus AGN, whereby there is no known evidence for the presence of ionized gas along the direct line of sight. Here deep ($>400$ ks exposure), high resolution X-ray spectroscopy of Ark 120 is presented, from XMM-Newton observations which were carried out in March 2014, together with simultaneous Chandra/HETG exposures. The high resolution spectra confirmed the lack of intrinsic absorbing gas associated with Ark 120, with the only X-ray absorption present originating from the ISM of our own Galaxy, with a possible slight enhancement of the Oxygen abundance required with respect to the expected ISM values in the Solar neighbourhood. However, the presence of several soft X-ray emission lines are revealed for the first time in the XMM-Newton RGS spectrum, associated to the AGN and arising from the He and H-like ions of N, O, Ne and Mg. The He-like line profiles of N, O and Ne appear velocity broadened, with typical FWHM widths of $\sim5000$ km s$^{-1}$, whereas the H-like profiles are unresolved. From the clean measurement of the He-like triplets, we deduce that the broad lines arise from gas of density $n_{\rm e}\sim10^{11}$ cm$^{-3}$, while the photoionization calculations infer that the emitting gas covers at least 10 percent of $4π$ steradian. Thus the broad soft X-ray profiles appear coincident with an X-ray component of the optical-UV Broad Line Region on sub-pc scales, whereas the narrow profiles originate on larger pc scales, perhaps coincident with the AGN Narrow Line Region. The observations show that Ark 120 is not intrinsically bare and substantial X-ray emitting gas exists out of our direct line of sight towards this AGN.

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arXiv2016

Imprints of a high velocity wind on the soft x-ray spectrum of PG 1211+143

An extended XMM-Newton observation of the luminous narrow line Seyfert galaxy PG 1211+143 in 2014 has revealed a more complex high velocity wind, with components distinguished in velocity, ionization level, and column density. Here we report soft x-ray emission and absorption features from the ionized outflow, finding counterparts of both high velocity components, v ~ 0.129c and v ~ 0.066c, recently identified in the highly ionized Fe K absorption spectrum. The lower ionization of the co-moving soft x-ray absorbers imply a distribution of higher density clouds embedded in the main outflow, while much higher column densities for the same flow component in the hard x-ray spectra suggest differing sight lines to the continuum x-ray source.

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