arXiv subjects

L. C. Gallo

Publications and source records attributed to L. C. Gallo.

86 records · Page 5Linked to original sources

arXiv2007

A longer XMM-Newton look at I Zwicky 1: physical conditions and variability of the ionised absorbers

We present a spectral analysis of the narrow-line Seyfert 1 galaxy I Zwicky 1, focusing on the characteristics of the ionised absorbers as observed with XMM-Newton in 2005. The soft X-ray spectrum shows absorption by two components of ionised gas with a similar column density (N_H~10^{21} cm^{-2}) and ionisation parameters logξ~0 and 2.5. Comparing this observation with a 2002 XMM-Newton data set, we see a clear anti-correlation between the X-ray ionisation parameter xi_X and the 0.1-10 keV luminosity. Viable explanations for this effect include transient clouds or filaments crossing the line of sight in a complex geometry or a gas observed in non-equilibrium. The outflow velocity of the X-ray low-ionisation absorber is consistent with the outflow of the UV absorber detected in a past Hubble Space Telescope observation. In addition, the ionic column densities of CIV and NV derived from the X-ray model are consistent with the UV values. This suggests that the low-ionisation outflowing gas may survive for many years, despite large changes in flux, and that there is a tight connection between the X-ray and UV absorbers that can only be confirmed with a simultaneous UV and X-ray observation.

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arXiv2007

A longer XMM-Newton look at I Zwicky 1: Variability of the X-ray continuum, absorption, and iron Kalpha line

We present the second XMM-Newton observation (85 ks) of the narrow-line Seyfert 1 galaxy (NLS1) I Zw 1 and describe its mean spectral and timing characteristics. On average, I Zw 1 is ~35 per cent dimmer in 2005 than in the shorter (20 ks) 2002 observation. Between the two epochs the intrinsic absorption column density diminished, but there were also subtle changes in the continuum shape. Considering the blurred ionised reflection model, the long-term changes can be associated with a varying contribution of the power law component relative to the total spectrum. Examination of normalised light curves indicates that the high-energy variations are quite structured and that there are delays, but only in some parts of the light curve. Interestingly, a hard X-ray lag first appears during the most-distinct structure in the mean light curve, a flux dip ~25 ks into the observation. The previously discovered broad, ionised Fe Ka line shows significant variations over the course of the 2005 observation. The amplitude of the variations is 25-45 per cent and they are unlikely due to changes in the Fe ka-producing region, but perhaps arise from orbital motion around the black hole or obscuration in the broad iron line-emitting region. The 2002 data are re-examined for variability of the Fe Ka line at that epoch. There is evidence of energy and flux variations that are associated with a hard X-ray flare that occurred during that observation.

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arXiv2006

The spectral energy distribution of PKS 2004-447: a compact steep-spectrum source and possible radio-loud narrow-line Seyfert 1 galaxy

(abridged) The spectral energy distribution (SED) of the compact steep spectrum (CSS) source and possible radio-loud narrow-line Seyfert 1 galaxy (NLS1), PKS2004-447, is presented. Five out of six well studied RL NLS1 share this dual classification (optically defined as a NLS1 with radio definition of a CSS or giga-hertz peaked spectrum (GPS) source). The SED is created from simultaneous observations at radio (ATCA), optical/NIR (Siding Spring) and UV/X-ray (XMM-Newton) wavelengths. The X-ray data show evidence of short-term variability, a possible soft excess, and negligible absorption. Together with the rest of the SED, the X-ray emission is excessive in comparison to synchrotron plus synchrotron self-Compton (SSC) models. The SED can be described with a two component model consisting of extended synchrotron/SSC emission with Comptonisation in the X-rays, though SSC models with a very high electron-to-magnetic energy density ratio cannot be excluded either. The peak emission in the SED appears to be in the near infrared, which can be attributed to thermal emission from a dusty torus. Analysis of a non-contemporaneous, low resolution optical spectrum suggests that the narrow-line region (NLR) is much more reddened than the X-ray emitting region suggesting that the gas-to-dust ratio in PKS2004-447 may be very different then in our own Galaxy. Long-term radio monitoring of PKS2004-447 shows a rather constant light curve over nearly a six month period with the exception of one outburst when the 6.65GHz flux increased by ~35% over 19 days. In comparison to general samples of GPS sources, which appear to be X-ray weak, NLS1-CSS/GPS sources possess stronger X-ray emission relative to radio, and lower intrinsic absorption than GPS sources of similar X-ray luminosity.

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arXiv2006

Investigating the nature of narrow-line Seyfert 1 galaxies with high-energy spectral complexity

(abridged) With the commissioning of XMM came the discovery of 2.5-10 keV spectral complexity in some narrow-line Seyfert 1 galaxies (NLS1). In this work, we define two samples of NLS1: a complex sample whose members exhibit high-energy complexity (C-sample), and a general sample of NLS1 whose 2.5-10 keV spectra do not strongly deviate from a simple power law (S-sample). Considering historical light curves of each object we find that the C-sample is representative of NLS1 in a low X-ray flux state, whereas the members of the S-sample appear to be in a typical flux state. Moreover, from measurements of alpha_ox with contemporaneous UV/X-ray data, we find that the C-sample of NLS1 appear X-ray weaker at the time of the observation. For two NLS1 in the C-sample multi-epoch measurements of alpha_ox are available and suggest that alpha_ox approaches more normal values as the complexity between 2.5-10 keV diminishes. This implies that a source could transit from one sample to the other as its X-ray flux varies. Secondly, there are indications that the C-sample sources, on average, exhibit stronger optical FeII emission, with the three most extreme (FeII/H_beta > 1.8) FeII emitters all displaying complexity in the 2.5-10 keV band. However, it is not clear if the possible connection between FeII strength and spectral complexity is due to the FeII producing mechanism or because strong FeII emitters may exhibit the greatest variability and consequently more likely to be caught in an extreme (low) flux state. Based on the current analysis it we can not straightforwardly dismiss absorption or reflection as the cause of the X-ray complexity.

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arXiv2005

X-rays from the radio-quiet quasar PG 1407+265: relativistic jet or accretion disc emission?

We present two XMM-Newton observations of the luminous (L_x > 10^46 erg/s), radio-quiet quasar, PG 1407+265, separated by eleven months. The data indicate two distinct states: a highly variable, bright state (first epoch); and a quiescent, low-flux one (second epoch). During the low-flux state the spectrum is consistent with a single, unabsorbed power law. However, during the brighter state a highly variable, steep component is statistically required. Contemporaneous UV data from the Optical Monitor allow an estimate of the optical-to-X-ray spectral index (alpha_ox), which appears typical of radio-quiet quasars during the low-flux state, but extremely flat during the high-flux state. The XMM-Newton data can be described as originating from a combination of jet and accretion disc processes, in which the (relativistic) X-ray jet only works intermittently. The scenario could help describe some of the complexities seen in the broadband spectral energy distribution of PG 1407+265, such as weak high-ionisation emission lines, strong Fe II, unbeamed continuum, and the weak radio emission relative to the optical.

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arXiv2005

An X-ray view of Mrk 705: A borderline narrow-line Seyfert 1 galaxy

Mrk 705 exhibits optical properties of both narrow- and broad-line Seyfert 1 galaxies. We examine the X-ray properties of this borderline object utilising proprietary and public data from Chandra, ASCA, ROSAT and RXTE, spanning more than twelve years. Though long-term flux variability from the pointed observations appears rather modest (about 3 times), we do find examples of rare large amplitude outbursts in the RXTE monitoring data. There is very little evidence of long-term spectral variability as the low- and high-energy spectra appear constant with time. A 6.4 keV emission line is detected in the ASCA spectra of Mrk 705, but not during the later, higher flux state Chandra observation. However, the upper limit on the equivalent width of a line in the Chandra spectrum is consistent with a constant-flux emission line and a brighter continuum, suggesting that the line is emitted from distant material such as the putative torus. Overall, the X-ray properties of Mrk 705 appear typical of BLS1 activity.

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arXiv2005

XMM-Newton observations of bright ROSAT selected active galactic nuclei with low intrinsic absorption

We present a sample of twenty-one ROSAT bright active galactic nuclei (AGN), representing a range of spectral classes, and selected for follow-up snap-shot observations with XMM-Newton. The typical exposure was between 5-10 ks. The objects were primarily selected on the bases of X-ray brightness and not on hardness ratio; thus the sample cannot be strictly defined as a `soft' sample. One of the main outcomes from the XMM-Newton observations was that all of the AGN, including eleven type 1.8-2 objects, required low levels of intrinsic absorption (nh < 10^21 cm^-2). The low absorption in type 2 systems is a challenge to account for in the standard orientation-based unification model, and we discuss possible physical and geometrical models which could elucidate the problem. Moreover, there does not appear to be any relation between the strength and shape of the soft excess, and the spectral classification of the AGN in this sample. We further identify a number of AGN which deserve deeper observations or further analysis: for example, the LINERs NGC 5005 and NGC 7331, where optically thin thermal and extended emission is detected, and the narrow-line Seyfert 1 II Zw 177, which shows a broad emission feature at 5.8keV.

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arXiv2005

A possible line-like emission feature at 8 keV in the Seyfert 1.2 UGC 3973

Two short X-ray exposures (< 3600 s each) of the radio-quiet Seyfert 1.2 galaxy UGC 3973 (Mrk 79) were conducted with XMM-Newton as part of an AGN snap-shot survey. In this paper we concentrate on the significance and possible origin of a narrow 8 keV (rest-frame) line-like emission feature detected with the pn instrument during the second observation. Simulations show that the feature is significant at 96.0-98.4% confidence, depending on what a priori assumptions are made. The feature cannot be attributed to background contamination and appears to be variable (or transient) since it was not detected in the first observation of UGC 3973 six months earlier. However, a constant feature cannot be completely dismissed, based on the 90% upper-limit on the flux from the first observation. There is some indication that the feature is variable over the duration of the second observation as well. We discuss various models (e.g. Ni emission, recombination edges, outflows, disc lines) which could potentially produce an emission feature at such energies.

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arXiv2004

The narrow-line quasar NAB 0205+024 observed with XMM-Newton

The XMM-Newton observation of the narrow-line quasar NAB 0205+024 reveals three striking differences since it was last observed in the X-rays with ASCA. Firstly, the 2-10 keV power-law is notably steeper. Secondly, a hard X-ray flare is detected, very similar to that seen in I Zw 1. Thirdly, a strong and broad emission feature is detected with the bulk of its emission redward of 6.4 keV, and extending down to ~5 keV in the rest frame. The most likely explanation for the broad feature is neutral iron emission emitted from a narrow annulus of an accretion disc close to the black hole. The hard X-ray flare could be the mechanism which illuminates this region of the disc, allowing for the emission line to be detected. The combination of effects can be understood in terms of the `thundercloud' model proposed by Merloni & Fabian.

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arXiv2004

Long-term spectral changes in the partial-covering candidate NLS1 1H 0707-495

We compare two XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy 1H 0707-495 separated by two years, and discuss the results in terms of the partial-covering phenomenon. The second longer observation once again displays a sharp (< 200 eV) spectral drop above 7 keV; however, in contrast to the first observation, the edge depth and energy have changed significantly. In addition to changes in the edge parameters, the high-energy spectrum appears steeper. The changes in the high-energy continuum can be adequately explained in terms of a partial-covering absorber outflowing from the central region. The low-energy spectrum also shows significant long-term spectral variability, including: a substantial increase in the disc temperature; detection of a ~0.9 keV emission feature; and the presence of warm absorption that was also detected during the ASCA mission, but not seen during the first XMM-Newton observation. The large increase in disc temperature, and more modest rise in luminosity, can be understood if we consider the slim-disc model for 1H 0707-495. In addition, the higher disc luminosity could be the driving force behind the outflow scenario and the re-appearance of a warm medium during the second XMM-Newton observation.

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arXiv2004

An intense soft-excess and evidence for light bending in the luminous narrow-line quasar PHL 1092

The narrow-line quasar PHL 1092 was observed by XMM-Newton at two epochs separated by nearly thirty months. Timing analyses confirm the extreme variability observed during previous X-ray missions. A measurement of the radiative efficiency is in excess of what is expected from a Schwarzschild black hole. In addition to the rapid X-ray variability, the short UV light curves (< 4 hours) obtained with the Optical Monitor may also show fluctuations, albeit at much lower amplitude than the X-rays. In general, the extreme variability is impressive considering that the broad-band (0.4-10 keV rest-frame) luminosity of the source is ~10^45 erg/s. During at least one of the observations, the X-ray and UV light curves show common trends, although given the short duration of the OM observations, and low significance of the UV light curves it is difficult to comment on the importance of this possible correlation. Interestingly, the high-energy photons (> 2 keV) do not appear highly variable. The X-ray spectrum resembles that of many narrow-line Seyfert 1 type galaxies: an intense soft-excess modelled with a multi-colour disc blackbody, a power-law component, and an absorption line at ~1.4 keV. The ~1.4 keV feature is curious given that it was not detected in previous observations, and its presence could be related to the strength of the soft-excess. Of further interest is curvature in the spectrum above ~2 keV which can be described by a strong reflection component. The strong reflection component, lack of high-energy temporal variability, and extreme radiative efficiency measurements can be understood if we consider gravitational light bending effects close to a maximally rotating black hole.

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arXiv2003

IZw1 observed with XMM-Newton: Low-energy spectral complexity, iron lines, and hard X-ray flares

We present a 20 ks XMM-Newton observation of the prototypical Narrow-Line Seyfert 1 galaxy IZw1. The best-fit model to the data is a double blackbody plus a dominant power-law, on which complex soft absorption (possibly a blended edge or absorption lines) and/or OVII emission are superimposed, as well as strong Fe Kalpha emission. The iron feature in the high-energy spectra appears broad; however, on close examination of the EPIC pn data, there exists the possibility that the broad emission feature can be attributed to a neutral Fe Kalpha line in addition to a blend of He- and H-like Fe Kalpha lines. The light curve shows a strong, hard X-ray flare concentrated in the 3-12 keV band. The flare appears to induce spectral variability, showing spectral hardening to be occuring as the flare intensifies. A detailed examination suggests that the spectral variability is most likely due to an increase in the 3-12 keV flux relative to the soft flux during the flare. A difference spectrum and complete modelling of the flare and non-flare spectra show intrinsic changes only in the normalisation of the continuum components and not in their shape parameters. The timing results are consistent with the flare originating in the accretion disc corona. The iron emission line(s) do not appear to respond to changes in the continuum flux during the flare; the iron lines are stronger in equivalent width during the low-flux (non-flare) states, and weaker during the flare.

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arXiv2003

The X-ray variability of the Narrow-Line Seyfert 1 galaxy IRAS 13224-3809 from an XMM-Newton observation

We report on the XMM-Newton timing properties of the most X-ray variable, radio-quiet, NLS1 IRAS 13224-3809. IRAS 13224-3809 continues to display the extremely variable behavior that was previously observed with ROSAT and ASCA; however, no giant, rapid flaring events are observed. We detect variations by a factor as high as ~8 during the 64 ks observation, and the variability is persistent throughout the light curve. Dividing the light curve into 9 minute segments we found almost all of the segments to be variable at > 3 sigma. When the time-averaged cross-correlation function is calculated for the 0.3-0.8 keV band with the 3-10 keV band, the cross-correlation profile is skewed indicating a possible smearing of the signal to longer times (soft band leading the hard). A correlation between count rate and hardness ratio is detected in four energy bands. In three cases the correlation is consistent with spectral hardening at lower count rates which can be explained in terms of a partial-covering model. The other band displays the reverse effect, showing spectral hardening at higher count rates. We can explain this trend as a more variable power-law component compared to the soft component. We also detect a delay between the 0.3-1.5 keV count rate and the 0.8-1.5 keV to 0.3-0.8 keV hardness ratio, implying flux induced spectral variability. Such delays and asymmetries in the cross correlation functions could be suggesting reprocessing of soft and hard photons. In general, much of the timing behavior can be attributed to erratic eclipsing behavior associated with the partial covering phenomenon, in addition to intrinsic variability in the source. The variability behavior of IRAS 13224-3809 suggests a complicated combination of effects which we have started to disentangle with this present analysis.

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arXiv2002

Mrk 1014: An AGN Dominated ULIRG at X-rays

In this paper we report on an XMM-Newton observation of the ultraluminous infrared QSO Mrk 1014. The X-ray observation reveals a power-law dominated (photon index of about 2.2) spectrum with a slight excess in the soft energy range. AGN and starburst emission models fit the soft excess emission equally well, however, the most plausible explanation is an AGN component as the starburst model parameter, temperature and luminosity, appear physically unrealistic. The mean luminosity of Mrk 1014 is about 2 times 10^44 erg s^-1. We have also observed excess emission at energies greater than 5 keV. This feature could be attributed to a broadened and redshifted iron complex, but deeper observations are required to constrain its origin. The light curve shows small scale variability over the 11 ks observation. There is no evidence of intrinsic absorption in Mrk 1014. The X-ray observations support the notion of an AGN dominated central engine. We establish the need for a longer observation to constrain more precisely the nature of the X-ray components.

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