arXiv subjects

K. A. Pounds

Publications and source records attributed to K. A. Pounds.

At least 37 records · Page 2Linked to original sources

arXiv2004

Exploring the complex X-ray spectrum of NGC 4051

Archival XMM-Newton data on the nearby Seyfert galaxy NGC 4051, taken in relatively high and low flux states, offer a unique opportunity to explore the complexity of its X-ray spectrum. We find the hard X-ray band to be significantly affected by reflection from cold matter, which can also explain a non-varying, narrow Fe K fluorescent line. We interpret major differences between the high and low flux hard X-ray spectra in terms of the varying ionisation (opacity) of a substantial column of outflowing gas. An emission line spectrum in the low flux state indicates an extended region of photoionised gas. A high velocity, highly ionised outflow seen in the high state spectrum can replenish the gas in the extended emission region over ~10^3 years, while having sufficient kinetic energy to contribute significantly to the hard X-ray continuum.

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arXiv2004

An XMM-Newton observation of the Seyfert 1 galaxy 1H 0419-577 in an extreme low state

Previous observations of the luminous Seyfert 1 galaxy 1H 0419-577 have found its X-ray spectrum to range from that of a typical Seyfert 1 with 2-10 keV power law index Gamma ~ 1.9 to a much flatter power law of Gamma ~ 1.5 or less. We report here a new XMM-Newton observation which allows the low state spectrum to be studied in much greater detail than hitherto. We find a very hard spectrum (Gamma ~ 1.0), which exhibits broad features that can be modelled with the addition of an extreme relativistic Fe K emission line or with partial covering of the underlying continuum by a substantial column density of near-neutral gas. Both the EPIC and RGS data show evidence for strong line emission of OVII and OVIII requiring an extended region of low density photoionised gas in 1H 0419-577. Comparison with an earlier XMM-Newton observation when 1H 0419-577 was 'X-ray bright' indicates the dominant spectral variability occurs via a steep power law component.

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arXiv2003

The XMM-Newton Iron Line Profile of NGC 3783

We report on observations of the iron K line in the nearby Seyfert 1 galaxy, NGC 3783, obtained in a long, 2 orbit (240 ks) XMM-Newton observation. The line profile obtained exhibits two strong narrow peaks at 6.4 keV and at 7.0 keV, with measured line equivalent widths of 120 and 35 eV respectively. The 6.4 keV emission is the K-alpha line from near neutral Fe, whilst the 7.0 keV feature probably originates from a blend of the neutral Fe K-beta line and the H-like line of Fe at 6.97 keV. The relatively narrow velocity width of the K-alpha line (<5000 km/s), its lack of response to the continuum emission on short timescales and the detection of a neutral Compton reflection component are all consistent with a distant origin in Compton-thick matter such as the putative molecular torus. A strong absorption line from highly ionized iron (at 6.67 keV) is detected in the time-averaged iron line profile, whilst the depth of the feature appears to vary with time, being strongest when the continuum flux is higher. The iron absorption line probably arises from the highest ionization component of the known warm absorber in NGC 3783, with an ionization of logxi=3 and column density of 5x10^{22}cm{-2} and may originate from within 0.1pc of the nucleus. A weak red-wing to the iron K line profile is also detected below 6.4 keV. However when the effect of the highly ionized warm absorber on the underlying continuum is taken into account, the requirement for a relativistic iron line component from the inner disk is reduced.

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arXiv2003

Evidence of a high velocity ionised outflow in a second narrow line quasar PG0844+349

Following the discovery of X-ray absorption in a high velocity outflow from the bright quasar PG1211+143 we have searched for similar features in XMM archival data of a second (high accretion rate) quasar PG0844+349. Evidence is found for absorption lines in both the EPIC and RGS spectra, whose identification with resonance transitions in H-like Fe, S, and Ne implies an origin in highly ionised matter with an outflow velocity of order ~0.2c. The line equivalent widths require a line-of-sight column density of N_H ~ 4 x 10^23 cm^-2, at an ionisation parameter of log(xi) ~ 3.7. Assuming a radial outflow being driven by radiation pressure from the inner accretion disc, as suggested previously for PG1211+143, the flow is again likely to be optically thick, in this case within ~ 25 Schwarzschild radii. We suggest that a high velocity, highly ionised outflow is likely to be a significant component in the mass and energy budgets of many AGN accreting at or above the Eddington rate.

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arXiv2003

Black Hole Winds

We show that black holes accreting at or above the Eddington rate probably produce winds which are optically thick in the continuum, whether in quasars or X-ray binaries. The photospheric radius and outflow speed are proportional to \mo^2 and \mo^-1 respectively, where \mo is the mass outflow rate. The outflow momentum rate is always of order L_Edd/c. Blackbody emission from these winds may provide the big blue bump in some quasars and AGN, as well as ultrasoft X-ray components in ULXs.

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arXiv2003

Fe K emission and absorption features in XMM-Newton spectra of Mkn 766 - evidence for reprocessing in flare ejecta

We report on the analysis of a long XMM-Newton EPIC observation in 2001 May of the Narrow Line Seyfert 1 galaxy Mkn 766. The 3-11 keV spectrum exhibits a moderately steep power law continuum, with a broad emission line at ~6.7 keV, probably blended with a narrow line at ~6.4 keV, and a broad absorption trough above ~8.7 keV. We identify both broad spectral features with reprocessing in He-like Fe. An earlier XMM-Newton observation of Mkn 766 in 2000 May, when the source was a factor ~2 fainter, shows a similar broad emission line, but with a slightly flatter power law and absorption at a lower energy. In neither observation do we find a requirement for the previously reported broad 'red wing' to the line and hence of reflection from the innermost accretion disc. More detailed examination of the longer XMM-Newton observation reveals evidence for rapid spectral variability in the Fe K band, apparently linked with the occurrence of X-ray 'flares'. A reduction in the emission line strength and increased high energy absorption during the X-ray flaring suggests that these transient effects are due to highly ionised ejecta associated with the flares. Simple scaling from the flare avalanche model proposed for the luminous QSO PDS 456 (Reeves etal. 2002) confirms the feasibility of coherent flaring being the cause of the strong peaks seen in the X-ray light curve of \mkn.

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arXiv2003

A simultaneous XMM-Newton and BeppoSAX observation of the archetypal Broad Line Seyfert 1 galaxy NGC 5548

We report the spectral analysis of a long XMM-Newton observation of the well-studied, moderate luminosity Broad Line Seyfert 1 galaxy NGC 5548. The source was at an historically average brightness and we find the hard (3-10 keV) spectrum can be well fitted by a power law of photon index gamma ~ 1.75, together with reflection. The only feature in the hard X-ray spectrum is a narrow emission line near 6.4 keV, with an equivalent width of ~ 60 eV. The energy and strength of this line is consistent with fluorescence from `neutral' iron distant from the central continuum source. We find no evidence for a broad Fe K line, with an upper limit well below previous reports, suggesting the inner accretion disc is now absent or highly ionised. The addition of simultaneous BeppoSAX data allows the analysis to be extended to 200 keV, yielding important constraints on the total reflection. Extrapolation of the hard X-ray power law down to 0.3 keV shows a clear `soft excess' below ~ 0.7 keV. After due allowance for the effects of a complex warm absorber, measured with the XMM-Newton RGS, we find the soft excess is better described as a smooth upward curvature in the continuum flux below ~ 2 keV. The soft excess can be modelled either by Comptonised thermal emission or by enhanced reflection from the surface of a highly ionised disc.

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arXiv2002

The X-ray afterglows of gamma-ray bursts GRB 001025A and GRB 010220 observed with XMM-Newton

The X-ray afterglows of GRB 001025A and GRB 010220 were detected by XMM-Newton with an average 0.2--10.0 keV flux of 4.4 and 3.3x10^{-14} erg cm^{-2} s^{-1} respectively; the afterglow of GRB 001025A is observed to decay. Afterglows at other wavelengths were not detected for either burst. A set of broadened soft X-ray emission lines are detected in the afterglow of GRB 001025A, at 5.0 sigma significance above a Galactic-absorbed power-law continuum. The spectra of both afterglows are significantly better fit by a variable abundance thermal plasma model than by an absorbed power-law and are consistent with the observations of GRB 011211, indicating that thermal emission from light elements may be common in the early X-ray afterglows of GRBs.

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arXiv2002

Soft X-ray emission lines in the afterglow spectrum of GRB 011211:- a detailed XMM-Newton analysis

We report on an XMM-Newton observation of the X-ray afterglow of the Gamma Ray Burst GRB 011211, originally detected by Beppo-SAX on 11th December 2001. The early afterglow spectrum obtained by XMM-Newton, observed 11 hours after the initial burst, appeared to reveal decaying H-like K emission lines of Mg, Si, S, Ar and Ca, arising in enriched material with an outflow velocity of order 0.1c (Reeves et al. 2002). This was attributed to matter ejected from a massive stellar progenitor occurring shortly before the burst itself. Here, we present a detailed re-analysis of the XMM-Newton EPIC observations of GRB 011211. In particular, we show that the detection of the soft X-ray line emission appears robust, regardless of detector background, calibration, spectral binning, or the spectral model that is assumed. We demonstrate that thermal emission, from an optically thin plasma, is the most plausible model that can account for the soft X-ray emission, which appears to be the case for at least two burst afterglow spectra observed by XMM-Newton. The X-ray spectrum of GRB 011211 evolves with time over the first 12 ksec of the XMM-Newton observation, the observations suggest that thermal emission dominates the early afterglow spectrum, whilst a power-law component dominates the latter stages. Finally we estimate the mass of the ejected material in GRB 011211 to be of the order 4-20 solar masses.

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arXiv2002

The Signature of Supernova Ejecta Measured in the X-ray Afterglow of Gamma-Ray Burst 011211

Since their identification with cosmological distances, Gamma-ray bursts (GRBs) have been recognised as the most energetic phenomena in the Universe, with an isotropic burst energy as high as 10^54 ergs. However, the progenitors responsible for the bursts remain elusive, favoured models ranging from a neutron star binary merger, to the collapse of a massive star. Crucial to our understanding of the origins of GRBs is the study of the afterglow emission, where spectroscopy can reveal details of the environment of the burst. Here we report on an XMM-Newton observation of the X-ray afterglow of GRB 011211. The X-ray spectrum reveals evidence for emission lines of Magnesium, Silicon, Sulphur, Argon, Calcium, and possibly Nickel, arising in enriched material with an outflow velocity of order 0.1c. This is the first direct measurement of outflowing matter in a gamma ray burst. The observations strongly favour models where a supernova explosion from a massive stellar progenitor precedes the burst event and is responsible for the outflowing matter.

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arXiv2001

The highly variable X-ray spectrum of the luminous Seyfert 1 galaxy 1H 0419-577

An XMM-Newton observation of the luminous Seyfert 1 galaxy 1H 0419-577 is presented. We find that the spectrum is well fitted by a power law of canonical slope (gamma ~ 1.9) and 3 blackbody components (to model the strong soft excess). The XMM data are compared and contrasted with observations by ROSAT in 1992 and by ASCA and BeppoSAX in 1996. We find that the overall X-ray spectrum has changed substantially over the period, and suggest that the changes are driven by the soft X-ray component. When bright, as in our XMM-Newton observation, it appears that the enhanced soft flux cools the Comptonising corona, causing the 2-10 keV power law to assume a `typical' slope, in contrast to the unusually hard (`photon-starved') spectra observed by ASCA and BeppoSAX four years earlier.

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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

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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arXiv2000

The variable XMM-Newton spectrum of Markarian 766

The narrow-line Seyfert 1 galaxy Markarian 766 was observed for 60 ks with the XMM-Newton observatory. The source shows a complex X-ray spectrum. The 2-10 keV spectrum can be adequately represented by a power law and broad Fe Ka emission. Between 0.7 and 2 keV the spectrum is harder and exhibits a flux deficit with respect to the extrapolated medium energy slope. Below 0.7 keV, however, there is a strong excess of emission. The RGS spectrum shows an edge-like feature at 0.7 keV; the energy of this feature is inconsistent with that expected for an OVII edge from a warm absorber. Markarian 766 varies by a factor of ~ 2 in overall count rate in the EPIC and RGS instruments on a timescale of a few thousand seconds, while no significant flux changes are observed in the ultraviolet with the OM. The X-ray variability is spectrally dependent with the largest amplitude variability occurring in the 0.4-2 keV band. The spectral variability can be explained by a change in flux and slope of the medium energy continuum emission, superimposed on a less variable (or constant) low energy emission component.

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arXiv2000

XMM-Newton monitoring of X-ray variability in the quasar PKS 0558-504

We present the temporal analysis of X-ray observations of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558-504 obtained during the XMM-Newton Calibration and Performance Verification (Cal/PV) phase. The long term light curve is characterized by persistent variability with a clear tendency for the X-ray continuum to harden when the count rate increases. Another strong correlation on long time scales has been found between the variability in the hard band and the total flux. On shorter time scales the most relevant result is the presence of smooth modulations, with characteristic time of ~ 2 hours observed in each individual observation. The short term spectral variability turns out to be rather complex but can be described by a well defined pattern in the hardness ratio-count rate plane.

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arXiv2000

XMM-Newton observation of an unusual iron line in the quasar Markarian 205

XMM-Newton observations of the low luminosity, radio-quiet quasar Markarian 205 have revealed a unique iron K emission line profile. In marked contrast to the broad and redshifted iron K line commonly seen in ASCA observations of Seyfert 1 galaxies, we find that a substantial amount of the line flux in Mrk 205 occurs above the neutral line energy of 6.4 keV. Furthermore, we find that the iron line emission has two distinct components, a narrow, unresolved neutral line at 6.4 keV and a broadened line centred at 6.7 keV. We suggest that the most likely origin of the 6.7 keV line is from X-ray reflection off the surface of a highly ionised accretion disk, whilst the 6.4 keV component may arise from neutral matter distant from the black hole, quite possibly in the putative molecular torus. Overall this observation underlines the potential of XMM-Newton for using the iron K line as a diagnostic of matter in the innermost regions of AGN.

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arXiv2000

XMM-Newton Detection of a Comptonized Accretion Disc in the Quasar PKS0558-504

We present XMM-Newton observations of the bright quasar PKS0558-504. The 0.2-10 keV spectrum is dominated by a large, variable soft X-ray excess. The fastest flux variations imply accretion onto a Kerr black hole. The XMM-Newton data suggest the presence of a `big blue bump' in PKS0558-504 extending from the optical band to ~3 keV. The soft X-ray spectrum shows no evidence for significant absorption or emission-line features. The most likely explanation for the hot big blue bump is Comptonization by the multi-temperature corona of a thermal accretion disc running at a high accretion rate.

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arXiv2000

The first XMM-Newton spectrum of a high redshift quasar - PKS 0537-286

We present XMM-Newton observations of the high redshift z=3.104, radio-loud quasar PKS 0537-286. The EPIC CCD cameras provide the highest signal-to-noise spectrum of a high-z quasar to date. The EPIC observations show that PKS 0537-286 is extremely X-ray luminous (Lx=2x10^47 erg/s), with an unusually hard X-ray spectrum (Gamma=1.27+/-0.02). The flat power-law emission extends over the whole observed energy range; there is no evidence of intrinsic absorption, which has been claimed in PKS 0537-286 and other high z quasars. However, there is evidence for weak Compton reflection. A redshifted iron K line, observed at 1.5 keV - corresponding to 6.15 keV in the quasar rest frame - is detected at 95% confidence. If confirmed, this is the most distant iron K line known. The line equivalent width is small (33eV), consistent with the `X-ray Baldwin effect' observed in other luminous quasars. The reflected continuum is also weak (R=0.25). We find the overall spectral energy distribution of PKS 0537-286 is dominated by the X-ray emission, which, together with the flat power-law and weak reflection features, suggests that the X-radiation from PKS 0537-286 is dominated by inverse Compton emission associated with a face-on relativistic jet.

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