arXiv subjects

P. T. O'Brien

Publications and source records attributed to P. T. O'Brien.

At least 163 records · Page 9Linked to original sources

arXiv2004

XMM-Newton observations of high-luminosity radio-quiet QSOs

XMM Newton observations of five high-luminosity radio-quiet QSOs (Q 0144-3938, UM 269, PG 1634+706, SBS 0909+532 and PG 1247+267) are presented. Spectral energy distributions were calculated from the XMM-Newton EPIC (European Photon Imaging Camera) and OM (Optical Monitor) data, with bolometric luminosities estimated in the range from 7 x 10^45 to 2 x 10^48 erg s^-1 for the sample, peaking in the UV. At least four of the QSOs show a similar soft excess, which can be well modelled by either one or two blackbody components, in addition to the hard X-ray power-law. The temperatures of these blackbodies (~100-500 eV) are too high to be direct thermal emission from the accretion disc, so Comptonization is suggested. Two populations of Comptonizing electrons, with different temperatures, are needed to model the broad-band spectrum. The hotter of these produces what is seen as the hard X-ray power-law, while the cooler (~0.25-0.5 keV) population models the spectral curvature at low energies. Only one of the QSOs shows evidence for an absorption component, while three of the five show neutral iron emission. Of these, PG 1247+267 seems to have a broad line (EW ~ 250 eV), with a strong, associated reflection component (R ~ 2), measured out to 30 keV in the rest frame of the QSO. Finally, it is concluded that the X-ray continuum shape of AGN remains essentially constant over a wide range of black hole mass and luminosity.

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arXiv2004

XMM-Newton observations of seven soft X-ray excess QSOs

XMM-Newton observations of seven QSOs are presented and the EPIC spectra analysed. Five of the AGN show evidence for Fe K-alpha emission, with three being slightly better fitted by lines of finite width; at the 99 per cent level they are consistent with being intrinsically narrow, though. The broad-band spectra can be well modelled by a combination of different temperature blackbodies with a power-law, with temperatures between kT ~ 100-300 eV. On the whole, these temperatures are too high to be direct thermal emission from the accretion disc, so a Comptonization model was used as a more physical parametrization. The Comptonizing electron population forms the soft excess emission, with an electron temperature of ~ 120-680 eV. Power-law, thermal plasma and disc blackbody models were also fitted to the soft X-ray excess. Of the sample, four of the AGN are radio-quiet and three radio-loud. The radio-quiet QSOs may have slightly stronger soft excesses, although the electron temperatures cover the same range for both groups.

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arXiv2004

A very low luminosity X-ray flash: XMM-Newton observations of GRB 031203

GRB031203 was observed by XMM-Newton twice, first with an observation beginning 6 hours after the burst, and again after 3 days. The afterglow had average 0.2-10.0keV fluxes for the first and second observations of 4.2+/-0.1x10^-13 and 1.8+/-0.1x10^-13 erg/cm^2/s respectively, decaying very slowly according to a power-law with an index of -0.55+/-0.05. The prompt soft X-ray flux, inferred from a detection of the dust echo of the prompt emission, strongly implies that this burst is very soft and should be classified as an X-ray flash (XRF) and further, implies a steep temporal slope (<~-1.7) between the prompt and afterglow phases or in the early afterglow, very different from the later afterglow decay slope. A power-law (Gamma=1.90+/-0.05) with absorption at a level consistent with the Galactic foreground absorption fits the afterglow spectrum well. A bright, low-redshift (z=0.105) galaxy lies within 0.5 arcsec of the X-ray position and is likely to be the GRB host. At this redshift, GRB031203 is the closest GRB or XRF known after GRB980425. It has a very low equivalent isotropic gamma-ray energy in the burst (~3x10^49 erg) and X-ray luminosity in the afterglow (9x10^42 erg/s at 10 hours), 3-4 orders of magnitude less than typical bursts, though higher than either the faint XRF020903 or GRB980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB980425, indicating that GRB031203 may be representative of low luminosity bursts.

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arXiv2004

The discovery of an evolving dust scattered X-ray halo around GRB 031203

We report the first detection of a time-dependent, dust-scattered X-ray halo around a gamma-ray burst. GRB 031203 was observed by XMM-Newton starting six hours after the burst. The halo appeared as concentric ring-like structures centered on the GRB location. The radii of these structures increased with time as t^{1/2}, consistent with small-angle X-ray scattering caused by a large column of dust along the line of sight to a cosmologically distant GRB. The rings are due to dust concentrated in two distinct slabs in the Galaxy located at distances of 880 and 1390 pc, consistent with known Galactic features. The halo brightness implies an initial soft X-ray pulse consistent with the observed GRB.

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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

Gamma-Ray Bursts observed by XMM-Newton

Analysis of observations with XMM-Newton have made a significant contribution to the study of Gamma-ray Burst (GRB) X-ray afterglows. The effective area, bandpass and resolution of the EPIC instrument permit the study of a wide variety of spectral features. In particular, strong, time-dependent, soft X-ray emission lines have been discovered in some bursts. The emission mechanism and energy source for these lines pose major problems for the current generation of GRB models. Other GRBs have intrinsic absorption, possibly related to the environment around the progenitor, or possible iron emission lines similar to those seen in GRBs observed with BeppoSAX. Further XMM-Newton observations of GRBs discovered by the Swift satellite should help unlock the origin of the GRB phenomenon over the next few years.

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arXiv2003

XMM-Newton observations of 3C 273

A series of nine XMM-Newton observations of the radio-loud quasar 3C 273 are presented, concentrating mainly on the soft excess. Although most of the individual observations do not show evidence for iron emission, co-adding them reveals a weak, broad line (EW ~ 56 eV). The soft excess component is found to vary, confirming previous work, and can be well fitted with multiple blackbody components, with temperatures ranging between ~40 and ~330 eV, together with a power-law. Alternatively, a Comptonisation model also provides a good fit, with a mean electron temperature of ~350 eV, although this value is higher when the soft excess is more luminous over the 0.5-10 keV energy band. In the RGS spectrum of 3C 273, a strong detection of the OVII He-alpha absorption line at zero redshift is made; this may originate in warm gas in the local intergalactic medium, consistent with the findings of both Fang et al. (2003) and Rasmussen et al. (2003).

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arXiv2003

The X-ray, optical and radio evolution of the GRB030329 afterglow and the associated SN2003dh

Extensive X-ray, optical and radio observations of the bright afterglow of the Gamma Ray Burst GRB 030329 are used to construct the multi-frequency evolution of the event. The data are fitted using the standard fireball shock model to provide estimates of the initial energy, epsilon = 6.8 x 10^52 ergs sr^-1, the density of the ambient medium, n_0 = 1 cm^-3, the electron and magnetic energy density fractions, epsilon_e = 0.24 & epsilon_B = 0.0017, the power law index of the relativistic electron spectrum, p = 2.25, and the opening angle of the jet, theta_j = 3 degrees. Deviations from the standard model seen in the optical and radio are most likely attributable to the concurrent hypernova SN2003dh. Peaks at 0.23 and 1.7 days in the R-band are much brighter than expected from a standard SN, and there is a large radio excess over the expected afterglow flux for t>2 days. No deviation from the best-fit afterglow model is seen in the X-ray decline, indicating that the excess optical and radio flux from 1-5 days arises from a later injection of slower electrons by the central engine.

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arXiv2003

XMM-Newton observation of the Seyfert 1 ESO 198-G24

We present the results from an XMM-Newton observation (January 24, 2001) of the bright Seyfert 1 galaxy ESO 198-G24 (z=0.045). We found that this Seyfert has an intrinsic 2-10 keV luminosity of about 10^44 erg s-1. This source shows no intrinsic absorption in addition to the Galactic absorption (N_{H}~3 x 10^20 cm-2). We found both with EPIC and RGS that this source possesses significantly steeper spectra below ~ 1.5-2 keV than observed at higher X-ray energies, the so-called soft X-ray excess. The RGS spectra reveal no significant narrow absorption lines suggesting that if there is a warm absorber, it either has a relatively low column density, or a very high ionization parameter. The RGS data are well described by the combination of a power-law, a modified black body continuum, and weak relativistic lines of O VIII, and C VI Lyalpha. However other interpretations are not definitely excluded. The 2-10 keV energy band is well fitted by an absorbed power-law with a photon spectral index of Gamma=1.77 +/- 0.04 (consistent with the typical Gamma~1.7 found in Seyfert 1 galaxies). We found the presence of a narrow Gaussian emission line at 6.41 keV (i.e < Fe XVII) with a moderate equivalent width of about 60-70 eV, and we found an upper limit for a broad component, if any, of 75 eV. We also found a weak absorption edge associated with cold iron with an optical depth of about 0.2.

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arXiv2003

An X-ray Baldwin effect for the narrow Fe K-alpha lines observed in Active Galactic Nuclei

The majority of Active Galactic Nuclei (AGN) observed by XMM-Newton reveal narrow Fe K-alpha lines at ~ 6.4 keV, due to emission from cold (neutral) material. There is an X-ray Baldwin effect in Type I AGN, in that the equivalent width of the line decreases with increasing luminosity, with weighted linear regression giving EW ~ L^{-0.17+/-0.08} (Spearman Rank probability of > 99.9%). With current instrumental capabilities it is not possible to determine the precise origin for the narrow line, with both the Broad Line Region and putative molecular torus being possibilities. A possible explanation for the X-ray Baldwin effect is a decrease in covering factor of the material forming the fluorescence line.

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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

Detection of type-2 quasars in the radio galaxies B3 0731+438 and 3C 257

We present XMM-Newton observations and spectral fitting of two highly redshift, [OIII]-luminous, narrow-line radio galaxies, B3 0731+438 and 3C 257. Their X-ray continua are well fitted by a partial covering model with intrinsically unabsorbed and absorbed powerlaw components. The spectral models indicate that both objects harbour highly obscured nuclei, with N_H ~ 0.5 - 2 x 10^23 cm^-2. Correcting for this absorption we find large intrinsic luminosities in the range L_X ~ 0.2 - 1 x 10^45 erg/s. Thus, both sources are type-2 quasars.

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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

An XMM-Newton observation of the Narrow Line Seyfert 1 Galaxy, Markarian 896

XMM-Newton observations of the NLS1 Markarian 896 are presented. Over the 2-10 keV band, an iron emission line, close to 6.4 keV, is seen. The line is just resolved and has an equivalent width of ~170 eV. The broad-band spectrum is well modelled by a power law slope of gamma ~ 2.03, together with two blackbody components to fit the soft X-ray excess. Using a more physical two-temperature Comptonisation model, a good fit is obtained for an input photon distribution of kT ~ 60eV and Comptonising electron temperatures of ~0.3 and 200 keV. The soft excess cannot be explained purely through the reprocessing of a hard X-ray continuum by an ionised disc reflector.

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arXiv2002

The X-ray afterglow of GRB 020322

The spectrum of the afterglow of GRB 020322 is the highest-quality X-ray spectrum of a GRB afterglow available to date. It was detected by XMM-Newton in an observation starting fifteen hours after the GRB with a mean 0.2--10.0 keV observed flux of 3.5+/-0.2x10^-13 erg cm^-2 s^-1, making it the brightest X-ray afterglow observed so far with XMM-Newton. The source faded; its lightcurve was well fit by a power-law with a decay index of 1.26+/-0.23. The spectrum is adequately fit with a power-law absorbed with neutral or ionised gas significantly in excess of the foreground Galactic column, at redshift 1.8_{-1.1}^{+1.0} or with low metal abundances. No spectral line or edge features are detected at high significance, in particular, a thermal emission model fits the data poorly, the upper limit on its contribution to the spectrum is 3.7x10^-14 erg cm^-2 s^-1, or at most ~10% of the total flux. No spectral variability is observed.

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arXiv2002

Serendipitous AGN in the XMM-Newton fields of Markarian 205 and QSO 0130-403

The X-ray spectra of serendipitously observed AGN in the XMM-Newton fields of Mrk 205 and QSO 0130-403 are analysed. The sample consists of 23 objects, none of which is detected at radio frequencies, with a median X-ray luminosity of ~ 4 x 10^44 erg s^-1 and redshifts ranging from ~0.1 to just over 3. The mean photon index was found to be 1.89 +/- 0.04. In contrast to past ASCA and ROSAT observations of high-redshift radio-loud quasars, we find little evidence for excess intrinsic absorption in these radio-quiet objects, with only three sources requiring a column density in excess of the Galactic value. Comparing the measured spectral indices over the redshift range, we also find there is no X-ray spectral evolution of QSOs with time, up to redshift 3. Within the sample there is no evidence for evolution of the optical to X-ray spectral index, alpha-ox, with redshift, the mean value being -1.66 +/- 0.04. However, upon comparing the values from the Bright Quasar Survey at low redshift (z < 0.5) and high redshift QSOs detected by Chandra (z > 4), a slight steepening of alpha-ox is noted for the more distant objects. In most of the sources there is no significant requirement for a soft excess, although a weak thermal component (< 10% of Lx) cannot be excluded. There is an indication of spectral flattening (by delta gamma=0.2) at higher energies (> 3 keV, QSO rest frame) for the sample as a whole. This is consistent with the presence of a Compton reflection component in these radio-quiet AGN, with the scattering medium (such as an accretion disc or molecular torus) occupying a solid angle of 2 pi steradians to the X-ray source.

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arXiv2002

The X-ray spectrum of the Seyfert I galaxy Mrk 766: Dusty Warm Absorber or Relativistic Emission Lines?

Competing models for broad spectral features in the soft X-ray spectrum of the Seyfert I galaxy Mrk766 are tested against data from a 130 ks XMM-Newton observation. A model including relativistically broadened Ly-alpha emission lines of OVIII, NVII and CVI is a better fit to 0.3-2 keV XMM RGS data than a dusty warm absorber. Moreover, the measured depth of neutral iron absorption lines in the spectrum is inconsistent with the magnitude of the iron edge required to produce the continuum break at 17-18Angstroms in the dusty warm absorber model. The relativistic emission line model can reproduce the broad-band (0.1-12 keV) XMM-EPIC data with the addition of a fourth line to represent emission from ionized iron at 6.7 keV and an excess due to reflection at energies above the iron line. The profile of the 6.7 keV iron line is consistent with that measured for the low energy lines. There is evidence in the RGS data at the 3sigma level for spectral features that vary with source flux. The covering fraction of warm absorber gas is estimated to be ~12%. Iron in the warm absorber is found to be overabundant with respect to CNO compared to solar values.

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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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