arXiv subjects

D. J. Walton

Publications and source records attributed to D. J. Walton.

193 records · Page 11Linked to original sources

arXiv2012

A blurred reflection interpretation for the intermediate flux state in Mrk 335

As part of a long term monitoring campaign of Mrk 335, deep XMM-Newton observations catch the narrow-line Seyfert 1 galaxy (NLS1) in a complex, intermediate flux interval as the active galaxy is transiting from low- to high-flux. Other works on these same data examined the general behaviour of the NLS1 (Grupe et al.) and the conditions of its warm absorber (Longinotti et al.). The analysis presented here demonstrates the X-ray continuum and timing properties can be described in a self-consistent manner adopting a blurred reflection model with no need to invoke partial covering. The rapid spectral variability appears to be driven by changes in the shape of the primary emitter that is illuminating the inner accretion disc around a rapidly spinning black hole (a > 0.7). While light bending is certainly prominent, the rather constant emissivity profile and break radius obtained in our spectral fitting suggest that the blurring parameters are not changing as would be expected if the primary source is varying its distance from the disc. Instead changes could be intrinsic to the power law component. One possibility is that material in an unresolved jet above the disc falls to combine with material at the base of the jet producing the changes in the primary emitter (spectral slope and flux) without changing its distance from the disc.

astro-ph.HE↗

arXiv2012

Long XMM observation of the Narrow-Line Seyfert 1 galaxy IRAS13224-3809: rapid variability, high spin and a soft lag

Results are presented from a 500ks long XMM-Newton observation of the Narrow-Line Seyfert 1 galaxy IRAS13224-3809. The source is rapidly variable on timescales down to a few 100s. The spectrum shows strong broad Fe-K and L emission features which are interpreted as arising from reflection from the inner parts of an accretion disc around a rapidly spinning black hole. Assuming a power-law emissivity for the reflected flux and that the innermost radius corresponds to the innermost stable circular orbit, the black hole spin is measured to be 0.988 with a statistical precision better than one per cent. Systematic uncertainties are discussed. A soft X-ray lag of 100s confirms this scenario. The bulk of the power-law continuum source is located at a radius of 2-3 gravitational radii.

astro-ph.HE↗

arXiv2012

The nature of the bright ULX X-2 in NGC3921: a Chandra position and HST candidate counterpart

We report on Chandra observations of the bright ultra-luminous X-ray (ULX) source in NGC3921. Previous XMM observations reported in the literature show the presence of a bright ULX at a 0.5-10 keV luminosity of 2x10^40 erg/s. Our Chandra observation finds the source at a lower luminosity of ~8x10^39 erg/s, furthermore, we provide a Chandra position of the ULX accurate to 0.7" at 90% confidence. The X-ray variability makes it unlikely that the high luminosity is caused by several separate X-ray sources. In 3 epochs of archival Hubble Space Telescope (HST) observations we find a candidate counterpart to the ULX. There is direct evidence for variability between the two epochs of WFPC2 F814W observations with the observation obtained in 2000 showing a brighter source. Furthermore, converting the 1994 F336W and 2000 F300W WFPC2 and the 2010 F336W WFC3 observations to the Johnson U-band filter assuming a spectral type of O7I we find evidence for a brightening of the U-band light in 2000. Using the higher resolution WFC3 observations the candidate counterpart is resolved into two sources of similar color. We discuss the nature of the ULX and the probable association with the optical counterpart(s). Finally, we investigate a potential new explanation for some (bright) ULXs as the decaying stages of flares caused by the tidal disruption of a star by a recoiled supermassive black hole. However, we find that there should be at most only 1 of such systems within z=0.08.

astro-ph.HE↗

arXiv2012

Searching for Massive Outflows in Holmberg IX X-1 and NGC 1313 X-1: The Iron K Band

We have analysed all the good quality XMM-Newton data publicly available for the bright ULXs Holmberg IX X-1 and NGC 1313 X-1, with the aim of searching for discrete emission or absorption features in the Fe K band that could provide observational evidence for the massive outflows predicted if these sources are accreting at substantially super-Eddington rates. We do not find statistically compelling evidence for any atomic lines, and the limits that are obtained have interesting consequences. Any features in the immediate Fe K energy band (6-7 keV) must have equivalent widths weaker than ~30 eV for Holmberg IX X-1, and weaker than ~50 eV for NGC 1313 X-1 (at 99 per cent confidence). In comparison to the sub-Eddington outflows observed in GRS 1915+105, which imprint iron absorption features with equivalent widths of ~30 eV, the limits obtained here appear quite stringent, particularly when Holmberg IX X-1 and NGC 1313 X-1 must be expelling at least 5-10 times as much material if they host black holes of similar masses. The difficulty in reconciling these observational limits with the presence of strong line-of-sight outflows suggests that either these sources are not launching such outflows, or that they must be directed away from our viewing angle.

astro-ph.HE↗

arXiv2012

Re-examining the XMM-Newton Spectrum of the Black Hole Candidate XTE J1652-453

The XMM-Newton spectrum of the black hole candidate XTE J1652-453 shows a broad and strong Fe K-alpha emission line, generally believed to originate from reflection of the inner accretion disc. These data have been analysed by Hiemstra et al. (2011) using a variety of phenomenological models. We re-examine the spectrum with a self-consistent relativistic reflection model. A narrow absorption line near 7.2 keV may be present, which if real is likely the Fe XXVI absorption line arising from highly ionised, rapidly outflowing disc wind. The blue shift of this feature corresponds to a velocity of about 11100 km/s, which is much larger than the typical values seen in stellar-mass black holes. Given that we also find the source to have a low inclination (i < 32 degrees; close to face-on), we would therefore be seeing through the very base of outflow. This could be a possible explanation for the unusually high velocity. We use a reflection model combined with a relativistic convolution kernel which allows for both prograde and retrograde black hole spin, and treat the potential absorption feature with a physical model for a photo-ionised plasma. In this manner, assuming the disc is not truncated, we could only constrain the spin of the black hole in XTE J1652-453 to be less than ~ 0.5 Jc/GM^{2} at the 90% confidence limit.

astro-ph.HE↗

arXiv2012

Investigating the reflection contribution to the X-ray emission of Ton S180

There is now growing evidence that the soft X-ray excess is almost ubiquitous among unobscured active galaxies. In spite of the various interpretations that have been considered in the last years, the nature of this foremost spectral feature is not firmly established yet. In this context, we review from a reflection perspective the three highest-quality X-ray observations of the narrow-line Seyfert 1 galaxy Tonantzintla S180, obtained by XMM-Newton and Suzaku. The X-ray spectrum of Ton S180 shows only moderate variations over a time span of several years, suggesting that the same physical process accounts for the bulk of the broad-band X-ray emission at the different epochs, and that the properties of the X-ray source are fairly stable. We have successfully applied in our spectral analysis a dual-reflector model, consisting of two separate components: one arises from the surface of the accretion disc, is highly ionized and blurred by relativistic effects; the other is cold, quite faint, and can be associated with a distant reprocessor. Due to the strength and the nearly power-law shape of its soft excess emission, Ton S180 is one of the most challenging sources to test the X-ray reflection scenario. In this work we provide a clear illustration of the great potential and spectral flexibility of blurred reflection models, while discussing some of their current limitations and possible shortcomings.

astro-ph.HE↗

arXiv2012

The Similarity of Broad Iron Lines in X-ray Binaries and Active Galactic Nuclei

We have compared the 2001 XMM-Newton spectra of the stellar mass black hole binary XTE J1650-500 and the active galaxy MGC-6-30-15, focusing on the broad, excess emission features at ~4--7 keV displayed by both sources. Such features are frequently observed in both low mass X-ray binaries and active galactic nuclei. For the former case it is generally accepted that the excess arises due to iron emission, but there is some controversy over whether their width is partially enhanced by instrumental processes, and hence also over the intrinsic broadening mechanism. Meanwhile, in the latter case, the origin of this feature is still subject to debate; physically motivated reflection and absorption interpretations are both able to reproduce the observed spectra. In this work we make use of the contemporaneous BeppoSAX data to demonstrate that the breadth of the excess observed in XTE J1650-500 is astrophysical rather than instrumental, and proceed to highlight the similarity of the excesses present in this source and MGC-6-30-15. Both optically thick accretion discs and optically thin coronae, which in combination naturally give rise to relativistically-broadened iron lines when the disc extends close to the black hole, are commonly observed in both class of object. The simplest solution is that the broad emission features present arise from a common process, which we argue must be reflection from the inner regions of an accretion disc around a rapidly rotating black hole; for XTE J1650-500 we find spin constraints of 0.84 < a* < 0.98 at the 90 per cent confidence level. Other interpretations proposed for AGN add potentially unnecessary complexities to the theoretical framework of accretion in strong gravity.

astro-ph.HE↗

arXiv2011

X-ray spectral variability in NGC 3783

NGC 3783 was observed for approximately 210ks by Suzaku and in this time showed significant spectral and flux variability at both short (20ks) and long (100ks) time scales. The full observation is found to consist of approximately six "spectral periods" where the behaviour of the soft (0.3-1.0keV) and hard (2-10keV) bands are somewhat distinct. Using a variety of methods we find that the strong warm absorber present in this source does not change on these time scales, confirming that the broad-band variability is intrinsic to the central source. The time resolved difference-spectra are well modelled with an absorbed powerlaw below 10keV, but show an additional hard excess at ~20keV in the latter stages of the observation. This suggests that, in addition to the variable powerlaw, there is a further variable component that varies with time but not monotonically with flux. We show that a likely interpretation is that this further component is associated with variations in the reflection fraction or possibly ionization state of the accretion disk a few gravitational radii from the black hole.

astro-ph.HE↗

arXiv2011

2XMM Ultraluminous X-Ray Source Candidates in Nearby Galaxies

Ultraluminous X-ray sources (ULXs) are some of the most enigmatic X-ray bright sources known to date. It is generally accepted that they cannot host black holes as large as those associated with active galaxies, but they appear to be significantly more luminous than their better understood Galactic X-ray binary (XRB) cousins, while displaying an intriguing combination of differences and similarities with them. Through studying large, representative samples of these sources we may hope to enhance our understanding of them. To this end, we derive a large catalogue of 650 X-ray detections of 470 ULX candidates, located in 238 nearby galaxies, by cross correlating the 2XMM Serendipitous Survey with the Third Reference Catalogue of Bright Galaxies. The presented dedicated catalogue offers a significant improvement over those previously published both in terms of number and the contribution of background contaminants, e.g. distant quasars, which we estimate to be at most 24 per cent, but more likely ~17 per cent. To undertake population studies, we define a `complete' sub-sample of sources compiled from observations of galaxies with sensitivity limits below 10^39 erg/s. The luminosity function of this sample is consistent with a simple power-law. Although we do not find any statistical requirement for a high luminosity cut-off, as has been reported previously, we are not able to rule out its presence. Also, we find that the number of ULXs per unit galaxy mass decreases with increasing galaxy mass for ULXs associated with spiral galaxies, in broad agreement with previous results, but is consistent with being constant with galaxy mass for sources associated with elliptical galaxies.

astro-ph.HE↗

arXiv2011

Comparing Spectral Models for ULXs with NGC 4517 ULX1

We present the previously unanalysed high quality XMM-Newton spectrum of an ultraluminous X-ray source candidate in NGC 4517. As with other high quality ULX spectra, a downturn in the spectrum is observed at ~6 keV. Both of the recent disc reflection and Comptonisation interpretations of this feature are applied, in order to present a direct comparison, and are found to provide statistically equivalent representations of the current data. We find that the reflection model requires the accretion disc to have a highly super-solar iron abundance, while the Comptonisation model requires low temperature Comptonising electrons, and for the corona to be optically thick. These physical requirements are discussed in detail, and physically motivated scenarios are highlighted in which each model can be considered a viable explanation for the observed emission. By extending our consideration of these two interpretations to high energies, we demonstrate that observations of ULXs at energies above ~10 keV should be extremely useful when attempting to distinguish between them. With current instrumentation, it is only viable to perform these observations for M82 X-1, but future high angular resolution hard X-ray imaging spectrometers, such as the Hard X-ray Imaging System due to fly on Astro-H, should go a long way to resolving this issue.

astro-ph.HE↗

arXiv2010

A Large Catalogue of Ultraluminous X-ray Source Candidates in Nearby Galaxies

Since their discovery, Ultraluminous X-ray sources (ULXs) have attracted attention due to their combination of extreme luminosities and extra-nuclear locations. However, they are a fairly rare phenomenon, and attempts to investigate the general properties of the population have been hindered by a relative lack of known sources. Here, we present a large catalogue of ULX candidates including 655 detections of 475 discrete sources, based on the 2XMM Serendipitous Survey. To demonstrate the potential of such a resource, we present some scientific analysis of this population, focusing on the spectral turnover seen, often at ~6 keV, in the highest quality ULX data. We also demonstrate how the recent reflection and Comptonisation interpretations of this feature may be distinguished observationally in the future, specifically using NGC 4517 ULX1, a previously unanalysed source with high quality data, as an example case.

astro-ph.HE↗

arXiv2010

A reflection origin for the soft and hard X-ray excess of Ark 120

Over the last few years several models have been proposed to interpret the widespread soft excess observed in the X-ray spectra of type 1 active galactic nuclei (AGN). In particular, reflection from the photoionized accretion disc blurred by relativistic effects has proven to be successful in reproducing both the spectral shape and the variability pattern of many sources. As a further test to this scenario we present the analysis of a recent ~100 ks long Suzaku observation of Arakelian 120, a prototypical 'bare' Seyfert 1 galaxy in which no complex absorption system is expected to mimic a soft excess or mask the intrinsic properties of this key component. We show that a reflection model allowing for both warm/blurred and cold/distant reprocessing provides a self-consistent and convincing interpretation of the broadband X-ray emission of Ark 120, also characterized by a structured iron feature and a high-energy hump. Although warm absorbers, winds/outflows and multiple Comptonizing regions may play significant roles in sources with more spectral complexity, this case study adds evidence to the presence of blurred disc reflection as a basic component of the X-ray spectra of type 1 AGN.

astro-ph.HE↗

arXiv2010

Explaining the Hard Excesses in AGN

A common model invoked to describe the X-ray spectra of active galaxies includes a relativistically blurred reflection component, which in some cases can be the dominant contributor to the received flux. Alternative interpretations are often based around complex absorption, and to date it has proven difficult to determine between these two viable models. Recent works on SUZAKU observations of the active nuclei in NGC 1365, 1H 0419-577 and PDS 456 have found the presence of strong X-ray emission at high (~10-50 keV) energies, referred to as 'hard excesses', and it has been claimed this emission cannot be explained with simple disc reflection models. Here we investigate the high energy emission in these sources by constructing disc reflection models and show that they can successfully reproduce the observed spectra. In addition, we find the behaviour of NGC 1365 and 1H 0419-577 in these observations to be broadly consistent with previous work on disc reflection interpretations.

astro-ph.HE↗