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This page written by Graziella Branduardi-Raymont (gbr@mssl.ucl.ac.uk).
Last modified 2nd July 2006
Talks abstracts
| Session 1: Clusters, galaxies and AGN | |||||||
Matt Hilton (Liverpool John Moores)
The XMM Cluster Survey
The XMM Cluster Survey (XCS) is an ambitious project designed to extract over 1,000 clusters from the XMM-Newton Science Archive. The goals of XCS are: (1) to measure cosmological parameters, (2) to study the evolution of the properties of X-ray gas, and (3) to study the evolution of the cluster galaxy population. In this talk I describe the discovery by XCS of the most distant cluster of galaxies found to date - XMMXCS J2215.9-1738 at z=1.45 (Stanford et al. 2006). I also provide an update on the ongoing NOAO-XCS (NXS) optical-follow-up survey, aimed at providing photometric redshifts for clusters in ~330 XMM pointings.
Tracing gas motions in the Centaurus Cluster
Determining the scale and velocities of gas motions in cluster cores is of critical importance in probing the microphysics of these environments and thus in understanding the heat balance in cool-core clusters. In the absence of direct measurements of the relevant velocity scales, it is necessary to use an indirect means. We have applied a method developed by Rebusco et. al. (2005), in which the observed iron distribution is used as a tracer of the underlying gas motions, to the special case of the Centaurus cluster, which is unusual in showing a highly super-solar central abundance. Our work suggests that the level of turbulent motion is about a factor of 5 lower than that found by Rebusco et. al. for the Perseus cluster and provides insight into the degree to which turbulent heating may be operating in this cluster.
The evolution of the ISM in post-merger galaxies
Massive halos of hot plasma exist around some, but not all, elliptical galaxies. There is evidence that this is related to the age of the galaxy. New X-ray observations are presented for three early-type galaxies that show evidence of youth in their stellar populations (from optical spectra). These results are set in context with other data drawn from the literature, for galaxies with average ages estimated from dynamical or spectroscopic indicators. We examine the X-ray evolution with age, normalised by B and K band luminosities, both during and after a major galaxy merger. Low values of Log(LX/LB) and Log(LX/LK) are found for all galaxies with ages between 1 and 4 Gyr. It is remarkable that luminous X-ray emission only appears in older galaxies. We discuss possible reasons for this apparent delay in producing the hot gas halo, in terms of feedback, and possible future tests. The ISM properties are also dependant on the distribution of mass in elliptical galaxies, which is hotly debated. Here we set limits on the mass distribution in one of these galaxies observed with XMM-Newton, NGC 4382, which contains significant hot gas.
Relating accretion rate and jet power in elliptical galaxies
Using Chandra X-ray observations of nine nearby, X-ray luminous elliptical galaxies with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi accretion rates calculated from the observed gas temperature and density profiles and estimated black hole masses, and the power emerging from these systems in relativistic jets. Our results show that a significant fraction of the energy associated with the rest mass of material entering the Bondi accretion radius (around 2%) eventually emerges in the relativistic jets. We discuss the implications of this result on accretion flows and the environment surrounding the galaxies.
AGN heating in cores of galaxy groups
We present gas temperature, density, entropy and cooling time profiles for the cores of a sample of 15 galaxy groups observed with Chandra. We find that the entropy profiles follow a power-law profile down to very small fractions of R_500. We also find no significant differences between the gas profiles of groups with radio loud BGGs and those with radio quiet BGGs. There is only a small difference between the L_X:T_X relations for the centres of the radio-loud and radio-quiet objects in our sample, in contrast to the much larger difference found on scales of the whole group in earlier work. We argue that, to first-order, radio-sources do not have a significant effect on the gas in the cores of groups. However, as a second-order effect, AGN do displace the gas on a local level and may prevent catastrophic cooling in group cores.
AGN line-width and black hole timing
For some years we have been measuring the break timescales, Tb, seen in the X-ray powerspectra of AGN and comparing them with those seen in Galactic Black Hole X-ray binary systems (GBHs). It is also known that Tb scales approximately with mass (M) but with considerable spread. Here we show that, in fact, Tb ~M/mdot (mdot=accretion rate), thereby explaining much of the uncertainty in the use of Tb as a mass indicator. We also show that the width of the permitted optical lines (eg Hbeta) in AGN, which has been known for some time to be smaller in more variable AGN, is much more strongly correlated with Tb than with any other indicator of variability, eg rms X-ray variability or fractional optical variability. We find that Tb~V^4. Using simple physics, ie the locally optimised condition for the production of optical emission lines, and simple virial motion, we expect that V^4 ~M/mdot. Thus we link X-ray timing with AGN linewidth and show that the simple assumptions underlying our explanation of the Tb~V^4 relationship are correct and, moreover, apply in broad as well as narrow-line AGN. Thus the so-called Narrow Line Seyfert 1 galaxies aren't any different to other AGN except that they have smaller ratios of M/mdot. We don't require particular orientations (eg pole-on) to explain the narrow lines and their observed small masses are simply a selection effect as mdot cannot easily exceed the Eddington limit.
| Session 2: AGN, galaxies and un-identified objects | |||||||
X-ray and optical variability of AGN as observed with XMM-Newton
The Optical Monitor onboard XMM-Newton allows simultaneous X-ray and optical/UV observations of sources to be made. This capability provides the opportunity to examine variability in different wavebands of AGN. Using a sample of 8 nearby Seyfert 1 galaxies from the XMM-Newton Science Archive, we examine X-ray and optical/UV lightcurves to determine whether there is a connection between the emission regions of the two bands e.g. reprocessing of X-ray to optical photons.
Bolometric corrections for AGN and studies of the X-ray background
The X-ray background (XRB) is understood to be the total emission from AGN. The XRB intensity can in principle be used to estimate the supermassive black hole density in the universe as detailed by Soltan (1982), and in the process it is possible to infer important parameters such as the typical AGN accretion efficiency and Eddington luminosity. However, in scaling up the XRB intensity to a bolometric intensity due to accretion in AGN, the bolometric correction factor required is the source of many uncertainties. Following on from the work of Elvis et al (1994) and later studies such as Marconi et al (2004), I present an observational study into the variation of the bolometric correction in a sample of AGN, along with a discussion of the systematic effects identified so far and their potential implications for XRB studies.
Antonis Georgakakis (Imperial College)
The environment of AGN at z~1
I will present results on the environment of X-ray selected AGN at z~1 using data from the Extended Groth strip International Survey (AEGIS) which combines (among others) deep (200ks per field) wide-area (0.5deg2) Chandra observations with extensive optical spectroscopy to R~24mag. The 3-D information is exploited to quantify the local density in the vicinity of an X-ray source by measuring the projected surface density of galaxies within a radius defined by the 3rd nearest neighbour. The main result is that X-ray selected AGN at z~1 avoid underdense regions at the 99.89% confidence level. Moreover, although the overall AGN population shares the same (rich) environment with optical galaxies of similar U-B and M_B, there is also tentative evidence (96%) that AGN with blue host-galaxy colors (U-B<1) reside in denser environments compared to optical galaxies. The results above can be understood as a consequence of the whereabouts of massive galaxies, capable of hosting supermassive black holes at their centers, with available cold gas reservoirs, the fuel for AGN activity. At z~1 an increasing fraction of such systems are found in dense regions.
The evolution and demographics of obscured AGN
The evolution and demographics of the obscured AGN population remain poorly constrained. However, an understanding of this population over a wide range of redshift and luminosity space is key to reconstructing the Universe's accretion history. The best way to constrain AGN absorption properties is by examining the extragalactic X-ray population. In the very deep XMM-Newton observations of the CDF-S field, there are >300 X-ray detections, 84 of which have redshift estimates, and nearly all of which are AGN. The absorbing column densities and intrinsic luminosities of these AGN are determined using a Monte-Carlo method, which exploits the wide bandpass, and high thoughput of XMM-Newton. A detailed simulation scheme is then used to compare the X-ray properties of the sources with the predictions of a number of theoretical AGN population models. We find that the observed AGN absorption distribution exhibits no strong dependence on either luminosity or redshift. The best matching population model contains AGN having a wide range of absorbing columns, with the heavily absorbed AGN outnumbering the unabsorbed objects by 3:1. In particular, our results are consistent with there being a large population of luminous but heavily absorbed AGN at high redshift. We discuss the implications for AGN torus models, and contrast our results to some recent Chandra surveys which find a much smaller population of luminous absorbed AGN.
XMM-Newton observations of unidentified gamma-ray objects
The majority of the galactic gamma-ray sources discovered by GRO/EGRET are still waiting for identification. While most of them are suspected to be associated with isolated neutron stars, the only certified class of galactic gamma-ray sources, straight identification via gamma-ray timing is prevented by the presently low count statistics and by the uncertain source position which hampers precise timing analysis. Until the launch of GLAST, the only viable identification strategy is through dedicated multi-wavelength observing campaigns. Here I present the strategy and the results of XMM-Newton and ESO X-ray/optical follow-up observations of two unidentified gamma-ray sources. In both cases, interesting neutron star candidates have been found.
Robin Barnard (Open University)
Artificial variability in XMM-Newton observations of M31 X-ray sources, and beyond
Power density spectra (PDS) that are characteristic of low mass X-ray binaries (LMXBs) have been previously reported for M31 X-ray sources observed by XMM-Newton. However, we have recently discovered that these PDS result from the improper addition/subtraction of non-simultaneous lightcurves. The lightcurves produced by the XMM-Newton Science Analysis Software (SAS) package evselect are non-synchronised by default. This affects not only the combination of lightcurves from the three EPIC detectors (MOS1, MOS2 and pn), but also background subtraction in the same CCD. It is therefore imperative that all SAS-generated lightcurves are synchronised by time filtering, even if the whole observation is to be used. While previous timing results from M31 have been proved wrong, and also the broken power law PDS in NGC 4559 ULX-7, XMM-Newton was able to detect aperiodic variability in just 3 ks of observations of NGC 5408 ULX1. Hence XMM-Newton remains a viable tool for analysing variability in extra-galactic X-ray sources.
| Session 3: Binary objects | |||||||
Catherine Brocksopp (UCL/MSSL)
The 2005 Outburst of GRO J1655-40 - Swift observations of a black hole X-ray transient
After nearly a decade of quiescence, the black hole X-ray transient, GRO J1655-40, became active again in 2005 February. This was perfectly timed for the new Swift satellite, which has monitored the outburst using all three instruments on board. Therefore we have obtained X-ray spectroscopy for the energy ranges 0.3-10 keV and 15-150 keV, plus optical and ultraviolet photometry (in U, B, V, UVW1, UVM2, UVW2 filters). We use these data to study the broad-band spectral evolution of the outburst. In particular we find that comparison of the lightcurves at different bands reveals different components to the variability. Preliminary, qualitative study suggests that these components are related to the power-law and disc-blackbody, commonly used to model the X-ray spectra, and therefore reflect the behaviour of the jet/corona and accretion disc respectively.
The nature of the close magnetic white dwarf + brown dwarf binary SDSS J121209.31+013627.7
The magnetic white dwarf SDSS J121209.31+013627.7 has a close, cool companion - most likely a brown dwarf - in a ~90 minute orbit. Schmidt et al. (2005) suggested the system is a progenitor of a magnetic cataclysmic variable (Polar), since they found no obvious evidence for ongoing accretion. We present optical light curves obtained with Ultracam and the Faulkes Telescope North that unexpectedly resemble those of Polars in a low state, with a hot spot on the surface of the white dwarf. We also present an X-ray spectrum obtained with Swift which confirms that the white dwarf is indeed accreting, at a low rate of 10e-12 to 10e-13 solar masses per year.
Hidden magnetic accretors in Cataclysmic Variables: latest results from XMM-Newton
Cataclysmic variables (CVs) are close binary stars in which a white dwarf accretes material from a Roche-lobe filling late-type companion. If the magnetic field of the white dwarf is strong enough, it can dominate the accretion flow from the inner accretion disk. A surprising result emerged from our analysis of the entire sample of non-magnetic cataclysmic variables observed with ASCA (Baskill, Wheatley & Osborne, 2005, MNRAS, 358, 626). We found two extremely hard systems that appeared spectrally distinct from the rest of the sample (these are V426 Oph & LS Peg). These are probably weakly magnetic accretors, but there is no strong evidence of any periodic variability - currently the only accepted method of identifying such a system. We have now performed follow-up observations of both these targets with XMM-Newton, in order to discover the true nature of these objects. In this talk, I present the fascinating results of both the 45ks observation of LS Peg (observed in June 2005), and the 37ks observation of V426 Oph (March 2006).
Ultra-compact binaries
Binary systems which have orbital periods less than ~70 mins must have a degenerate or semi-degenerate secondary star. Those binaries which are composed of two white dwarfs are called ultra-compact binaries or AM CVn systems. Their astrophysical importance lies in the fact that they are predicted to be strong sources of gravitational radiation and are expected to be numerous in our galaxy. I will present recent X-ray and UV observations of these systems, with emphasis on the 2 most compact binary systems, RX J0806+15 (orbital period 321 sec) and RX J1914+24 (569 sec).
Michael Bode (Liverpool John Moores)
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi
The recurrent nova RS Ophiuchi underwent its latest outburst in February this year - its first since 1985. The central system comprises a white dwarf near the Chandrasekhar mass limit in orbit with a late-type giant. As in Classical Novae, outbursts arise from a thermonuclear runaway (TNR) on the WD surface. High velocity ejecta then impact the pre-existing red giant wind setting up shock systems. Swift XRT observations began only 3 days after outburst and we detected a rapidly evolving, very bright X-ray source. Subsequently, we found that the outburst itself had been detected in the Swift BAT and we later deployed the Swift UV and optical grisms. For the first month, our X-ray data were consistent with the evolution of shock systems analogous to that in supernova remnants, but evolving over months rather than millennia. Subsequently, there was a rapid brightening of a super-soft source, which was at first highly variable, and that may be ascribed to the emergence of emission from the the TNR in the X-ray band. The Swift observations provide an unrivalled data set on this type of explosion and allow us to investigate the outburst itself and the environment into which it has occured in unprecedented detail.