New results in X-ray astronomy 2009


List of submitted posters

David Alexander, Durham University

Black-hole growth and Feedback in a Distant Protocluster

I will present the latest results from the z=3.09 Chandra Deep Protocluster survey in the SSA22 field, where we have found evidence for enhanced black-hole growth and evidence for feedback between AGNs and the larger-scale protocluster environment.

Heather Campbell, ICG, Portsmouth


Naomi Dubois, University of Sussex

Modelling the Point Spread Function of XMM-Newton

The point spread function of XMM-Newton is not currently well characterised. The most accurate current model is one-dimensional and spherical, whereas the actual PSF becomes highly elongated at the edge of the field of view. The development of a new 2D model, an elliptical beta model, is attempted here by fitting to XMM images of selected point sources from the XCS catalogue (>137,000 total sources). We present the results of these fits and attempt to quantify how accurately the resulting model represents the XMM PSF.

Poshak Gandhi, Riken

Very fast optical activity in X-ray binaries: discovery and implications

In one of the fastest multi-wavelength timing studies of black hole X-ray binaries (BHBs) to date, we have found correlated optical and X-ray variability in the low/hard state of two sources: GX 339-4 and SWIFT J1753.5-0127. Our simultaneous VLT/Ultracam and RXTE data reveal intriguing patterns with characteristic peaks, dips and lags down to the shortest timescales probed (~50 milli-seconds). Such rapid optical flickering is very rare amongst BHBs. Reprocessing can be ruled out as the origin of the aperiodic optical power. Instead, synchrotron emission from the inner accretion flow regions and jet seems to drive the fluctuations. We also show that both the optical and X-ray light curves are intrinsically non-linear, in the sense that the absolute source variability r.m.s. amplitude linearly increases with flux, and that the flares have a log-normal distribution. The implication is that variability at both wavelengths is not due to local variations alone, but rather arises as a result of coupling of perturbations over a wide range of radii and timescales. These 'optical and X-ray rms-flux relations' thus provide fundamental new constraints connecting the outer disc with the inner flow and jet.

Jonathan Gelbord, Durham University

X-ray selected active galaxies from the Sloan Digital Sky Survey

We have established a sample of low redshift active galaxies selected by a cross correlation between the SDSS spectroscopic catalog and the 2XMMi, including over 430 Seyfert galaxies at z < 0.40. We use the SDSS data for both spectroscopic classifications and to identify weak spectral features. In the optical band, we fit models to the emission features, including high-ionization lines (such as [Fe X] 6374, with an ionization potential of 234 eV). To interpret the X-ray spectra, we are developing a pipeline to fit a sequence of progressively more complex models to the XMM EPIC data. We will present a status report on this programme including preliminary results on the X-ray and optical properties of the sample, paying particular attention to the correlations between optical high-ionization lines and soft X-ray features.

Martin Hardcastle, University of Hertfordshire

The X-ray nuclei of powerful radio sources

A large fraction of the 3CRR sample of radio-loud AGN has been observed with Chandra and XMM-Newton. We present some highlights from a series of papers, including the nature of accretion in radio-loud AGN, the relationship between X-ray and mid-IR emission, the evidence for a jet-related nuclear X-ray component, and the distribution of absorption columns in narrow-line radio galaxies.

Floyd Jackson, Durham University Physics Department

Revealing X-ray luminous AGN in apparent starburst galaxies

It is commonly assumed in deep survey work that all galaxies with X-ray luminosities above about 1042 erg s-1 harbour AGN, regardless of their optical spectrum, due to the absence of starburst galaxies in the local Universe reaching such extreme luminosities. We investigate the validity of this assumption using new Chandra and XMM-Newton observations of three relatively nearby (z ~ 0.1) starburst galaxies, chosen on the basis of their SDSS optical spectrum, and their coincidence with a bright X-ray source from the 2XMM catalogue. If originating from the galaxy in question, the X-ray luminosity is in each case greater than 1042 erg s-1 (and in one case reaches 1043 erg s-1), meaning these would be the most X-ray luminous starburst galaxies yet discovered if no AGN were present. However, the new X-ray data shows two of the three sources to display X-ray spectral and temporal variability between observations, with all three appearing point-like at Chandra resolution. Furthermore they are spectrally dissimilar to starburst X-ray emission, possessing power-law dominated spectra more reminiscent of AGN. We therefore conclude that otherwise innocuous AGN are indeed fueling the X-ray emission from these galaxies, and that the assumption starburst galaxies do not exceed 1042 erg s-1 remains observationally justified.

Sandra Raimundo, Institute of Astronomy

AGN properties in a cosmological evolution scenario

To understand how AGN accrete, evolve and affect the surrounding galaxy, it is necessary to investigate how their intrinsic properties change with time. We consider different AGN populations, accounting for their physical properties, obscuration and spectral features, and trace their redshift evolution. In particular, we investigate the role of Eddington ratio and accretion efficiency in the black hole mass function, and build a scenario where objects with a high spread in Eddington ratios, including low values (10^-3 - 10^-2) are consistent with the observed local mass function. We find that in general, the black holes in our solutions are spinning rapidly, especially the ones with low hydrogen column densities. An evolutionary model will be presented.

Dr Curtis Saxton, MSSL/UCL

X-ray & radio emissions at the clumpy interface of jet and ISM

We conduct 2D and 3D simulations of light, supersonic AGN jets propagating through a dense ISM and calculate the time-dependent X-ray and polarized radio emissions from the jets and the disturbed ISM. We investigate the transient behaviour of the X-rays and the radio polarization. Our simulations indicate that the clumpy jet-agitated medium is thermally unstable. We characterise its passive evolution during a subsequent phase of jet quiescence. C.J. Saxton(1), J.H. Beall(2), K. Wu(1), M.T. Wolff(2) (1) Mullard Space Science Laboratory, University College London (2) Naval Research Laboratory, Washington, USA

Amy Scott, University of Leicester

The X-ray continuum slope of quasars revisited

We present the X-ray properties of a large, optically selected sample of quasars, from a cross-correlation of the SDSS DR5 quasar catalogue and the X-ray serendipitous source catalogue, 2XMMi. The distribution of power law slopes is considered, including the dependence of photon index on X-ray luminosity, redshift and Eddington ratio. In general our results agree with those of Young et al. (2009), however we offer a different interpretation of the correlation of photon index and X-ray luminosity. We find evidence for a correlation of photon index and redshift, (decreasing photon indices for higher redshift sources), and propose that this may be due to a Compton reflection component being redshifted into the XMM-Newton observed energy band, making the power law slope of the sources appear flatter. We show that these apparent trends can be removed when the spectra are analysed over a reduced energy range that would exclude any contribution from such a reflection component.

Klaas Wiersema, University of Leicester

The jet trail nebula of low mass X-ray binary SAX J1712.6-3739

In a multiwavelength program dedicated to identifying optical counterparts of faint persistent X-ray sources in the Galactic Bulge, we find an accurate X-ray position of low mass X-ray binary SAX J1712.6-3739 through Chandra observations, and discover its faint optical counterpart using our data from the ESO 3.6m telescope. We serendipitously discover a bright and large bow shock shaped nebula in our deep narrowband H alpha imaging, associated with the X-ray binary. A nebula like this has never been observed before, and provides a unique laboratory to study the energetics of accretion and jets. We perform a series of deep follow-up observations at multiple wavelengths, including optical spectroscopy with Gemini-South. Through these observations we confirm the nebula to be a jet trail nebula, formed when jets from the high proper motion X-ray binary interact with the ISM. We use optical line fluxes and line profiles to probe the physics of jet-ISM interaction, and constrain the history of accretion of this LMXB.

Dr Silvia Zane, MSSL, UCL

Charges acceleration in the magnetar's magnetosphere.

Over the last decade a large body of evidence accumulated in favor of the existence of (isolated) neutron stars (NSs) with magnetic fields much stronger that those typical of radio-pulsars. The magnetar scenario provided up to now a quite coherent and complete interpretation of the observational properties of SGRs/AXPs, in particular of both their bursting and persistent emission at X/gamma-ray energies. It has been suggested that the (soft) X-ray spectrum of SGRs/AXPs arises from resonant cyclotron upscattering (RCS) of thermal surface photons onto mildly relativistic particles flowing along the closed field lines in the twisted magnetosphere. The RCS model was further explored in several investigations. In particular, we developed a 3D Monte Carlo code and implemented computed models in XSPEC, obtaining successful fits to numerous SGRs/AXPs spectra in the ~0.5-10 keV range. Nevertheless, we caveat that, at the present status of art, is the physical structure of the magnetosphere which is still an open problem. All our fits require mildly relativistic electrons (Lorentz factor ~ a few), therefore the resonant scattering model works well provided that a braking mechanism exists to maintain electrons in a non-relativistic regime. In this poster we present a simple, stationary model for the motion of the charges along a 1D flux tube accounting for both the acceleration by the electric field and Compton losses induced by resonant cyclotron scatterings. We develop a simplified model for the electrodynamic of the magnetosphere, with potential implications for the observed emission from the IR to the hard X-ray range (Integral and Comptel band).