New results in X-ray astronomy 2009


List of submitted talks

Maria Caballero-Garcia, IoA

X-ray reflection in a sample of X-ray bright Ultraluminous X-ray sources

We apply a reflection-based model to the best available XMM-Newton spectra of X-ray bright Ultraluminous X-ray (ULX) sources (NGC 1313 X-1, NGC 1313 X-2, M 81 X-6, Holmberg IX X-1, NGC 5408 X-1 and Holmberg II X-1). A spectral drop is apparent in the data of all the sources at energies 6-7 keV. The drop is interpreted here in terms of relativistically blurred ionized reflection from the accretion disk. A soft-excess is also detected from these sources (as usually found in the spectra of AGN), with emission from O K and Fe L, in the case of NGC 5408 X-1 and Holmberg II X-1, which can be understood as features arising from reflection of the disk. Remarkably, ionized disk reflection and the associated powerlaw continuum provide a good description of the broad-band spectrum, including the soft-excess. With exception of NGC 1313 X-2 and M81 X-6, there is no requirement for thermal emission from the inner disk in the description of the spectra. The black holes of these systems must then be highly spinning, with a spin close to the maximum rate of a maximal spinning black hole. The results require the action of strong light bending in these sources.We discuss our results in terms of the masses of the black holes.

Giulio Del Zanna, University of Cambridge

Measuring electron temperatures using X-ray spectroscopy

I will briefly review the few diagnostic possibilities for measuring electron temperatures in the X-ray spectral range of Chandra HETG. The relevant atomic data are discussed and presented, and a few examples provided.

Poshak Gandhi, Riken

Resolving AGN tori in the mid-infrared: a new unbiased probe of intrinsic accretion power

We show that resolved mid-infrared (mid-IR) imaging of active galactic nuclei (AGN) is an isotropic probe of their intrinsic luminosities. We have obtained the largest sample to date of 8-m telescope diffraction-limited mid-IR imaging of radio-quiet AGN (42 sources so far), using which we find a strong correlation between mid-IR (12 micron) and intrinsic X-ray (2-10 keV) luminosities. The relation holds true for Seyferts of all types, including unobscured, obscured, as well as heavily Compton-thick sources. Thus, the mid-IR directly probes the intrinsic accretion power irrespective of obscuration, and provides a new and accurate proxy of AGN luminosity (complementing other tools such as the forbidden [OIII]5007 emission line). The relation may also be extended smoothly to the higher luminosity quasar regime. Most star formation around the nuclei is resolved out, and the remaining unresolved mid-IR flux is dominated by the AGN alone. This enables clean estimates of bolometric luminosities to be derived for the first time. The dispersion in the mid-IR:X-ray relation is tight enough to provide sensitive discrimination between physical models of clumpy vs. smooth dusty tori geometries. In addition, our relation also provides a simple pathway for estimating the intrinsic powers of Compton-thick AGN.

Jeanette Gladstone, Durham University

The Ultraluminous State

The spectra from ultraluminous X-ray sources have been described by several different models which give very different results for the inferred black hole mass, ranging from intermediate (>100 solar masses) to standard mass (~ 5 - 20 solar masses) black holes. Here we revisit the question of the nature of ULX through a detailed investigation of their spectral shape using the highest quality X-ray data currently available in the XMM-Newton public archives. We apply phenomenological models to characterise the spectra of these objects and more physically motivated models in order to explore the physical processes underlying these characteristics. Results show that the spectra of these sources are fundamentally different to that of Galactic X-ray binaries, whilst the application of physical models indicates a more extreme version of the highest known luminosity state, the very high state. We therefore speculate that in observing ULXs we are observing stellar-mass black holes residing in a new ‘ultraluminous’ state.

Joanna Goodger, University of Hertforshire

Long Term Monitoring of the Jet in Centaurus A

I will present new and archival multi-frequency radio and X-ray data for Centaurus A obtained over more than a decade at the VLA and with Chandra. I have used radio and X-ray flux variability, polarization variability, spectral indices and proper motions of the jet knots in Cen A to rule out impulsive particle acceleration in this jet and I will discuss the most likely knot formation processes and the related particle acceleration mechanisms.

Andy Goulding, Durham University

The present-day growth rates of a large population of heavily obscured AGNs

Deep X-ray surveys have shown that the space density of high-luminosity AGNs peaked at higher redshifts than lower luminosity AGNs, suggesting that the most massive SMBHs (M_BH ~ 10^8 - 10^9 M_sun) grew first. The implication of these results is that the most rapidly growing SMBHs in the local Universe should be of comparatively low mass (M_BH < 10^7 M_sun). We will show that this is indeed the case by providing X-ray and mid-IR spectral constraints of the most complete census of AGN activity to D<15 Mpc (Goulding & Alexander 2009). We find that ~50% of the AGNs in the local Universe are missed in even the most sensitive optical surveys, and many of these AGNs host relatively low-mass SMBHs (M_BH ~ 10^6 M_sun). Arguably, the most direct method for measuring the true intrinsic luminosity of the central source is through sensitive hard X-ray analyses. By combining X-ray data from Chandra, XMM-Newton, Suzaku, Swift and Beppo-SAX with mid-IR spectroscopy from Spitzer-IRS we will show that these AGNs appear to be intrinsically luminous objects, many of which are heavily obscured at X-ray energies (< 10 keV). Furthermore, we constrain the relative mass accretion rates, and hence present-day growth rates of these AGNs and conclude that AGNs hosting SMBHs with M_BH ~ 10^6 M_sun are amongst the most rapidly growing in the local Universe.

Lucy Heil, University of Leicester

Short term variability from some ULXs: Missing in action?

We present results from a study of short term variability in 19 archival observations by XMM-Newton of 16 Ultraluminous X-ray Sources (ULXs). Eight observations (six sources) showed intrinsic variability with power spectra in the form of either a power law or broken power law-like continuum and in some cases quasi-periodic oscillations (QPOs) seen in previous studies of these observations. Seven observations (seven sources) yielded upper limits comparable to, or higher than, the values measured from those observations with detectable variations. These represented the seven faintest sources all with f_x < 3x10^-12 erg cm^-2 ~ s^-1.In contrast there are four observations (three sources) that gave upper limits significantly lower than both the values measured from the ULX observations with detectable variations, and the values expected by comparison with luminous Galactic black hole X-ray binaries (BHBs) and Active Galactic Nuclei (AGN). The lack of detectable variability from these bright sources, in the observed frequency bandpass (10^-3 - 1 Hz), is at odds with the expectations based on observations of Galactic BHBs and AGN. We present an analysis of these results in terms of the fluxes and X-ray luminosities from the observations, and discuss solutions for suppression of variability from these sources.

Adam Ingram, University of Durham

A physical basis for the timing properties of X-ray binaries

The low frequency Quasi Periodic Oscillation has been known in black hole binaries and neutron stars for over 20 years but there is still no consensus in the literature as to its physical origin. I show how this can be naturally interpreted in the context of the truncated disc/hot inner flow models designed to explain the spectral evolution. Lense-Thirring (vertical) precession of the hot inner flow can explain the QPO frequencies and their observed correlation with spectral softness. It also explains the why the QPO spectrum is hard as the modulation is of the Comptonised emission from the hot flow rather than the thin disc. I then show how this model can be tested in atoll sources and extended to produce the broad band continuum noise from MRI fluctuations in the hot flow. This gives the first complete model for the power spectral components seen in black hole binaries and neutron stars.

Elise Laird, Imperial College London

The AEGIS-X survey of the Extended Groth Strip

The AEGIS-X survey is a 3.4Ms Chandra survey of the Extended Groth Strip (EGS) region, designed primarily for studying the co-evolution of black holes and their host galaxies. The original phase of the survey comprised of 8 contiguous ACIS-I pointings, each with nominal exposure 200ks, covering a total area of approximately 0.67 deg$^{2}$ in a strip of length 2 degrees. A total of 1325 band-merged sources were detected to a Poisson probability limit of $4 \times 10^{-6}$, with limiting fluxes of $5.3\times 10^{-17}$ erg cm$^{2}$ s$^{-1}$ in the soft (0.5--2 keV) band and $3.8 \times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$ in the hard (2--10 keV) band. These observations cover a region with excellent supporting ground and space-based panchromatic data (e.g. GALEX, HST/ACS, CFHTLS, Spitzer/IRAC, Spitzer/MIPS, VLA) and $\sim$20 000 spectroscopic redshifts from the DEEP2 and DEEP3 surveys. The second phase of AEGIS-X has recently been completed and comprises of an additional 1.6Ms in the central 3 pointings in the strip, where the supporting data is strongest. I will present the first results from the new, deeper AEGIS-X survey as well as reviewing some of the key results from the survey to date. In particular I will focus on the optical spectral properties, environment, stellar mass function and morphology of the X-ray sources.

Ed Lloyd-Davies, University of Sussex

The XMM Cluster Survey: First results for cluster scaling relations

The XMM Cluster Survey (XCS) will exploit the entire XMM-Newton data archive to find clusters. XCS already covers 275 square degrees and has found more than 1500 cluster candidates with > 300 counts. We have more than 400 have been optically confirmed clusters so far; z_max=1.45. Here we present the first results for cluster X-ray scaling relations from an initial XCS sample and discuss their implications for cluster physics and cosmology. Unlike most previous measurements of scaling relations, XCS has both a well characterised selection function and a large sample size, which will allow much more accurate inferences about the underlying cluster relations to be made from the measurements. To this end we also explore the effects of selection and measurement errors on our ability to measure scaling relations by folding clusters drawn from n-body/hydro simulations through the XCS selection function to infer selection and measurement biases.

Missagh Mehdipour, MSSL-UCL

Investigating the warm absorber and X-ray variability of the Seyfert 1 galaxy NGC 3516 with XMM-Newton

We present new analysis of the X-ray spectra of the Seyfert 1 galaxy NGC 3516 as seen by the XMM-Newton RGS and EPIC-pn in recent observations, with the emphasis put on high resolution spectroscopy. NGC 3516 shows interesting spectral and flux variability during four observations in 2006; we investigate whether this is due to absorption by the warm absorber and/or is intrinsic to the source emission. We show that the passage of an absorbing cloud, part of a clumpy disk wind model, is unable to account for this variability. Intrinsic changes in the source continuum are found to play a much more significant role in explaining the observed flux and spectral variability than originally thought for this AGN.

Matt Middleton, University of Durham

The XRB analogy to the first AGN QPO

The discovery of the first confirmed QPO in an AGN (REJ1034+396) is the missing link in the timing properties of X-ray Binaries (XRBs) and AGN and provides a slow-motion look at the much shorter timescale variability seen in XRBs. By testing the shape of the rapid variability against potential spectral models we can deduce that the QPO has it's orgins in the low-temperature comptonisation of the seed disc photons. This model fits the energy spectrum extremely well and naturally reproduces the large soft-excess seen in the source. The optical-X-ray SED of REJ1034+396 together with a low mass estimate suggests that the source is accreting at super-Eddington rates, from which we speculate that the QPO is the analogy of the 67Hz QPO of GRS1915+105, seen in a super-Eddington mass accretion rate state.

James Mullaney, University of Durham

X-rays and Infrared: A complementary study of AGNs.

Recent, deep X-ray observations have provided profound insights into the intrinsic properties of AGNs at all observable redshifts. However, it is becoming increasingly clear that the complete understanding of these cosmologically important objects will only be achieved when we can comprehend their energy output at all observable wavelengths. In this talk I will focus on how X-ray and infrared observations can be used in a complementary manner to paint a more complete picture of AGNs in their host environments (i.e., from accretion disks and dusty tori to their host galaxies). To demonstrate this, I will present the results from my recent infrared study of high redshift X-ray identified AGNs, which shows tentative evidence of higher dust covering factors and/or increased levels of star formation around AGNs in the distant universe (i.e., z=1-2).

Richard Owen, University of Leicester

Diffuse X-ray Emission in the Extended Disk of M33

We present results from a study of extended X-ray emission in M33 based on archival XMM-Newton observations. We investigate the spatial and spectral properties of diffuse X-ray emission in the inner galactic disk and explore the connection between this emission and recent star formation in the galaxy. We derive the X-ray to SFR ratio in the inner disk of M33 to be 4 x 10^39 (erg/sec)/(Msun/yr), consistent with results from a sample of nearby spiral galaxies. Comparison of the X-ray images with FUV and K band images shows the diffuse X-ray emission to be equally divided between two components: one strongly correlated with star formation in the inner spiral arms, the other more uniformly distributed across the galactic disk. We show this emission to be well modelled by a clumpy thin-disk distribution, with bubbles of hot gas tracing the spiral arms.

Kathy Romer, University of Sussex

X-ray Clusters: Still relevant for Cosmology?

I will review the work we have done with XCS (XMM Cluster Survey) towards cosmological parameters. In particular I will review Sahlen et al. 2009, which shows how important it is to understand scaling relations before attempting cosmology with clusters. I will go on to discuss the relevance of X-ray clusters to cosmology in light of new optical and SZ surveys.

Jeremy Sanders, IoA, Cambridge

Cool gas in cluster cores

XMM-Newton RGS observations of the cores of galaxy clusters show much less material than would be expected from a simple cooling flow picture. Deep observations do, however, show that cool X-ray emitting gas does exist and there is a wide range of temperatures in many clusters. By examining deep RGS and Chandra observations of cluster cores we examine the nature of this very cool gas. We discuss the implications for AGN feedback in these objects and show that the coolest gas must exist in the form of cool blobs in a hotter medium.

Klaas Wiersema, University of Leicester


On the 23d of April 2009 a gamma-ray burst was detected by Swift, with unremarkable gamma-ray and X-ray properties. Our UKIRT rapid response programme for GRB follow-up was triggered by an automatic (eStar) system, which revealed a relatively bright source in J, H and K bands. Observations with Gemini-North were obtained soon after, revealing no source to very deep limits at wavelengths lower than ~1 micron, indicating a photometric redshift well above 7. We activated our VLT spectroscopy program for high redshift bursts, and acquired spectroscopy with ISAAC and SINFONI. The spectra give a spectroscopic redshift of 8.23, making GRB 090423 by some distance the most distant object known to date. I will show how the gamma-ray and X-ray optical properties gave little indication of such an extreme redshift, which highlights the need to follow-up every burst with Swift and groundbased facilities. I will discuss how we may be able to use the current and future X-ray, optical and radio facilities to use extremely high redshift GRBs to probe the reionisation and chemical enrichment history of the Universe.

Tony Wilkinson, University of Southampton

The 'voice' of the accretion disc in black hole X-ray binaries

The accretion disc is a key component in the X-ray spectra of black hole X-ray binaries, but until now the disc has not made itself heard above the noise variations of the power-law continuum component on time-scales of seconds or less. At least within the soft states, the disc appears to be relatively stable on these short time-scales. Here, I will present evidence for accretion disc variability in the hard states of black hole X-ray binaries. I will show how reflection signatures can be used to disentangle intrinsic disc variations from the variable blackbody emission due to heating by the power-law. Using this approach and including new data on GX 339-4 from XMM-Newton, I will demonstrate that the disc shows intrinsic variations on time-scales of seconds, corresponding to the low-frequency Lorentzian seen in the power spectrum. This correspondence suggests that the disc itself produces this timing signature, and hence connects the disc to all other timing signatures whose frequencies correlate with that of the low-frequency Lorentzian. I will also describe possible links between the unstable disc and the production of jets in the hard states, and discuss the energy-dependent time-lags associated with the intrinsic disc variability, which can constrain the radial drift time-scales in the inner disc.

Dr Silvia Zane, MSSL, UCL

Modelling the magnetars quiescent spectra

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are peculiar X-ray sources which are believed to be magnetars: ultra-magnetized neutron stars with surface field in excess of 1E14 G, i.e. well above the QED threshold. Spectral analysis is an important tool in magnetar astrophysics since it can provide key information on the emission mechanisms. The first attempts at modelling the persistent (i.e. outside bursts) soft X-ray (<10 keV) spectra of AXPs proved that a model consisting of a blackbody (kT ~ 0.3-0.6 keV) plus a power-law (photon index ~ 2-4) could successfully reproduce the observed emission. Moreover, recent INTEGRAL observations have shown that, while in quiescence, magnetars emit substantial persistent radiation also at higher energies, up to a few hundreds of keV. Their high energy spectra are tipically modelled by a further power law which in some cases has been proved to exhibit a strong dependence on the spin phase. However, a convincing physical interpretation of the various spectral components is still missing and our knowledge of the broadband emission is severely hampered by the impossibility, so far, to carry out simultaneous observations in the whole energy range. In this talk I will focus on the interpretation of magnetar spectral properties during quiescence. I will summarise the present status of the art and our first attempts to model the broadband persistent emission of magnetars within a self consistent, physical scenario. I will then present the predictions of a synthetic model that we calculated with a new magnetic montecarlo 3D radiative code. Our code accounts for resonant cyclotron upscattering of soft thermal photons (emitted by the star surface), by a population of relativistic electrons threated in the magnetosphere. Polarization and QED effects are consistently accounted for, as well different configurations for the magnetosphere. I will discuss the predicted spectral and polarization properties in the 0.1-100 keV range, and the application to existing magnetars data.

Abdu Zoghbi, IoA

1H0707-495 ... Beyond Spectral Fitting

A detailed study of more than 600 ks XMM data on the NLS1 1H0707 is presented. The variability is explored thoroughly. The spectrum is well described by a relativistically blurred reflection from ionised material. The variability indicates the spectrum is composed of two components, interpreted as a power-law between 1-4 keV, and a blurred reflection outside. A significant soft lag is seen between the two, which is interpreted as reverberation from within 8 gravitational radii.