The Square Kilometre Array (SKA)

Abstract:

Three years of optical monitoring of the low-mass X-ray binary (LMXB) 4U 1957+11 is presented. The source was observed in V, R and i bands using the Faulkes Telescopes North and South. The light curve is dominated by long-term variations which are correlated (at the >3σ level) with the soft X-ray flux from the All Sky Monitor on board the Rossi X-ray Timing Explorer. The variations span 1 mag in all three filters. We find no evidence for periodicities in our light curves, contrary to a previous short-time-scale optical study in which the flux varied on a 9.3-h sinusoidal period by a smaller amplitude. The optical spectral energy distribution is blue and typical of LMXBs in outburst, as is the power-law index of the correlation β= 0.5, where Fν,OPT∝F^βX. The discrete cross-correlation function reveals a peak at an X-ray lag of 2-14 days, which could be the viscous time-scale. However, adopting the least-squares method we find the strongest correlation at a lag of 0 ± 4 d, consistent with X-ray reprocessing on the surface of the disc. We therefore constrain the optical lag behind X-ray to be between -14 and +4 d. In addition, we use the optical-X-ray luminosity diagram for LMXBs as a diagnostic tool to constrain the nature of the compact object in 4U 1957+11, since black hole and neutron star sources reside in different regions of this diagram. It is found that if the system contains a black hole (as is the currently favoured hypothesis), its distance must exceed ∼20 kpc for the optical and X-ray luminosities to be consistent with other soft-state black hole systems. For distances <20 kpc, the data lie in a region of the diagram populated only by neutron star sources (black hole systems are 10 times optically brighter for this X-ray luminosity). 4U 1957+11 is unique: it is either the only black hole LMXB to exist in an apparent persistent soft state or a neutron star LMXB which behaves like a black hole. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

Aims. We aim to demonstrate that the Herschel-ATLAS (H-ATLAS) is suitable for a blind and unbiased survey for debris disks by identifying candidate debris disks associated with main sequence stars in the initial science demonstration field of the survey. We show that H-ATLAS reveals a population of far-infrared/sub-mm sources that are associated with stars or star-like objects on the SDSS main-sequence locus. We validate our approach by comparing the properties of the most likely candidate disks to those of the known population. Methods. We use a photometric selection technique to identify main sequence stars in the SDSS DR7 catalogue and a Bayesian Likelihood Ratio method to identify H-ATLAS catalogue sources associated with these main sequence stars. Following this photometric selection we apply distance cuts to identify the most likely candidate debris disks and rule out the presence of contaminating galaxies using UKIDSS LAS K-band images. Results. We identify 78 H-ATLAS sources associated with SDSS point sources on the main-sequence locus, of which two are the most likely debris disk candidates: H-ATLAS J090315.8 and H-ATLAS J090240.2. We show that they are plausible candidates by comparing their properties to the known population of debris disks. Our initial results indicate that bright debris disks are rare, with only 2 candidates identified in a search sample of 851 stars. We also show that H-ATLAS can derive useful upper limits for debris disks associated with Hipparcos stars in the field and outline the future prospects for our debris disk search programme. © 2010 ESO.

Abstract:

SS 433 is a high-mass X-ray binary system (XRB) and one of the most persistent sources of relativistic jets in the Milky Way. The object has been intensively studied in radio at arcsecond scales, however the high-resolution observations (i.e. VLBI) are relatively scarce. In 2008 November the system was in outburst. Using the e-VLBI capabilities of the European VLBI Network (EVN) we observed SS 433 for three epochs during the active phase. The data offered a detailed view of the system’s behaviour in outburst at milliarcsecond scales. We used the “kinematic model” (which predicts the position along the jet of any knot ejected at some particular time in the past) to investigate the dynamic parameters of SS 433 and we examined the polarization properties of the ejected material. We report here the preliminary results.

Abstract:

We present the results of our search for high-redshift Lyman-break galaxies over the GOODS-South field. We use Hubble Space Telescope (HST)-ACS data in B, V, i′ & z′, Very Large Telescope (VLT)-ISAAC J and Ks, Spitzer-Infrared Array Camera (IRAC) 3.6, 4.5, 5.8 and 8.0 μm data in conjunction with the new HAWK-I Y-band science verification data to search for dropout galaxies in the redshift range 6 < z < 9. We survey ≈119 arcmin² to YAB = 25.7 (5σ), of which 37.5 arcmin² reaches YAB = 25.9. Candidate z′ and Y dropouts were selected on the basis of a colour cut of (Y - J)AB > 0.75 mag and (z′ - Y)AB > 1.0 mag, respectively. We find no robust Y-drops (z ≈ 9) brighter than JAB < 25.4. In our search for z′-band dropouts (z ≈ 6.5-7.5), we identify four possible candidates, two with z′-drop colours and clear Spitzer-IRAC detections and two less likely candidates. We also identify two previously known Galactic T-dwarf stellar contaminants with these colours, and two likely transient objects seen in the Y-band data. The implications if all or none of our candidates is real on the ultraviolet galaxy luminosity functions at z > 6.5 are explored. We find our number of z′-drop candidates to be insufficient based on the expected number of z′ drops in a simple no-evolution scenario from the z = 3 Lyman-break galaxy luminosity function but we are consistent with the observed luminosity function at z ≈ 6 (if all our candidates are real). However, if one or both of our best z′-drop candidates are not z > 6.5 galaxies, this would demand evolution of the luminosity function at early epochs, in the sense that the number density of ultraviolet luminous star- forming galaxies at z > 7 is less than at z ~ 6. We show that the future surveys to be conducted with the European Southern Observatory VISTA telescope over the next 5 yr will be able to measure the bulk of the luminosity function for both z′ and Y dropouts and thus provide the strongest constraints on the level of star-formation within the epoch of reionization. © 2010 The Authors. Journal compilation. © 2010 RAS.

Abstract:

E-VLBI (electronic very long baseline interferometry) is a new implementation of the VLBI technique consisting in transferring the data from the radio telescopes to the correlator over the internet and correlating them in real-time. Time-wise this is a major improvement over the traditional method. e-VLBI is thus offering new opportunities for radio transient studies. Its capability of rapid response enables a more efficient decision making process with respect to potential followup observations. The rapid feedback time also permits to quickly modify the observing strategy to best track the development of the transient phenomena. The results summarized here have been obtained with the EVN (European VLBI Network) in the past few years within a transient ToO programme. The targets were XRBs (X-ray binaries) undergoing periods of enhanced activity (outbursts). The EVN observations were performed at 5 GHz and were often complemented by quasi-simultaneous (within one day) data at other wavelengths (X-ray and optical). The findings reveal a complex behaviour of the accretion/ejection phenomena in the systems investigated and offer insights into the extreme physics close to a compact object.