Search

A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (X-ray) observations of the Galactic black hole binary GX 4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the and u′ filters show slow (∼20 s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (∼50 ms in r′ and g′) and with maximum strengths of more than twice the local mean. Power spectral analysis over ∼0.004–10 Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical versus X-ray flux ratio, higher than that seen on previous occasions when the source was fully jet dominated. Such a large ratio cannot be easily explained with a disc alone. Studying the optical–X-ray cross-spectrum in Fourier space shows a markedly different behaviour above and below ∼0.2 Hz. The peak of the coherence function above this threshold is associated with a short optical time lag with respect to X-rays, also seen as the dominant feature in the time-domain cross-correlation at ≈150 ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal irradiated disc fraction of 20 per cent around 5000 Å. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency quasi-periodic oscillation is seen in the optical but not in X-rays, and is associated with a low coherence. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ∼10 s and strong coherence in the blue u′ filter. Consistent with this, simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is unclear. However, canonical disc reprocessing cannot dominate the optical power easily, nor explain the fast variability. [ABSTRACT FROM AUTHOR]

We present the discovery of two brown dwarfs in the UK infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Deep Extragalactic Survey (DXS) Data Release 2 (DR2). Both objects were selected photometrically from 6 deg2 in DXS for their blue colour and the lack of optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82. Additional optical photometry provided by the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS) corroborated the possible substellarity of these candidates. Subsequent methane imaging of UDXS and UDXS J221903.10+002418.2 has confirmed them as T7 ± 1 and T6 ± 1 dwarfs at photometric distances of 81 (52–118 pc) and 60 (44–87 pc; 2σ confidence level). A similar search in the DR2 of the Ultra-Deep Survey over a smaller area (0.77 deg2) and shallower depth did not return any late-T dwarf candidate. The numbers of late-T dwarfs in our study are broadly in line with a declining mass function when considering the current area and depth of the DXS and UDS. These brown dwarfs are the first discovered in the Visible Imaging Multi-Object Spectrograph (VIMOS) 4 field and among the few T dwarfs found in pencil-beam surveys. They are valuable to investigate the scaleheight of T dwarfs. [ABSTRACT FROM AUTHOR]

We present high-speed optical photometry of the anomalous X-ray pulsar 1E obtained with ULTRACAM on the 8.2-m Very Large Telescope in 2007 June. We detect 1E at a magnitude of , consistent with the values found by Wang et al. and hence confirming their conclusion that the source was approximately 1 mag brighter than in 2003–06 due to an on-going X-ray flare that started in 2007 March. The increased source brightness enabled us to detect optical pulsations with an identical period (6.458 s) to the X-ray pulsations. The root-mean-square (rms) pulsed fraction in our data is per cent, approximately the same as the 2–10 keV X-ray rms pulsed fraction. The optical and X-ray pulse profiles show similar morphologies and appear to be approximately in phase with each other, the latter lagging the former by only cycles. The optical pulsations in 1E are very similar in nature to those observed in 4U 0142+61. The implications of our observations for models of anomalous X-ray pulsars are discussed. [ABSTRACT FROM AUTHOR]

We have conducted an extensive observational campaign of SWIFT J1753.5−0127 during 2007 June after its bright outburst episode in 2005. We have performed multiband optical photometry, optical spectroscopy, X-ray spectroscopy and timing and ULTRACAM optical photometry simultaneously in three bands. Both the optical spectrum and the X-ray spectrum along with enhanced brightness in broad-band photometry point to recent increased activity. We analyse the different spectral regions, finding a smooth optical continuum with a remarkable lack of lines and a very blue component modulated with a period of 3.2 h and a hard power-law X-ray spectrum. Both the X-ray and optical power spectra are flat at low frequencies up to the 0.1 Hz (10 s) range, then decreasing roughly as a power law consistent with flickering. Furthermore, the optical data show quasi-periodic oscillations near 0.08 Hz (13 s). Together with a dynamical and autocorrelation analysis of the light curves we attempt to construct a complete physical picture of this intriguing system. [ABSTRACT FROM AUTHOR]

We present the discovery of optical/X-ray flux correlations on rapid time-scales in the low/hard state of the Galactic black hole GX 339−4. The source had recently emerged from outburst and was associated with a relatively faint counterpart with mag 17. The optical [Very Large Telescope (VLT)/ULTRACAM] and X-ray ( Rossi X-ray Timing Explorer) data show a clear positive cross-correlation function (CCF) signal, with the optical peak lagging X-rays by ∼150 ms, preceded by a shallow rise and followed by a steep decline along with broad anticorrelation dips. An examination of the light curves shows that the main CCF features are reproduced in superpositions of flares and dips. The CCF peak is narrow and the X-ray autocorrelation function (ACF) is broader than the optical ACF, arguing against reprocessing as the origin for the rapid optical emission. X-ray flaring is associated with spectral hardening, but no corresponding changes are detected around optical peaks and dips. The variability may be explained in the context of synchrotron emission with interaction between a jet and a corona. The complex CCF structure in GX 339−4 has similarities to that of another remarkable X-ray binary XTE J1118+480, in spite of showing a weaker maximum strength. Such simultaneous multiwavelength, rapid timing studies provide key constraints for modelling the inner regions of accreting stellar sources. [ABSTRACT FROM AUTHOR]