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We identify "red clump stars" -core helium-burning giants -among 2MASS stars and use them to measure the run of reddening with distance in the direction of each of the Galactic Anomalous X-ray Pulsars (AXP). We combine this with extinction estimates from X-ray spectroscopy to infer distances and find that the locations of all AXP are consistent with being in Galactic spiral arms. We also find that the 2-10 keV luminosities implied by our distances are remarkably similar for all AXP, being all around ∼ 1.3 × 10 35 erg s −1 . Furthermore, using our distances to estimate effective black-body emitting radii, we find that the radii are tightly anti-correlated with pulsed fraction, and somewhat less tightly anti-correlated with black-body temperature. We find no obvious relationship of any property with the dipole magnetic field strength inferred from the spin-down rate.

A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (Xray) observations of the Galactic black hole binary GX 339−4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the r , g 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

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 V≈ 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.