Search

We present a search for excess mid-IR emission due to circumbinary material in the orbital plane of cataclysmic variables (CVs). Our motivation stems from the fact that the strong braking exerted by a circumbinary (CB) disc on the binary system could explain several puzzles in our current understanding of CV evolution. Since theoretical estimates predict that the emission from a CB disc can dominate the spectral energy distribution (SED) of the system at wavelengths > 5 microns, we obtained simultaneous visible to mid-IR SEDs for eight systems. We report detections of SS Cyg at 11.7 microns and AE Aqr at 17.6 microns, both in excess of the contribution from the secondary star. In AE Aqr, the IR likely originates from synchrotron-emitting clouds propelled by the white dwarf. In SS Cyg, we argue that the observed mid-IR variability is difficult to reconcile with simple models of CB discs and we consider free-free emission from a wind. In the other systems, our mid-IR upper limits place strong constraints on the maximum temperature of a putative CB disc. The results show that if any sizeable CB disc are present in these systems, they must be self-shadowed or perhaps dust-free, with the peak thermal emission shifted to far-IR wavelengths., Comment: 14 pages, 6 figures, accepted for publication in MNRAS

The X-ray emission from X-ray binaries may originate in flares occurring when magnetic loops anchored in the disc reconnect. In analogy with our Sun, H alpha emission should arise as the accelerated electrons thermalize in the optically emitting disc, perhaps leading to correlated variability between X-rays, H alpha and the optical continuum. We present simultaneous X-ray and optical high speed photometry of the neutron star low-mass X-ray binary Cyg X-2 to search for such correlations. The highest time resolution achieved is 5 ms in white light and 100 ms with a 3 nm filter centred on H alpha. We find power on timescales > 100s (flickering) in optical with a total r.m.s. of a few %, about an order of magnitude less than that seen in X-rays. We do not find significant correlations between the X-ray and optical fluxes on short timescales, hence cannot conclude whether magnetic flares contribute significantly to the optical emission., Comment: 8 pages, 3 figures, accepted for publication in MNRAS

We have observed the optical pulse profile of PSR B0656+14 in 10 phase bins at a high signal-to-noise ratio, and have measured the linear polarization profile over 30% of the pulsar period with some significance. The pulse profile is double-peaked, with a bridge of emission between the two peaks, similar to gamma-ray profiles observed in other pulsars. There is no detectable unpulsed flux, to a 1-sigma limit of 16% of the pulse-averaged flux. The emission in the bridge is highly (~ 100%) polarized, with a position angle sweep in excellent agreement with the prediction of the Rotating Vector Model as determined from radio polarization observations. We are able to account for the gross features of the optical light curve (i.e., the phase separation of the peaks) using both polar cap and outer gap models. Using the polar cap model, we are also able to estimate the height of the optical emission regions., Comment: 27 pages, 11 figures, accepted by ApJ (scheduled v597 n2, November 10, 2003)