Multicolour flickering studies of X1822 − 371

Abstract

We report on the analysis of high-speed multicolour photometry of the eclipsing X-ray binary X1822 − 371. We used new eclipse timings to derive a revised optical ephemeris. A quadratic fit to the eclipse timings is not statistically significant, but suggests that the orbital period is increasing on a time-scale of P0/| ˙ P |= (4.2 ± 1.4) × 10 6 yr. We find no systematic delay or advance of the optical timings with respect to the X-ray timings. Average UBVRI light curves show the deep eclipse of the disc by the secondary star superimposed on the broader and shallower occultation of the inner-disc regions by the outer disc (dip), and an orbital hump centred at phase +0.25 which is mostly seen in the U and B bands. There is no evidence of a secondary eclipse at phase +0.5 or of an ellipsoidal modulation of the light of the secondary star. The starting phase of the dip occurs earlier for shorter wavelengths, while the egress occurs at the same phase in all bands. This suggests that the thickening of the outer, occulting disc rim is gradual with azimuth at ingress but decreases sharply at egress. We fit synthetic photometry to the extracted colours of the inner and outer disc regions to estimate their effective temperatures. We find Teff = (9 ± 5) × 10 7 K and Teff = (6 ± 2) × 10 4 K for the inner and outer disc regions, respectively. The orbital dependence of the flickering activity is derived from the mean scatter of the individual light curves with respect to the average UBVRI light curves. The flickering curves show a broad eclipse at the dipping phases, the depth of which decreases with increasing wavelength. The blue, eclipsed flickering component is associated with the inner-disc regions and can be fitted by a blackbody spectrum of Teff = (2.1 ± 0.8) × 10 8 K, whereas the uneclipsed flickering component probably arises from the outermost disc regions and is well described by a blackbody of Teff = (9.6 ± 0.7) × 10 3 K.