Astronomy and Astrophysics, volume 493, 145-157 (2009/1-1)
Variations in the dip properties of the low-mass X-ray binary XB1254-690 observed with XMM-Newton and INTEGRAL.
DIAZ TRIGO M., PARMAR A.N., BOIRIN L., MOTCH C., TALAVERA A. and BALMAN S.
Abstract (from CDS):
We have analysed data from five XMM-Newton observations of XB1254-690, one of them simultaneous with INTEGRAL, to investigate the mechanism responsible for the highly variable dip durations and depths seen from this low-mass X-ray binary. Deep dips were present during two observations, shallow dips during one and no dips were detected during the remaining two observations. At high (1-4s) time resolution ``shallow dips'' are seen to include a few very rapid, deep dips whilst the ``deep'' dips consist of many similar very rapid, deep fluctuations. The folded V-band Optical Monitor light curves obtained when the source was undergoing deep, shallow and no detectable dipping exhibit sinusoid-like variations with different amplitudes and phases. We fit EPIC spectra obtained from ``persistent'' or dip-free intervals with a model consisting of disc-blackbody and thermal Comptonisation components together with Gaussian emission features at 1 and 6.6keV modified by absorption due to cold and photo-ionised material. None of the spectral parameters appears to be strongly correlated with the dip depth except for the temperature of the disc blackbody which is coolest (kT∼1.8keV) when deep dips are present and warmest (kT∼2.1keV) when no dips are detectable. We propose that the changes in both disc temperature and optical modulation could be explained by the presence of a tilted accretion disc in the system. We provide a revised estimate of the orbital period of 0.16388875 ±0.00000017day.