X-ray observations of 4U/MXB 1735 – 44

• Published in: Monthly notices of the Royal Astronomical Society 1980-11, Vol.193 (1), p.15-28
• Main Authors: Lewin, W. H. G., van Paradijs, J., Cominsky, L., Holzner, S.
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/193.1.15

MXB 1735–44 was observed with the SAS-3 X-ray observatory on five occasions in 1977 and 1978. It was always ‘burst active’. Fifty-three bursts were observed in a total of about 20 days. The intervals between bursts were highly irregular, varying from ∼ 2 to more than 50 hr. In the framework of the thermonuclear flash model for X-ray bursts, the irregular burst behaviour of MXB 1735 – 44 (and of MXB 1837 + 05) may be related to its high intrinsic X-ray luminosity. We have looked for possible correlations between burst frequency and properties of the associated persistent source (flux and hardness of the persistent spectrum). No such correlations were found. There is probably a correlation between the size of the bursts and the burst intervals: the bursts tend to be smaller when they occur more frequently. The distributions of the burst sizes in terms of their integrated burst flux Eb and maximum burst flux Fmax both have standard deviations (normalized to the mean value) of ∼ 37 per cent. Taking the smallest and largest burst, the total range in both Eb and Fmax is about a factor of 7. However, the ratio Eb/Fmax is approximately constant. We also found that there is no statistically significant evidence for a variation in blackbody radii with burst size. No periodic pulsations were found in the persistent emission: for periods between 1.5 and ∼ 1000 s the 3 σ upper limit to the pulsed fraction is 2 per cent.