H. Pedersen, European Southern Observatory; the Hakucho Team of the University of Tokyo (principal investigator M. Oda); and L. Cominsky, J. Doty, G. Jernigan, J. van Paradijs and W. H. G. Lewin, Massachusetts Institute of Technology, report the detection of simultaneous x-ray/optical bursts from MXB1636-53 (McClintock et al. 1977, Nature 270, 320). The optical observations were made with the 1.5-m Danish telescope at E.S.O., the x-ray observations with the Japanese satellite Hakucho, in the periods June 20-July 2 and July 20-Aug. 2. A total of 15 optical bursts were observed. The largest observed increase in the stellar brightness was a factor of 4, and the duration of the tail of the longest optical bursts was 2 min. During the times that simultaneous optical and x-ray observations were made, five bursts were detected simultaneously by both observatories. In one case an optical burst was observed but no x-ray burst, but the reverse was never the case. Though unlikely, the possibility that the optical event not accompanied by an x-ray burst was caused by a passing moonlit cloud cannot be excluded. The integrated x-ray burst fluxes show a range of size that differ by a factor of ~ 3. If the upper limit for the event where an optical but not an x-ray burst was observed is included, this range is at least a factor of 8. The optical bursts show a range of sizes by a factor of ~ 4. The risetimes were about 1 s in most optical and x-ray bursts. However, in at least one case the risetimes for both x-ray and optical bursts were ~ 5 s. In the case that has so far been analyzed in some detail, the optical burst is delayed by ~ 2.5 s. This delay is comparable to those observed for MXB 1735-44 (McClintock et al. 1979, Nature 279, 47) and MXB1837+05 (Hackwell et al. 1979, Ap. J. Lett. in press).
International Astronomical Union Circular
- Pub Date:
- September 1979