EX Hydrae Outburst – Observations Requested

The outburst will probably be short but measures should be made. Please submit to CBA (see http://cbastro.org/ and links to EX Hya reports therein, especially http://cbastro.org/communications/news/messages/0749.html). Please also submit to AAVSO (see their Special Notice at http://www.aavso.org/publications/specialnotice/211.shtml).

This star has been observed by the RASNZ VSS and the old RASNZ Photometry Section since 1968 and a number of important discoveries made and papers published. It’s important that the present outburst be monitored – either unfiltered or using a B or V filter and, if possible, BVRI or UBV measures made every hour or two. Good timing is essential for the reasons discussed below. There was a brief note about it in the February VSS Newsletter.

George Mumford began high precision measures of the is star in the 1950s hoping to fing some indications of gracvitational slowing of the orbit. Robert Kraft and others observed it in the 1960s and one of the first international multi-site programmes in 1978 saw it observed from Auckland, Brisbane, Cerro Tololo and South Africa by Marino, Kennedy, Walker, Page as amateurs and Vogt and Stercken as professionals. This resolved the problem of the puzzling and 67 and 98 minute periodicities. All measures are corrected as much as possible for time, even allowing for the Earth’s slowing rotation which affects our normal time.

EX Hydrae is still a puzzle. The 67 minute spin period is changing, although whether continuously or in jumps is not clear. As well, the 98 minute eclipse period seems to have a sinusoidal variation. Altogether, a most interesting system.

Once the outburst is over we suggest that unfiltered CCD or PEP measures for 3-5 hours each night for a month or so would be very valuable. Timing should be good, preferably to the nearest 2-5 seconds. We would also request that any measures, either during outburst or in the month or so thereafter, be copied to Variable Stars South. For this exercise send them to Stan Walker. A useful paper is certain to arise and all substantial contributors will be coauthors.

For any further details, or information about this interesting star, contact Stan Walker at the above address.