THE AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS 25 Birch Street, Cambridge, MA 02138 USA INTERNET: aavso@aavso.org Tel. 617-354-0484 FAX 617-354-0665 AAVSO ALERT NOTICE 257 (April 28, 1999) REQUEST TO MONITOR 2138+43 SS CYGNI FOR MULTIWAVELENGTH OBSERVATIONS We are collaborating with astronomers from Lawrence Livermore National Laboratory, Livermore, CA, in the scheduling of multiwavelength simultaneous observations, particularly with the Hubble Space Telescope's (HST) Imaging Spectrometer (STIS) and the Rosse X-Ray Timing Explorer (RXTE), and possibly the Extreme Ultraviolet Explorer (EUVE) and ground-based resources, of SS Cyg during outburst. One of the goals of this observing run is to carry out a multiwavelength search for very small-amplitude and very short-period oscillations of the white dwarf. A similar, highly successful campaign on SS Cygni was carried out in 1996 (see AAVSO Alert Notices 221 and 230 for information on this campaign; the campaign and results are summarized in the Annual Report of the Director in Volume 25, Number 2, of the Journal of the AAVSO). However, although EUVE and RXTE satellites were scheduled and had very successful runs, HST could not be scheduled at that time. In 1997 a second campaign was scheduled (see AAVSO Alert Notice 241), but SS Cygni unfortunately did not have suitable outbursts during times when HST observations could be scheduled. The present campaign is extremely important, as it is the LAST CHANCE for these astronomers to observe SS Cygni with HST under this proposal. Their observing window closes June 30. In this year's campaign, the investigating astronomers have requested that we inform them immediately when SS Cyg starts to go into outburst, i.e., brightens, and to continue to keep them informed throughout the outburst, particularly during the rise and at maximum so that the satellites and other observing resources may be scheduled to observe SS Cyg. Timely notification of the onset of the outburst and up-to-the-minute information on the behavior of SS Cyg throughout the outburst are essential for these multiwavelength observations. Presently, SS Cyg is near minimum, at approximate visual magnitude 11.5, according to observations received at the AAVSO. Its last outburst, a narrow one, occurred in late March 1999, when it began to brighten from magnitude approximately 11.8 on March 19-20, reached magnitude 8.3 on March 24, and declined to magnitude approximately 11.5 by April 4, where it has remained to date. We predict the next outburst to occur sometime between now and mid-May. SS Cyg has had several "minioutbursts" between real outbursts recently, when the star brightens to about magnitude 10.5 - 11 and fades to minimum after several days. Your continuous observations and alerts as the star brightens will be crucial to determining the type of the next outburst. AAVSO "b", "c", "d", and "e" scale standard charts for SS Cyg have been posted on our ftp site; see below for details. Those needing paper copies of these charts should contact AAVSO Headquarters. Please monitor SS Cyg closely, and inform us immediately by phone, fax, or email when it starts to brighten, i.e., becomes brighter than 11.5. Once SS Cyg goes into outburst, please send in your observations daily so we may keep the astronomers informed of the star's behavior and they may schedule the observations. The success of these observations very much depends on your observations and the early alerts and continuous information we can provide to our colleagues. And remember - this is the last chance for these astronomers to observe SS Cyg with HST under their current proposal. CHARTS AVAILABLE ON AAVSO WEB AND FTP SITES Electronic copies of the AAVSO charts mentioned in this Alert Notice, as well as the reversed versions prepared by Marc Biesmanns, are available through our web site at the following address: http://www.aavso.org These charts and the image may also be obtained directly from our FTP site: ftp.aavso.org (198.116.78.5), in /alerts/alert257) The answering machine at AAVSO Headquarters is on nights and weekends for your convenience. Please call our charge-free number (888-802-STAR = 888-802-7827) to report your observations. We also encourage observers to send observations by fax to 617-354-0665 or by e-mail through the Internet to observations@aavso.org. Many thanks for your valuable astronomical contributions and your efforts. Good observing! Janet A. Mattei Director