Circular No. 2879 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION Postal Address: Central Bureau for Astronomical Telegrams Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. Cable Address: SATELLITES, NEWYORK Western Union: RAPID SATELLITE CAMBMASS INTENSE X-RAY BURSTS FROM A GLOBULAR CLUSTER J. Grindlay, Center for Astrophysics, Harvard and Smithsonian Observatories; and J. Heise, Laboratory for Space Research, Utrecht, report the detection of two intense x-ray bursts from the x-ray source 3U 1820-30 associated with the globular cluster NGC 6624. These events appear to be qualitatively different from short-term variations previously reported in cosmic x-ray sources. The bursts were detected by both the 1-30 keV (HXX) Cambridge detectors and the 1-6 keV (SXX) Utrecht experiments on ANS on Sept. 28 at 1h31m51s and 9h49m40s UT. In the first event, the source intensity increased by a factor of 20 in about 1 s and decayed approximately exponentially over the next 10 s. The second burst was detected in a region of very low and constant background as a factor of 30 increase (less than 0.5-s rise time) in source intensity, lasting about 2 s and followed by a decay-time constant nearly identical with that of the first event. They can exclude the possibility that these were particle-induced events since the background counters showed no detectable increase and the 10' x 3o collimators of the HXX experiment yielded a direction for the bursts within 1' of 3U 1820-30 and the nucleus of NGC 6624. During the total exposure of three hours in pointings on the source between Sept. 27.4 and 30.7, the source intensity was otherwise constant at about 3.5 cts/s or about one-quarter of the similarly constant flux observed during three hours of ANS pointings between Mar. 24.4 and 28.0. No bursts were detected during this first observation period, during which the source x-ray luminosity was near the Uhuru value of about 10**38 erg/s. The peak luminosity during the bursts was thus near l0**39 erg/s. Although the time profile resembles that of gamma bursts the spectra were much softer (~ 10 keV exponentials). Given the detection of two such events in six hours of observing, further monitoring of globular-cluster x-ray sources at x-ray, optical and radio wavelengths is needed. CYGNUS X-1 S. S. Holt, L. J. Kaluzienski, E. A. Boldt and P. J. Serlemitsos, Goddard Space Flight Center, report that the present Cyg X-1 increase (IAUC 2863) shows no sign of decay after more than one month of continuous interrogation by the Ariel 5 all-sky monitor. Previous increases in May and September did not last much longer than one week, and the x-ray intensity started decaying after a few days. Cyg X-1 may therefore have returned to a long-term high-intensity level similar to that prior to the 1971 transition. Its present average intensity is approximately 1.5 times that of the Crab Nebula in the energy range 3-6 keV. PERIODIC COMET KOHOUTEK (1975c) The following precise positions were obtained by G. Schwartz, C. Y. Shao and R. E. McCrosky at the Harvard Observatory's Agassiz Station. Measurer: Shao. The Apr. 1 image was trailed. On Apr. 29 and May 9 there was a strong condensation embedded in a small, diffuse coma. 1975 UT R. A. (1950) Decl. m2 Observer Apr. 1.07554 6 02 03.88 +20 18 30.4 Schwartz 29.08907 7 15 32.16 +18 34 06.7 14.5-15 Shao May 9.06368 7 40 16.68 +17 34 08.5 15.5 McCrosky 30.08625 8 29 36.09 +14 56 12.6 " PERIODIC COMET GEHRELS 1 (1973 I) E. Roemer, University of Arizona, communicates the following precise positions. Measurers: D. J. Granrath (first eight plates) and C. C. McCarthy (last two plates). 1972/73 UT R. A. (1950) Decl. Note Dec. 3.07917 0 48 42.11 + 9 00 43.0 1 3.12292 0 48 42.33 + 9 00 50.7 1 13.06806 0 50 50.84 + 9 33 30.5 2 13.08472 0 50 51.15 + 9 33 34.4 2 Jan. 1.10069 1 00 37.00 +11 02 59.3 1 1.14583 1 00 38.79 +11 03 14.1 1 29.09850 1 26 10.82 +14 06 05.4 2 29.12176 1 26 12.34 +14 06 15.4 2 Sept.23.43420 7 42 57.84 +30 21 38.5 2 23.48038 7 43 00.90 +30 21 35.4 2 Note 1: Lunar and Planetary Laboratory's 154-cm reflector. Note 2: Steward Observatory's 229-cm reflector. L. M. Vaughn assisted on Dec. 13, J. Q. Latta on Jan. 29, G. Reskin on Sept. 23. COMET KOHOUTEK (1973 XII) C. Torres, University of Chile, provides the following precise position, obtained with the Maksutov astrograph at Cerro El Roble: 1973 UT R. A. (1950) Decl. Dec. 5.34332 14 05 38.58 -21 48 29.9 1975 December 8 (2879) Brian G. Marsden
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