Persistent X-Ray Emission from MXB1659-29

W. Lewin, J. Hoffman, H. Marshall, F. Primini, W. Wheaton, L. Cominsky, G. Jernigan and W. Ossman, Massachusetts Institute of Technology, report the discovery of persistent x-ray emission from a source located within 5' of a 0o.6-long line with end points R.A. = l6h57m.6, Decl. = -30o08' and R.A. = 16h59m.8, Decl. = -29o42' (equinox 1950.0). The highly variable source was observed by SAS 3 between Mar. 6d23h08m and 8d13h43m UT. On two occasions (1.6 hr apart) the source showed a rapid (in ~ 40s) decrease in intensity (by a factor of 4) to a low state that lasted ~ 15.5 min in both cases. The transition to the high state also took place in ~ 40s. In the high state the flux is ~ 8 percent that of the Crab (1-10 keV). It is likely that the persistent emission comes from MXB1659-29 (cf. IAUC 2994). This burst source has been observed by SAS 3 several times. On each occasion it produced about 10 bursts per day at very regular intervals, however, and no persistent emission was ever observed (Lewin 1977, Ann. N.Y. Acad. Sci. 302, 213). This time only one burst (on Mar. 8d07h47m32s UT) was detected in the entire 39-hr observing period. G. Share, K. Wood, D. Yentis, N. Johnson, S. Shulman, J. Meekins, W. Evans, E. Byram, T. Chubb and H. Friedman, Naval Research Laboratory, report that the source was observed by the large-area survey instrument on HEAO 1 during seven quick-look scans made during Mar. 6-10. Its 1-10-keV intensity was ~ 5 x 10**-10 erg cm**-2 s**-1 during six of the ~ 5s scans but was < 10**-10 erg cm**-2 s**-1 during the scan on Mar. 9d10h09m54s UT. The corners of the HEAO A1 error box are: R.A. = 16h58m05s, Decl. = -29o45'.8; 16h57m59s, -29o57'.7; 16n59m48s, -30o00'.2; 16h59m54s, -29o48'.3 (equinox 1950.0). R. Griffiths, M. Johnston, H. Bradt, R. Doxsey, H. Gursky, D. Schwartz and J. Schwarz, Center for Astrophysics and Massachusetts Institute of Technology, report that HEAO-1 scanning modulation collimator (A3) observations near Mar. 7d12h and 9d10h UT of the persistent x-ray source reported above by Lewin et al. and by Share et al. result in a small (~ 1.3 arcmin**2) error box having a center at R.A. = 16h58m54s.8, Decl. = -29o49'55" (equinox 1950.0) and corners at R.A. = 16h58m54s.5, Decl. = -29o52'02"; 16h58m53s.5, -29o49'00"; 16h58m55s.1, -29o47'48"; 16h58m56s.1, -29o50'50". Of the several possible A3 positions this is the only one that falls entirely within the HEAO A1 error box. It is also within the SAS-3 error ellipse for the burster MXB1659-29 (IAUC 2994). Three other possible A3 positions fall near but outside the edges of the A1 box. The globular cluster NGC 6266, which falls within the SAS-3 error box for the persistent source, lies just outside the A1 box and is independently excluded by the A3 positions. It can be noted that the above source locations are consistent with the 4o-long error box for 4U 1704-30 (3.1 Uhuru cts/s). Another possible example of a similar type of persistent or transient emission from a burster is that of MX1608-52 (Fabbiano et al. 1978, Astrophys. J. Lett. Apr. 15).