MK 421 = 2A 1102+384

G. J Fishman, National Aeronautics and Space Administration, writes that the HEAO-1 spacecraft is scheduled to observe the BL Lac object Mk 421 = 2A 1102+384 during May 28d03h-28d12h UT. Optical and radio monitoring of the source near this time would be desirable. HEAO-1 pointed observations are continuing to be made about 5 times per week for intervals of 3 to 9 hours. F. Walter and K. O. Mason, University of California at Berkeley; and G. Garmire, California Institute of Technology, report that the HEAO low-energy detectors have also observed a soft source near Mk 421 on serveral scans during 1977 Nov. 23-25 (cf. IAUC 3212). The 0.2-2.0-keV flux was ~ 10**-10 erg cm**-2 s**-1, while a thermal bremsstrahlung spectral fit yielded a temperature in the range 2 to 5 x 10**6 K and NH = 1 to 4 x 10**20 atoms cm**-2. The 94-percent error-box coordinates are: R.A. = 11h02m00s, Decl. = +38o29'; 11h01m26s, +38o16';10h59m10s, +38o29'; 10h59m43s, +38o42' (equinox 1950.0). Mk 421 lies on the edge of this box.