Electronic Telegram No. 176 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://cfa-www.harvard.edu/iau/cbat.html X-RAY FLASH FROM M51 S. Immler, Goddard Space Flight Center, NASA, and USRA; and A. Kong and W. H. G. Lewin, Massachusetts Institute of Technology, on behalf of a larger collaboration, report the detection of an x-ray flash in a 2136-s SWIFT XRT observation of M51 from July 6.231 UT (sequence 00030083011): "The significance of the detection is 6 sigma. Given an offset of the source from the tentative identification of the SN 2005cs progenitor (Richmond, IAUC 8555) of 8", and an XRT point-spread function of 18" (half-power diameter at 1.5 keV), this new source is probably unrelated to the supernova; however, based on the position alone, we cannot exclude the possibility that the emission is from SN 2005cs. The (background-subtracted) count rate of the source is (7.4 +/- 1.5) x 10**(-3) count/s, which corresponds to an unabsorbed 0.3-10-keV-band flux (averaged over the observation) of (3.3 +/- 0.7) x 10**(-14) erg cm**(-2) s**(-1) and an average luminosity of (2.8 +/- 0.6) x 10**(39) ergs/s, assuming a 10-keV thermal bremsstrahlung spectrum, an absorbing column density of 1.57 x 10**(20) cm**(-2) (Dickey and Lockman 1990, ARAA 28, 215), and a distance of 8.4 Mpc (Feldmeier et al. 1997, Ap.J. 479, 231). No x-ray source was detected 96 min later (sequence 00030083012, exposure 2031 s), as well as in all other SWIFT XRT observations of M51 on June 30, July 3, 5, 6, and 7, and in a previous 4800-s Chandra observation on 2003 Aug. 7. We encourage observations at other wavelengths." NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the formal IAU Circulars. (C) Copyright 2005 CBAT 2005 July 8 (CBET 176) Daniel W. E. Green