2005CBET..133....1Q

R. Quimby, P. Mondol, F. Castro, M. Sellers, P. Hoeflich, and J. C. Wheeler, University of Texas; and C. Gerardy, Imperial College, report the discovery of a supernova in unfiltered CCD images taken with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory on Mar. 28.17 (with the new object at mag about 16.2) and Apr. 2.16 UT (mag about 16.1). SN 2005bg is located at R.A. = 12h17m17s.18, Decl. = +16o22'17".6 (equinox 2000.0), which is 0".6 north and 0".4 east of the core of MCG +03-31-93. The new object was found by subtracting a co-addition of images taken between 2004 Dec. 11 and 2005 Jan. 17 (limiting mag about 18.6); a similar subtraction showed nothing at this location on Mar. 20.19 (limiting mag about 17.2). A spectrum (range 430-890 nm) of the new object, obtained on Apr. 7.13 with the 9.2-m Hobby/Eberly Telescope (+ Marcario Low-Resolution Spectrograph) by B. Roman and S. Rostopchin, shows it to be a type-Ia supernova. Assuming the NED host-galaxy recession velocity of 6921 km/s, the expansion velocity derived from the minimum of the Si II 635.5-nm feature is 11000 km/s. The spectrum also shows Mg II 448.1-nm, Fe II 508.3-nm, and S II 545.4- and 564.0-nm lines; however, the Ca II infrared triplet is not detected. Additional unfiltered CCD magnitudes for SN 2005bg: Apr. 3.16 UT, 15.8; 5.15, 15.6.