Discovery of an extremely bright submillimeter galaxy at z=3.93

Serendipitously we have discovered a rare, bright submillimeter galaxy (SMG) with a flux density of 30 +/- 2 mJy at lambda = 1.2 mm, using MAMBO2 at the IRAM 30-m millimeter telescope. Although no optical counterpart is known for MM18423+5938, we were able to measure the redshift z = 3.92960 +/- 0.00013 from the detection of CO lines using the IRAM Eight MIxer Receiver (EMIR). In addition, by collecting all available photometric data in the far-infrared and radio to constrain its spectral energy distribution, we derive the FIR luminosity 4.8 x 10(14)/m L-circle dot and mass 6.0 x 10(9)/m M-circle dot for its dust, allowing for a magnification factor m caused by a probable gravitational lens. The corresponding star-formation rate is 8.3 x 10(4)/m M-circle dot/yr. The detection of three lines of the CO rotational ladder, and a significant upper limit for a fourth CO line, allow us to estimate an H-2 mass of between 1.9 x 10(11)/m M-circle dot and 1.1 x 10(12)/m M-circle dot. The two lines C-I(P-3(1)-P-3(0)) and C-I(P-3(2)-P-3(1)) were clearly detected and yield a [C-I]/[H-2] number abundance of between 1.4 x 10(-5) and 8.0 x 10(-5). Upper limits are presented for emission lines of HCN, HCO+, HNC, H2O, and of other molecules. The moderate excitation of the CO lines is indicative of an extended starburst, and excludes the dominance of an AGN in heating this high-z SMG.