Discovery of an extremely bright submillimeter galaxy at z = 3.93

International audienceSerendipitously we have discovered a rare, bright submillimeter galaxy (SMG) with a flux density of 30 ± 2 mJy at λ = 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 × 1014/m L_⊙ and mass 6.0 × 109/m M_⊙ for its dust, allowing for a magnification factor m caused by a probable gravitational lens. The corresponding star-formation rate is 8.3 × 104/m M_⊙/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 H2 mass of between 1.9 × 1011/m M_⊙ and 1.1 × 1012/m M_⊙. The two lines C I(^3P1-3P_0) and C I(^3P2-3P_1) were clearly detected and yield a [ C I]/[ H2] number abundance of between 1.4 × 10-5 and 8.0 × 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