Radial velocity follow-up of GJ1132 with HARPS. A precise mass for planet 'b' and the discovery of a second planet

GJ1132 is a nearby red dwarf known to host a transiting Earth-size planet. After its initial detection, we pursued an intense follow-up with the HARPS velocimeter. We now confirm the detection of GJ1132b with radial velocities only. We refined its orbital parameters and, in particular, its mass ($m_b = 1.66\pm0.23 M_\oplus$),density($\rho_b = 6.3\pm1.3$g\mathrm{.}cm$^{-3}$) and eccentricity ($e_b<0.22$; 95%). We also detect at least one more planet in the system. GJ1132c is a super-Earth with period $P_c=8.93\pm 0.01$ days and minimum mass $m_c\sin i_c=2.64\pm 0.44M_{\oplus }$. Receiving about 1.9 times more flux than Earth in our solar system, its equilibrium temperature is that of a temperate planet ($T_{eq}=230-300$ K for albedos $A=0.75-0.00$) and places GJ1132c near the inner edge of the so-called habitable zone. Despite an a priori favourable orientation for the system, $Spitzer$ observations reject most transit configurations, leaving a posterior probability $<1\mathrm{\%}$ that GJ1132c transits. GJ1132(d) is a third signal with period $P_d=177\pm 5$ days attributed to either a planet candidate with minimum mass $m_d \sin i_d = 8.4^{+1.7}{-2.5}~M\oplus$ or stellar activity. (abridged)