On the optical extinction and distance of GRS 1915 + 105

Based on new millimeter and radio observations, we reevaluate the total hydrogen column density along the line of sight to the microquasar GRS 1915+105 to NH = (3.5 ± 0.3) × 10 22 cm −2. Our value is consistent with the one derived from X-ray measurements, namely (3.8 ± 0.3) × 10 22 cm −2 (Ebisawa et al. 1998). Using the empirical law between the visual extinction and the total column density of hydrogen, Av is found to 19.6 ± 1.7 mag. This result is ∼ 7 mag lower than previously thought, and therefore, a reevaluation of infrared fluxes after derredening is needed. The revisited kinematic study allows to give a lower limit to the distance of GRS 1915+105 , namely 6.0 kpc. Taking into account the most accurate upper limit of distance inferred from radio data (11.2 ± 0.8 kpc; Fender et al. 1999) as well as this lower limit, this implies a distance of 9.0 ± 3.0 kpc. 1. GRS 1915+105 and its optical extinction GRS 1915+105 is a Galactic X-ray binary in the Aquila constellation (l=45.37 o , b=-0.22 o), discovered on 15 august 1992 by the WATCH all-sky X-ray monitor on board the GRANAT satellite (Castro-Tirado et al. 1994). Based on follow-up observations with the VLA, GRS 1915+105 was shown to be the first Galactic source with apparent superluminal motion (Mirabel & Rodríguez 1994). Using the proper motion properties of relativistic radio ejecta observed with MERLIN and under the assumption of a symmetric ejection, the more constraining upper limit of the distance was found to 11.2 ± 0.8 kpc (Fender et al. 1999). CO absorption lines in the infrared spectrum clearly identified the donor as a K-M III star (Greiner et al. 2001a,2001b). Due to the relative motion of the companion star around the center of mass of the system, these lines were modulated by Doppler effect. The period of the system (33.5 day) and its mass function (9.5 ± 3.0 M ⊙) were obtained. The mass of the compact object was measured to 14 ± 4M ⊙ , which unambiguously identify the compact object as a black hole (Greiner et al. 2001b). During a monitoring observation of GRS 1915+105 by the Ryle telescope at 15 GHz, quasi-periodic oscillations were found and associated with the soft-X-ray variations on the same timescale (Pooley & Fender 1997). Fast infrared oscillations on timescales of less than an hour as …