A Parallax Distance to the Microquasar Grs 1915+105 and a Revised Estimate of Its Black Hole Mass

Abstract

Using the Very Long Baseline Array, we have measured a trigonometric parallax for the microquasar GRS 1915+105, which contains a black hole and a K-giant companion. This yields a direct distance estimate of 8.6 +2.0 −1.6 kpc and a revised estimate for the mass of the black hole of 12.4 +2.0 −1.8 M⊙. GRS 1915+105 is at about the same distance as some H II regions and water masers associated with high-mass star formation in the Sagittarius spiral arm of the Galaxy. The absolute proper motion of GRS 1915+105 is −3.19 ± 0.03 mas y−1 and −6.24 ± 0.05 mas y−1 toward the east and north, respectively, which corresponds to a modest peculiar speed of 22 ± 24 km s−1 at the parallax distance, suggesting that the binary did not receive a large velocity kick when the black hole formed. On one observational epoch, GRS 1915+105 displayed superluminal motion along the direction of its approaching jet. Considering previous observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet inclination to the line of sight of 60◦ ± 5 ◦ and a variable flow speed between 0.65c and 0.81c, which possibly indicates deceleration of the jet at distances from the black hole & 2000 AU. Finally, using our measurements of distance and estimates of black hole mass and inclination, we provisionally confirm our earlier result that the black hole is spinning very rapidly.