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 yr -1 and -6.24 0.05 mas yr -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.