2013ApJ...776...76P

The Galactic bulge shows a double red clump in sightlines at|b| ≳ 5° and -3° ≲ l ≲ 4°. This dump is interpreted as the signature of an X-shaped structure seen almost edge-on. We measure the proper motions of the stars belonging to the closer and the further arms of the X-shaped structure. The intrinsic kinematic properties of the two arms are found by incorporating information taken from the luminosity function. At b = -5°, we find that the proper motion difference between the two arms is a linear function of Galactic longitude for -0.°1 < l < 0.°5, which we interpret as a streaming motion of the stars within the X-shaped structure. A streaming motion was previously reported based on radial velocity data, not the proper motions. The proper motion difference in longitude is constant for -0.°8 < l < -0.°1, which provides an estimate of the bulge rotational speed of 87.9±8.2 km/s/kpc.