Small‐Scale Structure in Galactic H i Absorption toward GRS 1915+105

The existence of small-scale variations in the interstellar medium, between sight lines separated by 10-100 AU, is well established from pulsar, VLBI, MERLIN, and VLA observations. The angular resolutions are in the range 10-100 mas. During an outburst of the microquasar GRS 1915+105, the proper motion of the approaching component is ~250 AU (23 mas) day -1 . Using the VLA to observe the H I line at 21 cm, we have measured the time-variable H I absorption along the changing line of sight during such an outburst. We detect opacity changes of Δτ ~ 0.67 ± 0.16 in gas at v LSR of 55 km s -1 , near the velocity of the tangent point at 65 km s -1 , over ~6 days, for lines of sight ~1000 AU apart. Another detection at v LSR of 5 km s -1 has Δτ ~ 0.24 ± 0.06, on lines of sight ~40 AU (or 440 AU) apart, depending on the near (or far) distance to the absorbing gas. From the several days of spectra, 3 σ upper limits are derived: Δτ ≤ 0.5 for gas at 55 km s -1 , on scales ~150-900 AU; and Δτ ≤ 0.25 at 5 km s -1 , on scales ~25-150 AU (gas at 1 kpc) or ~275-1650 AU (gas at 11 kpc). We thus probe the H I structure in distant Galactic material, using a Galactic background source with rapid structural changes. Our approach complements those using extragalactic sources and nearby pulsars. Our results are consistent with extrapolation of the power-law index of 0.375 for the structure function of H I opacity (equivalent to an index of 2.75 for the power spectrum of H I), as measured by others from 4 pc to 0.02 pc toward Cassiopeia A.