Small-Scale Structure in Galactic H I Absorption toward GRS 1915+105

Details Small-Scale Structure in Galactic H I Absorption toward GRS 1915+105 Small-Scale Structure in Galactic H I Absorption toward GRS 1915+105

Sep 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000apj...540..863d&link_type=abstract

The Astrophysical Journal, Volume 540, Issue 2, pp. 863-868.

Astronomy and Astrophysics

Astronomy

17

Ism: Atoms, Ism: Clouds, Radio Lines: Ism, Stars: Individual (Grs 1915+105)

Scientific paper

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 vLSR 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 vLSR 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.

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