Time variations in the high-redshift 21 centimeter absorption spectrum of the BL Lacertae object AO 0235+164

Details Time variations in the high-redshift 21 centimeter absorption spectrum of the BL Lacertae object AO 0235+164 Time variations in the high-redshift 21 centimeter absorption spectrum of the BL Lacertae object AO 0235+164

Aug 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982apj...259..495w&link_type=abstract

Astrophysical Journal, Part 1, vol. 259, Aug. 15, 1982, p. 495-521.

Astronomy and Astrophysics

Astronomy

46

Absorption Spectra, Astronomical Models, Bl Lacertae Objects, Radio Sources (Astronomy), Red Shift, Centimeter Waves, Flux Density, Monte Carlo Method, Spectrum Analysis, Time Dependence

Scientific paper

We present evidence for time variations of the redshifted (Z =0.524)21 cm absorption spectrum of the BL Lac object AO 0235+164. A 4 year monitoring program with the 305 m telescope at Arecibo shows that the line depths of the four deepest features (A → D) vary in time. The strongest changes occur in feature D, which was observed to double its line depth in 0.5 yr. Weaker variations (between 15 and 35%) with durations between 0.25 and 0.5 yr are observed in all four features, and there is tentative evidence for a monthly flickering with amplitudes of ∼10%. Whereas each of the features evolves independently during some epochs, they appear to vary synchronously during others. We have also monitored the velocity centroids and velocity widths of the features and find that neither varies by more than ∼1 km s-1 km 5 1 We have placed a 3 σ upper limit of 0.75 km s-1 for centroid variations of feature D. But there is tentative evidence for systematic and correlated variations of ∼0.2 km s-1 in the velocity centroids of features A, B, and C. Throughout the monitoring program the continuum flux density at the observed frequency (932 MHz) of the 21 cm line has remained nearly constant.
We interpret the line variations to stem from activity in AO 0235+164. The extrinsic model assumes that 0235, which has a redshift Z > 0.85, is at its redshift distance. When viewed through a nonuniform distribution of foreground clouds, variations in the brightness distribution of the background source cause apparent changes in line depth. This model predicts larger and more frequent shifts in velocity centroid than are observed. The intrinsic model assumes that 0235 is located near the absorbing gas, and that the redshift difference is attributed perhaps to an ejection velocity. Variations of the 21 cm continuum produce changes in spin temperature, and hence in 21 cm optical depth. While this model explains the synchronous changes in line depth and the absence of velocity variations rather naturally, the lack of a significant increase in 932 MHz flux during those epochs when the line depths decreased considerably is difficult to understand. We describe future tests which can rule out either of these models conclusively.

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