The redshift of the highly variable BL Lac object H0323+022

Abstract

New optical spectra are used to derive the redshift of H0323 + 022, a bright, highly variable x-ray source whose nature has been the subject of controversy. Most of the spectra are dominated by a featureless nonstellar continuum, as found in BL Lac objects. Photometric variations of ≳2 mag have been seen over a time interval of only 2 days. The nonstellar light is sometimes faint enough to allow the detection of absorption lines at a redshift z = 0.1471 + 0.0005. These lines are clearly due to stars in the host galaxy, which is easily visible in deep images of the object. The galaxy is probably a relatively normal elliptical with M_v ≈ -21 to -22, except that a noticeable extension is evident in one quadrant. On 24.15 February 1985 UT, the nonstellar continuum could be described by a power law of index a ≈ 0.6 (where f_v ∝ v -α); its relative strength in the nucleus (2" X 5.2") was ~62% at 5460 Å, while M_v ≈ -22 to -23. When the power-law component fades, its slope appears to steepen. There is now no doubt that H0323 + 022 is a BL Lac object, rather than a peculiar Galactic star. As discussed by Feigelson et al. (1986), its extremely rapid variability (ΔL ≈ 1 X 10^(45) erg s^(-1) in Δt ≈ 30 s) is matched only by that of OJ 287, and the hard x rays must come from a region roughly a factor of 10^4 smaller than that which produces the radio emission.