Is 0406 + 121 the reddest BL Lac object?

Abstract

0406 + 121 is a flat-spectrum variable radio source first discovered by Shimmins et al.¹ and later studied by Condon et al.². While no optical counterpart was identified, similar sources were often found to be either BL Lac-type objects or high redshift QSOs. Recently, Rieke et al. detected the object in the near-IR, where it was observed to vary in luminosity over the six-month period between August 1978 and March 1979. A Kitt Peak 4-m plate taken on 29 November 1978 and published by Rieke et al. revealed a stellar image (V = 20.5 ± 0.4 mag) at the radio (and IR) position, which lies ∼30 arc s north of an anonymous spiral galaxy. Rieke et al. suggested that 0406 + 121 and some similar sources they detected having steep power law spectra were a possible new type of QSO discriminated against in conventional optical searches. To clarify the nature of 0406 + 121 further, we obtained a spectrum in good seeing conditions on the night of 27 February 1979 using the Steward Observatory 2.25-m telescope and intensified Reticon spectrograph. The object, clearly visible on an intensified TV guider, was wobbled every 80 s between two circular apertures 2.5 arc s in diameter spaced 20 arc s apart. Eighty-five minutes of integration time, calibration with the spectroscopic standard EG 67 (ref. 4) and a HeAr comparison lamp, and eight point smoothing of the 10 A resolution spectrum gave the results shown in Fig. 1.