Spectropolarimetric observations of OJ 287 during outburst

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Session 17 -- Radio Galaxies and High Luminosity AGNs
Oral presentation, Monday, 30, 1994, 10:00-11:30

[17.07] Spectropolarimetric observations of OJ 287 during outburst

JosÚ A. de Diego, JosÚ N. Gonzalez-Perez (Instituto de Astrofisica de Canarias), Phil Charles (Department of Astronomy, Oxford University), Mark Kidger (Instituto de Astrofisica de Canarias)

The BL Lac object OJ287 has been studied with the 4.2m William Herschel Telescope (WHT) from the Spanish Observatory of La Palma on six nights in December 1993 and January 1994, as part of the OJ-94 International Campaign of monitoring of OJ287. At the time of the first observations, in mid-December the object was in the decline phase of a moderate outburst, reaching minimum in early January. In order to get both spectral and polarimetric observations, we used the ISIS double spectropolarimeter, giving us a spectral resolution of 3┼ over the range from 5500-9000┼. This permits both the rapid continuum polarisation variations to be examined from individual exposures (resolution 40 minutes, provided by four individual integrations with different positions of the calcite retarder) and , by summing all the exposures of a full night (a total integration time of up to 8 hours), the weak emission lines may be studied at a resolution far higher than has ever previously been possible. A period or pseudoperiod of 11.65 yr has been claimed for the outbursts of this object (Sillapaa et al 1988, Kidger et al 1992), which has been interpreted as due to a binary black hole (Sillapaa et al 1988). This model predicts an outburst at some time in 1994. If the Winter 1993-94 eruption corresponds to the expected 1994 outburst, the rise in brightness would be associated to accretion processes and, therefore, we hope to find a polarized spectrum different of those originated by synchrotron sources. The WHT observations are, we believe, the first of their kind of a blazar and these data will help to elucidate this question, apart from supplying valuable information on the emission processes in active galactic nuclei.

References: Sillanpaa A., Haarala S., Valtonen M., Sundelius B., Byrd G.G, 1988, ApJ, 325, 628 Kidger M., Takalo L., Sillanpaa A., 1992, A&A, 264, 32

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