Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...278..391s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 278, no. 2, p. 391-405
Astronomy and Astrophysics
Astronomy
65
Blazars, Gravitational Lenses, Light Curve, Radio Jets (Astronomy), Visible Spectrum, Astronomical Photometry, Autocorrelation, Charge Coupled Devices, Emission Spectra, Fourier Transformation, Power Spectra, Spectral Resolution
Scientific paper
We present photometric measurements of the blazar 3C 345 (1641+399, z = 0.595) in Johnson R, V and B covering the time since Sep. 1988. The errors in our CCD photometry relative to stars in the frames range from 0.005 to 0.02 mag for good to medium observing conditions. Until the end of 1990, 3C 345 was relatively quiet; in 1991/92 three rapid flares (Delta R greater than or approx. = 1 mag, Rmin less than or approx. = 14.8) have been recorded. The blazar 3C 345 is redder during active phases, probably due to a less variable component of the continuum, the so-called 3000 A bump which is also obvious in an optical spectrum obtained at R = 15.9. We calculate the power spectrum and the auto-correlation function of the light curve, including literature data since 1965, and confirm that the variability of 3C 345 is essentially non-periodic, at best quasi-periodic concerning the appearance of long-term outbursts every 5-10 years and the appearance of short-term flares every 5-10 months during the outbursts. We also report on radio monitoring data at 22 and 37 GHz taken between 1980 and 1992. Long-term outbursts in the optical and mm-regime occur almost simultaneously; however, the short-term behavior is different. Assuming that the 1991/92 outburst is due to the formation of a new knot in the jet, we predict the appearance of a new superluminal VLBI component. The mean apparent knot velocity is probably upsilonapp approx. = 0.22 mas/yr starting at approx. Jan. 1991. We discuss whether the short-term optical flares could be the result of relativistic beaming, gravitational microlensing or a combination of both. Recently, the detection of a probable foreground galaxy has been reported; one may therefore assume that microlensing contributes in some part to the optical variability of 3C 345. However, neither the time-scale nor the frequency of the flares are in agreement with a microlensing model having reasonable parameters. We conclude that the flares more probably occur through variable relativistic beaming of the new knot moving on a helical orbit in the jet, producing a 'lighthouse effect'. A theoretical light curve based on this model shows a satisfactory correspondence with the observational data. Some deviations, however, indicate either that our actual model is too simple or that other variability mechanisms are at work as well.
Bade Norbert
Borgeest Ulf
Camenzind Max
Dreissigacker O.
Heidt Jochen
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