Astron. Astrophys. 351, 21-30 (1999)

1. Introduction

The peculiar galaxy NGC 1275 contains a peculiar nucleus. According to the emission-line characteristics of its optical spectrum, the nucleus is of Sy2 (Marziani et al., 1996), or Sy 1.5 (Veron-Cetty & Veron, 1998) type. But, considering the strong and rapid variability of the total and polarized flux in the optical and radio region the nucleus is suggested to be a blazar (Angel & Stockman, 1980; Kinney et al., 1991; P. Veron, 1978; Courvoisier et al., 1992 and others). Readhead et al. (1990) argued that the discrepancies between the emission lines and the continuum spectrum characteristics of the nucleus are a result of observational selection. The distance to the galaxy NGC 1275 is less by an order of magnitude than those to other galaxies of the same high-level luminosity 10⁴⁴ ergs/s. This gives rise to observational increase of the equivalent widths of the emission lines, compare to those of other blazars.

The nature of the processes causing the high blazar luminosity is not exactly clear. The mechanisms of the continuum emission are open to discussion, too. The spectrum of the nucleus of NGC 1275 was fitted by many authors, e.g. for the infrared by Hildebrand et al. (1977), Impey & Neugebauer (1988), Knapp et al. (1990), Knapp & Patten (1991); from the optical to the infrared by Rieke (1978), from optical to radio by Babadzhanjants et al. (1972), Longmore et al. (1984) and others. Unfortunately there is no common opinion for these fittings. Martin (1978) investigated the spectrum of the nucleus of NGC 1275 from the X - ray to radio and concluded that "NGC 1275 is a fascinating and important object. Understanding it would surely signify progress towards understanding active nuclei and quasars. All observational efforts to this end should be encouraged".

Variability measurements are a tool for exploring the physics of blazars, quasars and other Active Galactic Nuclei (AGNs). The high variability of the NGC 1275 nucleus was pointed out in the far ultraviolet (Treves & Girardi, 1991). It is in the list of the three most variable blazars among 8 observed with IUE from 1978 to 1984. The characteristics of the optical variability of the NGC 1275 nucleus were investigated over 30 years after the first study by Barnes (1968). The longest runs of observations were made by Lyuty (references in Lyuty, 1977, 1987; Nesterov et al., 1995), and Babadjaniants, Hagen-Thorn (see Babadjaniants et al., 1972; Babadjaniants & Hagen-Thorn, 1975; Hagen-Thorn, 1987). The extreme active flaring period was mentioned during the sixties and seventies, but from the end of the seventies the nucleus was at a minimum of activity (Nesterov et al., 1995). Lyuty & V. Pronik (1975) revealed two components in the U light curve in 1967-1974: a slow component with a time scale of 1.5 years and an amplitude 0.^m4, and a rapid one - with a time scale of 20 days and an amplitude 0.^m8. When the sampling became dense enough, the time scale of a rapid component decreased (Dibaj & Lyuty, 1976; Lyuty, 1977, 1987; Hagen-Torn, 1987; Basko & Lyuty, 1977 and others). The highest number of components in the light curve of the nucleus of NGC 1275 was revealed by Lyuty (1987) - 5 components with time scales of from 1 day to 16 years.

The variability in the optical on a time scale of about one day was investigated by Martin et al. (1976). The intranight variability of the nucleus was studied by Merkulova & Pronik (1985), Merkulova et al. (1987, 1988, 1992, 1993), Pronik et al. (1990, 1998), and Merkulova & Metik (1995, 1996). It was shown that in 1989-1994 the nucleus's intranight UBVRI variations at a level SD 3 (SD - the standard deviation of the variable flux averaged by night, and - the error of a single flux measurement) have a duty cycle of 0.4.

The optical variability of the nucleus of NGC 1275, with its diverse morphological characteristics and seemingly stochastic nature, is still a problem, with which no current model can adequately deal. In this paper, we examine the character of the optical variability of the nucleus of NGC 1275 using two samples of photoelectric observations obtained in 1982-1994. The examination led to the supposition that the variation of the nucleus is caused by two or more sources. Time evolution of the observed characteristics of the nucleus was suspected.

© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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