I

The Blazar 0235+164:

A Study of the Correlation Between Optical and Radio Variations

Elizabeth J. McGrath ’01, Vassar College (elmcgrath@vassar.edu)

Advisor: Thomas J. Balonek, Colgate University

Optical observations of the blazar 0235+164 over the past nine years have been taken at Colgate’s Foggy Bottom Observatory. We have reduced the data between July of 1992 and March of 1998 using standard IRAF procedures, and present our light curve here. 0235+164 has shown extreme variability, varying more than 3 magnitudes within a couple months, and almost a full magnitude within only a couple days. In comparing the R filter optical flux density of our observations with the two centimeter wavelength radio flux density obtained by the University of Michigan Radio Astronomy Observatory, we show that there are significant correlations between the optical and radio variations of 0235+164.

I. Introduction

BL Lacertae type quasars, or blazars, are known for their violent variability at all observable wavelengths. BL Lac objects show neither absorption nor emission lines in their spectra and are believed to be the centers of relatively gas free galaxies. Quasars are also thought to be the centers of galaxies, at a distance of many billion light years away, but differ from BL Lac objects in that they do show emission lines. 0235+164, initially classified as a BL Lac object because it showed no observed emission lines, was found by Cohen et al. (1987) to have faint emission lines at z=0.94. Also, there appears to be no elliptical galaxy surrounding 0235+164 as in other BL Lac objects. 0235+164 is highly polarized in optical wavelengths, however, as is typical of blazars. Thus, the designation, "blazar," a combination of BL Lac object and quasar, seems the most appropriate classification for 0235+164.

In addition to the faint emission-line redshift, z=0.94, calculated by Cohen et al. (1987), two absorption-line redshifts of z=0.524, 0.851 were determined by Burbidge et al. (1976) and Rieke et al. (1976). It was apparent that these lines were not coming directly from the quasar because the quasar was at a higher redshift, so they concluded that the z=0.524 emission must be from a "companion" galaxy approximately 2 arcsec S of 0235+164 (hereafter, companion A). Yanny et al. (1989) found another "companion" galaxy 1.3 arcsec E of 0235+164 (hereafter, companion A1). This could account for the observed redshift z=0.851 near 0235+164, but it is more likely that this smaller "companion" is at the same redshift as companion A (z=0.524) and that the two may be interacting with each other.

Study of 0235+164 at Colgate’s Foggy Bottom Observatory has consisted mainly of R filter observations since 1989. Our intent is to follow the fluctuations of this blazar carefully and compare the optical activity with radio data obtained from colleagues. This blazar has a history of striking correlations between the two wavelengths, the first such noted in a 1975 outburst, by Rieke et al. (1976). Balonek and Dent (1980), following another outburst in 1979, showed a second correlation between radio and optical wavelengths. We have evaluated optical data over the past nine years and compared it to radio data obtained by the University of Michigan. In presenting this data, we hope to prove that it is seemingly impossible for these correlations to be entirely coincidental.

II. Observations and Data Reduction

Quasar observations at Colgate’s Foggy Bottom Observatory (hereafter FBO) were obtained using a sixteen inch Newtonian-Cassegrain telescope equipped with a Photometrics CCD camera cooled by liquid nitrogen to a temperature of » -94° C. The effective Cassegrain focal ratio f/13, when combined with the CCD, gives a field of view of 8.1’ x 5.4’ with » 0.8" pixels. Observations of 0235+164 at FBO were taken over a nine year period, since 1989, in I, R, and V filters. Exposure times varied between 2 to 5 minutes. For our purposes, we ignored the two "companion" galaxies as they were too faint for FBO to detect, so any light contributed by these galaxies to the total flux of the blazar is minimal and within our error bars. Figure 1a shows the field of 0235+164–the result of 9 combined images from Colgate, compared with two combined Hubble images of the blazar obtained by Burbidge et al. (1996) (Figures 1b and 1c).

Figure 1a. FBO image of 0235+164 and
the surrounding field.
8.1’ x 5.4’

Figure 1b. HST image of
0235+164.
10" x 10"

Figure 1c. HST image
of "companion" galaxies.
0235+164 was removed
from figure 1b.

Our data set includes R filter observations from July 1992 to March 1998. Preliminary reductions have been made for the data prior to July 1992 and for the data in the month of July 1998 for comparison purposes only. We focused on R filter observations primarily because the CCD chip is most responsive to light at these wavelengths (5700 Å — 7525 Å).

Reductions of the data were made using Colgate scripts in IRAF (Image Reduction and Analysis Facility). Bias subtraction and flatfielding were included in these scripts. Dark current was negligible, and therefore not included in the reduction process. The main problem area we encountered during analysis occurred when the blazar was faint. Skies at Colgate tend to be around 19^th magnitude per square arc second in R during clear summer nights, so for the nights when 0235+164 was near its minimum brightness (the end of 1993 to the beginning of 1997), error bars are high. Changing these values to flux density, shows the minimal fluctuations of 0235+164 more precisely during this time period. Takalo, et al. (1998) have studied in depth the fluctuations during 0235+164’s faint state. Our results are consistent with theirs during periods of low brightness.

Discussion

In an attempt to better understand what a redshift of z=0.94 means, we calculated the distance to 0235+164 to be approximately 2600 Mpc, or 8.4 billion light years, using a recent value of the Hubble constant, H₀=68±6 km/s/Mpc obtained by Baum et al. (1997). To put things in perspective, the age of the universe using the same Hubble constant would be approximately 14.5 billion years.

In figure 2, we present our light curve from July 1992 to March 1998 for 0235+164. The values are nightly averages of typically half a dozen images, and the error bars are the IRAF calculated errors for each measurement added in quadrature.

Figure 2. FBO R filter light curve for 0235+164.

An important discovery we made during our analysis was that IRAF photometry calculations were underestimating the error on the measurements for Colgate data. We compared the errors predicted by photon statistics with IRAF to the scatter in the individual nightly measurements of several "nonvariable" pairs of stars in the 0235+164 field. We found that the correct photon statistics error should be 30% higher than IRAF calculations predict.

In our final analysis, we compared our optical data to 14.5 GHz radio data obtained from the University of Michigan Radio Astronomy Observatory. In order to do this, we had to convert our magnitudes into flux density, so that the units would be consistent with radio flux density. In figure 3, our final results are combined with our preliminary reductions and the results of Takalo et al. (1998) to make a complete light curve from 1989 through 1998. Above the optical light curve is the radio plot of flux density versus time. Comparing the two, we can see that the optical data consistently peaks before the radio, which is predicted by synchrotron emission models. This is due to the fact that light emitted by the quasar must travel through a gaseous "superluminal" jet which is expanding over time. The shorter wavelengths, with a higher energy, can penetrate the dense gas, while the longer wavelengths must wait until the gas expands, thus decreasing its particle density, so that the longer wavelengths can escape. In addition, shorter wavelengths not only vary sooner, but also faster than longer wavelengths. Taking this into account, the shape of both optical and radio curves are strikingly similar. Of particular notice, the flare in mid-1992 (as well as the minimum that precedes it) seen at optical wavelengths is seen a couple months later in radio wavelengths. Also, the two peaks in optical during 1990-1991 could correlate to the peaks in radio from mid 1990-beginning 1991. At higher radio frequencies, even smaller features show similarities to optical events, such as the slight rise seen in 1996. These correlations give us reason to believe that whatever is causing the optical variations, could also be causing the radio variations, and that the two wavelengths are more closely related than once thought.

Figure 3. Radio and optical flux density variations for 0235+164. The curve on top is radio data from the University of Michigan. In the bottom curve, the circles are preliminary reductions from FBO, the dots are our completed reductions, and the triangles are from Takalo, et al. (1998).

Acknowledgments

I would like to thank my advisor, Thomas Balonek, for working side-by-side with me this summer on this research. I thank him for all his encouragement and words of wisdom, as well as his constant enthusiasm for observing. In addition, I would like to thank two of my coworkers, Stacey Benson and Sandra Black, for their help with initial data reductions. I thank my two astronomy professors at Vassar, Frederick Chromey and Debra Elmegreen, for their constant support and instruction. I would also like to thank the W. M. Keck Foundation for providing me with the opportunity to do summer undergraduate astronomy research, and for providing the grant that made it possible.

This research has made use of data from the University of Michigan Radio Astronomy Observatory which is supported by the National Science Foundation and by funds from the University of Michigan.

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