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Physics & Astronomy Department

Astr212 Project: Variability of Active Galaxy BL Lac

Contents: You are looking at the mysterious BL Lac. This name originates from its location in the Lacerta constellation. Located near the Degoba system, Lacerta is just to the right of Pegasus’ front hooves. With a redshift of 0.0686, BL Lac is approximately 900 million lightyears away (assuming Hubble parameter 75 km/s/Mpc). The object we studied was the very first of a sub-group of objects known as BL Lac types. These objects amount to the quasi-stellar objects in the cosmos. About 120 BL Lac specimens have seen intense study and catalogued, however, scant information is known concerning the surrounding galaxies.

Our BL Lac object is an Active Galactic Nuclei (AGN), which is characterized by activity from a super massive black hole in the center of a galaxy. A few astronomical units in diameter (the distance between the earth and the sun), this enormous rotating black hole emits free-free and synchrotron radiation from infalling material and hot gasses at all wavelengths. As matter gets pulled into the dense region of gas surrounding the black hole (accretion disk), it gets compacted and highly pressurized, producing high amounts of blackbody radiation. Furthermore, the infalling material varies and so our object varies in its luminosity as well. A counter-example of a galaxy that is not an AGN, is our Galaxy, the Milky Way. Unlike BL Lac, the black hole at the center of our Galaxy does not continually "feed" off of the matter surrounding it and so it does not vary in brightness over time. As a result, our Galaxy’s black hole and others like it are known as "dormant".

By taking many images overnight for five sets of data within one month, we have captured certain moments of BL Lac varying in brightness over time. Our data shows both continuous and sharp peaks of radiation for at least one set of images. Other sets of data showed very little variation, if none at all. In the end, we combined the average brightness of BL Lac per data set (i.e. per night of observations) to obtain a graph of its variability in brightness over a period of one month.

Light Curve of BL LAC Over 5 Sets of Data:

The above graph illustrates the variance of our object over all five sets of data that were taken. This range extends over a one month period. As can be seen, variability over such a large time interval is quite large, extending over 1.5 magnitudes. This range covers about one third of the expected variability of 4.8 magnitudes. The error bars for these data points show that our range of variability is quite reasonable. In the future, observations that are more evenly spaced as well as within about a week apart, would greatly enhance our data set. In particular, it would more clearly show gradual changes in variability over a large interval of time rather than just sharp ‘peaks’ that cannot account for all the variability in between.

Highest Variability the Course of 1 Night:
The following observations were taken on October 10, 2002 over a 1 hour range. As can be seen, very high variability in our object is observed, ranging over 0.5 magnitudes. We also see that much of this variation does not consist of sharp spikes, but gradual changes. Such information tells us that the material falling into the black hole during those times gradually heated up and fell into the event horizon or "point of no return". The sharp spike near the end of our graph tells us quite a bit of information. First, this change in magnitude of 0.5 magnitudes occurred in over 2 minutes, which indicates that the light emitting region has a size of 23% of the distance from the Earth to the Sun. For such a distant object that is practically point like, a change in brightness so quickly tells us that the energy being heated by this object must be extroadinarily huge. From what we currently know, only a black hole sucking in a large amount of material down its gravitational potential well can produce such an effect. Over all five sets of data, this set showed the most variability over one night.

Lowest Variability Over the Course of 1 Night:
This next graph denotes very little change in the brightness of BL Lac. Although the error bars are very small, the range of magnitudes is no greater than 0.3. Since BL Lac is reported to change in brightness by an average of 4.8 magnitudes, this data does not give us very much information over such a scale. However, there is some noticeable variability, especially near the earlier observations that night.

Processing:
Over the five sets of data that were taken, very little variability is observed. However, within the range of data, about a 1.5 magnitude variability is observed. The images taken (including the one above), were taken using a Celestron 16-inch telescope with a St-8 CCD camera and 16.6 mm eyepiece focal length. A clear filter was used with 15 second exposure times. Dark images were taken for each set of data. The photographs were processed in CCD-soft and the Dark images were subtracted in order to increase our signal/noise ratio. Astrophotometric methods were then used within the CCD software to determine the magnitude of our object relative to the stars of fixed magnitude near it.

In summary, observations of BL Lac were taken the following nights, with the recorded values as shown:

Date Approximate period of observations (hours) Number of Images Used Max changes in brightness overnight (Mag.)
10-10-2002 1.38 17 0.574
10-15-2002 1.67 64 0.292
10-25-2002 2.29 56 0.292
10-29-2002 0.91 26 0.159
11-16-2002 0.99 38 0.421

Suggestions for future observations:
For aspiring astronomers who are interested in conducting such a project, suggestions are as follows:

  1. Obtain as sharp of focus as possible. This would decrease your error bars by a significant factor.
  2. Take your data sets approximately 5-7 days apart. This would show more gradual changes of brightness over time.
  3. Each data set should ideally cover a time period of an entire night. However, if no variability is observed, at least two hours of observations would be sufficient.

Team: Chris Walker, Jason Hartley, and Benjamen R. Meyer