Interpretation of Blazar Flux Variations as Music

Dr. James R. Webb

Department of Physics

Florida International University

Abstract

Blazars are believed to be distant galaxies in the process of formation. They emit electromagnetic radiation (light) over the entire electromagnetic spectrum from radio waves to gamma-rays. The emission varies with time in most frequency ranges and the causes for the variation are yet to be adequately explained. Astronomers have been monitoring these objects with optical telescopes for over 50 years now and we have collected a large database of brightnesses over these fifty years. This paper presents some of these light curves, and adopts a computational method to translate the brightness fluctuations into musical tones. These tones are then converted to sound using a midi synthesizer on a PC.

The music you are hearing is the converted light curve from Active Galactic Nucleus 3C 120. This source is extremely energetic and variable and is located about 1 billion parsecs from Earth.

Click here to download the 3C 120 quasar music in mp3 format.

Introduction

The study of Quasars began in 1962 when Marteen Schmidt (1962) interpreted the broad emission lines of 3C273 as Doppler shifts. The extreme redshifts were indications that the objects were extremely distant, literally at the edges of the visible universe. Subsequent observations of these sources showed that they were unresolved or "star-like", and emitted radiation over the entire electromagnetic spectrum. Some of these sources varied in brightness unpredictably and with large amplitudes (Webb et al 1988). This subset of objects were called Blazars. There are hundreds of Blazars now known, but only a handful of them have been targets of constant monitoring. Fourier analysis and other time series analytic methods have been used in an attempt to find periods or structure in the optical variations. This effort is difficult because of the irregular monitoring data available. The monitoring observations were made with ground-based telescopes and effects such as bad weather, annual motion of the Earth, monthly interruptions by the Moon, and funding cycles all combine to modulate the observing sequences. Because of the irregularity of the sampling, unequal interval Fourier transforms were developed (Deeming 19 ) and used on several sources (Webb et al. 1988, Webb and Smith 1989). No consistent periodicities were found, but the analysis showed that the variations are definitely not white noise. Simulations of 1/f noise sampled identically to the Blazar light curves also indicated there was more structure to the variations. The radiation in the optical through radio region of the spectrum of Blazars is almost certainly synchrotron emission. This radiation comes from relativistic electrons injected into a strong magnetic field. The basic model that is accepted by virtually every worker in the field is a massive black hole (107 Solar Masses) surrounded by an accretion disk that supplies the hole with mass and magnetic fields. The energetics of the sources and detailed radio observations indicate that the optical emission is probably emitted from a relativistic jet pointed toward us. This implies there are many more Blazars we don't see because their jets are not pointed in our direction. The reasons for the fluctuations in brightness could be due to many things. The leading candidates are: (1) shock waves propagating down the jets, (2) uneven accretion rates, and (3) hot spots on the accretion disk surrounding the central core.

Observations

The light curves used in this project are observations of 5 well observed Blazars extending back to around the mid 1970's. Most of the data were gathered with the 0.76 meter telescope at the University of Florida's Rosemary Hill Observatory (see Webb et al. 1988 and references within). Photographic plates with limited bandpass filters were used to collect the data which were then reduced using an Iris photometer and calibrated with nearby stars. Figure 1 shows the optical light curves.

Conversion to Musical Tones

In order to convert the light curve magnitudes to musical notes, we developed a simple algorithm and programmed it in IDL to run on a PC. This program reads in the optical magnitude and the time of observation and converts them to musical notes in the key of C. We decided to use the range of 2.5 octaves from the A below middle C to the C two octaves above middle C. These notes are generally accessible on the guitar. The maximum and minimum magnitudes of the observations are first determined and the range is calculated. This range is used to determine the magnitude increment that will be assigned to each of the 17 musical notes. The program tests each observation to see which note is associated with its magnitude, and prints out the results as A1 through C3. Figure 2 shows this association schematically.

The timing of the observations is actually a convolution of the actual magnitude variations and the observing times, thus this is actually not what the Blazar "sounds" like, but a window into the character of its variations. Due to the sparse sampling, it is interesting only from the musical standpoint. We used the following prescription to determine the duration of the notes where Dt is the time between the current and subsequent observations:

7 If Dt < 5 days - sixteenth note

7 5< Dt < 7 days - eigth note

7 7< Dt < 10 days - quarter note

7 10< Dt < 15 days - half note

7 15< Dt < 20 days - whole note

7 Dt > 20 days - whole note + rest

The timing was somewhat modified by the Home Studio when inputting the calculated notes into the Staff window, but the resultant timing is close to the timing derived from the data spacing. Once the data was input for each object, the Home Studio midi voice FX (goblins) was used to play back the music. The CD accompanying this paper is the result of this process.

References

Webb et al. 1988, Astronomical Journal 145.


For a powerpint presentation of the creation of blazar music, click here

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