The Correlation of Optical,

By: J. L. JENSEN

Supervised by:  J. P. Finley and R. W. Lessard

Department of Physics, Purdue University,

West Lafayette, Indiana

1.   Introduction

Markarian 421 has a red shift of .03 making it the closest BL Lacertae object astronomers and astrophysicists study.  The redshift is related to the distance to the object and is derivable from Hubble's Law.  Mkn 421 is an Active Galactic Nucleus, which means that the center of the galaxy is a region of variable emission over a wide range of frequencies.  We are making progress in correlating gamma ray, X-ray, and optical emission.  Correlation of the emission in the various wavebands can determine where the emission is being produced in the system and what the sizescale of the emitting region is. The Whipple Collaboration first detected TeV gamma rays from Mkn 421 in 1992 (Mc Enery and Buckley, 1996).  This is where we get the gamma ray data from for this project.  We are also using results from the All-Sky Monitor (ASM) of the Rossi X-ray Timing Explorer (RXTE) for the X-ray data.  Purdue's own Meade LX200 acquired optical data at the Cumberland Observatory.  For data collection, we utilized the SBIG CCD Camera model ST-6.

2.   Background Information

Active Galactic Nuclei, or AGN's, are a class of quasars and emit very low to very high energy photons (i.e. radio through gamma-ray).   Quasar is short for quasi-stellar object and is so named because the object appears stellar or point-like. AGN's are differentiated into subgroups called Radio-Quiet which have a low flux in the radio band and Radio-Loud (R-L) which have a high flux in the radio band.  R-L AGN's include Quasars, Blazars, and Radio Galaxies.   Blazars are characterized by emission from a jet emanating from the core of the galaxy that is pointed almost directly at the observer. The emission from Blazars is variable, and highly polarized  (Mc Enery, 1.2.4). This is why Blazars are often the focus of studies of the correlation between high, medium and low frequency radiation.  Blazars are further broken down into BL Lacertae objects and flat spectrum radio quasars or FSRQs (Mc Enery, 1.2.4). Markarian 421 is a BL Lac object given that its EM flux is directed towards the line of sight of the observer.

2.2.   Cumberland Observatory

Cumberland Observatory, located in West Lafayette, houses Purdue's Meade LX-200 16-inch telescope.  In addition to the telescope, Purdue utilizes an ST-6 CCD camera by Santa Barbara Instruments Group to acquire data.  The CCD detects the incoming photon and turns it into a digital signal that a computer can read for further analysis. Data are post-processed at Purdue using various analysis packages and relative magnitudes are calculated for various standard stars on each data frame.

2.3.   The Whipple Collaboration

Whipple Observatory on Mt. Hopkins in southeast Arizona is a center for gamma-ray research for the Whipple Collaboration.  The collaboration includes the following university research groups: Harvard-Smithsonian Center of Astrophysics, Purdue University, Iowa State University, University College Dublin, and University of Leeds.

2.4.   The Rossi X-ray Timing Explorer (RXTE)

On December 30, 1995, The RXTE satellite was put into space via a Delta II rocket into low-earth orbit. The satellite is monitored by the Goddard Space Flight Center (Lochner, 1998).

The RXTE makes sensitive measurements of x-ray variability over time scales ranging from milliseconds to years. The time variability is a source of important information about processes and structures in white-dwarf stars, X-ray binaries, neutron stars, pulsars, black holes and AGNs. Observations are acquired over an extended period with instruments sensitive to X-ray energies from 2-100 keV. The different missions of the RXTE are: periodic, transient, burst and white noise fluctuations in X-ray emissions, relaxation times for the interior structure of

neutron stars, binary stellar masses, separation distances, orbit eccentricities and mass exchange between ordinary and degenerate stars in binary systems (Lochner).

We accessed the data from the RXTE via a publicly accessible website. The sight has a number of x-ray sources which are monitored daily including Mkn421.

3.   Observations

On this graph large bursts are observed around 50860 and 50875 MJD. However, the simultaneous coverage is from 50900 and 50920 MJD because that is when we have optical and gamma ray data to compare with. As can be seen the x-ray flux is on the rise during this time period.

3.2. Whipple gamma ray data

Using optical data from Poyner, Hanson, Watanabe, and Gill, we see variability of the absolute magnitude. The differences appear small but magnitude is a logarithmic quantity and slight changes in magnitude mean significant changes in brightness. The Cumberland data is sparse due to the inclement weather during the winter and spring in West Lafayette. We are fortunate to have other optical data that we can utilize. The boxes represent the Cumberland data while the circles represent the data of Poyner, Hanson, Watanabe, and Gill. The Cumberland magnitude is originally measured as a relative magnitude and has been normalized to the other optical data. It is good to remember that the magnitude scale is inverted to the normal scale. A high magnitude corresponds to a low intensity and brightness.

4. Conclusions

Using Whipple gamma ray data and RXTE data from 50820 to 50940 MJD, we can see a correlation between the peaks in the latter part of the graph.  The goal of this project is to determine whether the optical data corresponds to these peaks with optical outbursts of energy. The data shows no evidence of a lag between the x-ray and gamma ray bursts. The optical intensity is increasing at the same time as the gamma ray bursts. Likewise, when there is minimum in the x-ray flux at 50900, the magnitude of Mkn 421 is the highest at 13, which means that it is at low brightness and intensity.

5. Mkn 421 Images

Mkn 421 (in box) taken March 26, 1998.

6. Other Images

The following images were taken at Cumberland Observatory using the Meade LX200 and the ST-6 CCD camera by J. L. Jensen, J. P. Finley, and R. W. Lessard.

M13 Globular Cluster in Hercules     Whirlpool Galaxy with its on March 26, taken on March 26,1998.                      eliptical companion, M52                                                                      taken on March 26, 1998.

The Owl Nebula taken on                      The same image after digitizing

March 26, 1998.

7. References

1. All Sky Monitor-RXTE, Massachusetts Institute of Technology. 1998, RXTE data, http://space.mit.edu/XTE/ASM_lc.html.

2. Catanese, M., Iowa State University.  1998, http://egret.sao.arizona.edu.

3. Lochner, Goddard Space Flight Center (NASA). 1998, http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/what_is_RXTE.html.

4. Mc Enery, J. E., University  College Dublin. 1997, Ph.D. Thesis.

5. Poyner, G.; Hanson, G.; Watanabe, T.; Gill, D. 1998, Optical Data.