Skip to main content

135. Photometric Monitoring of MRK 501

A Model For Measuring The Optical Variability Of BL Lacs

Nathan J. McGregor, Dr. Daryl J. Macomb

McGregor final poster - view content on posts page
Select to view full poster image


BL Lacertae Objects (BL Lacs)

  • As gas and dust accrete onto the supermassive black holes in galaxies, they give rise to active galactic nuclei (AGN)
  • Some of the most luminous and highly variable of AGNs are BL Lacs, whose relativistic jets are oriented close to our line of sight, producing rapid and large-amplitude flux variability and significant optical polarization
Event horizon telescope with figures, contact presenter for details


  • All VRI observations were taken with the automatic 0.40 m telescope at the Challis Astronomical Observatory
  • The telescope is equipped with a CCD camera and UBVRI filters
  • VRI observations were collected from 2011 through 2015


  • Images that were recognized and astrometrically calibrated formed the data for our analysis.


  • Artifacts were detected and removed by resampling and co-adding 10 FITS images in each band for each observation


  • A catalog of objects was generated for each co-added image

Cone Search

  • Identified Mrk 501 and reference stars by computing great-circle distance (angular on-sky separation) between SExtractor sources and Mrk 501 using Mrk 501’s known ICRS RA and Dec


  • Because the magnitudes of stars 1 and 4-6 have been measured in all bands in other works, they were selected to calculate Mrk 501’s magnitude
  • The average measured magnitude for each band for reference star 1 was used to determine a zero point
  • Mrk 501’s flux and magnitude were plotted from 2011 to 2015
Contact presenter for additional details about this image
R-bankd image, co-added image, Reference Stars


Mrk 501 and Reference Stars

  • Mrk 501 and the reference stars exhibit little variability in magnitude and flux; however, further calibration is needed in order to better assess Mrk 501’s flux variability
  • The measured magnitude for the reference stars are within ±0.2 of their published values
Mrk 501 graph, Contract presenter for details about this data

Mrk 501 graph, Contract presenter for details about this data

Future Research


  • Calculate relationship between observation ambient temperature and image noise, so the noise corresponding to the ambient temperature during the observation period could be subtracted
  • Use darks and bias frames to subtract noise

Error and Statistical Analysis

  • Calculate parameters for data processing (gain for converting counts to flux and zero point for calibrating the magnitude scale)
  • Calculate statistical values and errors

Correlate with Chandra and Fermi Data

  • Correlate photometric measurements with x-ray and gamma ray observations during same observation period to assess relationship. X-ray observations provided by the Chandra X-ray Observatory. Gamma ray observations provided by the Fermi Gamma-ray Space Telescope.

Apply Model to Other BL Lacs

Mrk 421, OJ 287, BL Lacertae, 1ES 1959+650, 3C 66A, S5 0716+71, W Com, RXS J1543+613, OT 081, PKS 0048-097


  • Fiorucci, M., Tosti, G., & Rizzi, N. (1998). VRI photometry of stars in the fields of 16 blazars. Publications of the Astronomical Society of the Pacific, 110(744), 105–110. doi: 10.1086/316127
  • J.Y. Kim, T.P. Krichbaum, et al.: “Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution”, in: Astronomy & Astrophysics (April 07, 2020)
  • Villata, M., Raiteri, C. M., Lanteri, L., Sobrito, G., & Cavallone, M. (1998). BVR photometry of comparison stars in selected blazar fields: Astronomy and Astrophysics Supplement Series, 130(2), 305–310. doi: 10.1051/aas:1998415

Additional Information

For questions or comments about this research, contact Nathan McGregor at