Campaign on the blazars 3C 273 and 3C 279

Dr. Kirill Sokolovsky (MPIfR, Bonn) has requested the assistance of AAVSO observers to obtain optical photometry of the blazar-type quasars 3C 273 and 3C 279 during 2011.  The observations obtained by AAVSO observers will be combined with multiwavelength observations -- from radio to gamma-rays -- as well as with very long baseline interferometry with the VLBA to obtain a better picture of what these objects are and how they vary.  In particular, Sokolovsky and collaborators hope to find correlations between the spectral energy distributions (SEDs) of these objects and the behavior of the jets at sub-parsec spatial resolution.

What follows are the technical requirements and scientific justification provided to the AAVSO by Dr. Sokolovsky:


Technical requirements

We request multicolor CCD observations of two classical radio-loud quasars:

3C273 12:29:06.70 +02:03:08.6 J2000 V~12.7
3C279 12:56:11.17 -05:47:21.5 J2000 V~16.1

through the 2011 observing season. The observations are requested once per clear night in any subset of UBVRcIc filters with the priority given to V and R bands. We encourage observers to use comparison stars indicated on these finding charts: (3C273) (3C279) for magnitude calibration.

Unfiltered CCD observations, unfortunately, will not be useful for the campaign due to the complex spectral shape of the quasars in optical band. Changing color of the objects will prohibit direct comparison of unfiltered measurements obtained with different instruments and will make absolute flux calibration difficult. For these reasons, observations in standard photometric filters are needed.

Scientific justification

The requested observations will be a part of the coordinated multifrequency-VLBA, ground-based near-infrared and optical, Swift (optical, ultraviolet, X-ray) and Fermi (GeV gamma-ray) observing program aimed to study the structural, spectral and polarization evolution of the innermost parts of relativistic jets of 3C279 and 3C273 at sub-parsec spatial resolution (achieved in radio by VLBA) and high temporal resolution, and to correlate these with the evolution of the overall spectral energy distribution (SED). The VLBA and Swift observations of 3C273 and 3C279 are planned once every 20 days, however it would be very useful to obtain an optical lightcurve with much better sampling. The multicolor lightcurve may probe changes in synchrotron radiation of the relativistic blazar jet, line emission from the broad line region and thermal emission from the accretion disk which all contribute to the flux observed in broad-band optical filters. Good sampling of the optical lightcurve is also needed for a proper comparison with Fermi gamma-ray data. Fermi provides information about average gamma-ray flux on timescales of days (the exact integration time depends on the source brightness) while optical observations typically require only a few hundred seconds of integration time and therefore provide information about "instant" source brightness. To avoid introducing bias in the gamma-ray/optical correlation analysis, one needs a number of optical observations obtained during each gamma-ray integration so the average gamma-ray flux may be compared with the average optical flux during the gamma-ray integration time, not with the instant optical flux measured at some point during the long gamma-ray observation.


Blazars are a class of  Active galactic nuclei which includes Flat-Spectrum Radio Quasars (FSRQ) and BL~Lacertae-type objects. One of the most remarkable things about blazars is their ability to radiate across the entire electromagnetic spectrum: from radio to GeV and TeV gamma-rays. This emission is believed to be produced by a relativistic jet pointing roughly towards the observer.

The SED of a blazar has two broad components: one peaking between far-IR and X-ray wavelengths and the other peaking at gamma-rays. For recent reviews of the blazar emission mechanisms and energetics one may consult Celotti et al. (2008, MNRAS, 385, 283); Ghisellini et al. (2009, MNRAS, 397, 985); Boettcher (2010, arXiv:1006.5048). The radio to UV (sometimes up to X-ray) emission of blazars is believed to be dominated by synchrotron radiation of relativistic electrons while radiation at higher energies could be due to the inverse Compton scattering of synchrotron photons emitted by the electrons themselves (the synchrotron self-Compton process, SSC, Jones et al. 1974, ApJ, 188, 353; Ghisellini & Maraschi 1989, ApJ, 340, 181) and/or photons from external sources (External Compton, EC, Sikora et al. 1994, ApJ, 421, 153; Dermer & Schlickeiser 2002, ApJ, 575, 667). The sources of the external seed photons for the EC process include accretion disc,  broad line region (BLR) clouds, warm dust, and the cosmic microwave background (CMB), with their relative contribution varying for different blazars.

Despite the wide acceptance of the picture outlined above, the exact location, geometry and physical properties of the regions responsible for blazar emission (the ``blazar zone'') at different bands remain a controversial issue. It is critical to identify the location of gamma-ray emitting regions in order to distinguish between SSC and EC models and determine the origin of seed photons for the EC process. Moreover, the blazar phenomenon can be explained in the framework of the hadronic models, where the relativistic protons in the jet are the primary accelerated particles, emitting gamma-ray radiation by means of photo-pair and photo-pion production and synchrotron radiation (see Mucke & Protheroe 2001, APh, 15, 121; Mucke et al. 2003, APh, 18, 593 for reviews on hadronic models).

The proposed observing campaign should provide a unique multiwavelength dataset which will enable further exploration of blazar emission physics.