Boston University  Blazar Group

MOBPOL:   Multi-Optical-Band Polarization of Selected Blazars

Above: Multi-optical-band linear polarization of the quasar CTA102 measured by S. Jorstad at the Perkins Telescope of Lowell Observatory over 5 nights in late October/early November 2016. RJD is Julian date minus 2450000. Black: I filter; red: R filter; green: V filter; blue: B filter. F is the total flux density, PF is the polarized flux density, plotted at the bottom of the top left figure and by itself in the top right figure. P is the degree of polarization in percent, while χ is the position angle of the polarization electric vector. The lines are intended only to highlight the variations; they do not necessarily indicate the actual behavior in between successive data points. Note especially the lower graphs, which show pronounced night-to-night changes in the polarization, but similar behavior in the different colors. More data of this type can potentially determine whether turbulence or magnetic reconnections are responsible for the  variations in flux and polarization.

Below: The same quasar during an extraordinarily bright outburst in visible light and gamma rays. Here the brightness is measured in magnitudes, an astronomer's system where the object is brighter by 2.5 times when its magnitutde *decreases* by 1. In this outburst, the quasar was more highly polarized when it was brighter,  meaning that its magnetic field was more highly ordered. As the brightness faded, the magnetic field became less well ordered and its mean direction (which equals the EVPA + 90 degrees) rotated by 120 degrees. The behavior was similar at different colors. This behavior is quite different from that expected from turbulence and suggests that the outburst occurred in a place in the jet where the magnetic field is twisted into a spiral pattern.

Above: Multi-optical-band linear polarization of the BL Lac object OJ287 measured by S. Jorstad at the Perkins Telescope of Lowell Observatory during the night of 29 January 2017. RJD is Julian date minus 2450000.   Black: I filter; red: R filter; green: V filter; blue: B filter. F is the total flux density, PF is the polarized flux density, plotted at the bottom of the top left figure and by itself in the top right figure. P is the degree of polarization in percent, while χ is the position angle of the polarization electric vector. Note especially the lower left graph, which shows that the degree of polarization is a changing function of color. A turbulent magnetic field can explain this.

Description of the MOBPOL program: We observe blazars - the most luminous objects in the universe that last longer than a few minutes - with the 1.83-meter-diameter Perkins Telescope of Lowell Observatory (Flagstaff, AZ) in 4-5 seven-night sessions per year. The first 1-2 nights of observations determine which blazars are flaring that week. We then observe 1 or 2 of the flaring blazars multiple times each night for the rest of the session. The main objective is to determine differences in the polarization both as a function of time and wavelength (as measured with filters, each restricting the wavelength range to a particular color).

The light from the high-energy plasma (charged particles and magnetic field) of a blazar jet is from synchrotron radiation - electrons executing spiral motions at velocities very close to the speed of light. Light is actually a wave of oscillating electric and magnetic field that moves through space, with the electric and magnetic fields perpendicular to each other and also perpendicular to the direction of motion. (You can find an animation of polarized light on a YouTube video.) The light is said to be unpolarized if the different waves passing the observer have have polarizations that are equally in one direction as in the perpendicular direction. This can happen, for example, if the polarization orientation has utterly random directions of the electric field, or if 50% of the waves have polarization in one direction and the other 50% (coming from a different region in the source) have polarization perpendicular to this. This is because mutually perpendicular polarizations cancel each other. In contrast, the light is 100% linearly polarized if the electric field is always in the same direction. (Actually, the field oscillates back and forth along that direction.) Detailed calculations find that synchrotron light can have linear polarization as high as about 75%. The synchrotron light is polarized at less than 75% if only some fraction of the waves have the same electric field direction.

A key feature of synchrotron light is that the net electric field of the polarization (the electric vector position angle, or EVPA, often represented by the Greek letter χ), is perpendicular to the direction of the magnetic field as viewed by the observer (i.e.,  as projected onto the plane of the sky). (Note that this is not always true for synchrotron light at radio wavelengths, whose EVPA can be changed by an effect called Faraday rotation.)

So, measuring the polarization of optical light can tell us quite a bit about the magnetic field. Is it mixed up, in a spaghetti-like pattern, as one might expect from turbulence? Or does it have a high level of order, for example in a helical (spiral) pattern wrapped around the jet or perhaps either parallel or perpendicular to the jet? This is important because the magnetic field is thought to play an important role in the dynamics of the jet, including its formation, acceleration to near-light speed, and constriction into a very narrow cone shape. The powerhouse of a blazar is a black hole with a mass about a billion times the mass of the Sun, accreting gas from its surroundings, but the main tool that the black hole uses to make two oppositely-directed jets is the magnetic field brought toward it by the accreted ionized gas. The field is wound up into a helical shape by the rotation of the black hole and the orbits of the infalling gas. The magnetic field is also likely to play the major role in the energization of the electrons whose radio, infrared, visible, UV, X-ray, and gamma-ray light we observe from blazars. Changes in velocity of the plasma flowing down the jet, away from the black hole at the center of the host galaxy, can form shock waves. If the magnetic field lies nearly perpendicular to the shock wave's front, some of the electrons (as well as protons) can pass across the shock and back numerous times, gaining a lot of energy in the process. Or, in regions where the magnetic field is mixed up, regions of oppositely directed field can come together, annihilating some of the field in a process called magnetic reconnection, and transferring the lost magnetic energy into energy of the electrons.

By measuring the polarization of a blazar's light and how it changes with time and wavelength, we can determine the pattern of the magnetic field in the jet and compare the data with the predictions of the different hypotheses for how the electrons gain the energies needed to radiate the optical (and gamma-ray, as well as X-ray in some blazars) light. To do this, we need to collect enough data to determine whether repeatable patterns, or just random fluctuations, occur. We expect to be able to determine this in 2019 after 3 years of observations.

List of publications from this program

For more information on blazars, see our research page.

Back to the blazar group's home page

Go to the personal web pages of: Alan Marscher ---- Svetlana Jorstad