The centers of most galaxies in the Universe each contain a supermassive black hole with a mass of more than a hundred million suns. Owing to this massive amount of mass, the gravity from a black hole is so strong that not even light itself can escape from the black hole. In essence, black holes should be, well, black and invisible in space. However, when we look out into the Universe, the most luminous objects are “active galactic nuclei” (AGN) – galaxies who’s centers are so staggeringly bright we often cannot see the rest of the galaxy. The brightness of these AGN are related to the black hole at the center – but how can an object so massive that light cannot escape make so much light!?
That is the basic question driving my research. The term “active galactic nuclei” is actually an over-arching term for specific types of objects we see in space. In particular, I study blazars – objects similar to other AGN, but with enough differences that blazars can be considered on their own. In fact, the differences are similar to squares and rectangles – all squares are rectangles, but not all rectangles are squares. All blazars are AGN, but not all AGN are blazars. However, of all galaxies in the Universe, only 10% are “active”, and of those 10%, only one in a thousand AGN are a blazar.
What makes blazars unique is that a jet of high-energy charged particles, such as electrons and protons, are streaming away from the central black hole at speeds near the ultimate speed limit of the universe – the speed of light. These jets are oriented almost directly towards the Earth, similar to a flashlight beam pointing at you at night! Don’t worry, though! These objects are so far away we are only awash in the glow of light coming from the jet, and not actually being hit by anything dangerous!
Just how do these charged particles get going moving so fast? That’s the question!
In order to answer it, I use light across the entire electromagnetic spectrum. Just like you can take light and separate it into all the colors of the rainbow, the electromagnetic spectrum extends beyond colors that humans can see. At the lowest-energy end are radio waves, and at the highest energies are gamma rays. I use light from all parts of the spectrum to try to piece together what is causing the light from a blazar and how the charged particles are moving so quickly.
Blazars are a subclass of quasars, a type of active galactic nuclei (AGN). Blazars are relatively rare, comprising only 5-10% of all quasars, and yet they are the most luminous non-transient sources in the Universe. They produce highly variable broadband emission (from radio waves to gamma rays) dominated by emission from a relativistic jet of charged particles with Lorentz factors as high as 50.
The driving central engine for an AGN is the conversion of gravitational potential energy into radiation as gas and dust falls into the central supermassive black hole of galaxies.
The black holes usually have a mass of near a billion solar masses, and have large, likely thick accretion disks providing a thermal component of the radiation. For a reason yet unknown, relativistic jets start extremely close to the black hole near the axis of the accretion disk. However, the dominant components of the observed spectral energy distribution depend on the direction of our line of sight. For objects where our viewing angle is within 40° of the disk equator the object is classified as a non-blazar. If the viewing angle is within 20° of the relativistic jet, the object is classified as a blazar.
In the case of blazars, the emission is dominated by the relativistic jet, as light originating in the jet is relativistically beamed towards us. From other relativistic effects, this light can have extremely short variability timescales, on the range from months and years to as little as tens of minutes.
I have several active research projects:
I study the shortest timescales of variability across the electromagnetic spectrum. By observing the differences in brightness across the electromagnetic spectrum, we can begin to piece together what physical processes are occurring in the jet, and what the charged particle distribution is like. To this end, I utilize data obtained at radio wavelengths with the Very Long Baseline Array (VLBA), optical wavelengths with a variety of telescopes (such as the 14-inch telescope at the Colgate University Foggy Bottom Observatory, and the 72-inch Perkins Telescope and 4-meter Discovery Channel Telescope of Lowell Observatory), and gamma-ray wavelengths with the Fermi Gamma Ray Space Telescope. Recently, we have begun to use the Transiting Exoplanet Survey Satellite (TESS) to obtain high-time resolution observations of our blazars, such as with the blazar BL Lacertae.
I am also tracking and characterizing changes in the parsec-scale jets of blazars utilizing 43 GHz radio maps made using the Very Long Baseline Array (VLBA). Moving features, called knots, are visible in these radio maps as they are emitting light, and by making many observations over timescales of months to years we can figure out the speed and trajectory of these knots. Ultimately, we would like to correlate the appearance of these knots in the jets of blazars with the brightening of the sources at other wavelengths, called outbursts, to determine if an outburst always has an associated knot appear.
My most recent published paper is devoted to characterizing the 2016 multi-wavelength outburst of the blazar 3C454.3. In June 2016 the blazar increased in brightness by a factor of 10 over a week and a half, before decaying to pre-outburst levels over the span of 3 days. This decay was the most precipitous decay seen ever. This structure was seen at both optical and gamma-ray wavelengths. At optical wavelengths the source exhibited variability on the order of 1 magnitude over the course of a single night, dubbed intraday variability. Also, the source exhibited variability of the order 0.05 magnitude over the course of just hours, dubbed micro-variability, that has been exceedingly rare in the observed history of most sources. The findings from this research were published in April 2019, and are available from the Astrophysical Journal.
Astronomy is useful because it raises us above ourselves; it is useful because it is grand; …. It shows us how small is man’s body, how great his mind, since his intelligence can embrace the whole of this dazzling immensity, where his body is only an obscure point, and enjoy its silent harmony. – Henri Poincaré