By Krzysztof Nalewajko
Blazing Jets Beaming Straight Towards The Earth
For several decades now, astrophysicists have known of the existence of powerful jets of particles and magnetic fields that shoot out at nearly the speed of light (that is, “relativistically”) from the centers of certain “active” galaxies. Scientists have learned that the jets originate in the accretion disks surrounding supermassive black holes at the cores of these galaxies. Relativistic jets are sources of strongly beamed radiation characterized by broad and smooth (or “non-thermal”) spectra, therefore their orientation relative to the observer has dramatic effects on the observed characteristics of the active galaxies. In particular, when one of the jets happens to point towards us, the galaxy in which it originates is called a “blazar”.
Below: Artist’s view of an active galaxy with its main components - accretion disk (in yellow/red, some blue in central parts) and relativistic jet flowing out (in blue). A supermassive black hole sits in the middle of the accretion disk, and is generally thought to be responsible for launching and powering the jet. When we look down the axis of the jet, we observe a blazar with relativistically-boosted radiation from the jet. (Credit: Wolfgang Steffen, Cosmovision)
Probing Jets with Space Telescopes
Observations of rapid gamma-ray flares in some of these blazars with the orbiting Fermi Gamma-ray Space Telescope’s main instrument, the Large Area Telescope (LAT), enable us to probe the mysterious mechanism responsible for particle acceleration in relativistic jets. Recently, a collaboration led by Masaaki Hayashida detected a very peculiar gamma-ray event in the historically well-known blazar 3C 279, which was characterized by a lack of simultaneous activity in the visible wavelengths. These gamma-ray flares are thought to be produced through “Inverse Compton” scattering (where ambient visible light photons are boosted in energy by interactions with high energy electrons), and they are generally expected to be accompanied by some level of both visible and radio emissions due to “synchrotron radiation” (an energy loss mechanism which occurs when energetic electrons spiral around the magnetic field lines).
A Curiosity: Blazing Jets Without any Visible Light Accompaniment
The lack of any significant emissions in the visible light band indicates that the gamma ray-emitting region must have very weak magnetic fields. How, then, are the radiating particles accelerated to the extreme energies required in these blazing jets, since most of the theorized mechanisms to accelerate them rely on substantial magnetic fields confining the particle motions?
Below: Multi-band light curve (i.e. observed light intensity as a function of time) of blazar 3C 279 observed between Nov 2013 and Apr 2014. From top to bottom, this shows the gamma-ray, X-ray, visible light (labelled “optical flux” in the figure) and radio wave fluxes (in different units) as observed by the Fermi, Swift, SMARTS and SMA observatories, respectively, with four specific time epochs labelled. Note a sharp gamma-ray flare for Epoch B, and no corresponding visible light flare. (Adapted from Hayashida et al. 2015.)
The gamma-ray sky changes dramatically over time, because the “cosmic particle accelerators” that boost particles to the extreme energies needed to produce gamma rays are always passing through transient states. Quite often, this acceleration proceeds against serious odds – in the face of strong magnetic fields and a dense radiation background, the most energetic particles do not last long before they lose their energy to radiation. And because the connection between the energetic particles and the gamma rays is so intertwined, we hope to learn a great deal about these cosmic particle accelerator systems by studying the gamma rays emitted from their regions. To do this, we use the space-based Fermi as well as ground-based gamma-ray telescopes like VERITAS, MAGIC, and HESS, which are better at observing in the higher energy gamma-ray bands in part because of their very large collecting area.
Below: Detailed gamma-ray flight curve of blazar 3C 279 during Epoch B, indicating the transience of these events. The gamma-ray fluxes are calculated in two photon energy ranges: above 100 MeV (top) and above 1 GeV (bottom); for comparison the rest-mass energy of an electron is 0.5 MeV, and 1 GeV = 1000 MeV is about the rest-mass energy of a proton. The gamma-ray flare is more prominent above 1 GeV, which is a signature of the hard gamma-ray spectrum. The characteristic variability time scale is estimated at two hours. (Adapted from Hayashida et al.)
Blazing Jets Beaming Out from the Hearts of Galaxies
The above picture certainly applies to blazars, which are the most numerous discrete gamma-ray sources in the sky. Very high speed motion of the particles in the blazar jets has a dramatic effect on the appearance of cosmic radiation sources due to “relativistic beaming” – they can be thousands of times brighter for some observers, depending on how close the observer’s viewing position is to being directly in line with the jet, compared to observers who are off-axis.
Thus blazar jets outshine their host active galaxies, just as long-type gamma-ray bursts (another type of ultra-relativistic jet phenomenon) outshine their host supernovae (i.e. exploding giant stars at the end of their life).
How Does So Much Energy Pour Into the Jets?
But even correcting for the effects of Einstein's special relativity, the energy requirements behind the observed radiation are quite high. A significant fraction of the total power of relativistic jets must somehow be converted into the radiation that we observe. This total radiative efficiency can be considered in three stages:
- 1. A large fraction of the jet volume must participate in particle acceleration
- 2. A large fraction of the particles in the acceleration region must become accelerated
- 3. A large fraction of the energy of accelerated particles must be radiated away
Each of these three fractions must be lower than 100%, but their product must be high, of order of 10%. And therein lies the challenge.
Normal Jets from Blazars Share Energy with Strong Magnetic Fields
Fermi has observed thousands of blazars, some of them in very high detail. From these observations a standard picture has emerged. For the most powerful sources (known technically as “flat spectrum radio quasars”), the gamma-ray luminosity dominates by an order of magnitude over the infrared luminosity, and even more so over X-ray, visible, millimeter, and radio bands, all of which have been observed for multiple sources. The spectral peak is thought to lie in the mysterious medium-energy gamma-ray band (1-10 MeV), and the LAT observes a gently decaying (soft) gamma-ray spectrum above 100 MeV. From the modeling of multi-band emission of blazars we can estimate how much of the jet energy is shared between the particles and the magnetic fields, and usually we find those fractions to be similar to each other.
A Most Curious Flare
In the new multi-band investigation of blazar 3C 279 led by Masaaki Hayashida, we observed a very peculiar gamma-ray flare on December 20th, 2013. It had three unusual properties:
- A very sharp profile of very short duration – only a couple of hours
- A rising (hard) gamma-ray spectrum indicating a spectral peak around 5-10 GeV
- No simultaneous activity observed in the visible band
The last property indirectly indicates a very low magnetization of the gamma-ray emitting region. This is very puzzling because as we mentioned, in every model of particle acceleration in high-energy astrophysical sources we think that magnetic fields play a key role. And relativistic jets themselves are thought to owe their highly relativistic motion to a gradual conversion of magnetic energy to kinetic energy, which can only work if the initial magnetization is very high.
Below: Broad-band energy spectra (technically known as “spectral energy distributions”) of blazar 3C 279 calculated for the four epochs A, B, C and D (in the same units for each epoch) marked in the multi-band light curve (in addition to some historical data in black and gray). Note the stark difference in the gamma-ray spectra between Epoch B (red) and other epochs. Also note the modest differences between the visible spectra in all four epochs (labelled as “optical” below, in the 1015 Hz frequency region of the plot). (Adapted from Hayashida et al.)
What Might Be Happening
So the new observation suggests a complete destruction – if only locally – of the jet’s magnetic fields. Or perhaps there was a gap in the jet that was filled by surrounding interstellar gas. Or something completely different – perhaps an extra source of matter polluting the jet, perhaps from a large, unlucky star that straggled too close (such an interaction is thought to lead to at least a partial destruction of the star).
In fact, this is not the first case of extreme gamma-ray flares from blazars. Probably the first example was observed in blazar PKS 1510-089 and described by Shinya Saito and collaborators in 2013. However, in that case there were no visible light/infrared observations made that could constrain the counterpart luminosity. Such extreme gamma-ray flares may be rare, and we may have even overlooked some of them in the Fermi data, but we should pay close attention to them because they are ultra-luminous and because they tell us something new about the physics of particle acceleration.
In short – there is a real mystery here that we have yet to unravel.
For further reading:
M. Hayashida, K. Nalewajko, G. Madejski, et al., “Rapid Variability of Blazar 3C 279 during Flaring States in 2013-2014 with Joint Fermi-LAT, NuSTAR, Swift, and Ground-Based Multi-wavelength Observations”, 2015, submitted to the Astrophysical Journal, arXiv:1502.04699