Sky and Telescope - September 2018 - 35

ASTROPH YSICA L JE T: CASE Y REED / S&T;
BL L AC: POSS-II / CA LTECH / STSCI / PA LO M A R OBSERVATORY

tances. And while some unusual white dwarfs do exhibit flat
spectra, none matched the spectra of BL Lacs. Neutron stars
are also too faint and small. No theorist could fit the known
properties of neutron stars to the characteristics of BL Lacs.
Could BL Lacs be black holes?
Remember that theory proposed in 1974 by Shapiro and
Elliot of Cornell University? Well, it did fit just about everything regarding BL Lacs. But Shapiro and Elliot were considering black holes within our galaxy. BL Lac Objects were
presumed to be the cores of distant galaxies - could they
actually be black holes?
The problem was the extraordinary energy output arising
from processes other than fusion, assuming astronomers were
interpreting the spectra correctly. This implied a black hole
with an unheard of, even unimaginable, mass. If BL Lacs were
indeed black holes, they had to be the most massive objects
in the universe. The energy output demanded this. And "very
large mass" here means millions if not billions of suns.
Thanks to the work of Yakov Zel'dovich and Igor Novikov
in the USSR, Edwin Salpeter in the U.S, and Donald LyndenBell in the UK, who each independently demonstrated that
these gargantuan black holes were the only possible explanation for these enormous outpourings of energy, the idea
began to take hold.
And this scenario fit the observations. In time, as the
theory became established, these black holes at the centers of
galaxies earned the moniker "supermassive" to denote their
extraordinarily high masses (as opposed to "stellar-mass"
black holes, which are generally the remnants of stars after
supernovae events).
The model emerging from physics looked like this: BL Lac
Objects are composed of a supermassive black hole gobbling
up matter from its host galaxy. As this matter - gas, dust,
and disrupted stars - approaches the supermassive black
hole through a process known as accretion, it heats up as it
swirls inward in a flat structure surrounding the black hole
known as the "accretion disk." Eventually, as the matter
reaches maximal temperature near the black hole's event
horizon, it is ejected perpendicular to the disk, spewing out
in highly relativistic jets.
These jets are threaded by magnetic field lines that twist,
turn, and recombine. It's this magnetic field that polarizes
the jet's radiation. As the magnetic field contorts, the polarization varies too, producing changes on timescales that can
be as short as hours.
Eventually it dawned on astronomers that quasars and BL
Lacs (and their cousins, Seyfert galaxies) are all galaxies at
cosmological distances with central supermassive black holes
and highly relativistic jets viewed from different angles.
Different components of the supermassive black hole/
accretion disk/jets system contribute to the broadband spec-

E N

BL Lac

p A SPECK AMONG MANY BL Lac appears distinctly star-like in this
25′ × 20′ field of view. It is therefore not surprising that in the 1920s
astronomers originally thought the target was a star, and due to the frequent changes in brightness, it is even less surprising that they awarded
it a variable star designation.

trum, which extends from radio wavelengths to high-energy
gamma rays, to varying degrees depending on orientation. If
the orientation is such that one of the jets is pointed directly
at the observer along the line of sight, then the jet's brightness swamps the radiation arising from any of the system's
other physical components, resulting in the featureless spectrum characteristic of BL Lac.

Mystery Solved?
Since the seminal observations of the 1970s, BL Lac and its
companions - collectively known as blazars - have been
observed across the electromagnetic spectrum, from the radio
to the very high-energy gamma rays, confirming the basic
morphological structure of a supermassive black hole accreting matter from a host galaxy and subsequently ejecting this
matter via relativistic jets. However, many questions still
remain: How do supermassive black holes arise? How do jets
form? How do black holes power jets? How do the magnetic
field lines form and interact with their environment? Is there
a limit to the power of accretion?
This is currently where we stand with this detective story.
What will the next chapter look like? Only time will tell.

¢ DAVID NAKAMOTO is an aerospace engineer who worked
at JPL for two decades developing instruments and systems
for Hubble, Galileo, etc. He's been an amateur astronomer
for almost 50 years and a Los Angeles Astronomical Society
member since 1989. He can be reached at dinakamoto@
hotmail.com.

FURTHER READING: For more information on supermassive black holes, accretion disks, jets, and blazars, read "The Universal Jet
Set" by C. Renée James in the April 2010 issue of Sky & Telescope. To find some of these blazars, see "Blazar, Blazar, Burning Bright" by
Steve Gottlieb, in the same issue.
sk yandtelescope.com * S E P TE M B E R 2 018

35


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