Black-hole-powered
galaxies called blazars have powerful jets that are thought to be
fortuitously aimed directly toward Earth. Blazars emit at wavelengths
from the radio to the gamma-rays, but astronomers have now found two
objects that are blazar like in many ways but which are radio-quiet. Credit: NASA; M. Weiss/CfA
A blazar is a galaxy whose central nucleus is bright at wavelengths from the low energy radio band to high energy gamma rays (each gamma ray photon is over a hundred million times more energetic than the X-rays seen by the Chandra X-ray Observatory). Astronomers think that the blazar nucleus contains a supermassive black hole, similar to a quasar nucleus. The emission results when matter falls onto the vicinity of the black hole and erupts into powerful, narrow jets of radiating charged particles moving close to the speed of light. Two defining characteristics of blazars, strong radio emission and high variability, are results of the accretion and jets.
Although the nuclei of other galaxies also eject jets of particles,
the class of blazars is thought to result from our unique viewing angle:
staring directly down the throats of these jets. The orientation makes
these objects unique probes of exotic physical activity, with the
relative intensities of the radiation providing key diagnostics. In most
other galaxies, for example, infrared radiation comes from heated dust,
but in blazars the infrared colors indicate that it comes from jet
emission. Because the jet emission is so bright the underlying galaxy
light can be masked, with the result that in the class of BL Lac blazars
emission and absorption lines are not detected, making their distances
difficult to determine.
CfA astronomers Raffaele D'Abrusco and Howard Smith and their four
colleagues report discovering blazars that challenge this general
paradigm. They found two BL Lac blazars with no apparent radio emission:
"radio weak" BL Lacs. The astronomers discovered them by using the
Fermi catalog of very high energy sources to identify a set of possible
new blazars, and the WISE infrared sky catalog to reinforce the
categorization and to pinpoint the locations of the sources in the sky.
After searching radio catalogs for counterparts to the sources, they
discovered two that had no detected radio emission.
Since blazars are by definition highly variable, and since not all of
the wavelengths were measured at the same time, the scientists review
the possibility that the emission at one or more wavelengths varied
enough to account for the peculiar observations; they also examine some
other possibilities. In the end, they conclude that although
variability might be a possible explanation, if these candidates behaved
like other blazars, variability alone could not resolve the mystery of
the radio silence. If confirmed, these new Radio Weak BL Lac objects
challenge the basic explanation of blazars. How many radio weak BL Lacs
exist, how far away they are, and how they are formed and evolve -
indeed why they exist at all - are now pressing questions in
extragalactic astronomy.
Reference(s):
"Radio-weak
BL Lac Objects in the Fermi Era," F. Massaro, E. J. Marchesini, R.
D'Abrusco, N. Masetti, I. Andruchow & Howard A. Smith, ApJ 834, 113, 2017.
