New research hints at the presence of unconventional galaxies containing
2 black holes
An international team of astronomers identifies periodic gamma-ray
emissions from 11 active galaxies

Date:
June 19, 2020
Source:
Clemson University
Summary:
Astronomers have identified periodic gamma-ray emissions from 11
active galaxies, paving the way for future studies of unconventional
galaxies that might harbor two supermassive black holes at their
centers.



FULL STORY ==========================================================================
A Clemson University scientist has joined forces with an international
team of astronomers to identify periodic gamma-ray emissions from 11
active galaxies, paving the way for future studies of unconventional
galaxies that might harbor two supermassive black holes at their centers.


========================================================================== Among astronomers, it has long been well-established that most galaxies
host a black hole at their center. But galaxies hosting a pair of black
holes has remained theoretical.

The results of the team's research appeared in The Astrophysical Journal
on June 19, 2020 in a paper titled "Systematic search for gamma-ray
periodicity in active galactic nuclei detected by the Fermi Large Area Telescope." "In general, supermassive black holes are characterized by
masses of more than a million masses of that of our sun," said Pablo
Pen~il, lead author of the study and a Ph.D. student at Universidad
Complutense de Madrid in Spain. "Some of these supermassive black holes,
known as active galactic nuclei (AGN) have been found to accelerate
particles to near the speed of light in collimated beams called jets. The emission from these jets is detected throughout the entire electromagnetic spectrum, but most of their energy is released in the form of gamma rays." Gamma rays, which are the most extreme form of light, are detected by the
Large Area Telescope onboard NASA's Fermi Gamma-ray Space Telescope. AGN
are characterized by abrupt and unpredictable variations in brightness.

"Identifying regular patterns in their gamma-ray emission is like looking
at the stormy sea and searching for the tiny regular set of waves caused
by, say, the passage of a small boat," Pen~il said. "It becomes very challenging very quickly." The team accomplished the first difficult step
of identifying a large number of galaxies that emits periodically over
years and is trying to address the question of what is producing that
periodic behavior in these AGN. Several of the potential explanations
are fascinating.

"The next step will be the preparation of observational campaigns
with other telescopes to closely follow up on these galaxies and
hopefully unravel the reasons behind these compelling observations,"
said co-author Marco Ajello, an associate professor in the College of
Science's department of physics and astronomy at Clemson University. "We
have a few possibilities in mind -- from lighthouse effects produced
by the jets to modulations in the flow of matter to the black hole --
but one very interesting solution would be that periodicity is produced
by a pair of supermassive black holes rotating around each other.

Understanding the relation of these black holes with their environment
will be essential for a complete picture of galaxy formation." Thanks to
a decade of Fermi-LAT observations, the team was able to identify the repetition of gamma-ray signals over cycles of a few years. On average,
these emissions repeated about every two years.

"Our study represents the most complete work to date on the search for periodicity in gamma rays, a study that will be instrumental in deriving insights about the origin of this peculiar behavior," said co-author
Alberto Domi'nguez, Pen~il's Ph.D. supervisor in Madrid and also a former postdoctoral researcher in Ajello's group at Clemson. "We have used nine
years of continuous LAT all-sky observations. Among the more than two
thousand AGN analyzed, only about a dozen stand out for this intriguing cyclical emission." Enlarging the limited sample of periodic emitters constitutes an important leap forward for understanding the underlying
physical processes in these galaxies.

"Previously only two blazars were known to show periodic changes in
their gamma-ray brightness. Thanks to our study, we can confidently say
that this behavior is present in 11 other sources," said co-author Sara
Buson, a professor at University of Wu"rzburg in Germany. "In addition,
our study found 13 other galaxies with hints of cyclical emission. But
to confidently confirm this, we need to wait for Fermi-LAT to collect
even more data."

========================================================================== Story Source: Materials provided by Clemson_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. P. Pen~il, A. Domi'nguez, S. Buson, M. Ajello, J. Otero-Santos,
J. A.

Barrio, R. Nemmen, S. Cutini, B. Rani, A. Franckowiak, E. Cavazzuti.

Systematic Search for g-Ray Periodicity in Active Galactic Nuclei
Detected by the Fermi Large Area Telescope. The Astrophysical
Journal, 2020; 896 (2): 134 DOI: 10.3847/1538-4357/ab910d ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200619143440.htm

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