Fermi observations suggest possible years-long
cyclic changes in gamma-ray emission from the blazar PG 1553+113. The
graph shows Fermi Large Area Telescope data from August 2008 to July
2015 for gamma rays with energies above 100 million electron volts
(MeV). For comparison, visible light ranges between 2 and 3 electron
volts. Vertical lines on data points are error bars. Background: One
possible explanation for the gamma-ray cycle is an oscillation of the
jet produced by the gravitational pull of a second massive black hole,
seen at top left in this artist's rendering.Credits: NASA's Goddard Space Flight Center/CI Lab. Hi-res image
Astronomers using data from NASA's Fermi Gamma-ray Space Telescope
have detected hints of periodic changes in the brightness of a so-called
"active" galaxy, whose emissions are powered by a supersized black
hole. If confirmed, the discovery would mark the first years-long cyclic
gamma-ray emission ever detected from any galaxy, which could provide
new insights into physical processes near the black hole.
"Looking at many years of data from Fermi's Large Area Telescope
(LAT), we picked up indications of a roughly two-year-long variation of
gamma rays from a galaxy known as PG 1553+113," said Stefano Ciprini,
who coordinates the Fermi team at the Italian Space Agency's Science
Data Center (ASDC) in Rome. "This signal is subtle and has been seen
over less than four cycles, so while this is tantalizing we need more
observations."
Supermassive black holes weighing millions of times the sun's mass
lie at the hearts of most large galaxies, including our own Milky Way.
In about 1 percent of these galaxies, the monster black hole radiates
billions of times as much energy as the sun, emission that can vary
unpredictably on timescales ranging from minutes to years. Astronomers
refer to these as active galaxies.
More than half of the gamma-ray sources seen by Fermi's LAT are
active galaxies called blazars, like PG 1553+113. As matter falls toward
its supermassive black hole, some subatomic particles escape at nearly
the speed of light along a pair of jets pointed in opposite directions.
What makes a blazar so bright is that one of these particle jets happens
to be aimed almost directly toward us.
"In essence, we are looking down the throat of the jet, so how it
varies in brightness becomes our primary tool for understanding the
structure of the jet and the environment near the black hole," said Sara
Cutini, an astrophysicist at ASDC.
Motivated by the possibility of regular gamma-ray changes, the
researchers examined a decade of multiwavelength data. These included
long-term optical observations from Tuorla Observatory in Finland, Lick
Observatory in California, and the Catalina Sky Survey near Tucson,
Arizona, as well as optical and X-ray data from NASA's Swift spacecraft.
The team also studied observations from the Owens Valley Radio
Observatory near Bishop, California, which has observed PG 1553+113
every few weeks since 2008 as part of an ongoing blazar monitoring
program in support of the Fermi mission.
"The cyclic variations in visible light and radio waves are similar
to what we see in high-energy gamma-rays from Fermi," said Stefan
Larsson, a researcher at the Royal Institute of Technology in Stockholm
and a long-time collaborator with the ASDC team. "The fact that the
pattern is so consistent across such a wide range of wavelengths is an
indication that the periodicity is real and not just a fluctuation seen
in the gamma-ray data."
Ciprini, Cutini, Larsson and their colleagues published the findings
in the Nov. 10 edition of The Astrophysical Journal Letters. If the
gamma-ray cycle of PG 1553+113 is in fact real, they predict it will
peak again in 2017 and 2019, well within Fermi's expected operational
lifetime.
The scientists identified several scenarios that could drive periodic
emission, including different mechanisms that could produce a
years-long wobble in the jet of high-energy particles emanating from the
black hole.
The most exciting scenario involves the presence of a
second supermassive black hole closely orbiting the one producing the
jet we observe. The gravitational pull of the neighboring black hole
would periodically tilt the inner part of its companion's accretion
disk, where gas falling toward the black hole accumulates and heats up.
The result would be a slow oscillation of the jet much like that of a
lawn sprinkler, which could produce the cyclic gamma-ray changes we
observe.
PG 1553+113 lies in the direction of the constellation Serpens, and its light takes about 5 billion years to reach Earth.
NASA's Fermi Gamma-ray Space Telescope was launched in June 2008. The
mission is an astrophysics and particle physics partnership, developed
in collaboration with the U.S. Department of Energy and with important
contributions from academic institutions and partners in France,
Germany, Italy, Japan, Sweden and the United States.
Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland
Source: NASA/Galaxies
