David J. Thompson, Jean Ballet,
Isabelle Grenier, & Seth Digel talk with
ScienceWatch.com and answer a few questions about this
month's Fast Breaking Paper Paper in the field of Space
Article Title: Fermi/Large Area Telescope Bright
Gamma-Ray Source List
Authors: Abdo, AA;et al.
Journal: ASTROPHYS J SUPPL SER, Volume: 183, Issue: 1, Page:
46-66, Year: JUL 2009
* Stanford Univ, WW Hansen Expt Phys Lab, Dept Phys, Kavli Inst
Particle Astrophys & Cosmol, Stanford, CA 94305 USA.
* Stanford Univ, SLAC Natl Accelerator Lab, Stanford, CA 94305
* Univ Calif Santa Cruz, Santa Cruz Inst Particle Phys, Dept
Phys, Santa Cruz, CA 95064 USA.
(addresses have been truncated.)
Why do you think your paper is highly
The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is
providing the first new look at the high-energy gamma-ray sky in more than
a decade and with unprecedented sensitivity. Because gamma rays are the
most energetic form of light, they can only be produced in processes that
involve large transfers of energy.
The LAT is observing the extreme facets of the Universe: huge gravitational
pulls, intense magnetic and electric fields, jets, shocks that accelerate
particles to nearly the speed of light, and energetic nuclei colliding with
matter and light are some of the phenomena that produce gamma rays.
Bright Source List (BSL) is being cited because it
summarizes the early results from the LAT survey of the sky. It takes
advantage of the LAT's wide field of view (it surveys the entire sky
every three hours), high sensitivity, broad energy range, and good
resolution to produce the most detailed picture yet of the powerful
sources of gamma rays.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
Top to bottom: Coauthors:
Jean Ballet, Isabelle Grenier, & Seth Digel
In some respects, the LAT Bright Source List covers all three of these
1. The list includes many newly discovered astrophysical gamma-ray sources,
giving their locations in the sky, with some information about their
variability and their energy spectra.
2. The methodology is new, because no previous gamma-ray telescope has had
to deal with such a huge region of the sky and such a broad energy range at
3. Synthesis of knowledge is essential in astrophysics, because we always
learn the most about a phenomenon by combining knowledge from many
different types of observations and with theoretical interpretation.
New types of objects or new aspects of their activity and evolution are
found by comparing our gamma-ray data with observations from radio,
optical, and X-ray telescopes and then applying models of physical
processes to explain these multiwavelength studies.
Would you summarize the significance of your paper
in layman's terms?
Trying to understand what is out there in the Universe and how those things
work is a bit like the proverbial blind men examining an
elephant—different ways of sensing produce different impressions.
What the Fermi LAT Bright Source List does is to demonstrate that gamma
rays give us a valuable new way to explore the cosmos, and, in particular,
some of the powerful forces that shape the Universe.
For instance, the LAT Bright Source List is dominated by huge black holes
that have formed at the centers of galaxies during their cosmological
evolution. These black holes are both witnesses and significant actors of
the evolution of the Universe to its present state. Their intense gamma-ray
emission, produced in jets outside their event horizons, offers a new means
to explore their activity over billions of years.
The other sources in the List are found mostly in our own Galaxy. Observing
the high-energy side of their activity allows us to explore physical
processes that we could not even dream of producing in terrestrial
How did you become involved in this research, and
were there any problems along the way?
The four of us who are corresponding authors on this paper are just
representatives of a large collaboration that built and operates the Fermi
Large Area Telescope. In fact, the collaboration is large enough that we
produce our major papers with an alphabetic author list (hence, Abdo et
We four came from different backgrounds—Dave from the early days of
US gamma-ray research at Goddard, Isabelle from the European COS-B
gamma-ray telescope collaboration, Seth from radio astronomy, and Jean from
Many others in the LAT collaboration come from particle physics, since the
techniques of gamma-ray telescopes are similar to those used in high-energy
physics. What brought us together was the opportunity represented by the
Fermi Gamma-ray Space Telescope (originally called GLAST).
As with any large satellite project, especially one involving an
international collaboration, there were issues (we do not call them
problems) of designing the best possible instrument, dividing up
responsibilities, finding funding, meeting schedules, and planning for
contingencies. We are happy to report that the satellite worked well from
the beginning of the mission and is still going strong.
Where do you see your research leading in the
The Bright Source List is just a first step for the Fermi mission, which is
planned to run for at least five years. As observations continue, we have
two outstanding opportunities:
1. The gamma-ray sky is constantly changing, on time scales running from a
few hours to many years. The ability of the LAT to monitor the whole sky
several times a day enables us to alert observers at other wavelengths of
potential targets of interest.
2. As LAT scans the sky, it produces deeper and deeper images of the
high-energy Universe. Fainter sources become visible, and greater details
emerge of sources already known.
"The Large Area Telescope (LAT) on the Fermi Gamma-ray
Space Telescope is providing the first new look at the
high-energy gamma-ray sky in more than a decade and with
To build the Bright Source List, we have developed and tested the tools and
methods necessary to tackle the much larger datasets that are now
available. We have also evaluated the pitfalls of such an exercise.
The collaboration has prepared and submitted for publication a fuller
catalog with six times more sources that have become visible with data from
the first year of the mission.
One remarkable feature of the Fermi mission is the fact that all the data
are public. The Fermi Science Support Center at NASA Goddard Space
Flight Center in Maryland, USA, provides all the data in near-real-time,
along with software so that anyone can look to see whether a favorite
object in the sky is producing gamma rays at any given time.
Do you foresee any social or political
implications for your research?
It may sound puzzling to think of studying gamma rays coming from distant
parts of the Universe as having any social or political implications, but
here are several obvious ones:
1. The excitement of exploration. One defining characteristic of humans is
the drive to explore. With telescopes like Fermi, we explore the Universe,
not by going to distant places but by learning from what comes to us from
When we can tell listeners, for example, that we are seeing millisecond
pulsars—stars the size of a city that spin as fast as a kitchen
blender—or black holes that gobble and spew matter at bewildering
energies—we generate enthusiasm for exploration of all kinds. That
sort of excitement translates directly into support for scientific research
2. The challenge of the unknown. Everyone involved in basic research has to
deal with the question, "What good is your research?" That question is an
opportunity to remind the questioner that even when we have no immediate
idea where our research will lead; it may have long-term effects that we
cannot even imagine.
One prominent example is Einstein’s general theory of relativity.
Surely Einstein had no idea that this work on space and time would prove
useful, but anyone who uses a GPS system in an automobile should be aware
it would not work if the system did not take general relativistic effects
on the GPS satellites into consideration.
3. The importance of interdependence. The more we explore the Universe and
its content, the more we see that many phenomena, seemingly independent,
are in fact deeply connected.
Interplays between objects or between an object and its environment have
played key roles in the evolution of the Universe, whether the object is a
planet, a star, or an entire galaxy.
For instance, high-energy phenomena such as exploding stars and black holes
have been active in shaping our current Universe. In this context, modern
science has to face a great challenge: how to understand with mathematical
equations and computers the complexity of these interplays and feedbacks.
This challenge applies to all disciplines, such as cell biology, climate
evolution, seismology, or world finances. The Universe is both the simplest
and most complex system to explore new methods.
David J. Thompson
NASA Goddard Space Flight Center
Greenbelt, MD, USA
Jean Ballet and Isabelle Grenier
CEA/DSM-CNRS-Université Paris Diderot
SLAC National Accelerator Laboratory
Menlo Park, CA, USA