From the Special Topic of
According to our Special Topics analysis on Gamma-ray
Burst research over the past decade, the work of Dr. Peter
Mészáros ranks at #1 by total citations and
by total number of papers, based on 147 papers cited a
total of 5,074 times. In
Essential Science IndicatorsSM from
Reuters, his record includes 153 papers, the majority
of which are classified in Space Science, cited a total of
5,564 times between January 1, 1999 and February 28,
Dr. Mészáros is the Eberly Chair of Astronomy &
Astrophysics and Professor of Physics, as well as the Director of the
Center for Particle Astrophysics, at the Pennsylvania State University in
University Park, PA.
In the interview below, he
talks with ScienceWatch.com about his gamma-ray burst
Would you tell us a bit about your
educational background and research experiences?
My parents were war refugees from Hungary who settled first in Belgium and
later in Argentina, where as a boy on camping trips I slept under the stars
and wondered what makes them shine. I got an MS degree in Physics at the
University of Buenos Aires, and continued graduate studies at the
University of California, Berkeley, where I got my Ph.D. in 1972, working
on the astrophysics of the interstellar medium.
I became fascinated with high-energy astrophysics and cosmology during
postdoctoral stints at Princeton and Cambridge (UK). I worked on early
versions of cold dark matter cosmology and black hole accretion problems,
and until 1990 I concentrated mainly on black holes and magnetized neutron
stars in various forms. However, gamma-ray bursts (GRBs) were becoming an
increasingly attractive puzzle, which somehow was connected to those
interests of mine, but it was not clear exactly how.
What influenced your focus on gamma-ray
This illustrates (partial image shown) nicely the
point about the high energy pulse (last curve down)
coming 4 seconds later than the low energy pulse
(first curve), which is the basis of our argument
for constraining the quantum gravity energy scale.
View full image
The Compton Gamma Ray Observatory was launched by NASA in 1990, and
demonstrated that GRBs were isotropically distributed, so they had to be
either very nearby galactic objects, or very distant cosmological objects.
If the latter, they would be the most energetic explosions in the Universe,
and this definitely turned my attention towards GRBs, during a sabbatical
spent at Cambridge, which led to a long and fruitful line of work on the
Two mileposts of this were our fireball shock model, with Martin Rees,
where we described the GRB radiation as originating in external and
internal shocks in the relativistic outflow following the explosion, where
electrons are accelerated and radiate via the synchrotron and inverse
Compton mechanisms. Much new data and theory has been upcoming since then,
and many new questions have arisen, but these models continue being the
main workhorses used for fitting and interpreting the data.
One of your most influential papers is the 1997
Astrophysical Journal article "Optical and long wavelength
afterglow from cosmological gamma-ray bursts" (Mészáros
P, Rees MJ, 476: 232-7, 10 February 1997). Would
you talk about this paper and how it set the groundwork for future
work in this field?
In this paper we predicted, before the observations, that an afterglow from
the external shock was inevitable, leading to emission at increasingly
longer (and hence easier to observe) wavelengths, which would decay as a
power-law in time sufficiently slowly to allow accurate localizations with
X-ray and optical telescopes. This indeed was very soon afterwards achieved
when the Italian-Dutch satellite Beppo-SAX started to detect afterglows,
and optical redshifts proved that GRBs were at cosmological distance. Thus,
they were indeed the mightiest powerhouses in the Universe: they burn up as
much energy in a few seconds as the Sun does in ten billion years, or as an
entire galaxy does in 100 years.
The study of afterglows is what has allowed us to map the distribution with
redshift of gamma-ray bursts, i.e., how they fit in with the development of
the universe. It also led to the identifications of the host galaxy and
progenitor star types, as well as providing a closer and deeper look into
the afterglow mechanism and its interaction with the host galaxy medium.
These types of experimental data have provided the guidance and tests
needed to develop the contemporary theoretical views.
Your most-cited paper in our analysis is the 2004
Astrophysical Journal paper, "The Swift gamma-ray burst
mission." Would you tell us about this paper, its goals and
The Swift mission was designed to follow up burst afterglows in a prompt
fashion, so that one could point an X-ray and optical instrument at it
before the afterglow had time to fade much. This was made possible by
building Swift so that as soon as a burst was detected by an
omni-directional (but poorly focused) gamma-ray detector, the spacecraft
would slew (repoint) in less than 100 seconds to acquire a high angular
resolution x-ray and optical image, and communicate this to the ground in
5-10 seconds to alert large ground-based optical telescopes.
This is a key mission, with the principal investigator
Neil Gehrels from NASA and teams from Penn State,
NASA, Los Alamos, the UK, and Italy participating in the design,
building, and operation, which is controlled from the Mission Control
center at Penn State. My role in this was and is to serve as Lead of the
theory team participating in the analysis of the GRB data it produces.
Swift has resulted, as we hoped, in production-line generation of large
numbers of identifications, redshift distances, and multi-wavelength
studies of bursts. This has led to a much more detailed elucidation of
the physics of bursts, showing that they are much more complex than they
appeared from the much sketchier early observations. It has led to the
secure identification of the progenitors of some bursts, which are
massive stars whose explosion leads also to a supernova remnant, and
leaves behind a black hole.
It has also led to the demonstration that a second class of shorter bursts
is probably associated with the merger of compact binaries involving
neutron stars, which must also result in forming a black hole. And it has
led to the detection of the most distant object in the Universe, GRB
090429, whose extreme distance was first estimated by Derek Fox at Penn
State, and which was soon spectroscopically confirmed at a record redshift
of z=8.1. This distance corresponds to the burst having occurred only 630
million years after the Big Bang, when the Universe was just 1/22 of its
More recently, you were part of the team that
published the March 2009 Science paper, "Fermi Observations
of High-Energy Gamma-Ray Emission from GRB 080916C" (Abdo AA, et
al., 323: 1688-93, 27 March 2009). Could you tell our
readers something about this paper?
This burst was detected by the recently launched Fermi satellite, and it
was the first burst for which Fermi's LAT high-energy pair-conversion
counters detected with high confidence a large number of photons above GeV
energies. Fermi is a large international collaboration, led by principal
investigator Peter Michelson from Stanford, with contributions from many
universities and labs. Detecting a source with a high density of photons at
this energy is extremely interesting, since in order for photons of this
energy to avoid annihilating each other and converting into electron
positron pairs, the plasma jet where they originate must be moving at
extremely relativistic speeds.
Previously we had rough estimates of how close this speed had to be to the
speed of light, but with large numbers of photons and a good GeV spectrum
it was possible to demonstrate that the jet had to be moving with Lorenzt
factors Gamma of about 880, an extremely high value corresponding to a
velocity which is roughly 0.999999 of the speed of light.
The most interesting result, however, was that the highest energy photon of
13 GeV arrived about four seconds later than the lower, MeV energy photons
at the outset of the burst. Quantum gravity is a theory which is as yet
non-existent but which is the Holy Grail of 21st century physics, uniting
gravity and quantum mechanics in a Theory of Everything. One of its general
predictions is that it induces foam-like fluctuations in space-time which
cause a relative delay between the propagation of higher and lower energy
photons. The magnitude of the delay depends on a fundamental quantity of
physics, the Quantum Gravity energy scale, which is estimated to be around
1.3 x 1019 in GeV units.
From left to right: Nino Cucchiara (from Milano,
grad student of Fox at PSU), Dr. Derek Fox
(assistant prof. at PSU), Dr. Alessandra Corsi
(from Rome, postdoc of PM), Peter
Mészáros, Dr. Xuefeng Wu (from
Nanjing, postdoc of PM), Dr. Kenji Toma (from
Kyoto, postdoc of PM).
The delay observed in GRB 080916C allowed our team to set an experimental
lower limit to this scale, the highest obtained so far by any group, of
just one order of magnitude below the theoretical estimate, 1.5 x
1018 GeV. These unimaginably high energies are completely out of
reach of even the highest energy particle accelerators such as the LHC at
CERN, which aims to probe up to 14,000 GeV, but this burst allowed us to
set a robust experimental lower limit on this much higher, fundamental
What are some of the key things we now know about
gamma-ray bursts that we didn't know 10 years ago?
We confirmed 12 years ago that they had afterglows and they were at
cosmological distances, and in the last 10 years we have learned a number
of other new key facts. One is that within seconds after the gamma-ray
trigger, sometimes a very intense optical flash is also detected, and it is
still disputed whether it is due to a reverse external shock or it is
associated with the prompt gamma-ray mechanism. We know that some bursts
(those with gamma-ray durations above two seconds) are associated with the
collapse of young massive stars, sometimes showing a supernova remnant, and
we have detected what appears to be the breakout of the shock through the
We have learned that shorter-duration bursts appear to originate from
older, lower mass progenitors, probably the merger of neutron star
binaries. We have obtained evidence which indicates that the bursts signal
the endpoint of the evolution of certain types of stars, and signal the
formation of a black hole. This is a way in which galaxies seed themselves
with stellar mass black holes.
We have learned that bursts occur already at the earliest dates being
probed by telescopes, and are among the most distant objects in the
Universe. We have learned that the jets producing the radiation through
which burst are seen are more relativistic and have a more complex geometry
than had been originally considered in the absence of the data we have now.
In what directions do you see this field going in
the next decade?
We need to better understand the prompt gamma-ray emission mechanism, and
how it relates to the generation of magnetic fields and relativistic
particles. Missions such as Fermi will allow us to probe into the higher
energy range, which will help to understand the total energy budget,
including how much energy is involved in protons and magnetic fields. We
also need to know what the contribution of neutrinos and gravitational
waves is. These sources provide natural laboratories where conditions and
energies surpass anything that terrestrial laboratories can provide.
Experiments such as IceCube, Auger, LIGO and VIRGO, and air Cherenkov
telescopes will provide new information and constraints on questions posed
by GRBs, which could have important implications in other areas of science.
We also expect to see a large increase in the use of GRBs as cosmological
probes. They provide the most intense electromagnetic beacons at the
largest distances; they act as lighthouses shining through the fog. They
indicate where they are, what the burst's immediate environment consists
of, and also what is the distribution and chemical composition of the
material between them and the Earth. GRBs are basically time machines: we
can travel back in time, and see how the universe looked at the earliest
time when stars first started to form. This will allow probing how these
first sources of radiation reionized and lit up the Universe. The proposed
JANUS mission, which is in the last stages of review by NASA and in which
Penn State is involved, is designed to provide many of the answers to such
What lessons would you like lay people to remember
about your research?
Major questions of interest to all humanity, such as how the Universe looks
at the earliest times and the largest distances we can probe, can be
addressed with resources which require a minuscule fraction of the US
budget. International collaborations are invaluable in achieving such
goals. Universities, both public and private, coupled to the resources of
national labs and agencies, are ideal hothouses for providing the talent
and manpower which can lead to momentous scientific results.
Department of Astronomy & Astrophysics
Department of Physics
The Pennsylvania State University
University Park, PA, USA
KEYWORDS: GAMMA-RAY BURSTS, SWIFT GAMMA-RAY BURST MISSION,
X-RAY AFTERGLOW LIGHT CURVES, GAMMA-RAY BURST AFTERGLOWS, FIREBALL
SHOCK MODEL, REDSHIFT, HOST GALAXY, PROGENITOR STAR TYPES, FERMI
SATELLITE, PHOTONS, GEV SPECTRUM, OPTICAL FLASH, EMISSION MECHANISM,
MAGNETIC FIELDS, RELATIVISTIC PARTICLES, JANUS MISSION,