The following information is relevant to a press release made at the 191st Meeting of the American Astronomical Society that was held on January 6-10, 1998. The complete text of the press release is followed by links to postscript images.
FOR RELEASE: 2:00 PM EST, JANUARY 9, 1998
Astronomers are announcing today that they may have found evidence
for gamma-ray flare activity on an unusual white dwarf star.
The report is being presented by Dr. Mark McConnell of the University
of New Hampshire, Durham, NH to the American Astronomical Society
meeting in Washington, DC. The report represents work by McConnell
and his colleagues at the Institute for Earth, Oceans and Space
at the University of New Hampshire, and at the Max Planck Institute
in Munich, the Space Research Organization of the Netherlands
in Utrecht, Holland and the European Space Agency in Noordwijk,
Holland. "This result provides further support for the idea
that solar flares are a prototype for larger-scale phenomena elsewhere
in the galaxy," says McConnell, a Research Associate Professor.
"Our understanding of solar flares may therefore prove extremely
useful in understanding other astrophysical phenomena."
The existence of stellar flares has been known for many years,
but these are typically observed at optical, radio or X-ray wavelengths.
This is the first time that evidence has been found for gamma-ray
flares on an object other than the Sun. "Our understanding
of the solar flare phenomena may be directly applicable in our
efforts to better understand the environment of a more exotic
object like a white dwarf," says Dr. James Ryan (a co-investigator
and Professor at the University of New Hampshire).
The report is based on the detection of a gamma-ray source near
the south celestial pole. Although there are several candidate
X-ray sources that are consistent with the newly discovered source,
the best candidate is an extremely unusual white dwarf, RE J0317-853.
Not only is this the brightest of the X-ray candidate sources,
it is also the only one of the X-ray candidates seen at extreme
ultraviolet energies by both the ROSAT X-ray satellite and by
the Extreme Ultraviolet Explorer (EUVE) satellite.
A white dwarf represents the last stage in the life cycle a solar-type
star. It is an object about the size of the Earth, but with a
mass comparable to that of the Sun. This particular white dwarf
has one of the highest surface temperatures ever measured for
a white dwarf, about 50,000 K (roughly ten times the surface temperature
of the Sun). More importantly, it has one of the strongest white
dwarf magnetic fields ever measured, about 340 million Gauss (or
about a million times the typical solar magnetic field strength).
In addition, the white dwarf appears to be rotating once every
12-minutes. In astronomical terms, it is also relatively nearby,
about 114 light years from Earth.
The measured gamma-rays have an energy of 2.2 million electron
volts (MeV). This corresponds to the energy of a photon released
when a free neutron recombines with a proton to produce a nucleus
of deuterium (a nucleus of heavy hydrogen). The process by which
this takes place is also known as neutron capture.
The unusual nature of RE J0317-853 and its close proximity to
the Earth suggest that it may, in fact, be the source of the gamma-rays.
The gamma-ray emission could be explained by direct analogy with
solar flares. Neutron capture emission at 2.2 MeV is often the
dominant component of a solar flare gamma-ray spectrum. In solar
flares, a large release of stored magnetic energy takes place
by a process known as magnetic reconnection, when magnetic field
lines are disrupted and the magnetic field must reconfigure itself.
The release of magnetic energy results in the rapid acceleration
of charged particles. These accelerated particles interact in
the solar atmosphere to produce neutrons that are subsequently
captured by protons, thus generating the 2.2 MeV photons. The
very strong magnetic fields on RE J0317-853 certainly provide
sufficient magnetic field energy. The presence of gamma-ray emission
would further imply that some sort of magnetic reconnection process
is taking place.
Although the gamma-ray data do not themselves indicate flaring
activity, the presence of flares is implied by direct analogy
with the origin of 2.2 MeV emission from the Sun. "The data
are consistent with a steady source of emission," says McConnell,
"but some level of time variability cannot be ruled out.
We might be witnessing several very large flares that erupt only
ocassionally or we might be witnessing a continuous or nearly-continuous
eruption of smaller flares across the surface of the white dwarf."
In either case, the flares would be large by solar standards.
The measured level of gamma-ray emission implies an amount of
energy corresponding to the energy of a typical solar flare being
released once every second.
The results are based on work with the COMPTEL experiment on
the Compton Gamma-Ray Observatory (CGRO). COMPTEL images gamma-ray
photons in the energy range of 1-30 MeV. This corresponds to
the range in which many nuclear processes result in the emission
of gamma-ray photons at distinct energies (the so-called "nuclear
lines"). Previously, COMPTEL data have been used to map
the distribution of radioactive aluminum in the galaxy, a result
which sheds light on the origin of heavy elements.
In order to search for neutron capture emission, the COMPTEL
researchers assembled a map of the entire sky spanning a small
energy range centered on 2.2 MeV. The map was generated using
data accumulated over the first five years of the CGRO mission.
The data were processed so that the maps were sensitive specifically
to line emission, rather than any continuum emission that might
be present in the same energy range.
The initial motivation for the study was to search for emission
from X-ray binaries. In X-ray binary systems, matter is accreted
onto either a neutron star or a black hole. Several theories
predicted that nuclear reactions within the accretion flow would
lead to neutron production. The detection of neutron capture
radiation would provide evidence of these nuclear reactions and
allow for a detailed study of the heating processes within the
accretion flow. In the end, however, the researchers found no
evidence for neutron capture emission from any known X-ray binary.
Further confirmation of the gamma-ray emission is needed. Additional
observations with both COMPTEL and OSSE (the Oriented Scintillation
Spectrometer Experiment) on CGRO are planned for June. Observations
are also being planned for the near future with the Rossi X-Ray
Timing Explorer (RXTE) to search for hard X-ray emission. A detection
of RE J0317-853 by either OSSE or by RXTE would provide an important
link between the soft X-ray observations by ROSAT and the gamma-ray
observations of COMPTEL.
This work has been supported by NASA's Compton Gamma Ray Observatory
Program, the Deutsche Agentur fuer Raumfahrtgelenheiten (DARA),
the Netherlands Organization for Scientific Research and by the
European Space Agency.
A single figure is provided with this press release. It represents a map of the full sky at an energy of 2.2 MeV, as produced using data from the COMPTEL experiment on the Compton Gamma-Ray Observatory. It is presented in galactic coordinates. Several versions of this figure are available in postscript format:
A map of the full-sky as seen at an energy of 2.2 MeV by the COMPTEL
experiment on the Compton Gamma-Ray Observatory. The map is presented
in galactic coordinates. At the sensitivity level of this map,
the sky is generally featureless. For example, there is no evidence
for emission along the galactic plane. The only significant feature
is a single point source in the lower-right quadrant of the map.
The location of this source is close to the south celestial pole
and is consistent with the unusual white dwarf object RE 0317-853.
Here is the abstract for the technical presentation given at the AAS meeting. A copy of the viewgraphs used during the technical presentation is available here in pdf format.
M. McConnell, J. Ryan
University of New Hampshire, Durham, NH
R. Diehl, V. Schönfelder, A. Strong
Max Planck Institute for Extraterrestrial Physics, Garching, Germany
H. Bloemen, W. Hermsen
Space Research Organization of the Netherlands (SRON), Utrecht, The Netherlands
K. Bennett, R. van Dijk
Astrophysics Division, ESTEC, Noordwijk, The Netherlands
Los Alamos National Laboratory, Los Alamos, NM
The COMPTEL experiment on the Compton Gamma Ray Observatory, with its large FoV imaging capability, has successfully surveyed the entire sky in the 0.75-30 MeV energy range. Falling within this energy range is the neutron capture line at 2.2 MeV, emission from which might be expected in certain binary accretion models. We have therefore assembled an all-sky map for 2.2 MeV line emision following the successful approach used to image the 1.806 MeV line emission from 26Al. The resulting map is generally featureless, except for one possible point-like feature near (l,b) = (300.5°, -29.6°), which is significant at the 3.7 sigma level. Within the 3 sigma error box of this source, there are 11 objects catalogued by the ROSAT All-Sky Survey. None of these correspond to any known X-ray binary, cataclysmic variable or active galactic nucleus. The most interesting of the ROSAT sources is RE J0317-853, a source that is also catalogued both by the ROSAT Wide Field Camera and by EUVE. This source is identified as a nearby white dwarf star (only 35 pc away) with some rather unique characteristics. Most notably, RE J0317-853 is one of the hottest known white dwarfs (with a temperature of 50,000 K) and it has one of the strongest white dwarf magnetic fields (340 MG). Although the association of a 2.2 MeV point source with RE J0317-853 is speculative, such an association remains an intriguing possibility. Here we shall review the analysis of the COMPTEL data and the evidence for a point source of 2.2 MeV emission. We then shall discuss the implications of a possible association with the white dwarf object RE J0317-853.
For more information:
Dr. Mark McConnell
Space Science Center
Morse Hall, Rm 312
39 College Road
University of New Hampshire
Durham, NH 03824
Phone: (603) 862-2047
Fax: (603) 862-4685