JANUARY 9, 1998
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:
* False-color map with galactic coordinate overlay. * False-color map without galactic coordinate overlay. * Grey-scale map with galactic coordinate overlay. * Grey-scale map without galactic coordinate overlay.
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.