Unlike the peaceful nighttime
sky we see with our eyes, the cosmos is violent and variable
at X-ray and gamma ray energies. One such high-energy eruption
that occurred during the last few months has been monitored by
the Naval Research Laboratory's (NRL's) Oriented Scintillation
Spectrometer Experiment (OSSE) on NASA's Compton Gamma Ray Observatory
(CGRO).
OSSE observations of this totally
new X-ray source -- believed to be located near the center of
our galaxy -- show that it is pulsating at half second intervals,
similar to a flashing lighthouse beam. It bursts at rates ranging
from 20 bursts per hour soon after its discovery to one or two
per hour now. Pulsation and bursting behaviors have been previously
associated with X-ray sources, but never before from the same
object. The new bursting X-ray pulsar has been officially named
GROJ1744-28 after the discovering mission and its celestial
coordinates.
Pulsed X-rays from such high-energy
eruptions are thought to be produced when a neutron star pulls
matter from its binary companion star. A neutron star is the
remnant of the collapse of an ordinary star. Outbursts of X-ray
emission from a neutron star occur as matter flows from a companion
star onto the surface of a neutron star and heats to such high
temperatures, on the order of one hundred million degrees, that
it glows in X-rays. A magnetic field a trillion times stronger
than that found at the surface of the Earth is needed to funnel
the matter onto hot spots at the poles of the neutron star.
The discovery of this unusual
object was made by the Burst and Transient Source Experiment
(BATSE) on CGRO, which continuously monitors the sky at X-ray
and soft gamma-ray energies. When BATSE first detected this object
in December, 1995 the instrument was "seeing" bursting
rates of roughly 20 per hour, each lasting between 5 and 20 seconds.
After several days, the bursting rate slowed considerably, but
the source then began producing bright, persistent emission and
became one of the brightest X-ray stars in the sky. At this point,
CGRO was reoriented for other instruments, including OSSE, to
begin observations.
If this object is near the Galactic
Center, as the OSSE observations suggest, then the X-ray power
of this source exceeds the total energy output of our Sun by
a factor of nearly one million, with the bursts being even brighter.
This would be equivalent to packing the mass of the Sun and the
radiant power of one million suns into a region not more than
20 miles
across. With this much power, most astronomers think that matter
would be blown away by the force of the radiation, so that the
X-ray fuel source would be shut off. Thus, say scientists,either
its direction is a coincidence, or our understanding of extremely
luminous sources in incomplete.
OSSE has observed GROJ1774-28
on three separate occasions, making several interesting discoveries.
For example, the phase of the pulsed flux during an outburst
lags that of the flux during steady emission by 20%. This is
a phenomenon never before seen in X-ray pulsars and may indicate
that the location of the X-ray emitting region moves from one
point of the neutron star surface to another between steady emission
and outburst.
Analysis of the OSSE data from
this source showed that the spectra of the steady and burst emissions
were not significantly different, suggesting that the emission
mechanisms for the two intervals are the same. This mechanism
is thought to be the conversion of gravitational energy into
X-rays when falling matter hits the surface of the neutron star.
This is significantly different from standard X-ray burst sources,
which are thought to undergo thermonuclear explosions during
outbursts.
From the OSSE observations, the
NRL researchers have concluded that GROJ1744-28 has a magnetic
field about 10 times smaller than most X ray pulsars and about
100 times larger than most X-ray bursters. By having a field
just in the middle of these regimes, the bursting X-ray pulsar
shares properties of both types of objects. The scientists, now
studying different models for the bursts, hope to solve this
problem before it fades away completely, which could happen within
the next few weeks.
OSSE was developed by the Gamma
and Cosmic Ray Astrophysics Branch of NRL's Space Science Division,
under the leadership of principal investigator, Dr. James Kurfess.
OSSE team members studying GROJ1744 28 include, Jim Kurfess,
Mark Strickman, Chuck Dermer, Eric Grove, Neil Johnson and Gerry
Share of NRL; Greg Jung and Bernard Philips of Universities Space
Research Associates; Danny Messina of SFA, Inc.; Steve Matz of
Northwestern University; and Steve Sturner, NRC/NRL research
associate.
OSSE is one of four instruments
launched on CGRO in 1991 to provide continuous coverage of the
most energetic phenomena in nature and can respond to targets
of opportunity such as this bursting X-ray pulsar. The instrument
was designed to make comprehensive observations of astrophysical
sources in hard X-ray and gamma ray ranges and uses four identical
detector systems to observe the high-energy processes occurring
in compact sources in our galaxy such as pulsars, neutron stars,
and suspected black holes, and explosive phenomena such as supernovae,
novae and gamma ray bursts.
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