WFC Scientific highlights
Although at least 253 distinct source were detected with it
(see catalog
by Verrecchia et al. (2007)), WFC excelled in the ability to detect
fast transient cosmic X-ray phenomena with time scales of seconds to
hours. The scientific highlights concentrate on transients with time
scales of less than a minute which are referred to as 'bursts'. Two
types of bursts with widely different mechanisms can be distinguished:
gamma-ray and X-ray bursts.
Gamma-ray bursts
Gamma-ray burst last from a fraction of a second to at least 15 minutes.
Their spectra peak at a few hundred keVs and are very broad.
Photons have been detected of GeV energies down to, very recently, optical
wavelengths. During a brief moment, a gamma-ray burst (GRB) can be the
brightest gamma-ray object in the sky. Since their discovery in the late
sixties until the launch of BeppoSAX, the cause of GRBs has been
speculative. The main reason for this was not knowing a distance to
any GRB.
WFC has the unique capability to swiftly (within hours) and accurately
(to a few arcminutes) determine the positions of bursts. This resulted
in a revolution of gamma-ray burst (GRB) research in 1997. The reason is
that GRB localizations through WFC enabled for the first time the discovery
of afterglow emission in X-rays, optical and radio wavelengths. In particular
the optical spectrum is very instrumental in diagnosing a GRB because this
enables the determination of its distance. This happened for the first time
for a GRB which occurred ob May 8, 1997. Since then, the distance of 15 more
GRBs has been determined. It turns
out that GRBs are at truly cosmological distances. The WFC accomplishment
has earned the BeppoSAX GRB team and Jan van Paradijs (University of
Amsterdam) the prestiguous Rossi
Prize of the American Astronomical Society in 1998.
The WFC sky coverage of ~4% and its sensitivity make possible
that, on average, once every 9 weeks a gamma-ray burst is in the field of
view and leaves a detectable signal. To be able to discriminate a gamma-ray
burst from other burst phenomena in the sky (see below), the Gamma-Ray Burst
Monitor on board BeppoSAX is used. Thus, the BeppoSAX observatory is capable to
localize and diagnose GRB independent from other observatories such as
the Compton Gamma-Ray Observatory. Since the data is down-loaded every
1.5 hour BeppoSAX orbit, the data can be analyzed within a couple of hours
and this is done so by duty scientists at the BeppoSAX Science Operation
Center (SOC). The SOC is staffed 24 hours each day. If a GRB is detected,
an alert procedure is initiated which involves quick dissemination of the
position to the astronomical community and planning of X-ray follow-up
studies with the narrow-field instruments on BeppoSAX.
Up to March 2001, WFC detected 45 GRBs. Some interesting cases follow:
- The event on Oct. 30, 2001, was a peculiar burst
with the longest duration (23 min) and a low gamma-ray content
- GRB 010222 is the burst with the highest
X-ray fluence and has evidence for a high-density circumburst medium.
- GRB 990123 is the most energetic
GRB observed ever. Also, it is the
first burst for which contemperaneous optical emission was detected.
- GRB 980519 is a bright X-ray
counterpart with the strong suggestion of
an X-ray precursor to the gamma-ray event. It is the second-best
documented case.
- GRB 980425 has a positional and
timing association with a
supernova in a nearby (z=0.0085) galaxy. This suggests that
there are at least two kinds of GRBs: those at cosmological
distances with total energy outputs of ~1052-54 ergs, and
those due to SN with outputs of order 1048 ergs.
- GRB 971214 is the farthest GRB so far (at z=3.14)
- GRB 970508 is the first GRB ever with a distance estimate through
a measurement of the cosmological redshift (z=0.835)
- GRB 970228 is the first GRB
with an unambiguously identified optical afterglow
X-ray bursts
The main drive for building the WFC was to point it at the center of the
Galaxy and study low-mass X-ray binaries (LMXBs). More than 50% of the
population of Galactic low-mass X-ray binaries is contained in this field. A
substantial fraction of LMXBs exhibits X-ray bursts. Such bursts are
caused by thermonuclear helium flashes on the hard surfaces of neutron
stars and they are an extremely simple and effective diagnostic to
determine the nature of the compact object in the binary to be a neutron
star (it could either be a white dwarf, neutron star or black hole).
A 2 to 8 keV combined image of the 40 by 40 square degrees
WFC field of view pointed at the Galactic center. The field contains
usually about 30 active sources of which one or two are transient.
Most of these sources are low-mass X-ray binaries.
WFC has a large observation program on the Galactic center (in fact,
all WFC primary mode observations are contained in this program). Up
to November 2000, WFC has been exposed to the Galactic center for a
total of over 4 million seconds. During this exposure, WFC detected at
least 2200 X-ray bursts from at least 53 LMXBs. It has expanded the
population of LMXB bursters by 40% through its discovery of 23 new
LMXB bursters - the best burster discovery track record of any instrument.
Photon rate in 2 to 25 keV versus time for one
observation on the Galactic bulge in March 1997. The observation
is interrupted at 110~min intervals by Earth occultations.
This plot illustrates the large amount of X-ray bursts and science that derives
from that. The following burst sources were identified:
GRO~J1744-28 (26 bursts), SAX~J1750.8-2900 (4), H~1724-307 (2),
GX~354-0 (4), GS~1826-24 (2), KS~1731-26 (2)
A few important scientific results from this program are:
- An extraordinary long and rare X-ray burst was discovered from
4U 1735-44, which provided a
new testing ground for the theory of nuclear burning on neutron
stars. It turned out that this is the prototype of a new type of
burst called 'superburst' which is due to ignition of carbon (the
same element that is thought to be responsible for type Ia
supernovae) at a depth of 100 m in the neutron star atmosphere
instead of ignition of helium at 1 m. Subsequent archival WFC
searches have revealed 6 more superbursts: 1 from
KS 1731-260 and Ser X-1 and 4
from GX 17+2. The total number of superbursts is now (2016) about
20. See first Paper
- Another subclass of X-ray bursters was discovered after the
emission: that of the 'intermediate duration X-ray bursts'. In
contrast to all other bursts, these happen on relatively cool
neutron stars due to low average mass accretion rates. To reach
ignition conditions for helium, the lower temperature needs to be
compensated by larger densities, ergo thicker piles of helium. This
results in the most powerful X-ray bursts (all way over the Eddington
limit) with the thick piles taking longer to cool of, so longer burst
durations. These bursts are by many considered the most opportune X-ray
bursts to find absorption features that may help constrain neutron star
radii. So far, they have not been detected with Chandra or XMM-Newton. See
Paper
- The prototypical accretion-powered millisecond pulsar SAX
J1808.4-3658 was discovered with WFC (the pulsation was
discovered with RXTE). Also, the first burst oscillation from
such a system was discovered with the WFC, confirming that the
many other burst oscillations were due to neutron star
rotation. See Paper 1
and paper 2
- X-ray bursts were detected from the globular cluster sources
NGC 6652
and NGC 6640, thus reducing the
number of non-bursters among the twelve bright X-ray sources in
clusters to only 1.
- Over the course of 2.5 years GS 1826-238 (a former suspected BHC)
has exhibited about 80 X-ray bursts which
occur quasi-periodic with a period of 5.75
hrs. See Paper
Jean in 't Zand, SRON, April 8, 2016