WFC Scientific highlights

BeppoSAX reentered Earth's atmosphere April 29, 2003

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:

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:

Jean in 't Zand, SRON, April 8, 2016