A new cosmic phenomenon has recently been discovered in the x-ray sky. Short sudden outbursts of x-rays appear as new sources in the sky, lasting only a few minutes. They are termed X-Ray Flashes (XRFs).

Today Dr. John Heise of the SRON National Institute for Space Research Netherlands and Utrecht University will present further evidence about the nature of these events. We now know that at least some originate in very powerful explosions seen throughout the universe and are most likely related to the death of stars and the formation of black holes as the stellar remnant.

The X-ray Flashes were discovered with the Wide Field Cameras on board the Italian-Dutch X-ray satellite BeppoSAX in observations performed in collaboration with Dr. Jean in 't Zand. John Heise, the Principle Investigator of this instrument: "I am happy we could guide astronomers for the second time to a new kind of explosive phenomenon in the sky. A wide view combined with precise positions is difficult to achieve in x-rays, but essential for such discoveries. The random flashes occur almost daily somewhere in the sky, but at unknown positions, so one needs a broad view to catch a few. The most important part is to localise them fast and accurately."

The Wide Field Camera is the same instrument that for the first time localised quickly and precisely (in 1997) Gamma Ray Bursts in the sky, by employing their x-rays counterpart. This and subsequent discoveries immediately revolutionised the ideas on the origin of Gamma Ray Bursts and proved the cosmological origin of such events.

Astronomers know a dramatic explosion marks the endpoint of the life of a massive star. At present two types of manifestations of this explosion are known from observations: Supernovae and Gamma Ray Bursts. Evidence shows that most likely X-Ray Flashes represent a third type. In particular the detection of "afterglow radiation" after the event points towards a dramatic explosion. The detailed properties of X-ray Flashes show them to be close relatives to Gamma Ray Bursts.

The first type of bright transient sources in the sky, the Supernovae ("super-bright new stars") appear suddenly in the optical sky. They are powered by the collapse of a massive star at the end of its life. Large amounts of mass are ejected at relatively low velocities. These remnants can be observed with optical telescopes.

The second type of bright transient sources in the sky, Gamma Ray Bursts, appear during a few seconds or minutes as the brightest objects in the gamma ray sky. They are equally well related to the death of stars and are similarly powerful. Astronomers now understand that in Gamma Ray Bursts only small amounts of mass are ejected but at extremely large velocities. Such extreme relativistic (near-speed-of-light) velocities boost the photons into gamma rays, to be observed with satellites.

In the new third type of bright transient sources in the sky, the X-ray Flashes, most of the photons come out as x-rays. Such events are expected (and even were predicted) to occur when the mass from the explosive ejection is larger than the mass inferred from Gamma Ray Bursts, but not as large as in Supernovae. X-ray Flashes probably are mildly relativistic and in this sense intermediate between Supernovae and Gamma Ray Bursts. The exact difference in origin between Gamma Ray Bursts en X-ray Flashes remains unclear so far. In another line of reasoning, outbursts in x-rays are expected when Gamma Ray Bursts occur at distances even larger than of classical Gamma Ray Bursts, near the edge of the observable universe. The expansion of the universe causes such events to be shifted to lower energies (called cosmological red shift). They would then manifest themselves as X-ray Flashes. Future Gamma Ray Burst instruments should look in x-rays in search for the most distant Gamma Ray Bursts to find out about the first stars ever formed in the early universe, the first stars that lighted the previously dark universe.

John Heise: "It is a fascinating thought that the first stars formed after the Big Bang, can now potentially be seen as X-ray Flashes".

Figure 1: A Gamma Ray Burst is shown in the left pannel, where the intensity of both gamma rays (lower part) and x-rays (upperpart) is shown in the cource of the half a minute that the bursts last. An X-ray Flash (right pannel) is shown on exactly the same scale. The X-rays also last about half a minute, but no gamma rays are detected. The X-ray data (2-28 keV) are from the Wide Field Camera and the gamma ray data from the Gamma Ray Burst Monitor (40-700 keV) both on board BeppoSAX.

For further information contanct:

Prof. Dr. John Heise
mailto:%20%20<a%20href="mailto:">j.heise@sron.nl</a>
Space Research Organization Netherlands (SRON )
Utrecht, The Netherlands

Tel: +31 30 2535727
Secr: +31 30 2535600
Fax: +31 30 2540860