WFC Science highlights - an extraordinary long and rare X-ray burst

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The source dubbed '4U 1735-44' originates in a close binary star system which is known since the earliest days of X-ray astronomy. The X-radiation is produced mainly in an accretion disk around a compact neutron star (i.e., a star as massive as the Sun but the size as the city of Amsterdam). This neutron star is also known to exhibit thermonuclear flashes which show themselves as bursts of X-rays. These bursts always last shorter than about 10 minutes, like for any other burster. They are always characterized by a thermal spectrum with a temperature of 20 million degrees, which cools during the decay.

On August 22nd, 1996, the WFC made detected an unusual event from 4U 1735-44: a flare with a temperature of 20 million degrees, which cools during the decay. Taking into account also the peak flux, the flare showed all characteristics of a thermonuclear flash except one: it lasted about 1000 times as long. No other physical mechanism for such a flare is known thus far, and it was proposed by Remon Cornelisse et al. (from SRON and Utrecht University) that this is an extraordinary case of a thermonuclear flash where perhaps not hydrogen or helium make up the fuel by higher-Z elements. Since their discovery, two more such flares have been discovered in two different sources.

Top: The nine day lightcurve of 4U 1735-44 as observed with the WFC in August 1996. Countrates are for channels 1-31 (energy range 2-28 keV). Each time bin corresponds to 15 minutes. A large enhancement in intensity starts near MJD 50318.1 and ends about 4.0 hours later. The vertical dotted lines indicate the time interval for which the countrate and hardness ratio are shown in the expanded view of the lower frames. The hardness ratio shown is the ratio of the countrate in channels 12-29 (5-20 keV) to that in channels 3-11 (2-5 keV). During the flux enhancement the exponential softening expected for a type-I X-ray burst is clearly visible. From Cornelisse et al., A&A 357, L21 (2000).

Jean in 't Zand, SRON, November 22, 2000