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Astron. Astrophys. 357, L21-L24 (2000)

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1. Introduction

Of the [FORMULA]150 low-mass X-ray binaries known in our galaxy, about 40% show occasional bursts of X-rays, in which a rapid rise, lasting from less than a second to [FORMULA]10 s, is followed by a slower decay, lasting between [FORMULA]10 s to minutes. During the decay the characteristic temperature of the X-ray spectrum decreases. An X-ray burst is explained as energy release by rapid nuclear fusion of material on the surface of a neutron star and thus an X-ray burst is thought to identify the compact object emitting it unambiguously as a neutron star. If the burst is very luminous, reaching the Eddington limit [FORMULA], the energy release may temporarily lift the neutron star atmosphere to radii of order 100 km. Reviews of observations of X-ray bursts are given by Lewin et al. (1993, 1995).

The properties of a burst depend, according to theory, on the mass and radius of the neutron star, on the rate with which material is accreted onto the neutron star, and on the composition of the accreted material. It is hoped that a detailed study of X-ray bursts can be used to determine the mass and radius of the neutron star, via the relation between luminosity, effective temperature and flux, and via the changes in the general relativistic correction to this relation when the atmosphere expands from the neutron star surface to a larger radius. However, the physics of the X-ray burst is complex. There is evidence that the emitting area does not cover the whole neutron star and changes with the accretion rate. Reviews of the theory of X-ray bursts are given by Bildsten (1998, 2000).

In this paper we describe a long flux enhancement that we observed with the Wide Field Cameras of BeppoSAX in the X-ray burst source 4U 1735-44, and argue that this event is the longest type I X-ray burst ever observed. In Sect. 2 we describe the observations and data extraction, in Sect. 3 the properties of the flux enhancement. A discussion and comparison with earlier long bursts is given in Sect. 4. In the remaining part of this section we briefly describe earlier observations of 4U 1735-44.

4U 1735-44 is a relatively bright low-mass X-ray binary. Smale et al. (1986) fit EXOSAT data in the 1.4-11 keV range with a power law of photon index 1.8 with an exponential cutoff above 7 keV, absorbed by an interstellar column [FORMULA]. The flux in the 1.4-11 keV range is [FORMULA]. Van Paradijs et al. (1988) show that a sum of thermal bremsstrahlung of [FORMULA] keV and black body radiation of [FORMULA] keV, absorbed by an interstellar column [FORMULA], adequately describes EXOSAT data in the same energy range and at a similar flux level, obtained one year later. A similar spectrum, with a higher absorption column [FORMULA], fits the Einstein solid-state spectrometer and monitor proportional counter data (Christian & Swank 1997). During GINGA observations, the source was somewhat brighter, at [FORMULA] in the 1-37 keV range (Seon et al. 1997).

Bursts were detected at irregular time intervals during each of the five occasions in 1977 and 1978 that SAS-3 observed 4U 1735-44, leading to a total of 53 detected bursts (Lewin et al. 1980). EXOSAT detected one burst in 1984 (Smale et al. 1986) and five bursts during a continous 80 hr observation in 1985 (Van Paradijs et al. 1988), one rather bright burst was detected with GINGA in 1991 (Seon et al. 1997), and five X-ray bursts with RXTE in 1998 (Ford et al. 1998). Burst intervals range from about 30 minutes to more than 50 hrs. Three of the bursts observed with EXOSAT and the single burst observed with GINGA were radius expansion bursts (Damen et al. 1990, Seon et al. 1997), and have been used to determine the distance to 4U 1735-44 as about 9.2 kpc (Van Paradijs and White 1995).

4U 1735-44 was the first X-ray burster for which an optical counterpart was found: V926 Sco (McClintock et al. 1977). From optical photometry an orbital period of 4.65 hrs was derived (Corbet et al. 1986).

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000