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Astron. Astrophys. 329, 965-970 (1998)

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1. Introduction

KS 1731-260 was discovered on October 1988 by the coded-mask imaging spectrometer TTM on the MIR-KVANT Observatory (Sunyaev et al. 1989; Sunyaev et al., 1990a,b). It is located in the Galactic center region; about five degrees from the Galactic nucleus ([FORMULA] 600 pc), and very close to the X-ray binary pulsar GX1+4 (less than 1 degree). In August 1989, TTM observed KS 1731-260 for 15 days while the source intensity varied from 50 to 100 mCrab in the 2-27 keV band (average around 80 mCrab). The corresponding time-averaged spectrum was best fit by a thermal Bremsstrahlung model of temperature (kT) [FORMULA] keV absorbed through a column density (NH) of [FORMULA]  H atoms cm-2. The averaged 1-20 keV flux was then [FORMULA] ergs s-1 cm-2  (Sunyaev et al., 1990a, In't Zand, 1992). Several X-ray bursts with intensities up to 1.2 Crab were also recorded during the same TTM observations (Sunyaev et al. 1990a, In't Zand, 1992). Since KS 1731-260 was not known prior to TTM, in particular it had not been detected by experiments such as UHURU, HEAO2, Ariel V, it was naturally classified as a soft X-ray transient (Sunyaev et al., 1990a).

However, following the TTM observations, the source was detected twice in 1990 (April 4 and August 23) by the ART-P X-ray telescope aboard GRANAT at a luminosity-level similar to that observed by TTM (Sunyaev et al., 1990c). KS 1731-260 was subsequently detected during the GINGA/LAC scans of the Galactic center region performed from March 1988 through March 1990 (Yamauchi and Koyama 1990) and during the ROSAT all sky survey (Predehl and Schmitt 1995, see below). On March 14, 1992, the source was detected by the SIGMA telescope at energies above 30 keV with a 40-150 keV luminosity of [FORMULA] ergs s-1 (at 8.3 kpc). KS 1731-260 then became one of the first X-ray bursters and hence neutron star systems detected in hard X-rays (Barret et al., 1992). Finally KS 1731-260 is also present in the KVANT-TTM maps of the Galactic Center region obtained in May 1994 (Aleksandrovitch et al. 1995), and September 26-28 1994 (Borozdin et al. 1995). All these detections spanning over six years clearly argued against the pure transient nature of the source. Not surprisingly KS 1731-260 is now being monitored by the All Sky Monitor (ASM) aboard the ROSSI X-ray Timing Explorer and appears as a persistent source in the 2 to 12 keV band (RXTE). Recently in a short pointed observation performed with the RXTE/Proportional Counter Array (PCA), a periodic signal at 524 Hz was detected during the peak of a type I X-ray burst. This result was interpreted as KS 1731-260 containing a rapidly rotating neutron star with a spin period of 1.91 milliseconds (Morgan and Smith 1996). Furthermore this burst showed photospheric radius expansion (Smith et al. 1997). Assuming that the burst peak luminosity corresponds to the Eddington limit for a 1.4 [FORMULA]  neutron star, Smith et al. (1997) derived a distance of [FORMULA] kpc for KS 1731-260. For the persistent X-ray emission, the PCA spectrum was found to be very similar to the TTM spectrum (a fit by a thermal Bremsstrahlung yields a temperature of 5.5 keV), the main difference being that the NH observed by the PCA ([FORMULA]  H atoms cm-2) is three times larger than the one observed by TMM.

Using the TTM error box (1 arcmin radius) Cherepashchuk et al. (1994) proposed two counterparts for KS 1731-260: the first one is a 18th B magnitude (the bluest object they found in the error box on the POSS E chart), the second one is a variable star designated as object No 2547 in Terzan and Gossett (1982). The latter lies also inside the error box of the IRAS source IRAS 1731-2604.

Having some doubts about the transient nature of the source, we proposed KS 1731-260 for a short ROSAT HRI observation in order to accurately determine its position and to test the optical counterparts proposed by Cherepashchuk et al. (1994). In this paper, we first present results from a refined analysis of the ROSAT PSPC all sky survey data (Sect. 2.1), already used by Predehl and Schmitt (1995) for their study of scattering halos around bright X-ray sources. Then we report on the results of our HRI observation (Sect. 2.2). We describe in Sect. 2.3 the results of an infrared investigation conducted in the ROSAT HRI error box. Finally in Sect. 3 we show that the HRI position rules out the two proposed optical counterparts, and discuss on the nature of the source in view of our recent infrared observations.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997