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Astron. Astrophys. 357, 501-506 (2000)

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1. Introduction

The transient X-ray pulsar GS 1843+00 was discovered on 1988 April 3 during a galactic plane scan observation near the Scutum region by the Gingasatellite (Makino et al. 1988a; Makino et al. 1988b). The Large Area Counter (LAC, Turner et al. 1989) on board Ginga(Makino et al. 1987) detected at a J2000 position of [FORMULA], [FORMULA], a coherent pulsation with a period of 29.5 s and an 2-37 keV X-ray intensity of 50 mCrab (Koyama et al. 1990a). On 1988 April 19 and 20 Gingacarried out a pointed observation of GS 1843+00, measuring a highly variable X-ray flux on a wide range of time scales, ranging from 30 to 60 mCrab. In addition to a coherent oscillation with P[FORMULA] s, an energy-dependent aperiodic variation was found (Koyama et al. 1990b).

On 1997 March 3, the Burst and Transient Source Experiment (BATSE ) on board CGRO detected a new outburst from this peculiar source (Wilson et al. 1997). The mean 20-50 keV rms pulsed flux was [FORMULA] mCrab while the mean barycentric pulse period at an epoch of March 6.0 was P[FORMULA] s. The P variation during this observation implies a spin-up rate, [FORMULA], of [FORMULA] s s-1. The data confirmed the low pulsed fraction ([FORMULA]7%) observed by Ginga .

Between 1997 February 1 and March 19 the All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE ) observed the source to be at a flux level of [FORMULA] mCrab (Takeshima 1997).

A pointed observation carried out on 1997 March 5 with the RXTE Proportional Counter Array (PCA) detected the source at a 2-60 keV flux level of 62 mCrab, measuring a barycentric pulse period of [FORMULA] s at an epoch of March 5.1712 UT (Takeshima 1997).

On 1997 April 4 the BeppoSAXNarrow Field Instruments (NFIs) performed a pointed observation of GS 1843+00 (Piraino et al. 1998). The source flux was [FORMULA]erg cm-2s-1 in the 0.3-100 keV energy range.

Using the capability of BeppoSAXimaging instruments, the 90% confidence J2000 position of GS 1843+00 was constrained to be within a [FORMULA] radius circular error region centered on [FORMULA], [FORMULA] (Santangelo et al. 1997). On the same day, the source was also observed by the ROSAT High Resolution Imager (HRI) which found a J2000 position for the source of [FORMULA], [FORMULA] (90% confidence error radius [FORMULA], Dennerl & Greiner 1997).

In this paper we present the results of both a timing and spectroscopic analysis of the BeppoSAXobservation of GS 1843+00.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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