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

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5. Discussion

After its discovery in 1988, GS 1843+00 was detected again in 1997 March as a bright (0.3-100 keV) [FORMULA] erg cm-2s-1 X-ray source. Due to the spatial capabilities of the BeppoSAXimaging instruments an improved position was obtained. The BeppoSAXposition is within the Ginga (Koyama et al. 1990b) and RXTE (Chakrabarty et al. 1997) error boxes, and is also consistent with that measured by the ROSAT HRI (Dennerl & Greiner 1997). Accurate measurement of the position of the source is important in order to carry out a systematic search for the still unidentified optical counterpart. Pulsations with a period [FORMULA] s together a mean pulse period change [FORMULA] yr-1, which is in good agreement with the one measured by Ginga were found. Koyama et al. (1989) suggested that such a high spin up rate could be due, at least partly, to an orbital Doppler motion. Pulse period variations observed in the 30 days monitoring obtained by combining data from BATSE , RXTE-PCA and BeppoSAX, confirmed the presence of a high intrinsic spin-up rate. Moreover, assuming a Be transient system having an orbital period between 50 and 60 days, inferred from the pulse-orbital periods relation of Corbet (1986), a possible Doppler effect may be overlapped to this intrinsic spin-up rate.

The source spectrum, which is well described by an absorbed power law with high energy cut-off, is typical of accreting X-ray pulsars. The very high absorption, [FORMULA] is consistent with that reported by Koyama et al. (1990b). The hypothesis that the absorption is mainly interstellar rather then circumstellar (Koyama et al. 1990a) is supported by the marginal detection of a fluorescent [FORMULA] iron line in the source spectrum.

Assuming a distance of 10 kpc (Koyama et al. 1990a; Hayakawa et al. 1977) the 0.3-100 keV luminosity is [FORMULA][FORMULA] erg s-1.

It is unclear if cyclotron resonance scattering features are present in the hard X-ray spectrum of the source. Koyama et al. (1990b) suggested that the cut-off in the spectrum observed at [FORMULA]18 keV could be related to a very intense magnetic field typical of this class of source. Moreover, Mihara (1995), fitting the phase resolved spectrum with an absorption-like feature at [FORMULA]20 keV, classified GS 1843+00 as a possible cyclotron source. Although the spectrum is observed with good statistics up to [FORMULA]100 keV, no evidence of any cyclotron feature is observed in the BeppoSAXpulse phase averaged spectrum of GS 1843+00. Also the "crab-ratio technique"(Dal Fiume et al. 1998), successfully exploited in detecting Resonance Cyclotron Features (RCFs) in other X-ray pulsars, does not display any sign of cyclotron features. Moreover no evidence of cyclotron absorption features was found in the phase resolved spectra below 100 keV. However, we found an upper limit on the depth of 0.15 for the possible 20 keV feature. This value is compatible with that found by Mihara (1995).

Manchanda (1999), using data from the LASE experiment, a balloon-born large area scintillation counter, recently suggested the possibility of an absorption feature around 100 keV or an emission at 140 keV. Unfortunately, statistics of BeppoSAXspectra is quite low at that energy and a much deeper analysis, which is underway, is required.

There are [FORMULA]80 known accreting X-ray pulsars (see Bildsten et al. 1997for a recent review). Until recently only the relatively bright nearby pulsars were visible due to the limited sensitivity of previous detectors. This is changing with the discovery by ASCA , ROSAT , BeppoSAXand RXTE of a population of faint, absorbed pulsars (e.g., Angelini et al. 1998; Kinugasa et al. 1998, Torii et al. 1998). The search for faint pulsars is one of the main scientific objectives of the ASCA galactic plane survey (e.g., Sugizaki et al. 1997; Torii et al. 1998).

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000