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Astron. Astrophys. 330, 175-180 (1998)

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1. Introduction

1E 2259+586 is an unusual pulsar with a pulsation period of 7 s and a very steep spectrum (photon index, [FORMULA], [FORMULA] 4). The source has exhibited a near constant spin-down trend at [FORMULA] 5 [FORMULA] s s-1 since its discovery. 1E 2259+586 is located in, or near, the supernova remnant (SNR) G109.1-1.0 which is also known as CTB 109. The SNR consists of an approximately hemispherical shell of X-ray and radio emission and a jet-like lobe located mid-way between the shell and the pulsar (Gregory & Fahlman 1980; Hughes et al. 1981; Morini et al. 1988; Hurford & Fesen 1995). There may be a compact (2-3 [FORMULA] radius) synchrotron nebula surrounding the pulsar (Rho & Petre 1997). The location and shape of the X-ray lobe led to speculation that it is a "jet" of material connecting the pulsar and the SNR (Gregory & Fahlman 1983). However, its thermal spectrum and detailed morphology do not support this view (Hurford & Fesen 1995; Rho & Petre 1997).

1E 2259+586 together with other sources such as 1E 1048.1-593 and 4U 0142+614 belong to a small group of pulsars with similar spin periods (around 6 s) and properties that clearly distinguish them from the "classical" pulsars in high-mass X-ray binaries (Mereghetti & Stella 1995). Koyama et al. (1987) argue that the slow spin-down rate implies that 1E 2259+586 contains an accreting neutron star rotating at close to its equilibrium period with a magnetic field of [FORMULA] [FORMULA]  G. However, there is no direct evidence that the pulsar is in a binary system. In particular, no optical or radio counterparts have been found (Coe & Jones 1992; Coe et al. 1994) and no Doppler shifts in pulse period have been detected (Koyama et al. 1989; Hanson et al. 1988; Morini et al. 1988).

The lack of an optical counterpart and orbital Doppler shifts argue against a binary models for 1E 2259+586, unless the companion has an extremely low mass. Models based on single stars have been proposed, such as spin-down of a white dwarf (Paczynski 1990; Usov 1994), loss of magnetic energy of a strongly magnetized neutron star (Thompson & Duncan 1993) and a neutron star accreting from a circumstellar disk (Corbet et al. 1995; van Paradijs et al. 1995). The detection by Mereghetti (1995) of a large change in the spin-down rate of the similar system 1E 1048.1-5937, as well as the long term pulse frequency fluctuations in 1E 2259+586 (Baykal & Swank 1996) support an accretion hypothesis.

We present a detailed study of the X-ray emission from 1E 2259+586 and G109.1-1.0 using the imaging capabilities of BeppoSAX to separate the contributions of the individual sources. This topic is particularly interesting because of the disparate spectral results reported for 1E 2259+586 as well as of the uncertain origin of the X-ray lobe in G109.1-1.0.

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998