The X-ray Counterpart to SGR 0526-66: Results from a monitoring campaign of N49 with ROSAT

The Soft Gamma-Ray Repeater SGR 0526--66 is famous for its extremely bright outburst on March 5, 1979. A persistent X-ray source was previously detected within the error box coinciding with the super nova remnant N49. Here we present the result of a 12 month monitoring program of N49 with the ROSAT HRI. Four interesting results emerge from our analysis. (1) The previously identified persistent source is composed of two sources, A and B separated by 7 arcseconds. (2) The spectrum of A is softer than the spectrum of N49; T_eff<0.22 keV. (3) Source A exhibits deep modulation (40%) with a period of about 95 days. [4] Source B, the fainter source, shows no month-to-month variation but underwent a flare in 1992. We argue that source B is most likely a background AGN and source A is the X-ray counterpart of SGR 0526--66. It's unusually soft spectrum is interpreted as X-ray emission from the surface of a hot neutron star. We note that following the 5 March 1979 a series of outbursts were observed from SGR 0526--66. A period of 164 days was identified within the series of the outburst start times. Due to poor sampling this period is possibly consistent with the periodicity seen in X-rays. This long period forces us to invoke a binary model for SGR 0526--66. In our favorite model the X-ray emission is from a magnetar, a highly magnetized neutron star in an eccentric binary with an orbital period of 95 days. The X-ray emission from the magnetar is fueled by steady magnetic field decay. Then, some magnetospheric interaction during periastron passage results in additional dissipation of magnetic energy. The alternative, less plausible model is an accretion powered binary system.