The Quiescent Counterpart of the Soft Gamma‐Ray Repeater SGR 0526−66

It is now commonly believed that Soft gamma-ray repeaters (SGRs) andAnomalous X-ray pulsars (AXPs) are magnetars -- neutron stars powered by theirmagnetic fields. However, what differentiates these two seemingly dissimilarobjects is, at present, unknown. We present Chandra observations of RXJ052600.3-660433, the quiescent X-ray counterpart of SGR 0526-66. Restrictingto a period range around 8-s, the period noted in the afterglow of the burst of5 March 1979, we find evidence for a similar periodicity in two epochs of data,obtained 20 months apart, implying a period derivative of $6.6(5)e-11$ s/swhich is similar to the period derivatives of the magnetars. The spectrum isbest fitted by a combination of a black body and a power law. However, thephoton index of the power law component, $\Gamma\sim 3$ -- intermediate tothose of AXPs and SGRs. This continuum of $\Gamma$ leads us to suggest that theunderlying physical parameter which differentiates SGRs from AXPs is manifestedin the power law component. Two decades ago, SGR 0526-66 was a classical SGRwhereas now it behaves like an AXP. Thus it is possible that the same objectcycles between SGR and AXP state. We speculate that the main difference betweenAXPs and SGRs is the geometry of the $B$-fields and this geometry is timedependent. [Abstract truncated]Comment: 7 pages, 3 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted by Ap