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

Abstract

It is now commonly believed that soft gamma-ray repeaters (SGRs) and anomalous X-raypulsars (AXPs) are magnetars-neutron stars powered by their magneticfields. 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, famous forthe intense burst of 1979 March 5. The source is unresolved at theresolution of Chandra. Restricting observations to a period range around 8s, the period noted in the afterglow of the burst of 1979 March 5,we find evidence for a similar periodicity in two epochs of dataobtained 20 months apart. The secular period derivative based onthese two observations is 6.6(5)�10 -11 s s -1,similar to the period derivatives of the magnetars. As is the case withother magnetars, the spectrum is best fitted by a combination of ablackbody and a power law. However, quite surprisingly, the photonindex of the power-law component is Gamma~3-intermediate to those ofAXPs and SGRs. This continuum of Gamma led us to suggest that theunderlying physical parameter that differentiates SGRs from AXPs ismanifested in the power-law component. Two decades ago, SGR 0526-66 wasa classical SGR, whereas now it behaves like an AXP. Thus, it ispossible that the same object cycles between the SGR and AXP states.We speculate that the main difference between AXPs and SGRs is thegeometry of the B fields, and this geometry is time dependent. Finally,given the steep spectrum of RX J052600.3-660433, the total radiatedenergy of RX J052600.3-660433 can be much higher than traditionallyestimated. If this energy is supplied by the decay of the magnetic field,then the inferred B field of RX J052600.3-660433 is in excess of10 15 G, the traditional value for magnetars. Independentof this discussion, there could well be a class of neutron stars,10 14 G 15 G, which are neither radio pulsars nor magnetars.