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-ray pulsars (AXPs) are magnetars-neutron stars powered by their magnetic fields. However, what differentiates these two seemingly dissimilar objects is, at present, unknown. We present Chandra observations of RX J052600.3À660433, the quiescent X-ray counterpart of SGR 0526À66, famous for the intense burst of 1979 March 5. The source is unresolved at the resolution of Chandra. Restricting observations to a period range around 8 s, the period noted in the afterglow of the burst of 1979 March 5, we find evidence for a similar periodicity in two epochs of data obtained 20 months apart. The secular period derivative based on these two observations is 6:6ð5Þ Â 10 À11 s s À1 , similar to the period derivatives of the magnetars. As is the case with other magnetars, the spectrum is best fitted by a combination of a blackbody and a power law. However, quite surprisingly, the photon index of the power-law component is À $ 3-intermediate to those of AXPs and SGRs. This continuum of C led us to suggest that the underlying physical parameter that differentiates SGRs from AXPs is manifested in the power-law component. Two decades ago, SGR 0526À66 was a classical SGR, whereas now it behaves like an AXP. Thus, it is possible that the same object cycles between the SGR and AXP states. We speculate that the main difference between AXPs and SGRs is the geometry of the B fields, and this geometry is time dependent. Finally, given the steep spectrum of RX J052600.3À660433, the total radiated energy of RX J052600.3À660433 can be much higher than traditionally estimated. If this energy is supplied by the decay of the magnetic field, then the inferred B field of RX J052600.3À660433 is in excess of 10 15 G, the traditional value for magnetars. Independent of this discussion, there could well be a class of neutron stars, 10 14 GdBd10 15 G, which are neither radio pulsars nor magnetars.

Physics of strongly magnetized neutron stars

“…Fits to the quiescent magnetar spectra with blackbody or with crude atmosphere models indicate that the thermal X-rays can be attributed to magnetar surface emission at temperatures of (3-7) ×10 6 K (see, e.g., Juett et al 2002;Tiengo et al 2002;Patel et al 2003;Kulkarni et al 2003;Tiengo et al 2005). One of the intriguing puzzles is the absence of spectral features (such as ion cyclotron line around 1 keV for typical magnetar field strengths) in the observed thermal spectra.…”

Anomalous X-ray pulsars: long-term monitoring and soft-gamma repeater like X-ray bursts

“…Specifically, the latter have similar pulse periods, are spinning down (Kouveliotou et al, 1998;Kouveliotou et al, 1999), and have X-ray spectra that are comparable to, though somewhat harder than, those of the AXPs, at least when not in outburst (Mereghetti et al, 2000;Kulkarni et al, 2002). Independent evidence for the ultra-high magnetic fields exists in SGRs; for example, a ∼ 10 15 G magnetic field is required to confine the radiation that is seen following major outbursts (Thompson and Duncan, 1995).…”

Unveiling the Multi-wavelength Phenomenology of Anomalous X-ray Pulsars

“…Taken at face value, these data suggest that AXPs evolve into SGRs. However, this hypothesis has at least two severe problems (Kulkarni et al 2003). First, the rotational periods of SGRs are similar to those of AXPs, about 10-s. Second, inferred magnetic field strengths of SGRs are similar to (and perhaps even larger than) those of AXPs (Mereghetti et al 2002).…”