Magnetic Dipole Moment of SGRs and AXPs Described as Massive and Magnetic White Dwarfs

Abstract

The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in the recent years. They are understood as neutron stars (NSs) with super strong magnetic fields, namely $B\gtrsim10^{14}$ G. However, in the last two years two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected. Moreover, three fast and very {\it magnetic} white dwarfs (WDs) have also been observed in the last years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole moment, characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. The pulsar magnetic dipole moment $m$ depending only on the momentum of inertia $I$, and observational properties, such as the period $P$ and its first time derivative $\dot{P}$, can help to identify the scale of $I$ for SGRs/AXPs. We analyze the pulsar magnetic dipole moment $m$ of SGRs and AXPs when a model based on a massive fast rotating highly magnetized white dwarf is considered. We show that the values for $m$ obtained for several SGRs and AXPs are in agreement with the observed range $10^{34}{\rm emu}\leq m \leq10^{36}{\rm emu}$ of isolated and polar magnetic white dwarfs. This result together with the fact that for {\it magnetic} white dwarfs $B\sim(10^6-10^8)$ G their magnetic dipole moments are almost independent of the star rotation period ($10^{4}\lesssim P \lesssim10^{6} {\rm s}$) - a phenomenology not shared by neutron stars pulsars - suggests a possible {\it magnetic} white dwarf nature for some of SGRs/AXPs that have much smaller periods ($P\sim 10$ s).