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 $B\gtrsim 10^{14}$ 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 $10^{34}\mathrm{emu}\le m\le 10^{36}\mathrm{emu}$ 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 ($10^4\lesssim P\lesssim 10^6\mathrm{s}$) - 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).

11 pages, 1 figure, accepted by Publications of the Astronomical Society of Japan (PASJ)

References in corpus (25)

• The strongest cosmic magnets: Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars
• A strong magnetic field around the supermassive black hole at the centre of the Galaxy
• Corona of Magnetars
• The Magnificent Seven: Magnetic fields and surface temperature distributions
• Magnetar-like Emission from the Young Pulsar in Kes 75
• NuSTAR discovery of a 3.76-second transient magnetar near Sagittarius A*
• Binary Star Origin of High Field Magnetic White Dwarfs
• Supernova remnant energetics and magnetars: no evidence in favour of millisecond proto-neutron stars
• Spectropolarimetric Survey of Hydrogen-rich White Dwarf Stars
• Magnetic White Dwarf Stars in the Sloan Digital Sky Survey
• Swift Discovery of a New Soft Gamma Repeater, SGR J1745-29, near Sagittarius A*
• Resonant cyclotron scattering in magnetars' emission
• The Compact X-ray Source 1E 1547.0-5408 and the Radio Shell G327.24-0.13: A New Proposed Association between a Candidate Magnetar and a Candidate Supernova Remnant
• An ultra-massive fast-spinning white dwarf in a peculiar binary system
• The fundamental plane for radio magnetars
• Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations
• New Close Binary Systems from the SDSS-I (Data Release Five) and the Search for Magnetic White Dwarfs in Cataclysmic Variable Progenitor Systems
• An accretion model for the anomalous X-ray pulsar 4U 0142+61
• The number of progenitors in the core - degenerate scenario for type Ia supernovae
• AXPs/SGRs: Magnetars or Quark-stars?
• Supernova remnants with magnetars: clues to magnetar formation
• A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?
• SGR 0418+5729, Swift J1822.3-1606, and 1E 2259+586 as massive fast rotating highly magnetized white dwarfs
• AE Aquarii represents a new subclass of Cataclysmic Variables
• What do exotic equations of state have to offer?

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• General Relativistic effects in the structure of massive white dwarfs
• Dynamical instability of white dwarfs and breaking of spherical symmetry under the presence of extreme magnetic fields
• Massive white dwarfs in $f(R,L_m)$ gravity
• Magnetars and White Dwarf Pulsars
• On the nature of some SGRs and AXPs as rotation-powered neutron stars
• Thermal X-ray emission from massive, fast rotating, highly magnetized white dwarfs
• Consequence of total lepton number violation in strongly magnetized iron white dwarfs
• A magnetic white-dwarf accretion model for the anomalous X-ray pulsar 4U 0142+61
• Orbital decay of double white dwarfs: beyond gravitational wave radiation effects
• Born-Infeld magnetars: larger than classical toroidal magnetic fields and implications for gravitational-wave astronomy
• An observational argument against accretion in magnetars
• Nonvalidity of I-Love-Q Relations for Hot White Dwarf Stars
• The double white dwarf merger progenitors of SDSS J2211+1136 and ZTF J1901+1458
• Gravitational waves from SGRs and AXPs as fast-spinning white dwarfs
• Time evolution of rotating and magnetized white dwarf stars
• Mass limits of the extremely fast-spinning white dwarf CTCV J2056-3014
• High-energy gamma-ray emission from SNR G57.2+0.8 hosting SGR J1935+2154
• On the optical transients from double white-dwarf mergers
• Equilibrium Configurations of Rotating White Dwarfs at Finite Temperatures
• Quasi-periodic oscillations during magnetar giant flares in the strangeon star model
• An updated view and perspectives on high-energy gamma-ray emission from SGR J1935+2154 and its environment
• Ultra-high energy cosmic rays from white dwarf pulsars and the Hillas criterion
• Prospects for the Observation of Continuous Gravitational Waves from Deformed Fast-spinning White Dwarfs
• SMWDs as SGRs/AXPs and the lepton number violation
• Data-Driven Constraints on Magnetar Population: No Evidence for a Distinct White Dwarf Channel