SGR J1745-2900 was detected from its outburst activity in 2013 April and it was the first soft gamma repeater (SGR) detected near the center of the Galaxy (Sagittarius A*). We use 3.5 yr Chandra X-ray light-curve data to constrain some neutron star (NS) geometric parameters. We assume that the flux modulation comes from hot spots on the stellar surface. Our model includes the NS mass, radius, a maximum of three spots of any size, temperature and positions, and general relativistic effects. We find that the light curve of SGR J1745-2900 could be described by either two or three hot spots. The ambiguity is due to the small amount of data, but our analysis suggests that one should not disregard the possibility of multi-spots (due to a multipolar magnetic field) in highly magnetized stars. For the case of three hot spots, we find that they should be large and have angular semiapertures ranging from 16° to 67°. The large size found for the spots points to a magnetic field with a nontrivial poloidal and toroidal structure (in accordance with magnetohydrodynamics investigations and Neutron Star Interior Composition Explorer’s (NICER) recent findings for PSR J0030+0451) and is consistent with the small characteristic age of the star. Finally, we also discuss possible constraints on the mass and radius of SGR J1745-2900 and briefly envisage possible scenarios accounting for the 3.5 yr evolution of SGR J1745-290 hot spots.

Context. Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slow rotating isolated pulsars whose energy reservoir is still matter of debate. Adopting neutron star (NS) fiducial parameters; mass M = 1.4 M ⊙, radius R = 10 km, and moment of inertia, I = 1045 g cm2, the rotational energy loss, Ė rot, is lower than the observed luminosity (dominated by the X-rays) L X for many of the sources.Aims. We investigate the possibility that some members of this family could be canonical rotation-powered pulsars using realistic NS structure parameters instead of fiducial values.Methods. We compute the NS mass, radius, moment of inertia and angular momentum from numerical integration of the axisymmetric general relativistic equations of equilibrium. We then compute the entire range of allowed values of the rotational energy loss, Ė rot, for the observed values of rotation period P and spin-down rate Ṗ. We also estimate the surface magnetic field using a general relativistic model of a rotating magnetic dipole.Results. We show that realistic NS parameters lowers the estimated value of the magnetic field and radiation efficiency, L X/Ė rot, with respect to estimates based on fiducial NS parameters. We show that nine SGRs/AXPs can be described as canonical pulsars driven by the NS rotational energy, for L X computed in the soft (2–10 keV) X-ray band. We compute the range of NS masses for which L X/Ė rot < 1. We discuss the observed hard X-ray emission in three sources of the group of nine potentially rotation-powered NSs. This additional hard X-ray component dominates over the soft one leading to L X/Ė rot > 1 in two of them.Conclusions. We show that 9 SGRs/AXPs can be rotation-powered NSs if we analyze their X-ray luminosity in the soft 2–10 keV band. Interestingly, four of them show radio emission and six have been associated with supernova remnants (including Swift J1834.9-0846 the first SGR observed with a surrounding wind nebula). These observations give additional support to our results of a natural explanation of these sources in terms of ordinary pulsars. Including the hard X-ray emission observed in three sources of the group of potential rotation-powered NSs, this number of sources with L X/Ė rot < 1 becomes seven. It remains open to verification 1) the accuracy of the estimated distances and 2) the possible contribution of the associated supernova remnants to the hard X-ray emission.

There is solid observational evidence on the existence of massive, M ∼ 1 M_⊙, highly magnetized white dwarfs (WDs) with surface magnetic fields up to B ∼ 10⁹ G. We show that, if in addition to these features, the star is fast rotating, it can become a rotation-powered pulsar-like WD and emit detectable high-energy radiation. We infer the values of the structure parameters (mass, radius, moment of inertia), magnetic field, rotation period and spin-down rates of a WD pulsar death-line. We show that WDs above the death-line emit blackbody radiation in the soft X-ray band via the magnetic polar cap heating by back flowing pair-created particle bombardment and discuss as an example the X-ray emission of soft gamma-repeaters and anomalous X-ray pulsars within the WD model.

The effect of strong magnetic fields on the properties of the pasta structure is calculated within a Thomas-Fermi approach using relativistic mean-field models to modulate stellar matter. It is shown how quantities such as the size of the clusters and Wigner-Seitz cells, the surface tension, and the transition between configurations are affected. It is expected that these effects may give rise to large stresses in the pasta phase if the local magnetic field suffers fluctuations.DOI: http://dx.doi.org/10.1103/PhysRevC.88.0358046 MoreReceived 19 June 2013Published 13 September 2013©2013 American Physical Society