Evidence for a Multipolar Magnetic Field in SGR J1745-2900 from X-Ray Light-curve Analysis
Rafael C. R. de Lima, Jaziel G. Coelho, Jonas P. Pereira, Claudia V. Rodrigues, Jorge A. Rueda
Abstract
Abstract 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.