The rotation-powered nature of some soft gamma-ray repeaters and anomalous X-ray pulsars
1 Div. Astrofísica, Instituto Nacional de Pesquisas Espaciais, Av. dos Astronautas 1758, 12227–010 São José dos Campos, Brazil
2 Dipartimento di Fisica and ICRA, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy
3 ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy
4 Universidade do Estado de Santa Catarina, Av. Madre Benvenuta, 2007 Itacorubi, 88. 035–901 Florianópolis, Brazil
5 Instituto Tecnológico de Aeronáutica, Praça Marechal Eduardo Gomes, 50 Vila das Acácias, 12228–900 São José dos Campos, Brazil
6 ICRANet-Rio, Centro Brasileiro de Pesquisas Físicas, Rua Dr. Xavier Sigaud 150, 2290–180 Rio de Janeiro, Brazil
Received: 12 August 2016
Accepted: 6 December 2016
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, Ėrot, is lower than the observed luminosity (dominated by the X-rays) LX 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, Ėrot, 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, LX/Ėrot, 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 LX computed in the soft (2–10 keV) X-ray band. We compute the range of NS masses for which LX/Ėrot< 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 LX/Ėrot > 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 LX/Ėrot< 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.
Key words: pulsars: general / stars: rotation / stars: neutron / stars: magnetic field
© ESO, 2017