Magnetic field decay in neutron stars: Analysis of general relativistic effects

Abstract

An analysis of the role of general relativistic effects on the decay of neutron star's magnetic field is presented. At first, a generalized induction equation on an arbitrary static background geometry has been derived and, secondly, by a combination of analytical and numerical techniques, a comparison of the time scales for the decay of an initial dipole magnetic field in flat and 1 curved spacetime is discussed. For the case of very simple neutron star models, rotation not accounted for and in the absence of cooling effects, we find that the inclusion of general relativistic effects result, on the average, in an enlargement of the decay time of the field in comparison to the flat spacetime case. Via numerical techniques we show that, the enlargement factor depends upon the dimensionless compactness ratio ǫ = 2GM c 2 R , and for ǫ in the range (0.3 , 0.5), corresponding to compactness ratio of realistic neutron star models, this factor is between 1.2 to 1.3. The present analysis shows that general relativistic effects on the magnetic field decay ought to be examined more carefully than hitherto. A brief discussion of our findings on the impact of neutron stars physics is also presented.

Are pulsars born with a hidden magnetic field?

“…This estimate has interesting implications: since it is likely that most neutron stars can undergo such an accretion process, and the field would only reemerge after a few thousand years (Geppert et al 1999;Viganò & Pons 2012), the CCO scenario is actually not peculiar at all and we expect that most very young NSs show actually an anomalously low value of the magnetic field. On the contrary, magnetar-like field strengths are much harder to screen and the required accreted mass is very large, in some cases so large that the neutron star would collapse to a black hole.…”

“…An alternative explanation is the hidden magnetic field scenario (Young & Chanmugan 1995;Muslimov & Page 1995;Geppert et al 1999;Shabaltas & Lai 2012). Following the supernova explosion and the neutron star birth, the supernova shock travels outwards through the external layers of the star.…”

“…From left to right the columns indicate the name of the CCO, the age, the distance d, the period P , the inferred surface magnetic field, Bs, the bolometric luminosity in X-rays, L x,bol , the name of the remnant, the characteristic age, and bibliographical references. References: (1) Hui & Becker (2006), (2) Gotthelf & Halpern (2013), (3) Zavlin et al (2000), (4) Mereghetti et al (2002), (5) Bignami et al (2003), (6) De Luca et al (2004), (7) , (8) Seward et al (2003), (9) Gotthelf et al (2005), (10) Halpern et al (2003), (11) Halpern & Gotthelf (2010) & Chanmugan 1995; Muslimov & Page 1995;Geppert et al 1999) and established that the timescale for the magnetic field reemergence is ∼ 1 − 10 7 kyr, critically depending on the depth at which the magnetic field is buried. More recent investigations have confirmed this result.…”

Spin period evolution of a recycled pulsar in an accreting binary

“…thermal effects speed up Ohmic dissipation of NS crust currents, which accounts for the field decay and spin evolution Geppert et al 1999;Romani 1990). The interactions between the accreted matter and magnetic field in the course of accretion phase have been studied to account for the field decay (Melatos & Phinney 2001;Payne & Melatos 2004;Konar & Bhattcharya 1999a,b;Konar & Choudhury 2004;Lovelace et al 2005).…”

Origin of Magnetar-Scale Crustal Field in PSR J1852+0040 and ‘Frozen’ Magnetars

“…First, despite its 2 Popov young age, the NS in Kes 79 is an analogue of so-called lowfield magnetars (see a review in Turolla & Esposito 2013). Second, both components are strong, and they have been significantly submerged due to a strong fall-back episode (Geppert, Page, & Zannias 1999 called such sources 'hidden magnetars'). Below, we analyse how this can be used to put constraints on the origin of magnetars' magnetic field.…”