IGR J17480-2446: a new class of accreting binaries?

Abstract

Context. The recent discovery of long-period, low magnetic field pulsars in low-mass X-ray binaries (LMXBs) represents a challenge for the standard evolutionary scenario. These pulsars have a magnetic field strength comparable to that of millisecond pulsars (~10<SUP>8</SUP>-10<SUP>9</SUP> G), but their period is at least an order of magnitude longer. <BR /> Aims: We discuss the origin of this new class of pulsars within the standard picture of LMXBs formation and apply our results to the case of IGR J17480-2446. <BR /> Methods: The magnetothermal evolution of the binary system is studied numerically by taking into account the effect of different accretion rates during the Roche-lobe overflow in the framework of the minimal cooling scenario. <BR /> Results: We show that, in addition to standard millisecond pulsars, long-period low magnetic field pulsars should also be expected as a possible outcome of the binary evolution, depending on the strength of the accretion rate during the Roche-lobe overflow. In particular, we argue that IGR J17480-2446 belongs to this new class of objects.