Radio transients from the accretion-induced collapse of white dwarfs

Abstract

It has long been expected that in some scenarios when a white dwarf (WD) grows to the Chandrasekhar limit, it can undergo an accretion-induced collapse (AIC) to form a rapidly rotating neutron star. Nevertheless, the detection of such events has so far evaded discovery, likely because the optical, supernova-like emission is expected to be dim and short-lived. Here we propose a novel signature of AIC: a transient radio source lasting for a few months. Rapid rotation along with flux freezing and dynamo action can grow the WD's magnetic field to magnetar strengths during collapse. The spin-down of this newly born magnetar generates a pulsar wind nebula (PWN) within the ∼10 -3-10-1 M⊙ of ejecta surrounding it. Our calculations show that synchrotron emission from the PWN may be detectable in the radio, even if the magnetar has a rather modest magnetic field of ∼2 × 1014 G and an initial spin period of ∼10 ms. An all-sky survey with a detection limit of 1 mJy at 1.4 GHz would see ∼4(f/10 -2) above threshold at any given time, where f is the ratio of the AIC rate to Type Ia supernova rate. A similar scenario may result from binary neutron stars if some mergers produce massive neutron stars rather than black holes. We conclude with a discussion of the detectability of these types of transient radio sources in an era of facilities with high mapping speeds. © 2013. The American Astronomical Society. All rights reserved.