Formation of Sub-millisecond Pulsars and Possibility of Detection

TL;DR

This paper explores the formation of sub-millisecond pulsars, especially quark stars formed via accretion-induced collapse, and discusses their potential detectability and distinguishing features from neutron stars.

Contribution

It introduces a new formation mechanism for sub-millisecond quark stars through accretion-induced collapse and compares their spin periods with those of neutron stars.

Findings

Almost all newborn quark stars could have initial spin periods around 0.1 ms.

Newborn quark stars can have high spin-down luminosity suitable for pair production.

Quark stars formed via AIC can have periods ≤ 0.5 ms, shorter than neutron stars formed by accretion spin-up.

Abstract

Pulsars have been recognized as normal neutron stars, but sometimes argued as quark stars. {\it Sub-millisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars.} We focus on the formation of such sub-millisecond pulsars in this paper. A new approach to form a sub-millisecond pulsar (quark star) via accretion induced collapse (AIC) of a white dwarf is investigated here. Under this AIC process, we found that: (1) almost all the newborn quark stars could have an initial spin period of $\sim 0.1$ ms; (2) the nascent quark stars (even with a low mass) have sufficiently high spin-down luminosity and satisfy the conditions for pair production and sparking process to be as sub-millisecond radio pulsars; (3) in most cases, the timescales of newborn quark stars in the phase of spin period $< 1$ (or $<0.5$) ms can be long enough to be detected. As a comparison, an accretion spin-up process (for both neutron and quark stars) is also investigated. It is found that, quark stars formed through AIC process can have shorter periods ($\leq$ 0.5 ms); while the periods of neutron stars formed in accretion spin-up process must be longer than 0.5ms. Thus if a pulsar with a period less than 0.5 ms can be identified in the future, it should be a quark star.