Extracting Information from the Gravitational Redshift of Compact Rotating Objects

Abstract

When rotation is not taken into account, the measurement of the Gravitational Redshift can provide unique information about the compactness ($M/R$) of the star. Rotation alters the gravitational redshift rendering thereby a unique determination of the compactness parameter impossible. Nevertheless, it can be shown that by using some theoretical input, useful information on, say, the radii of compact rotating objects can still be extracted. Moreover, by measuring the gravitational redshift one can infer the maximum angular velocity of the object. As it is well known, the minimum observed periods of rotation are found in millisecond pulsars. Here we show that millisecond periods are actually a semi-theoretical limit that can be found by General Relativistic arguments corresponding to the maximum angular velocity. We apply our method to compact objects such as pulsars, white dwarfs and neutron stars.