The original estimate of the remnant's age
Two different methods were used to estimate the age of SN N157B (which is
also the age of the pulsar). In the first method, X-ray images of the
outer shock of the remnant were compared to models of supernova remnant
evolution. By matching observed characteristics of the remnant with model
features, the scientists estimated the age of the remnant to be about
5,000 years. (see the original article by Wang et al. in Volume
494 of the Astrophysical Journal). This estimate of the pulsar's age is
fairly uncertain, so scientists were happy to find a pulsar that allowed
another calculation of the age.
Another Estimate of the Age of the Remnant
The discovery of the pulsar in N157B allows another way of estimating the
age, this time by finding the age of the pulsar that formed at the same
time as the remnant (for the original article, see Marshall et
al., Volume 499 of the
Astrophysical Journal Letters). In this technique, data from the RXTE and
satellites were used to calculate the age of the pulsar. This calculation
relies on the fact that the pulsar is slowing down over time. The
observed quantities of the spin period, P, and the rate that period is
changing as the pulsar spins down, Pdot, can be used to calculate a
'characteristic age at the present time'. This technique is derived in
detail in the book "Black Holes, White Dwarfs, and Neutron Stars", by
Shapiro and Teukolsky (p.278-279).
The scientists calculated the age in this way to be 5,000 yrs.
The fact that these two techniques give about the same answer for the age
of the pulsar (and of the remnant) makes the scientists confident that
this age is in the right ball park. At an age of 5 thousand years,
the remnant is a fairly young one....in astronomical terms!