## Finding the Age of Supernova Remnant N157B

### 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 ASCA 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.

### Conclusion

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!