Q&A: Supernova Remnants and Neutron Stars

Q:
The image you have posted of the Kepler SNR is said to be 14ly in diameter and expanding at 2 million meters per second. Is the rate of expansion slowing or has it slowed since the SN event? Is the rate constant since the event? What was the speed of expansion at the start of the event?

A:
While you have asked three simple questions, the answers are not so simple. Young supernova remnants are very complicated objects with different behaviors depending on the type of telescope you use to look at them. I'll give you an explanation based on expansion measurements with three of the world's best telescopes - the Very Large Array radio telescope in New Mexico, the Hubble Space Telescope, and the Chandra X-ray Observatory (also a space telescope in orbit around Earth).

(The recent press release for the birthday of Kepler's SNR also includes infrared images from the new Spitzer Space Telescope. Infrared motions have not been measured for Kepler's SNR as of yet but those of us who study young SNRs eagerly await a time when it will be feasible to measure infrared motions.)

Supernova remnants (SNRs) are made up of different components which have distinct motions and radiate in different wavelengths of light.

The blast wave of the explosion radiates in optical, X-ray, and radio light which is unique for SNRs. Most of the time only one or two of the three are present.

The stellar debris from the explosion, often called ejecta, radiate in X-rays and radio light. This is also unique for Kepler's SNR because the ejecta of all other young SNRs also radiates in optical light.

The material encountered by the blast wave, called circumstellar material (CSM), radiates in optical and X-ray light.

The expansion rate of 2000 km/s is a measurement of the blast wave using optical light. This measurement was made using both hydrogen spectral lines and actual sky motions.

The expansion of the CSM is also measured in optical light using actual sky motions. The CSM is only moving at 100-200 km/s. The interpretation is that this material was stellar wind material that was ejected before the star blew up. Although the blast wave has encountered this material, very little momentum was transferred and so the CSM still travels at the original wind velocity which is significantly slower than the blast wave velocity.

The expansion of the ejecta has been measured in both radio and X-ray light using sky motions. The X-ray data used was from the Einstein and ROSAT telescopes. The X-ray measurements show that the ejecta have experienced very little deceleration and are in what is called free expansion. The measurements of the radio ejecta indicate a velocity half that of the X-ray ejecta. This behavior is typical of young SNRs and it is not completely understood why this happens. Leading theories propose a density difference between the X-ray-emitting ejecta and the radio-emitting ejecta. I have used the term free expansion without indicating an actual velocity. This is because the ejecta expansions are homologous. This means that the rate of expansion is related to the radius from the explosion center. X-ray ejecta very near the edge of the SNR (radius=7 ly) are moving nearly 5000 km/s while X-ray ejecta halfway between the center and edge are moving nearly 2500 km/s.

So now to answer your questions:

1. Is the rate of expansion slowing or has it slowed since the SN event?

The blast wave has been slowed in some places and not in others. The X-ray ejecta has not been slowed while the radio ejecta has. The CSM material is moving at a slow rate consistent with stellar wind speeds and has not been accelerated or decelerated.

2. Is the rate constant since the event?

Typically, the blast wave does decelerate when CSM is encountered and it continues to decelerate as the SNR ages. The ejecta also typically decelerates. Since some parts of the blast wave and the X-ray ejecta have not decelerated yet, this indicates that Kepler's SNR is still in an early stage of evolution before significant deceleration has occured. So in some places, the rate has been constant since the explosion and in other places deceleration has occured.

3. What was the speed of expansion at the start of the event?

The blast wave was initially moving at about 5000 km/s. The ejecta speed ranged between 5000 km/s to 1000 km/s which accounts for some ejecta being near the edge and some closer to the center. The CSM was moving at 100-200 km/s when the explosion occured.