The Curious Case of SGRs

A star which ends with a bang, a supernova, leaves behind substantial evidence – a supernova remnant (SNR). The SNR will include a black hole if the progenitor star had a sufficiently high mass (at least 20 times the mass of the sun). Stars with less than the requisite mass will leave a neutron star behind.

A typical neutron star is 1.5 times as massive as the Sun but only 20 km in diameter. As implied by the name, the star consists mostly of neutrons (Chandra/Harvard Website). Their progenitor stars were rotating and now, like the ice skater performing a spin and pulling in their arms, they are spinning very fast – several times a second.

The fast rotational speed contributes to a magnetic dynamo effect. Radiation is beamed along the axis of the magnetic axis – close to but not quite the same as the axis of rotation. This beamed radiation may point in our direction at times and we see it as a pulse of electromagnetic radiation. It can be in almost any part of the EMR spectrum from radio waves up to gamma-rays. We call these stars pulsars. They were originally called LGM for Little Green Men because the period of time between pulses, seconds down to milliseconds, was so constant that it seemed they must be artificial. There are more than 1000 known pulsars.

Nearly all pulsars are detected in suboptical wavelengths: microwaves and radio waves. A few are higher but it must require a lot of energy to emit at the most energetic wavelengths. These higher energy pulsars are found in the optical wavelengths, X-rays and gamma-rays. Some are called DINs for Dim Isolated Neutron stars that may emit in optical wavelength. Another group is called the Anomalous X-ray Pulsars or AXPs. And then there are those that emit in gamma-rays, the Soft Gamma-Ray Repeaters or SGRs.

All of these higher energy pulsars appear to have extremely strong magnetic fields, possibly 1000 times stronger than the already strong fields around normal neutron stars. The strongest magnetic fields created in a laboratory is 10^5 gauss. Anything stronger and you will destroy the magnet and probably the entire laboratory. A magnetar has fields of at least 10^14 gauss. Perhaps 1 in 10 neutron stars wind up in the magnetar state. It is also possible that a neutron star may flip between being a typical pulsar and a magnetar.

They are also quite rare. There are 4 confirmed SGRs discovered so far and two other candidates are still being analyzed. About 10 AXPs have been discovered. DINs are a new category but an object named SWIFT J195509+261406 was recently discovered. This is the first star that acts somewhat like an SGR/AXP but in the lower energy levels. It could be an object that lies between SGRs/AXPs and DINs. After the optical bursts, some infrared bursts were detected and then the object ceased any bursting behavior. There are no signs of a SNR or massive stellar cluster near this object. (ESO Website)

But the history of SGRs is also quite strange. We are not surprised by the fact that astronomers found one, then a couple, then a few, then dozens of extrasolar planets. We kind of expect discoveries to be done this way – find one, perhaps by mistake – then find many. This has also been true for pulsars, black holes, gamma-ray bursts and supernovae – all things that would seem to be somewhat related to SGRs. But the first SGR was found in 1979 by the Russian Venera spacecraft that was then en route to Venus. Two other SGRs would be found that year. No other SGRs would be confirmed until 1998 and none have been confirmed since then.

One hypothesis regarding SGRs is that they are an older form of AXPs. Although gamma-rays are more energetic than X-rays, it might be the case that emitting gamma-rays is something a magnetar does just before it loses that extra strength magnetic field - a dying gasp if you will.

What evidence is there that SGRs are older than AXPs? First, the SGRs have been found farther away from an SNR, indicating that more time has elapsed between the supernova event and when the SGR is found. Second, the SGRs rate of rotation is slowing more quickly than is the case for AXPs. It is thought that the rotation speed decreases more quickly for older pulsars. Third, SGRs have a greater propensity for briefly flaring up - perhaps another indication of that last gasp of a magnetar. One such flare caused a significant disturbance in the earth’s ionosphere. (Paper by Mandea and Balsis)

And, of course, the rarity of SGRs might be an indication that it is just a transient state along the road to stellar senescence. But the number of such objects is so small that it is hard to draw solid conclusions.