Authors: Slavko Bogdanov, Adam T. Deller, James C. A. Miller-Jones, Anne M. Archibald, Jason W. T. Hessels, Amruta Jaodand, Alessandro Patruno, Cees Bassa & Caroline D’Angelo
First Author’s Institution: Columbia Astrophysics Laboratory, Columbia University, New York, NY
Status: Submitted to ApJ, open access
What’s more interesting than a rapidly spinning neutron star that emits electromagnetic radiation parallel to it’s magnetic poles? One that doesn’t exactly behave as expected of course. This weirdly acting pulsar, PSR J1023+0038 is a transitional millisecond pulsar (tMSP) which is fancy speak for a pulsar with a millisecond or so rotational period that switches between radio and x-ray emission on a several year timescale. However emitting in both x-ray and radio on these longer timescales isn’t what piques the interest of astronomers in the case of this astrobite.
Weird Pulsar Behavior
Beyond the basic idea of a pulsar, they typically can fall into one of the following categories. Radio pulsars are powered by exchanging rotational energy from the spinning neutron star into emitting radiation. This means that their rotation slows and their pulse length increases. Meanwhile, x-ray pulsars are accretion powered, meaning they turn infalling matter that is heated up into x-ray emission. What distinguishes PSR J1023+0038 from the background of pulsars that switch between accretion powered x-ray and rotation powered radio pulsars is that it has a simultaneous anti-correlated x-ray and radio emission. The authors looked at about 5 hours of overlapping and concurrent observations from the Chandra X-ray Observatory and the Very Large Array (VLA) to try and understand this weird relationship between the x-ray and radio emissions. This is very clearly shown in Fig. 1 where we can see a tiny sample of time of overlapping x-ray and radio flux measurements. The anti-correlation is quite strong, meaning that when the x-ray emissions are weakest, the radio emission is strongest.
But wait…there’s more! When we zoom out on the flux/time series observations (Fig. 2) we can not only see that the anti-correlation is persistent, but we can also see that the x-ray emission has at least 3 unique modes of operation. The author’s classify these x-ray emission modes as (1) sporadic flaring (~12.7 hrs in Fig. 2), (2) high, and (3) low modes. The difference between high and low in this context is the magnitude of the luminosity.
Trying to explain away the strangeness
This complex and weird behavior unfortunately does not come with an easy or readily available answer. What we know about pulsars and how we can model pulsar accretion doesn’t shed any new light on the situation. The authors do suggest that the switching between high and low modes might occur due to a changing unstable magnetosphere. They also propose that the increase in radio emission can be explained by an outflow of plasma which emits synchrotron radiation as it travels. Additionally when comparing PSR J1023+0038 to a low mass x-ray emitting binary black hole (BH LMXB) (Fig. 3), we can see that the low mode of this tMSP falls into the binary BH region. This is unusual because there is a pretty clear separation between the x-ray/radio luminosity relationship of neutron stars and BHs. Knowing this now, it may call into question whether some BH LMXBs could have been misidentified.
Now you may be asking, “so what did we actually discover?”, which is a completely valid question. Well for one we did learn that there in fact does exist strange and unique pulsars that exhibit odd behavior. But the more exciting result is that we may not have a great understanding of pulsars in general. This is exciting because it can spur new astrophysical theories and models; ones that can more generally explain even the weirdest behaviors. Like most of astronomy (and science in general) however, before we can fully claim any specific mechanism for causing the anti-correlated x-ray and radio emissions and the switching between emission modes, we’ll probably need more observations.