We start with a story from Monash University in Australia. Graduate student Adelle Goodwin has made the first-ever start-to-finish observations of a cataclysmic variable star. This system consists of a neutron star, a regular companion star, and a whole lot of gravity that is tearing material off the companion star and pulling it into a disk around the neutron star. As this disk grows, it will dump material onto the surface of the neutron star. This isn’t a continuous process but occurs in spurts. For decades, folks have tried to catch these systems at the moment an outburst begins, and campaigns involving amateur astronomers monitoring these variables have been common. Despite all these efforts, no one had been able to acquire detailed observations across multiple-wavelengths… at least not until now.
In a new paper in the Monthly Notices of the Royal Astronomical Society, this team describes the behavior of a system captured at the moment the outburst. Over the course of twelve days, they performed high sensitivity optical and X-Ray observations as material swirled inward to collide with the neutron star surface. This twelve-day duration was the first of their discoveries! It had previously been thought that outbursts consistently lasted two to three days! According to Goodwin, “Using multiple telescopes that are sensitive to light in different energies we were able to trace that the initial activity happened near the companion star, in the outer edges of the accretion disk, and it took twelve days for the disk to be brought into the hot state and for material to spiral inward to the neutron star, and X-rays to be produced”.
This science required five ground- and two space-based telescopes. This arrangement gave continuous coverage over many different wavelengths of light. This system is 11,000 lightyears away, and the neutron star is a special kind of neutron star – a pulsar that rotates 400 times a second!
This theory defying slow burn and burst pointed toward something being out of spec with this system. Thanks to the tremendous data set they acquired, it was possible to measure the composition of the material in the disc, and they discovered it had a much higher helium content than has been observed in other systems. While most accretion disks are primarily hydrogen, this disc was measured to be 50% helium, a gas that burns at a hotter temperature. Just as it can take longer to preheat your oven to higher temperatures, it takes longer to preheat a disk to detonation temperatures. This twelve-day duration reflects the higher temperature needed to trigger the helium.
While this paper largely focuses on the observations, we expect more to come as Adelle finishes her Ph.D. and, we hope, more systems receive this kind of intensive observation.
“The Binary Evolution of SAX J1808.4-3658: Implications of an Evolved Donor Star,” A. J. Goodwin & T. E. Woods, 2020 May 6, Monthly Notices of the Royal Astronomical Society (Preprint on arxiv.org)